STEAM POWERED ROBOTS

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                                   One of the most significant challenges in the development of an autonomous human-scale robot is the issue of power supply. Perhaps the most likely power supply/actuator candidate system for a position or force actuated human-scale robot is an electrochemical battery and dc motor combination. This type of system, however, would have to carry an inordinate amount of battery weight in order to perform a significant amount of work for a significant period of time.

A state-of-the-art example of a human scale robot that utilizes electrochemical batteries combined with dc motor/harmonic drive actuators is the Honda Motor Corporation humanoid robot model P3. The P3 robot has a total mass of 130 kg (285lb), 30 kg (66 lbs) of which are nickel-zinc batteries. These30 kg of batteries provides sufficient power for approximately 15–25 min of operation, depending on its workload. Operation times of this magnitude are common in self-powered position or force controlled human-scale robots, and represent a major technological roadblock for designing actuated mobile robots that can operate power-autonomously for extended periods of time.

1.1. Figure of Merit

Assuming that a given power supply and actuation system can deliver the requisite average and peak output power at a bandwidth required by a power-autonomous robot, three parameters are of primary interest in providing optimal energetic performance. These are the mass-specific energy density of the power source (Es), the efficiency of converting energy from the power source to controlled mechanical work(n) , and the maximum mass-specific power density of the energy conversion and/or actuation system(Ps) . A simple performance index is proposed by forming the product of these parameters

A.P = Es*n*Ps                    ------------- (1)

where A.P is called the actuation potential. Such a figure of merit is justified by the fact that a system with high power-source energy density, high conversion efficiency, and high actuator power density will be the lightest possible system capable of delivering a given amount of power and energy.    

 In the case of a battery-powered dc-motor-actuated robot, the energy density of the power source would be the electrical energy density of the battery, the conversion efficiency would be the combined efficiency of the (closed-loop controlled) dc motor and gear head, and the power density of the energy conversion and actuation system would be the rated output power of the motor/gear head divided by its mass. In the case of a gasoline-engine-powered hydraulically actuated system, the energy density of the power source would be the thermodynamic energy density of gasoline; the conversion efficiency would be the combined efficiency of the internal combustion engine(converting thermodynamic energy to shaft energy), hydraulic pump (converting shaft energy to hydraulic energy), and the hydraulic actuation system (converting hydraulic energy to controlled mechanical work); and finally, the power density of the energy conversion and actuation system would be the maximum output power of the hydraulic actuation system, divided by the combined mass of the engine, pump, accumulator, valves, cylinders, reservoir, and hydraulic fluid of the hydraulic system.

 With regard to this figure of merit, batteries and dc motors capable of providing the requisite power for a human scale robot offer reasonable conversion efficiency, but provide relatively low power-source energy density and a similarly low actuator/gear head power density. A gasoline-engine-powered hydraulically-actuated human-scale robot would provide a high power-source energy density, but a relatively low conversion efficiency and actuation system power density.

1.2. A Monopropellant Powered Approach

Liquid chemical fuels can provide energy densities significantly greater than power-comparable electrochemical batteries. The energy from these fuels, however, is released as heat, and the systems required to convert heat into controlled, actuated work are typically complex, heavy, and inefficient. One means of converting chemical energy into controlled, actuated work with a simple conversion process is to utilize a liquid monopropellant to generate a gas, which in turn can be utilized to power a pneumatic actuation system. Specifically, monopropellants are a class of fuels (technically propellants since oxidation does not occur) that rapidly decompose (or chemically react) in the presence of a catalytic material. Unlike combustion reactions, no ignition is required, and therefore the release of power can be controlled continuously and proportionally simply by controlling the flow rate of the liquid propellant. This results in a simple, low-weight energy converter system, which provides a good solution to the design tradeoffs between fuel energy density and system weight for the scale of interest.

Monopropellants, originally developed in Germany during World War II, have since been utilized in several applications involving power and propulsion, most notably to power gas turbine and rocket engines for underwater and aerospace vehicles. Modern day applications include torpedo propulsion, reaction control thrusters on a multitude of space vehicles, and auxiliary power turbo pumps for aerospace vehicles. This seminar describes the design of a monopropellant-powered actuation system appropriate for human-scale self-powered robots, and presents theoretical and experimental results that indicate the strong potential of this system for high energy density human-scale robot applications. Specifically, with regard to the figure of merit described before .The proposed approach is projected to provide a significantly greater power-source energy density and actuation power density relative to batteries and dc motors, and is projected to provide a higher conversion efficiency and significantly greater actuation system power density relative to a gasoline-powered hydraulic system.

2. DESCRIPTION OF MONOPROPELLANT ACTUATION SYSTEM

The monopropellant-powered actuation system is similar in several respects to a typical pneumatically actuated system, but rather than utilize a compressor to maintain a high-pressure reservoir, the proposed system utilizes the decomposition of hydrogen peroxide (H2O2) to pressurize a reservoir. Peroxide decomposes upon contact with a catalyst. This decomposition is a strongly exothermic reaction that produces water and oxygen in addition to heat. The heat, in turn, vaporizes the water and expands the resulting gaseous mixture of steam and oxygen. Since the liquid peroxide is stored at a high pressure, the resulting gaseous products are similarly at high pressure, and mechanical work can be extracted from the high-pressure gas in a standard pneumatic actuation fashion.

The conversion of stored chemical energy to controlled mechanical work takes place as follows. The liquid H2O2 is stored in a tank pressurized with inert gas (called a blow down tank) and metered through a catalyst pack by a solenoid-actuated control valve. Upon contact with the catalyst, the peroxide expands into oxygen gas and steam. The flow of peroxide is controlled to maintain a constant pressure in the reservoir, from which the gaseous products are then metered through a voice-coil-actuated four-way proportional spool valve to the actuator. Once the gas has exerted work on its environment, the lower energy hot gas mixture is exhausted to atmosphere.

3. MONOPROPELLANT ACTUATOR PROTOTYPE

3.1. Hardware

A prototype of the monopropellant-powered actuation system depicted in Fig. 1 was fabricated and integrated into a single degree-of-freedom manipulator, as shown in Fig. 2.
The primary objective of building the prototype was to demonstrate tracking control and to conduct experiments characterizing the actuation potential described by (1). The propellant is stored in a stainless steel blow down propellant tank, and is metered through a two-way solenoid-actuated fuel valve through a catalyst pack and into a stainless-steel reservoir. The catalyst pack consists of a 5-cm-long (2 in), 1.25-cm-diameter (0.5 in) stainless-steel tube packed with catalyst material. A pressure sensor measures the reservoir pressure for purposes of pressure regulation. The high-pressure hot gas is metered into and out of a 2.7 cm (1-1/16 in) inner diameter, 10 cm (3.9 in) stroke double-acting single-rod cylinder by a four-way spool valve, modified for proportional operation by replacing the solenoid actuator with a thermally isolated voice coil. The valve spool displacement is measured with a differential variable reluctance transducer (DVRT) in order to enable closed-loop control of the valve spool position. The pneumatic cylinder is kinematically arranged to produce a bicep-curling motion upon extension of the piston, as illustrated in Fig. 3.

3.2. Control

Control of the system is achieved using three separate control loops. The first and simplest is the pressure regulation of the reservoir. Pressure feedback from the pressure sensor switches the solenoid fuel valve with a thermostat-type on-off controller that regulates the reservoir pressure to 1515 kPa (220 psig). The second control loop provides a high-bandwidth (i.e., approximately10 Hz) position control of the valve spool. Finally, the valve spool position is commanded by an outer control loop, which controls the angular motion of the single-degree-of-freedom manipulator. The outer control loop utilizes a rotary potentiometer to provide arm angle measurement for a position, velocity, acceleration (PVA) feedback controller, which commands the valve spool position.

4. EXPERIMENTS

4.1. Load Profile

Since the actuator relies on gas as an energetic medium, and since the actuation system is not designed to utilize energy resulting from condensation of the steam (steam quality less than 100%), the energy required to vaporize the water will not be recovered and as a result the conversion efficiency is lower than if actuation system included partial condensation. The best possible efficiency would occur when partial condensation is allowed to occur within the actuator and also when the load profile of the piston is designed to allow isentropic expansion from high pressure down to the lowest pressure possible (atmospheric pressure). In particular the most efficient load profile is such that the expansion of the peroxide reaction products is isobaric until all propellant mass is in the actuator, at which point the expansion becomes isentropic and continues as such until the cylinder pressure reaches atmospheric. Partial condensation occurs as a result of this load profile, leaving 70% quality steam in the actuator. This load profile would yield a theoretical efficiency of 39 %( calculated theoretically) for the 70% peroxide solution at a supply pressure of 220 psig.

4.2. Uninsulated Experiments

Experiments were conducted to measure the previously calculated conversion efficiency. A 70% peroxide solution was used as the propellant to maintain acceptable temperatures for commercially available components. For these experiments, the single-degree-of-freedom manipulator was commanded to move the 11 kg mass through a 30-degree amplitude, 1-Hz sinusoidal motion. The work output was computed indirectly by measuring the angle and, in post-processing, computing the actuation torque using a model of the load. The instantaneous power and average power could then be calculated. The propellant mass consumption was measured indirectly by recording the pressure of the nitrogen gas in the blow down tank, assuming an isothermal process inside the constant-volume tank, and calculating the volume occupied by the nitrogen from the ideal gas equation, which in turn yields the volume of propellant in the tank. Since the propellant is a liquid, the mass of propellant used is easily computed from the known volume and density. The conversion efficiency is then computed over an integer number of cycles with the heat of decomposition of 70% hydrogen peroxide solution.

 Based on these measurements, the experimentally determined conversion efficiency was found to be 6.6%. Note that the electrical power required to operate the valves was neglected in this analysis. The measured average combined electrical power required by the fuel and gas valves was approximately 2 W. Since this is only about 3% of the average work delivered by the actuator, this electrical power can be legitimately omitted from the analysis. The significant discrepancy between the measured conversion efficiency of 6.6% and the calculated upper bound of 16% is due to two major factors. The first is inefficiency in control and the second is heat loss. Specifically, the thermodynamic model assumed that no gas was exhausted during a given monotonic segment of the trajectory, and that no energy was lost as heat. Regarding the former, any overshoot of the desired trajectory will violate the assumed monotonicity of the trajectory, and therefore will result in an intermittent exhaust of hot gas and a resulting decrease in the efficiency. The existence of such intermittent exhaust is evident in the oscillations exhibited in the power delivered to the load which is shown in Fig. 4 plotted against the theoretically required power .Regarding inefficiency due to heat loss, the external surfaces of the catalyst pack, reservoir, and actuator were hot during the experiments, thus indicating the presence of heat flow. In order to more quantitatively assess the degree of heat loss, the prototype was instrumented with thermocouples so that the rate of heat loss could be estimated from surface temperature measurements referenced to tables associated with heat loss from uninsulated steam piping . This measurement yielded an estimated heat loss rate of 140W. Note that the average measured mechanical power output was approximately 60 W. The prototype lost twice as much energy in the form of heat as it delivered in the form of work. Taking into account this heat loss, the conversion efficiency of the prototype was recalculated to be 10 %

4.3. Insulated Experiments

In order to improve the measured conversion efficiency, the catalyst pack, reservoir, and actuator were wrapped in insulating tape, as shown in Fig. 5, and measurement of the conversion efficiency was repeated. For the insulated case, the experimentally determined conversion efficiency was found to be 9 %.Thermocouple measurement of the surface temperatures, as previously described, yielded an estimated heat loss rate of 73 W, approximately half of the uninsulated case. Using this heat loss rate, the theoretically calculated efficiency was 12 %, the difference presumably due to control inefficiency (i.e., intermittent exhausts).

4.4. Experimentally Determined Actuation Potential

Having measured the conversion efficiency, the mass-specific power density of the actuator and the mass-specific energy density of the power source need to be determined in order to calculate the actuation potential (1). The former is found by determining the mass and the maximum output power of the energy conversion and actuation system. Though finding the mass is a trivial task, characterizing the maximum deliverable power is not as straightforward due to the dependence upon several factors, including the supply pressure, the valve flow coefficient of the proportional valve, and the nature of the load, among others. In order to base the actuator power density solely on measured data the maximum deliverable power was estimated by using the peak power consistently measured during the previously described efficiency experiments. As evidenced by the data in Fig. 4, the actuator can consistently generate peak power of 150W, as indicated by the dashed line overlaid on the plot. The mass of the actuation system was obtained by weighing the components of the actuator shown in Fig. 2. The mass of each component is summarized in Table 1

As indicated in the table, the total actuation system mass is 1.5 Kg, thus resulting in an actuation system power density of 100 W/Kg. This would increase for a multi-degree-of-freedom system, since such a system would only include a single fuel valve, catalyst pack, pressure reservoir, and pressure sensor. Having determined the actuator power density, only the power-source energy density need be found in order to calculate the actuator potential. As previously mentioned, the heat of decomposition of 70% hydrogen peroxide propellant is 2.0 MJ/Kg. The propellant must be stored, however, in a pressurized blow down propellant tank, and as such a legitimate characterization of the energy density should include the mass of a tank. Based on available data for a composite over wrapped propellant tank, the mass of a propellant tank for a volume on the order of 10 liters would conservatively decrease the mass-specific energy density of 70% peroxide from approximately 2.0 MJ/Kg to approximately 1.7 MJ/Kg. Based on this and the measured values of conversion efficiency and actuator power density previously described, the actuation potential for this single-degree-of-freedom system, as given by (1), would be 15.3 KJ KW/Kg2. As previously mentioned, the power density will increase for a multi-degree-of-freedom system, and thus so will the actuation potential. For a six-degree-of-freedom system, for example, the total actuation system mass would be 5.2 Kg, or 870 g per actuator. The reservoir used in the single-degree-of-freedom experiment was oversized, and is appropriately sized for a power-comparable six-degree-of-freedom system. The actuation system power density would therefore increase to 172 W/Kg, and the corresponding actuation potential to 26.4 KJ KW/Kg2 for the six-degree-of-freedom system.

 For purposes of comparison, the best commercially available rechargeable batteries have energy densities of approximately180 KJ/Kg (e.g., Evercel M40-12 nickel zinc, or SAFT 27 10 LAS silver zinc). A rare-earth permanent-magnet dc motor with a harmonic drive gear head with output characteristics capable of achieving the trajectory specified by Table I, has a power density of approximately 48 W/Kg. Note that this remains invariant, regardless of the number of degrees of freedom. Finally, one can assume that the overall conversion efficiency would be the combined efficiencies due to pulse width-modulation (PWM) control, the motor, and the gear head. The PWM efficiency was estimated to be 95%, the motor efficiency calculated for the desired trajectory to be 90% (i.e., the resistive power loss in the motor windings was calculated given the desired torque), and the harmonic drive gear head efficiency was estimated based on manufacturer data to be 65%. The resulting actuation potential for this type of system would therefore be 4.8 KJ KW/Kg. The poorly insulated single-degree-of-freedom experimental setup with 70% peroxide therefore exhibited an actuation potential more than three times a state-of-the-art battery/dc motor system. A similar six-degree-of-freedom system would exhibit an actuation potential over fives times the battery/dc motor system.

4.5. Projected Performance for High-Test Propellant

Though improvements can clearly be made with improved insulation and control performance, the most obvious means of improving the actuation system performance is to substitute a fully concentrated version of the propellant (i.e., 100%hydrogen peroxide) in place of the 70% solution used in the previously described experiments. Though procedurally quite simple, such experiments cannot be performed on commercially available pneumatic components, due to the high decomposition temperatures. Specifically, the adiabatic decomposition temperature of 100% peroxide is approximately 1000 0 C (1800 F), compared to approximately 230 0 C (450 F) for a 70% solution. Rather than conducting experiments using 100% peroxide, one can obtain a reasonable estimate of performance with projections based upon the experiments conducted with 70% solution. Upon replacing 70% propellant with 100% (technically 99.6%), at least two of the three parameters forming the actuation potential figure of merit would be expected to increase. Specifically, since the propellant contains more peroxide per unit mass, the heat of decomposition increases by a factor of 1.45 from 2.0 MJ/Kg to 2.9 MJ/Kg.

 Additionally, the relatively low conversion efficiencies observed earlier were primarily due to the heat required to vaporize the water in the reaction product. Since the 100% propellant contains less water, less energy is invested in vaporizing the reaction product. Recalculating the expected efficiencies accounting for the reduced water content, the conversion efficiency scales by a factor of 1.56. Assuming that the actuation system power density remains invariant (i.e., that it does not increase with the 100% propellant), the single-degree-of-freedom system shown in Fig. 2 with 100%propellant would be expected to have an actuation potential of 35 KJ KW/Kg2 , which is 7.3 times greater than the battery /dc motor system. A similar six-degree-of-freedom system would exhibit an actuation potential of 60.4 KJ KW/Kg2, more than an order of magnitude greater than the battery/dc motor system. The promise of such performance, which would presumably be further improved with better insulation and light weight components, justifies the fabrication of custom high-temperature pneumatic components.

5. CHALLENGES OF DESIGN

The biggest challenge in using monopropellant as a power source is providing adequate insulation to prevent the heat loss from the system. We have seen from the experimental results that the heat loss exceeds the power output obtained from the actuator. Finding a suitable method to contain this heat loss is the first and the biggest challenge in designing a monopropellant based power supply.

Another problem is the non availability of parts that can withstand the heat produced on the decomposition of 100% hydrogen peroxide. Due to this reason, a hydrogen peroxide solution of lower strength has to be used. Materials more resistant to heat are to be used to make the parts of the system so that it can withstand higher temperatures. This will aid the use of higher concentrations of peroxide thereby increasing the actuation potential of the system.

Monopropellants are highly reactive materials and are even toxic to humans. It has a tendency to catch fire if spilt on clothing. So the persons handling the fuel should be extremely cautious in order to avert possible danger of explosion and intoxication. The conventional power systems do not have such problems. Hence extra care must be taken while selecting the materials to be used in the system.

The selection or rejection of a proposed design depends heavily on its economic aspect. Hydrogen peroxide powered system when compared with battery operated power system is very costly in running condition. Also due to the presence of valves, maintenance costs of the system are high as well. The choice of valves can also influence the reliability of the system. The next challenge in designing monopropellant powered systems is attaining proper co ordination between the different control loops which controls the operation of the system. The emission of hot steam might be an inconvenience to other human workers if such robots are used along with humans. So it is better to limit the use of such robots to places inaccessible to humans. Controlling the emissions from the robot can make it usable along with humans. This can also improve the efficiency of the system.

6. CONCLUSION

A power supply and actuation system appropriate for a position or force controlled human-scale robot was proposed. The proposed approach utilizes a monopropellant as a gas generant to power pneumatic-type hot gas actuators. Experiments were performed that characterize the energetic behavior of the proposed system and offer the promise of an order-of-magnitude improvement in actuation potential relative to a battery powered dc-motor-actuated approach. Experiments also demonstrated good tracking and adequate bandwidth of the proposed actuation concept.

Steam powered robots are a possibility in the future provided the limitations of the existing prototype is done away with. A better actuation potential can be obtained by providing better insulation to the prototype thereby reducing the heat loss. Another challenge before researchers is to manufacture parts that can withstand the high temperatures generated on decomposition of 100% H2O2 .With the introduction of better controls, fuel and insulation, the se robots could function effectively and economically.

The proposed power supply was found to be a feasible solution to the problem of providing a long lasting power supply to robots that can actually work. Moreover the power output could be easily adjusted by controlling the rate of flow of the monopropellant. Although a full size human scale robot powered by a monopropellant is yet to be made, the experimental results obtained from a single degree of freedom manipulator proves the feasibility of such a system.

A Brief Description of NDT Techniques

Non-destructive testing (NDT) is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. The terms Nondestructive examination (NDE), Nondestructive inspection (NDI), and Nondestructive evaluation (NDE) are also commonly used to describe this technology. Because NDT does not permanently alter the article being inspected, it is a highly-valuable technique that can save both money and time in product evaluation, troubleshooting, and research.
Non-destructive Testing is one part of the function of Quality Control and is Complementary to other long established methods. By definition non-destructive testing is the testing of materials, for surface or internal flaws or metallurgical condition, without interfering in any way with the integrity of the material or its suitability for service.
The technique can be applied on a sampling basis for individual investigation or may be used for 100% checking of material in a production quality control system. Whilst being a high technology concept, evolution of the equipment has made it robust enough for application in any industrial environment at any stage of manufacture - from steel making to site inspection of components already in service. A certain degree of skill is required to apply the techniques properly in order to obtain the maximum amount of information concerning the product, with consequent feed back to the production facility. Non-destructive Testing is not just a method for rejecting substandard material; it is also an assurance that the supposedly good is good. The technique uses a variety of principles; there is no single method around which a black box may be built to satisfy all requirements in all circumstances.
What follows is a brief description of the methods most commonly used in industry, together with details of typical applications, functions and advantages.

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 The methods Covered are:
• Radiography
• Magnetic Particle Crack Detection
• Dye Penetrate Testing
• Ultrasonic Flaw Detection
• Eddy Current and Electro-magnetic Testing
However, these are by no means the total of the principles available to the N.D.T. Engineer. Electrical potential drop, Sonics, infra-red, acoustic emission and Spectrograph, to name but a few, have been used to provide information that the above techniques have been unable to yield, and development across the board Continues.

Definitions
The following definitions apply
Testing: Testing or examination of a material or component in accordance with this Classification Note, or a standard, or a specification or a procedure in order to detect, locate, measure and evaluate flaws.
Defect: One or more flaws whose aggregate size, shape, orientation, location or properties do not meet specified requirements and are rejectable.
Discontinuity: A lack of continuity or cohesion; an intentional or unintentional interruption in the physical structure or configuration of a material or component
Flaw: An imperfection or discontinuity that may be detectable by non-destructive testing and is not necessarily reject able.
Indication: Evidence of a discontinuity that requires interpretation to determine its significance
False indication: An indication that is interpreted to be caused by a discontinuity at a location where no discontinuity exists.
Non relevant indication: An indication that is caused by a condition or type of discontinuity that is not reject able. False indications are non-relevant
Imperfections: A departure of a quality characteristic from its intended condition.
Internal imperfections: Imperfections that are not open to a surface or not directly accessible.
Quality level: Fixed limits of imperfections corresponding to the expected quality in a specific object. The Limits are determined with regard to type of imperfection, their amount and their actual dimensions.
Acceptance level: Prescribed limits below which a component is accepted.
Planar discontinuity: Discontinuity having two measurable dimensions
Non-planar discontinuity: Discontinuity having three measurable dimensions.
Abbreviations
·         ET Eddy current testing
·         MT Magnetic particle testing
·         PT Penetrant testing
·         RT Radiographic testing
·         UT Ultrasonic testing
·         VT Visual testing
·         HAZ Heat affected zone
·         WPS Welding Procedure Specification
·         TMCP Thermo mechanically controlled processed
·         NDT Non-destructive testing.

Where to apply NDE Methods:
There are NDE application at almost any stage in the production or life cycle of a component.
To assist in product development
To screen or sort incoming materials
To monitor, improve or control manufacturing processes
To verify proper processing such as heat treating
To verify proper assembly
To inspect for in-service damage

Uses of NDE Methods
·         Flaw Detection and Evaluation
·         Leak Detection
·         Location Determination
·         Dimensional Measurements
·         Structure and Microstructure Characterization
·         Estimation of Mechanical and Physical Properties
·         Stress (Strain) and Dynamic Response Measurements
·         Material Sorting and Chemical Composition Determination




HISTORY OF NDT-INSTRUMENTATION

After World War II the emerging modern industry needed more and more testing equipment for the production of flawless components. Therefore, instruments for NDT were developed, produced in quantities and continuously improved. The first NDT-method coming into industrial application was the X-Ray Technique.

X-Ray Technique
Already 1895 Wilhelm Conrad Röntgen discovered "An Unknown Kind of Radiation" which were named in all German speaking countries after him. In his first publication he described all effects including possible flaw detection. At that time industry did not yet need this invention but medicine did. So medical equipment was developed, used and produced in quantities. The only effect Röntgen could not foresee was that X-rays harm human health. Before radiation protection became introduced, many persons lost their life. Early technical X-ray applications in Germany were realized by Richard Seifert around 1930. He improved medical equipment, cooperated with welding-institutes and built up the small company founded by his father to a world-wide respected name: Richard Seifert Hamburg 13. He got competition by Siemens and C.H.F. Muller, part of the Philips-organization, who already worked in the medical field. Seifert died in 1969, but his company kept leadership in technical X-ray-application under the direction of his youngest daughter Elisabeth Samish. Radiation testing can also be carried out with radioactive isotopes. This was discovered by Mme. Curie. She, born as Maria Sklodowska in Warscaw received the Nobel-prize for physics in 1903 together with her husband Pierre Curie and Henri Becquerel. This was the second award after Rontgen’s in 1901. Also radioactive isotopes were initially used for medical applications. In Germany Rudolf Berthold and Otto Vaupel applied them after 1933 to welded joints. After World War II Arturo Gilardoni in Italy, Drenk and Andreasen in Denmark developed X-ray-equipment, Kurt Sauerwein portable isotope-containers in Germany.

Magnetic particle crack detection
Magnetic particle crack detection was executed even earlier than X-ray testing. The Englishman S.M. Saxby already in 1868 and the American William Hoke in 1917 tried to find cracks in gun barrels by magnetic indications. Real industrial application was made by Victor de Forest and Foster Doane after 1929. They formed 1934 a company with the name Magnaflux in 1934, famous world-wide until today. The first European who built a magnetic particle crack detector was an Italian in 1932: Giraudi. His machine was named "Metalloscopio". In Germany Berthold and Vaupel applied MP-technique to welded constructions. Their equipment was produced by Ernst Heubach. Bruno Suschyzki sold this equipment. He invented swinging field MP-testing.In Berlin too E.A.W. Müller designed MP-testing machines for Siemens. In Prague the Seifert-representative Karasek began with similar production.After World War II Wilhelm Tiede, a former Seifert-employee, started his own company in Southern Germany.
Through the Seifert-organisation he had connections to Karasek who emigrated to Brazil in 1948 after the communistic revolution in Cechoslovakia. There he continued production of MP-machines.Starting with dry-powder methods two more companies entered this market in the late Fifties: Karl Deutsch in Germany and CGM (Carlo Gianni Milano) in Italy.

Penetrate Testing
Penetrant Testing started also in second half of 19th century. The first people who applied the "Oil and Whiting"-process for crack detection to railway-components are unknown. The method was replaced by the upcoming MP-technique. Just before and during World War II the fast growing aircraft-industry used more and more nonmagnetic light metals, which could not be tested with MP. So independent of each other Magnaflux together with the brothers Switzer in USA, Brent Chemicals in GB, Adler (Blohm & Voss) and Klumpf (Junkers) in Germany started production of fluorescent and dye penetrants.

Eddy Current Testing
Eddy Current Testing has old roots as well. The French Dominique Arago discovered the phenomenon during the first half of 19th century. The principle was explained by and named after Leon Foucault. Many proposals for application were made until Friedrich Förster brought this method to industrial use. He worked since 1933 for the Kaiser-Wilhelm-Institute and there he developed instruments for measuring conductivity and to sort out mixed-up ferrous components. In 1948 he founded his own company in Reutlingen which grew up with eddy current testing (ET). Institut Dr. Förster became market-leader worldwide for many decades. In Sweden Anders Arnelo started similar developments at Svenska Metalverken (SM). He solved the problem to test hot wires and invented the pre-magnetization for ET of ferritic bars.Other companies followed later: Magnaflux, Hentschel, Law and Zetec in the USA, Rohmann and Prüftechnik Busch & Partner in Germany, Bergstrand in Sweden and Hocking in Britain.

Ultrasonic Testing
Came latest into industrial use. The methods of exciting ultrasound were discovered already in 1847 by James Precott Joule and in 1880 by Pierre Curie and his brother Paul Jacques. Not earlier than 1912 a first application was proposed after the "Titanic" had sunk. The Englishman Richardson claimed the identification of icebergs by ultrasound in his patent applications. In France Chilowski and Langevin started their development to detect submarines by ultrasound during World War I. In 1929 the Russian Sokolov proposed to use ultrasound for testing castings. In Berlin Pohlman realized in 1937 an image-cell to indicate the differences of ultrasound-energy similar to a X-ray image-screen.
The detection of laminations in plates and fine non-metallic inclusions in hot-rolled profiles became necessary during World War II. The already existing NDT-methods - X-rays, MP, PT and ET - were unable to solve these problems.Industrial uses of ultrasonic testing started simultaniously in three countries: USA, GB and Germany. The key-persons, Floyd Firestone, Donald O. Sproule and Adolf Trost had no knowledge of each other as they worked strictly in secret. Not even their patent-applications were published. Sproule and Trost used transmission-technique with seperate transmitter- and receiver-probes. Trost invented the so-called "Trost-Tonge". The 2 probes were contacted on opposite sides of a plate, held in same axis by a mechanical device - the tonge - and coupled to both surfaces by continuously flowing water. Sproule placed the 2 probes on the same side of the workpiece. So he invented double-crystal probes. But it has to be mentioned that he used this combination also with variing distance from each other. Firestone was the first to realize the reflection-technique. He modified a radar instrument and developed a transmitter with short pulses and an amplifier with short dead-zone.Sproule and Firestone found industrial partners for their instruments: Kelvin-Hughes and Sperry Inc.In Germany 1949 two persons received information about the Firestone-Sperry-Reflectoscope by publications in technical papers: Josef Krautkrämer in Cologne and Karl Deutsch in Wuppertal. Both started developments - without knowledge of each other. Josef Krautkrämer and his brother Herbert were physicists, working in the field of oscilloscopes. They could develop ultrasonic instruments alone. Karl Deutsch, a mechanical engineer needed a partner for the electronics and found him with Hans-Werner Branscheid who had got some technical experience in radar-technique during the war. Within only one year both young and tiny companies could present their UT-flaw-detectors, starting a competition still existing today.Later on more UT-units came on the international markets: Siemens and Lehfeldt in Germany, Kretztechnik in Austria, Ultrasonique in France and Ultrasonoscope in Britain. They all stopped their production before the 70-ies, Kelvin-Hughes also stopped at the same time, Sperry was later renamed Automation Ind., around 1995.
Krautkrämer became world-wide market-leader in the early 60-ies and has kept this position until today. Besides Karl Deutsch new names came up: Nukem in Germany, Panametrics and Stavely (after Sonic and Harisonic) in USA, Sonatest and Sonomatic in GB, Gilardoni in Italy and Mitsubishi in Japan.Today more than 50 companies are active in industrial NDT. They are still working under strong competition to the benefit of their customers on their way to improve the quality of industrially produced parts.


                           A BRIEF DESCRIPTION OF NDT TECHNIQUES

Radiography
This technique is suitable for the detection of internal defects in ferrous and nonferrous metals and other materials. X-rays, generated electrically, and Gamma rays emitted from radio-active isotopes, are penetrating radiation which is differentially absorbed by the material through which it passes; the greater the thickness, the greater the absorption. Furthermore, the denser the material the greater the absorption. X and Gamma rays also have the property, like light, of partially converting silver halide crystals in a photographic film to metallic silver, in proportion to the intensity of the radiation reaching the film, and therefore forming a latent image. This can be developed and fixed in a similar way to normal photographic film. Material with internal voids is tested by placing the subject between the source of radiation and the film. The voids show as darkened areas, where more radiation has reached the film, on a clear background. The principles are the same for both X and Gamma radiography.
In X-radiography the penetrating power is determined by the number of volts applied to the X-Ray tube - in steel approximately 1000 volts per inch thickness is necessary. In Gamma radiography the isotope governs the penetrating power and is unalterable in each isotope. Thus Iridium 192 is used for 1/2" to 1" steel and Caesium 134 is used for 3/4" to 21/2" steel. In X-radiography the intensity, and therefore the exposure time, is governed by the amperage of the cathode in the tube. Exposure time is usually expressed in terms of milliampere minutes. With Gamma rays the intensity of the radiation is set at the time of supply of the isotope. The intensity of radiation from isotopes is measured in Becquerel’s and reduces over a period of time. The time taken to decay to half the amount of curies is the half life and is characteristic of each isotope. For example, the half life of Iridium 192 is 74 days, and Caesium 134 is 2.1 years. The exposure factor is a product of the number of curies and time, usually expressed in curie hours. The time of exposure must be increased as the isotope decays - when the exposure period becomes uneconomical the isotope must be renewed. As the isotope is continuously emitting radiation it must be housed in a container of depleted uranium or similar dense shielding material, whilst not exposed to protect the environment and personnel.
To produce an X or Gamma radiograph, the film package (comprising film and intensifying screens - the latter being required to reduce the exposure time – enclosed in a light tight cassette) is placed close to the surface of the subject. The source of radiation is positioned on the other side of the subject some distance away, so that the radiation passes through the subject and on to the film. After the exposure period the film is removed, processed, dried, and then viewed by transmitted light on a special viewer. Various radiographic and photographic accessories are necessary, including such items as radiation monitors, film markers, image quality indicators, darkroom equipment, etc. Where the last is concerned there are many degrees of sophistication, including fully automatic processing units. These accessories are the same for both X and Gamma radiography systems. Also required are such consumable items as radiographic film and processing chemicals.

 The part is placed between the radiation source and a piece of film.  The part will stop some of the radiation.  Thicker and denser area will stop more of the radiation  
 Advantages of Radiography
1.      Information is presented pictorially.
2.      A permanent record is provided which may be viewed at a time and place
3.      Distant from the test.
4.      Useful for thin sections.
5.      Sensitivity declared on each film.
6.      Suitable for any material.

Disadvantages of Radiography
1.       Generally an inability to cope with thick sections.
2.      Possible health hazard.
3.      Need to direct the beam accurately for two-dimensional defects.
4.      Film processing and viewing facilities are necessary, as is an exposure compound.
5.      Not suitable for automation, unless the system incorporates fluoroscopy with
6.      an image intensifier or other electronic aids
7.      Not suitable for surface defects.
8.      No indication of depth of a defect below the surface

Magnetic Particle Inspection
 This method is suitable for the detection of surface and near surface discontinuities in magnetic          material, mainly ferrite steel and iron. An Illustration of the Principle of Magnetic Particle Inspection
                                             

The principle is to generate magnetic flux in the article to be examined, with the flux lines running along the surface at right angles to the suspected defect. Where the flux lines approach a discontinuity they will stay out in to the air at the mouth of the crack. The crack edge becomes magnetic attractive poles North and South. These have the power to attract finely divided particles of magnetic material such as iron fillings. Usually these particles are of an oxide of iron in the size range 20 to 30 microns, and are suspended in a liquid which provides mobility for the particles on the surface of the test piece, assisting their migration to the crack edges. However, in some instances they can be applied in a dry powder form. The particles can be red or black oxide, or they can be coated with a substance, which fluoresces brilliantly under ultra-violet illumination (black light). The object is to present as great a contrast as possible between the crack indication and the material background. The technique not only detects those defects which are not normally visible to the unaided eye, but also renders easily visible those defects which would otherwise require close scrutiny of the surface. There are many methods of generating magnetic flux in the test piece, the simplest one being the application of a permanent magnet to the surface, but this method cannot be controlled accurately because of indifferent surface contact and deterioration in magnetic strength. Modern equipments generate the magnetic field electrically either directly or indirectly.
In the direct method a high amperage current is passed through the subject and magnetic flux is generated at right angles to the current flow. Therefore the current flow should be in the same line as the suspected defect. If it is not possible to carry out this method because of the orientation of the defect, then the indirect method must be used.
 This can be one of two forms:
1. Passing a high current through a coil that encircles the subject.
2. Making the test piece form part of a yoke, this is wound with a current carrying coil. The effect is to pass magnetic flux along the part to reveal transverse and circumferential defects.

If a bar with a length much greater than its diameter is considered, then longitudinal defects would be detected by current flow and transverse and circumferential defects by the indirect method of an encircling coil or magnetic flux flow. Subjects in which cracks radiating from a hole are suspected can be tested by means of the threading bar technique, whereby a current carrying conductor is passed through the hole and the field induced is cut by any defects. Detection of longitudinal defects in hollow shafts is a typical application of the threader bar technique. The electricity used to generate the magnetic flux in any of these methods can be alternating current, half wave rectified direct current or full wave rectified direct current. A.C. generated magnetic flux, because of the skin effect, preferentially follows the contours of the surface and does not penetrate deeply into the material. H.W.D.C. penetrates more deeply but is inclined not to follow sharp changes in section. H.W.D.C. is useful for the detection of slightly subsurface defects. The pulsing effect of A.C. and H.W.D.C. gives additional mobility to the indicating particles. D.C. penetrates even more deeply but does not have this facility. Furthermore, demagnetizing of the material after D.C. magnetizing is far more difficult than after A.C. magnetizing. Normally, to ensure that a test piece has no cracks, it is necessary to magnetize it in at least two directions and after each magnetizing - and ink application – visually examine the piece for crack indications. Since this double process, which would include adjustment of the magnetizing equipment controls in between each magnetizing takes time it is obviously advantageous to have the facility to reduce the time required.
            The recent development of the Swinging Field method of multi-directional magnetizing will indicate all defects, regardless of their orientation on the surface, with one magnetizing shot and therefore requires only one inspection. (Please refer to our paper entitled Faster Magnetic Crack Detection using the Multi-directional Swinging Field Method).
Basically magnetic crack detection equipment takes two forms. Firstly, for test pieces which are part of a large structure, or pipes, heavy castings, etc. which cannot be moved easily, the equipment takes the form of just a power pack to generate a high current. This current is applied to the subject either by contact prods on flexible cables or by an encircling coil of cable. These power packs can have variable amperages up to a maximum of 2000 Amps for portable units, and up to 10,000 Amps for mobile equipments. Both A.C. and H.W.D.C. magnetising current is available. The indicating material is applied by means of a spray and generally the surplus runs to waste. For factory applications on smaller more manageable test pieces the bench type of equipment, as represented by our EUROMAG range, is normally preferred. This consists of a power pack similar to those described above, an indicating ink system which recirculates the fluid, and facilities to grip the work piece and apply the current flow or magnetic flux flow in a more methodical, controlled manner. The work pieces are brought to the equipment and can be individually tested. Subjects up to approximately 100" long can be accommodated is such equipments and can be loaded by crane if necessary. This type of universal equipment is ideally suited to either investigative work or routine quality control testing. These bench type equipments often incorporate a canopy to prevent direct light falling on the subject so that ultra-violet fluorescent material can be used to the best effect. The indicating particles may be suspended in very thin oil (kerosene) or water. In some circumstances the indicating medium can be applied dry. These equipments are suited to production work and in certain circumstances can be automated to the extent of loading, magnetising, inking and unloading. The work pieces still have to be viewed by eye for defect indications. Specialised equipments are also frequently manufactured to test a particular size and type of test piece.

                     
Advantages of Magnetic Particle Crack Detection
1.      Simplicity of operation and application.
2.      Quantitative.
3.      Can be automated, apart from viewing. (Though modern developments in automatic defect recognition can be used in parts of simple geometry e.g. billets and bars. In this case a special camera captures the defect indication image and processes it for further display and action)

Disadvantages of Magnetic Particle Crack Detection
1.      Restricted to ferromagnetic materials.
2.      Restricted to surface or near surface flaws.
3.      Not fail safe in that lack of indication could mean no defects or process not carried out properly.

Dye Penetrate Testing
This method is frequently used for the detection of surface breaking flaws in non ferromagnetic materials. The subject to be examined is first of all chemically cleaned, usually by vapour phase, to remove all traces of foreign material, grease, dirt, etc. from the surface generally, and also from within the cracks. Next the penetrate (which is a very fine thin oil usually dyed bright red or ultra-violet fluorescent) is applied and allowed to remain in contact with the surface for approximately fifteen minutes. Capillary action draws the penetrate into the crack during this period.

The surplus penetrate on the surface is then removed completely and thin coating of powdered chalk is applied. After a further period (development time) the chalk draws the dye out of the crack, rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background. The process is purely a mechanical/chemical one and the various substances used may be applied in a large variety of ways, from aerosol spray cans at the most simple end to dipping in large tanks on an automatic basis at the other end. The latter system requires sophisticated tanks, spraying and drying equipment but the principle remains the same.

Advantages of Dye Penetrate Testing
1.      Simplicity of operation.
2.      Best method for surface breaking cracks in non-ferrous metals.
3.      Suitable for automatic testing, with reservation concerning viewing.
(See automatic defect recognition in Magnetic Particle Inspection)

 Disadvantages of Dye Penetrate Testing
1.      Restricted to surface breaking defects only.
2.      Decreased sensitivity.
3.      Uses a considerable amount of consumables.

Ultrasonic Flaw Detection
This technique is used for the detection of internal and surface (particularly distant surface) defects in sound conducting materials. The principle is in some respects similar to echo sounding. A short pulse of ultrasound is generated by means of an electric charge applied to a piezoelectric crystal, which vibrates for a very short period at a frequency related to the thickness of the crystal. In flaw detection this frequency is usually in the range of one million to six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this frequency have the ability to travel a considerable distance in homogeneous elastic material, such as many metals with little attenuation. The velocity at which these waves propagate is related to the Young’s Modulus for the material and is characteristic of that material. For example the velocity in steel is 5900 meters per second, and in water 1400 meters per second. Ultrasonic energy is considerably attenuated in air, and a beam propagated through a solid will, on reaching an interface (e.g. a defect, or intended hole, or the back wall) between that material and air reflect a considerable amount of energy in the direction equal to the angle of incidence. For contact testing the oscillating crystal is incorporated in a hand held probe, which is applied to the surface of the material to be tested. To facilitate the transfer of energy across the small air gap between the crystal and the test piece, a layer of liquid (referred to as ‘coolant’), usually oil, water or grease, is applied to the surface. As mentioned previously, the crystal does not oscillate continuously but in short pulses, between each of which it is quiescent. Piezo electric materials not only convert electrical pulses to mechanical oscillations, but will also transducer mechanical oscillations into electrical pulses; thus we have not only a generator of sound waves but also a detector of returned pulses. The crystal is in a state to detect returned pulses when it is quiescent. The pulse takes a finite time to travel through the material to the interface and to be reflected back to the probe. The standard method of presenting information in ultrasonic testing is by means of a cathode ray tube, in which horizontal movement of the spot from left to right represents time elapsed. The principle is not greatly different in digitized instruments that have a LCD flat screen. The rate at which the spot moves is such that it gives the appearance of a horizontal line on the screen. The system is synchronized electronically so that at the instant the probe receives its electrical pulse the spot begins to traverse the screen. An upward deflection (peak) of the line on the left hand side of the screen is an indication of this occurrence. This peak is usually termed the initial pulse.
While the base line is perfectly level the crystal is quiescent. Any peaks to the right of the initial pulse indicate that the crystal has received an incoming pulse reflected from one or more interfaces in the material. Since the spot moves at a very even speed across the tube face, and the pulse of ultrasonic waves moves at a very even velocity through the material, it is possible to calibrate the horizontal line on the screen in terms of absolute measurement. The use of a calibration block, which produces a reflection from the back wall a known distance away from the crystal together with variable controls on the flaw detector, allows the screen to be calibrated
in units of distance, and therefore determination of origins of returned pulses obtained from a test piece. It is therefore possible not only to discover a defect between the surface and the back wall, but also to measure its distance below the surface. It is important that the equipment is properly calibrated and, since it is in itself not able to discriminate between intended boundaries of the object under test and unintended discontinuities, the operator must be able to identify the origin of each peak. Further as the pulses form a beam it is also possible to determine the plan position of a flaw. The height of the peak (echo) is roughly proportional to the area of the reflector, though there is on all instruments a control, which can reduce or increase the size of an indication - variable sensitivity in fact. Not only is party of the beam reflected at a material/air interface but also at any junction where there is a velocity change, for example steel/slag interface in a weld. Probing all faces of a test piece not only discovers the three-dimensional defect and measures its depth, but can also determine its size. Two-dimensional (planar) defects can also be found but, unlike radiography, it is best that the incident beam impinges on the defect as near to right angles to the plane as possible.

To achieve this some probes introduce the beam at an angle to the surface. In this manner longitudinal defects in tubes (inner or outer surface) are detected. Interpretation of the indications on the screen requires a certain amount of skill, particularly when testing with hand held probes. The technique is, however, admirably suited to automatic testing of regular shapes by means of a monitor – an electronic device that fits into the main equipment to provide an electrical signal when an echo occurs in a particular position on the trace. The trigger level of this signal is variable and it can be made to operate a variety of mechanical gates and flaw warnings. Furthermore, improvements in computer technology allow test data and results to be displayed and out-putted in a wide variety of formats. Modern ultrasonic flaw detectors are fully solid state and can be battery powered, and are robustly built to withstand site conditions. Since the velocity of sound in any material is characteristic of that material, it follows that some materials can be identified by the determination of the velocity. This can be applied, for example in S.G. cast irons to determine the percentage of graphite Modularity.

 Advantages of Ultrasonic Flaw Detection
1.      Thickness and lengths up to 30 ft can be tested.
2.      Position, size and type of defect can be determined.
3.      Instant test results.
4.      Portable.
5.      Extremely sensitive if required.
6.      Capable of being fully automated.

Disadvantages of Ultrasonic Flaw Detection
1.      Access to only one side necessary.
2.      No consumables.
3.      Disadvantages of Ultrasonic Flaw Detection
4.      No permanent record available unless one of the more sophisticated test results and data collection systems is used.
5.      The operator can decide whether the test piece is defective or not whilst the test is in progress.
6.      Indications require interpretation (except for digital wall thickness gauges).
7.      Considerable degree of skill necessary to obtain the fullest information from the test.
8.     Very thin sections can prove difficult.

Eddy Current and Electro-Magnetic Methods
The main applications of the eddy current technique are for the detection of surface or subsurface flaws, conductivity measurement and coating thickness measurement. The technique is sensitive to the material conductivity, permeability and dimensions of a product. Eddy currents can be produced in any electrically conducting material that is subjected to an alternating magnetic field (typically 10Hz to 10MHz). The alternating magnetic field is normally generated by passing an alternating current through a coil. The coil can have many shapes and can between 10 and 500 turns of wire. The magnitude of the eddy currents generated in the product is dependent on conductivity, permeability and the set up geometry. Any change in the material or geometry can be detected by the excitation coil as a change in the coil impedance The most simple coil comprises a ferrite rod with several turns of wire wound at one end and which is positioned close to the surface of the product to be tested. When a crack, for example, occurs in the product surface the eddy currents must travel farther around the crack and this is detected by the impedance change                                

Coils can also be used in pairs, generally called a driven pair, and this arrangement can be used with the coils connected differentially. In this way ‘lift off’ (distance of the Probe from the surface) signals can be enhanced.

                                               
Coils can also be used in a transformer type configuration where one coil winding is a Primary and one (or two) coil windings are used for the secondary’s.
The detected eddy current signals contain amplitude and phase information and which can be displayed on CRT type displays – non digital displays. Signals can be displayed as the actual, i.e. absolute signal, or with appropriate electronics, only a signal change is displayed. The best results are obtained where only one product parameter is changes, e.g. the presence of a crack.
In practice changes in eddy current signals are caused by differences in composition, hardness, texture, shape, conductivity, permeability and geometry. In some cases the effects of the crack can be hidden by changes in other parameters and unnecessary rejection can occur. However, the coils can be selected for configuration, size and test frequency in order to enhance detection of cracks, conductivity, metal loss etc. as required.
The depth to which the eddy currents penetrate a material can be changed by adjusting the test frequency – the higher the frequency, the lower the penetration; however, the lower the frequency, the lower sensitivity to small defects. Larger coils are less sensitive to surface roughness and vice versa. The latest electronic units are able to operate a wide range of coil configurations in absolute or differential modes and at a wide range of frequencies. For surface testing for cracks in single or complex shaped components, coils with a single ferrite cored winding are normally used. The probe is placed on the component and ‘balanced’ by use of the electronic unit controls. As the probe is scanned across the surface of the component the cracks can be detected. See Fig.1 Where surfaces are to be scanned automatically the single coil windings are suitable only if the lift off distance is accurately maintained. Generally differential coil configurations are used with higher speed scanning systems where lift off effects, vibration effects, etc. can be cancelled out to an acceptable extent. See Fig.2. Tubes, bar and wire can be inspected using an encircling coil and these usually have a coil configuration with one primary and two secondary’s connected differentially. See Fig.3. Most eddy current electronics have a phase display and this gives an operator the ability to identify defect conditions. In many cases signals from cracks, lift off and other parameters can be clearly identified. Units are also available which can inspect a product simultaneously at two or more different test frequencies. These units allow specific unwanted effects to be electronically cancelled in order to give improved defect detection. The eddy current test is purely electrical. The coil units do not need to contact the product surface and thus the technique can be easily automated. Most automated Systems are for components of simple geometry where mechanical handling is simplified.

Advantages of Eddy Current Testing
1.      Suitable for the determination of a wide range of conditions of conducting material, such as defect detection, composition, hardness, conductivity, permeability etc. in a wide variety of engineering metals.
2.      Information can be provided in simple terms: often go/no go. Phase display electronic units can be used to obtain much greater product information.
3.      Extremely compact and portable units are available.
4.      No consumables (except probes – which can sometimes be repaired).
5.      Flexibility in selection of probes and test frequencies to suit different applications.
6.      Suitable for total automation.

Disadvantages of Eddy Current Testing
1.      The wide range of parameters which affect the eddy current responses means that the signal from a desired material characteristic, e.g. a crack, can be masked by an unwanted parameter, e.g. hardness change. Careful selection of probe and electronics will be needed in some applications.
2.      Generally tests restricted to surface breaking conditions and slightly subsurface flaws.

Visual Inspection
1.      Thermo graphic surveys
2.      Bore scope and fibre optic inspections
3.      Miniature fiberscope inspections form HGP components with complex geometries.
                                                                                   
Non-Destructive Testing Methods & Applications      

Current NDT methods applied to these ceramic components are expensive, cannot find all the defect of interest, rely on operator interpretations of signals, and frequently use chemicals with deleterious effects on the environment To increase the Probability of Detection (POD) the conventational NDT are advanced by some  method are listed as given below:-

1.      TOFD Technique (Time of Flight Diffraction).
2.     Ultrasonic Phased Array.
3.      Process Compensated Resonance Testing (PCRT).
4.      Optical NDT Technologies.

TOFD Technique (Time of Flight Diffraction)
The TOFD technique is a fully computerized system able to scan, store, and evaluate indications in terms of height, length, and position with a grade of coverage, accuracy and speed not achieved by other ultrasonic techniques. The TOFD technique is based on diffraction of ultrasonic energy from tips of discontinuities, instead of geometrical reflection on the interface of the discontinuities.

Principle of TOFD

 TOFD inspection employs two longitudinal wave (L-wave) angle beam transducers arranged symmetrically opposite facing each other, straddling the weld or base material under test. One probe acts like a transmitter of ultrasonic energy while the other probe receives the ultrasound energy. The transducer, pulser, and amplifier characteristics are selected to generate as broad distribution of energy as possible over the material under test providing full weld coverage. A single-axis scan (that is, along the weld), with a position encoder records the position of the weld and enables the display of digital images in real time.
Capabilities
1.      TOFD defect detection does not depend on the defect orientation, in contrast to the pulse echo technique.
2.      In contrast to the radiography method, planar defects and cracks, which are not perpendicular to the measured surface, can be detected.
3.      Defect height can be exactly determined.
4.      Higher POD improves risk reduction and calculation.
Applications
1.      New and existing welds
2.      Defect monitoring
3.      Stress Corrosion Cracking
4.      Weld root erosion surveys

Ultrasonic Phased Array
Conventional ultrasonic transducers for NDT commonly consist of either a single active element that both generates and receives high frequency sound waves, or two paired elements, one for transmitting and one for receiving. Phased array probes, on the other hand, typically consist of a transducer assembly with from 16 to as many as 256 small individual elements that can each be pulsed separately. These may be arranged in a strip (linear array), a ring (annular array), a circular matrix (circular array), or a more complex shape. As is the case with conventional transducers, phased array probes may be designed for direct contact use, as part of an angle beam assembly with a wedge, or for immersion use with sound coupling through a water path. Transducer frequencies are most commonly in the range from 2 MHz to 10 MHz. A phased array system will also include a sophisticated computer-based instrument that is capable of driving the multi-element probe, receiving and digitizing the returning echoes, and plotting that echo information in various standard formats. Unlike conventional flaw detectors, phased array systems can sweep a sound beam through a range of refracted angles or along a linear path, or dynamically focus at a number of different depths, thus increasing both flexibility and capability in inspection setups


How do they work?
In the most basic sense, a phased array system utilizes the wave physics principle of phasing, varying the time between a series of outgoing ultrasonic pulses in such a way that the individual wave fronts generated by each element in the array combine with each other to add or cancel energy in predictable ways that effectively steer and shape the sound beam.
This is accomplished by pulsing the individual probe elements at slightly different times. Frequently the elements will be pulsed in groups of 4 to 32 in order to improve effective sensitivity by increasing aperture, which reduces unwanted beam spreading and enables sharper focusing. Software known as a focal law calculator establishes specific delay times for firing each group of elements in order to generate the desired beam shape, taking into account probe and wedge characteristics as well as the geometry and acoustical properties of the test material. The programmed pulsing sequence selected by the instrument's operating software then launches a number of individual wave fronts in the test material. These wave fronts in turn combine constructively and destructively into a single primary wave front that travels through the test material and reflects off cracks, discontinuities, back walls, and other material boundaries like any conventional ultrasonic wave. The beam can be dynamically steered through various angles, focal distances, and focal spot sizes in such a way that a single probe assembly is capable of examining the test material across a range of different perspectives. This beam steering happens very quickly, so that a scan from multiple angles or with multiple focal depths can be performed in a small fraction of a second. The returning echoes are received by the various elements or groups of elements and time-shifted as necessary to compensate for varying wedge delays and then summed. Unlike a conventional single element transducer, which will effectively merge the effects of all beam components that strike its area, a phased array transducer can spatially sort the returning wavefront according to the arrival time and amplitude at each element. When processed by instrument software, each returned focal law represents the reflection from a particular angular component of the beam, a particular point along a linear path, and/or a reflection from a particular focal depth. The echo information can then be displayed in any of several formats.

What do the images look like?
In most typical flaw detection and thickness gauging applications, the ultrasonic test data will be based on time and amplitude information derived from processed RF waveforms. These waveforms and the information extracted from them will commonly be presented in one or more of four formats: A-scans, B-scans, C-scans, or S-scans. This section shows some examples of image presentations from both conventional flaw detectors and phased array systems.

A Scan Display
An A-Scan is a simple RF waveform presentation showing the time and amplitude of an ultrasonic signal, as commonly provided by conventional ultrasonic flaw detectors and waveform display thickness gages. An A-scan waveform represents the reflections from one sound beam position in the test piece. The flaw detector A-scan below shows echoes from two side-drilled holes in a steel reference block. The columnar sound beam from a common single-element contact transducer intercepts two out of the three of the holes and generates two distinct reflections at different times that are proportional to the depth of the holes.

A single-element angle beam transducer used with a conventional flaw detector will generate a beam along one angular path. While beam spreading effects will cause the beam diameter to increase with distance, the area of coverage or field of vision of a conventional angle beam will still basically be limited to one angular path. In the example below, a 45 degree wedge at one fixed position is able to detect two of the side-drilled holes in the test block because they fall within its beam, but it is not possible to detect the third without moving the transducer forward. A phased array system will display similar A-scan waveforms for reference, however in most cases they will be supplemented by B-scans, C-scans, or S-scans as seen below. These standard imaging formats aid the operator in visualizing the type and position of flaws in a test piece

B Scan display
A B-scan is an image showing a cross-sectional profile through one vertical slice of the test piece, showing the depth of reflectors with respect to their linear position. B-scan imaging requires that the sound beam be scanned along the selected axis of the test piece, either mechanically or electronically, while storing relevant data. In the case below, the B-scan shows two deep reflectors and one shallower reflector, corresponding to the positions of the side drilled holes in the test block. With a conventional flaw detector, the transducer must be moved laterally across the test piece.

C-Scan displays
A C-scan is a two dimensional presentation of data displayed as a top or planar view of a test piece, similar in its graphic perspective to an x-ray image, where color represents the gated signal amplitude at each point in the test piece mapped to its x-y position. With conventional instruments, the single-element transducer must be moved in an x-y raster scan pattern over the test piece. With phased array systems, the probe is typically moved physically along one axis while the beam electronically scans along the other. Encoders will normally be used whenever precise geometrical correspondence of the scan image to the part must be maintained, although unencoded manual scans can also provide useful information in many cases.
The images that follow show C-scans of a reference block made with a conventional immersion scanning system with a focused immersion transducer, and with a portable phased array system using an encoded hand scanner and a linear array. While the graphic resolution is not fully equivalent, there are other considerations. The phased array system is field portable, which the conventional system is not, and costs about one-third the price. Additionally, the phased array image was made in a few seconds, while the conventional immersion scan took several minutes.

S-Scan displays
An S-scan or sectorial scan image represents a two-dimensional cross-sectional view derived from a series of A-scans that have been plotted with respect to time delay and refracted angle. The horizontal axis corresponds to test piece width, and the vertical axis to depth. This is the most common format for medical sonograms as well as for industrial phased array images. The sound beam sweeps through a series of angles to generate an approximately cone-shaped cross-sectional image. It should be noted that in this example, by sweeping the beam the phased array probe is able to map all three holes from a single transducer position.

Where is phased array systems used?
Ultrasonic phased array systems can potentially be employed in almost any test where conventional ultrasonic flaw detectors have traditionally been used. Weld inspection and crack detection are the most important applications, and these tests are done across a wide range of industries including aerospace, power generation, petrochemical, metal billet and tubular goods suppliers, pipeline construction and maintenance, structural metals, and general manufacturing. Phased arrays can also be effectively used to profile remaining wall thickness in corrosion survey applications.
The benefits of phased array technology over conventional UT come from its ability to use multiple elements to steer, focus and scan beams with a single transducer assembly. Beam steering, commonly referred to sectorial scanning, can be used for mapping components at appropriate angles. This can greatly simplify the inspection of components with complex geometries. The small footprint of the transducer and the ability to sweep the beam without moving the probe also aids inspection of such components in situations where there is limited access for mechanical scanning. Sectorial scanning is also typically used for weld inspection. The ability to test welds with multiple angles from a single probe greatly increases the probability of detection of anomalies. Electronic focusing permits optimizing the beam shape and size at the expected defect location, thus further optimizing probability of detection. The ability to focus at multiple depths also improves the ability for sizing critical defects for volumetric inspections. Focusing can significantly improve signal-to-noise ratio in challenging applications, and electronic scanning across many groups of elements allows for C-Scan images to be produced very rapidly.

Process Compensated Resonance Testing (PCRT)
PCRT is an evolution of Resonant Ultrasound Spectroscopy (RUS), the analysis of the   resonant frequencies of a part to detect flaws. Developed by Los Alamos Nationals Lab and covered by ASTM Stand Guide E2001-08, RUS is based on the physics fundamental that rigid parts resonate at specific frequencies that are a function of mass, geometry, and material properties. PCRT combines RUS with proprietary pattern recognition algorithms, based on Mahalanobis- Taguchi System (MTS) statistical analysis, to identify the resonance spectra characteristics that represent acceptable parts and those caused by unacceptable defects. The operator is presented with a clear Pass/Fail result that requires no interpretation. This eliminates operator subjectivity and the need for expertise in analyzing the test results, enabling full automation. PCRT is the only NDT technology that can effectively compare the results of successive inspections to detect the accumulation of fatigue and damage.
Figure 1 shows examples of selected resonances for acceptable and unacceptable parts. The upper spectrum is a ceramic ball with no defects. The lower spectrum is for a ball with a C-spall crack as indicated by the resonant peak split. The structural defect in the part is directly reflected in the resonant spectra.
The PCRT System consists of a control computer, a transceiver, and a component nest. The computer, loaded with the PCRT software, provides the user interface and communicates with the transceiver. The transceiver is a precision signal generator and spectrum analyzer. The nest holds three lead-zirconate-titantate (PZT) contact transducers that interface with the part. One transducer excites the part while the other two measure the part’s resonant response. A PCRT system diagram is shown in Figure 3, and sample hardware is shown in Figure 4. The nest shown is for a ceramic ball. Testing a part is simple. The part is placed on the nest, the part temperature is recorded (so the software can compensate for frequency variation vs. temperature), and the testing software is run. Tests require no part preparation and are typically completed in seconds, yielding superior throughput and cost-effectiveness.

Optical NDT Technologies
Optical fibre NDT features the use of optical fibres to collect and sense the signal light from the
object under test (OUT). The optical fibre is resistant to electromagnetic interference and corrosion, which means that in can be used in extreme environments. The optical fibre is small and lightweight and could be either attached to the surface of the tested structure or be embedded inside, having little impact on the structure itself. Fibre optic sensors can be designed to measure or sense just about anything. A few examples include liquid level, chemical, pressure, electric field, vibration strain and temperature. There are many types of sensor designs, including Bragg, IFPI (Intrinsic Fabry Perot Interferometer) and EFPI (Extrinsic Fabry Perot Interferometer) [4]. While there are many types of optical sensors, the focus of this section is on the fibre Bragg grating sensor. When the physical quantities to be measured are strain, temperature, pressure, etc., the optical fibre itself can be the sensor. In these cases, the physical quantities to be measured can cause a change of the intensity, phase, polarization, wavelength, or transit time of the light in the fibre system, and NDT is implemented through monotoring these optical parameters.Light intensity monitoring is the simplest and most frequently used technology in practice. The most successful application of optical fibre NDT is the field of civil engineering.

Working
The optical setup for the polarization-based fibre optic sensor. It is formed by polarizing the light from a light source (polarized He-Ne laser) via a half-wave plate. The half wave plate here is used to rotate the polarization state of the plane polarized He-Ne light. That could be a length of polarization-preserving fibre. The fibre here serves as the sensing medium. Under external perturbation such as stress or strain, the phase difference between two polarization states is changed, and so the output polarization state is changed accordingly. Hence, by using PC analyzing the output polarization state at the exit end of the fibre, the external perturbation can be detected.

Fig(18)A structure of optical fiber polarization sensor detecting system
Optical fibre sensing is usually used as a non-destructive testing method at multiple locations for
long time monitoring, for single or multiple parameters like temperature and strain. To monitor the status of a large object or system, like a natural gas pipeline, multiple discrete optical fiber sensors are placed at different locations of the object or system, forming a distributed sensor network. Real-time parameters like temperature and strain are collected by the sensors and transmitted to a data processing center through an optical transmission network. This method uses the optical fibre as the sensing medium to monitor more than one physical parameters, as well as the transmission medium to combine the distributed sensor network. It has advantages of real-time monitoring, large data processing capability, high-integration, and excellent environment tolerance. Therefore, distributed fibre sensing technology is a hot research area today, and the multi-sensor network addressing (or multiplexing) and location (or demodulation) methods are important research fields in NDT.

Electro - Hydraulic Brake ( EHB )

Brake performance can be divided into two distinct classes:


1)      Base brake performance
2)      Controlled brake performance.

           A base brake event can be described as a normal or typical stop in which the driver maintains the vehicle in its intended direction at a controlled deceleration level that does not closely approach wheel lock.
All other braking events where additional intervention may be necessary, such as wheel brake pressure control to prevent lock-up, application of a wheel brake to transfer torque across an open differential, or application of an induced torque to one or two selected wheels to correct an under- or over steering condition, may be classified as controlled brake performance. Statistics from the field indicate the majority of braking events stem from base brake applications and as such can be classified as the single most important function. From this perspective, it can be of interest to compare modern-day Electro-Hydraulic Brake (EHB) hydraulic systems with a conventional vacuum-boosted brake apply system and note the various design options used to achieve performance and reliability objectives.

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INTRODUCTION

The next brake concept. This system is a system which senses the driver's will of braking through the pedal simulator and controls the braking pressures to each wheels. The system is also a hydraulic Brake by Wire system.

Many of the vehicle sub-systems in today’s modern vehicles are being converted into “by-wire” type systems. This normally implies a function, which in the past was activated directly through a purely mechanical device, is now implemented through electro-mechanical means by way of signal transfer to and from an Electronic Control Unit. Optionally, the ECU may apply additional “intelligence” based upon input from other sensors outside of the driver’s influence. Electro-Hydraulic Brake is not a true “by-wire” system with the thought process that the physical wires do not extend all the way to the wheel brakes. However, in the true sense of the definition, any EHB vehicle may be braked with an electrical “joystick” completely independent of the traditional brake pedal. It just so happens that hydraulic fluid is used to transmit energy from the actuator to the wheel brakes.  This configuration offers the distinct advantage that the current production wheel brakes may be maintained while an integral, manually applied, hydraulic failsafe backup system may be directly incorporated in the EHB system. The cost and complexity of this approach typically compares favorably to an Electro-Mechanical Brake (EMB) system, which requires significant investment in vehicle electrical failsafe architecture, with some needing a 42 volt power source. Therefore, EHB may be classified a “stepping stone” technology to full Electro-Mechanical Brakes.


HYDRAULIC DESIGN CONSIDERATIONS
FAILSAFE AND SYSTEM COMPLEXITIES
Analogous to a vacuum boosted system in base brake mode, EHB supplies a braking force proportional to driver input, which reduces braking effort. The boost characteristics also contribute to the pedal “feel” of the vehicle. If the boost source fails, the system resorts to manual brakes where brake input energy is supplied in full by the driver. As would be expected, the pedal forces vs. vehicle deceleration characteristics are significantly affected.

This is shown by the input pedal force vs. Brake line pressure output in Figure 1 of a typical vacuum boosted vehicle.
Looking at a comparison using the failsafe pedal force input limit of 500 N, the difference between the resulting    brake line pressure is 2.5 MPa unboosted vs. 8.5 MPa    boosted. This correlates to an approximately proportional difference in vehicle deceleration. In this example there approximately correlates to 0.3 g’s decel. Unboosted, and 0.9 g’s boosted. With EHB systems, there is room to improve this performance, but only at the expense of pedal travel, which becomes a hydraulic lever    arm of sorts. For example, to achieve a higher decel   from 0.3 g to 0.5 g in failed system, the pedal travel may    have to increase from 50 - 75 mm to perhaps 150 mm, which is about the practical limit for brake pedal travel.    Thus, due to the consequences of boost failure, careful    attention must be paid to component system design irrespective    of the type of mechanism employed.  
A comparison     between the conventional vacuum boosted system     and an EHB system is shown in Figure
The conventional system utilizes a largely mechanical link all the way from the brake pedal through the vacuum booster and into the master cylinder piston. Proportional     assist is provided by an air valve acting in conjunction with the booster diaphragm to utilize the stored vacuum    energy. The piston and seal trap brake fluid and transmit    the hydraulic energy to the wheel brake.  

Compare this to the basic layout of the typical EHB system.     First, the driver’s input is normally interpreted by up     to three different devices: a brake switch, a travel sensor, and a pressure sensor while an emulator provides the normal pedal “feel”. To prevent unwanted brake applications, two of the three inputs must be detected to     initiate base brake pressure. The backup master cylinder     is subsequently locked out of the main wheel circuit     using isolation solenoid valves, so all wheel brake pressure     must come from a high-pressure accumulator     source. This stored energy is created by pressurizing brake fluid from the reservoir with an electro-hydraulic pump into a suitable pre-charged vessel. The accumulator pressure is regulated by a separate pressure sensor     or other device. The “by-wire” characteristics now come     into play as the driver’s braking intent signals are sent to the ECU. Here an algorithm translates the dynamically changing voltage input signals into the corresponding     solenoid valve driver output current waveforms.
As the apply and release valves open and close, a pressure sensor at each wheel continuously “closes the loop” by   feeding back information to the ECU so the next series of current commands can be given to the solenoid   valves to assure fast and accurate pressure response.    

It is obvious the EHB system is significantly more complex in nature. To address this concern, numerous steps   have been taken to eliminate the possibility of boost failure   due to electronic or mechanical faults. In the ECU design, component redundancy is used throughout. This       includes multiple wire feeds, multiple processors and internal circuit isolation for critical valve drivers. The extra components and the resulting software to control them, does add a small level of additional complexity in itself. Thermal robustness must also carefully be designed       into the unit, as duty cycles for valves and motors will be higher than in add-on type system. Thus, careful attention must be given to heat sinking, materials, circuit designs, and component selection. Special consideration       must be given to the ECU cover heat transfer properties, which could include the addition of cooling fins. On the mechanical side there is redundancy in valves and wheel brake sensors in that the vehicle may still be braked with two or three boosted channels. In regards to the E-H pump and accumulator, backup components   are not typically considered practical from a size, mass, and cost viewpoint. However, these few components are extremely robust in nature and thoroughly tested to       exceed durability requirements.    

The second area used to evaluate potential failure concerns   is through the study of past warranty data of similar systems. The system chosen for comparison was an early ABS system integrated into a hydraulic booster. The data was collected from two different North American passenger vehicles built in the early 1990’s at a 12-month PPM level. Both vehicles utilized a central hydraulics unit that in turn supplied power to the hydraulic brake booster and ABS block. The data in Table 1 represents an approximation of warranty comparison based upon an averaging of returns from both vehicle lines. Note any vehicles requiring a vacuum pump (such as diesel) would also have to take those failures into consideration for the baseline calculation.
 
Although the total failure frequency is higher, many of the failures may illuminate the fault light on the dashboard, but would not affect the base brakes. For example, there is sufficient redundancy in sensors and in hydraulic valve block components that the vehicle would still maintain boosted braking on the unaffected wheels. As previously noted, multiple feed wires and grounds are being employed which could therefore negate many of the concerns related to wiring harness defects. In similar         fashion, many of the ECU failures would also not result in loss of the base brake boost function. Thus, when adding the E-H pump and some smaller percentage of wiring and ECU failures, the total combination that would affect base brake performance could be expected to be         closely the same or even less than the conventional system. This type of comparison using ten-year-old data is only a guideline since modern technology and manufacturing methods continue to make both electronic and mechanical components more reliable.
 

BASE NON-ISOLATED HYDRAULIC CIRCUITDESIGN

Designing for base brake systems poses a challenge to be able to utilize the same hydraulic components to  meet two extreme braking conditions. One is a panic mode situation, where an extreme amount of hydraulic          energy needs to be transmitted through the brake system in a very short amount of time in order to apply the wheel brakes as quickly as possible. Current specifications    typically call for reaching pressures at the wheel          brakes of approximately 8 MPa in 120 milliseconds or  less. For a typical midsize vehicle, this translates into average power requirements of 1,200          watts with flow rates in excess of 40 cm3/s at each wheel brake. The second challenge is to be able to modulate pressures in a very stiff system when the brakes are applied. Pressure resolution of approximately 30 kPa is required. The problem of control becomes apparent. Very small quantities of brake fluid must be sufficiently modulated to give a good base brake pedal feel. To meet the requirements the selected control valves must  be designed to have very good response time characteristics          (i.e. < 10 millisecond) with relatively unrestricted flow paths. The basic means to achieve wheel brake modulation comes from using two normally closed proportional control valves per wheel brake. The apply valve regulates flow from the high pressure central accumulator to the           wheel brake, while the release valve regulates flow from the wheel brake back to reservoir, which is maintained at  atmospheric pressure. A typical single wheel schematic is shown in Figure 3.  

For failsafe operation, it becomes necessary to include  an isolation valve between the pedal feel emulator -master cylinder (PFE-MC) assembly and wheel brake. Its functions include blocking the driver’s manual output pressure during a boosted apply as well as providing a vent path back to reservoir when the brakes are not activated. Additionally, a balance valve is placed between wheel brakes on each axle to prevent momentary pressure imbalance during panic-type base brake applies.           This design is especially well suited for front/rear (T-T) type of systems since the master cylinder circuits are also allocated to each axle. In this design the accumulator  circuit leads directly into the master cylinder and           wheel brake circuits through the apply valve as is shown  by the arrows on the graph.
Most EHB’s utilize brake fluid stored in a central, gas pressurized            accumulator. Typical sizes for a North American midsize vehicle may range from 200 to 300 cm3. A typical accumulator pressure operating range may be 16 MPa (pump turn on) to 18 MPa (pump turn off). The gas most commonly used is nitrogen due to its relatively low cost and relative inertness. The nitrogen gas is kept separated from the brake fluid by either an elastomeric or metallic membrane or diaphragm. Most            elastomeric membranes have a single, curved shape which folds back upon itself as the device fills with brake fluid. The all-metallic type of membrane is usually in the shape of a bellows with a number of folds (much like an accordion) and relies on thin plate bending with large            deformations and low stress levels to accomplish the task of displacing brake fluid. Due to the small size of the nitrogen molecules, permeation is also a factor to consider, particularly with elastomeric types of diaphragms. The nitrogen gas will typically find its way through most elastomeric materials, and enter the molecular “pores” within the spaces of the pressurized brake fluid volume until all of the voids are filled. At that time, equilibrium is re-established and finally permeation diminishes.
High temperatures may also accelerate this phenomenon. With the latest multi-layer proprietary materials being developed, certain accumulator
manufacturers are claiming improvements in permeation reduction of five to six times. Thus, estimated useful life is now in the range of 10 – 15 years.

Failure Mode Considerations – Non-isolated Circuit

Returning to considerations for the high pressure accumulator. This device stores significant amounts of energy, typically as much as 1,700 watt-seconds. This has the advantage of being able to supply numerous (i.e. 5 –15) reserve stops should the electro-hydraulic pump fail. It was previously noted the pressurized nitrogen gas was separated from the brake fluid by one of two types of diaphragms. Even though the latest versions of both these devices have become extremely reliable through years of development, it might not yet be possible to classify either of these types of units as a true zero defect type of device since manufacturing quality must always be considered. Therefore, the consequence of diaphragm failure must be investigated. A test was devised utilizing a non-isolated wheel brake circuit of the type shown in Figure 3. A carefully constructed accumulator with a small hole punctured in the diaphragm was installed in a vehicle. The brakes were subsequently applied and released at discreet intervals to study any change in operating characteristics. The graph in Figure 4 below shows the status of the measured brake pedal force and travel parameters after 100 powered base brake applies, where functionality was shown to be normal. (The unit was fully checked every 50 strokes.)
The pedal feel emulator-master cylinder in this test had a lockout feature. The bottom curve represents the system in normal powered mode showing the simulated pedal travel. The top curves shows brake performance in failsafe mode. At stroke number 114 of the brake pedal, the diagnostics of the ECU detected a “pressure out-of-bounds” failure indicating base brake output pressure was no longer able to follow the driver’s brake pedal input commands. The system immediately reverted to the hydraulic failsafe backup mode.

Figure 5 is a plot of the brake system performance just one stroke after the failure occurrence. In this case, in the failed system backup mode, the PFE-MC assembly achieved the full travel of 120 mm with an input force of only 45 N. This is the approximate force required to displace the master cylinder springs to full travel with no hydraulic load present and indicates there is minimal pressure output to the wheel brakes.
Attempts were made to find where the nitrogen gas might have migrated to in the system. The circuit was first partially re-bled just between the master cylinder and isolation valve inside the hydraulic control unit (refer again to Figure 3). The results are shown in Figure 6.

The brake pedal force reached approximately 37% of its previous full-travel pedal force value prior to failure. This indicates a significant percentage of the nitrogen also
made its way into the HCU-to-wheel brake circuit. As a final step the system was then bled between the HCU and wheel brakes. The results of the re-bleed are shown in Figure 7. Note this curve is nearly identical to that shown in Figure 4 indicating all of the escaped nitrogen gas had been removed from the brake circuits. The test was subsequently repeated with similar results. The gas expulsion could not be anticipated with the diagnostic methods utilized at the time.

These test results verify the severity level of nitrogen discharge due to a defective accumulator diaphragm. The total amount of gas which migrated into the wheel brake and master cylinder circuits was not measured but was at least equivalent to the aster cylinder volume. One technique to assure some level of braking can still be maintained with this type of failure is to allow the pump to run in a continuous mode to eventually compress the discharged gas and subsequently build wheel pressure. The effectiveness of this solution will be determined by pump flow rate and the quantity of gas discharged. For a pump with a nominal flow rate of 8 cm3/s any substantial quantity could result in relatively slow braking response times. If the gas permeates the master cylinder circuit, there could be limited or no force feedback from the pedal feel emulator, which would result in abnormal pedal feel.

 One method to maintain a non-isolated circuit is to employ accumulator travel sensing. This is accomplished by incorporating a suitable sensor to track the displacement of the accumulator membrane and works especially well with the metal bellows type of unit. In addition to the travel associated with the normal filling and release of fluid, the metal bellows also has an elastic memory. Thus a defect in the bellows, which allows brake fluid to begin to fill the accumulator can be immediately detected and shut down boosted braking to prevent the possibility of gas expulsion. It is also necessary to know temperature information with this approach to be able to account for pressure variations due to temperature changes. Since the open flow path to the wheel and master cylinder circuits are still present, the sensing method must be robust.

ISOLATED HYDRAULIC CIRCUIT DESIGN
An alternative approach is to utilize an isolated base brake circuit. A typical single-wheel circuit is shown in Figure 8.

There are some distinct differences between the isolated and non-isolated concepts. The first, and most obvious is the addition of an isolation piston assembly between the pump circuit and each wheel brake circuit which will positively stop nitrogen from entering the wheel brake circuit. The arrows in the graph highlight the restricted
flow path. For design simplification, cost reduction, and improved durability, a single seal design is shown although a dual seal, vented design may also be substituted.The other change is that the proportional release valve is normally open. This provides an open flow path back to reservoir, which is independent of the wheel brake circuit. Any escaping nitrogen from the accumulator will have an unrestricted path back to reservoir in failsafe mode. Also note that the balance valve is placed in the pump circuit and may now be a normally open valve for either the T-T or X type of hydraulic circuits.
         Another indirect benefit with this approach is the amount of nitrogen gas, which can be permanently trapped, is limited to the drilled holes in the HCU housing, the clearance volume behind the isolation piston, and the volumes around the proportional control valves. Therefore, running the pump to boost brake output in the event of accumulator diaphragm failure is likely to be more effective.

As isolation piston previously mentioned included only a single seal. Although this solution raises the question of introducing a latent (i.e. undetectable) failure, there are
means, both in plant and algorithmically, of detection. The many benefits of using a single seal include occupation of less space, fewer holes to drill, and fewer components, all of which translate to saving cost. In addition, seal stress loading is reduced by maintaining the seal in near pressure equilibrium. This has the added benefit of reducing wear and reducing frictional forces with the bore. The seal force values shown in Figure 9 were derived from prior generic seal testing.

To be able to detect a missing or defective seal, a suitable algorithm must be employed. State-of-the art assembly plant air testing can detect functionality of a seal
in a bore. However, even though this is assured as a new product, it does not assure seal functionality over the ten to fifteen year design life. A leak test procedure, which can be performed on the vehicle, however, may be implemented. The procedure is outlined in detail in Figure 10.
One other step required in assuring a failsafe approach to an EHB design which utilizes isolated wheel circuits, is understanding the volume relationships between the
three variable displacement devices: wheel brake, isolation piston, and master cylinder. The typical wheel cylinder and master cylinder may be simply represented as a piston inside a bore as shown in Figure 11.

By knowing the exact volumetric relationships between wheel brake, isolation piston, and master cylinder, a system can be designed that will assure failed system performance. For example, in the lower half of the diagram above, the piston and cylinder assemblies each graphically represent the volumetric comparisons between the three aforementioned devices. First, consider the brake circuit between the HCU and master cylinder. Note the compliance this circuit may be accurately assessed from knowing the travel position of the master cylinder piston. This is true even when pedal feel emulator displacement is considered, since that is also a fixed pressure-to volume relationship and is known from the component geometry. To further test for system effectiveness, the isolation piston may be applied to a target pressure by appropriate activation of the apply valve. If the wheel brake circuit is not able to achieve the requested pressure, then system compliance is excessive, and appropriate
warning can be issued. If, on the other hand, the target pressure is achieved at the wheel brake, the system is functioning properly. The isolation piston may also be utilized as a means to purge the master cylinder circuit as referred to in step no. 2 of Figure 10.
One additional factor for EHB failsafe braking must also be considered which can best be defined as “base brake system compliance allowances”. These are the factors
that can contribute to a soft or spongy brake pedal such as from brake pad warpage or distortion from aging or abuse. These allowances should be recognized and included in initial component sizing. In summary, careful employment of isolation pistons, with accompanying diagnostics can be an effective solution for accumulator isolation.

2-WHEEL VS. 4-WHEEL FAILSAFE MODE

Yet another area of failsafe performance which requires consideration is 2-wheel vs. 4-wheel manual operation. Past hydraulically boosted, backup systems in the field
have largely been configured with 2-wheel backup. However, utilizing a four-wheel failsafe approach offers more design flexibility, and potential stopping distance
reduction on certain classes of vehicles.
The first step is to evaluate suspension and vehicle dynamics. In some vehicles, such as those with front wheel drive and a higher center of gravity, there may be insufficient normal force on the rear wheels during a moderate braking stop in the range of 0.4 to 0.6 g’s that use of rear wheel brakes is not very efficient. However,
there are some classes of vehicles, particularly rear wheel- drive cars and trucks, as well as some of the larger front-wheel-drive cars, where there may be sufficient wheel-to-road braking torque available. For those vehicles use of all four wheels for emergency braking could be considered.
The next step is to measure is front vs. rear wheel brake relative efficiency. The variable in question is defined as:

WHEEL BRAKE RELATIVE EFFICIENCY

Rel.Efficiency = OUTPUT TORQUE T (p)
INPUT ENERGY = P * V (p)

In this instance, the input hydraulic energy is the brake line pressure capable of being generated by the driver times the displacement used to achieve that pressure. Thus, from an energy viewpoint, it is better to utilize the wheel brakes on a vehicle that have the highest specific torque output at the lowest displacement. Some rear wheel brakes (disc or drum) tend to have similar specific torques to the front brakes, yet require less fluid displacement. By taking instantaneous wheel brake torque, pressure, and displacement readings, the measure of wheel brake relative efficiency can be plotted, as shown in Figure 12.
In the example given, it will be more efficient to utilize front and rear brakes together to minimize stopping distances, provided vehicle dynamic conditions are met. Delphi utilizes a in-house computer program to estimate stopping distances. This tool takes into account all of the variables mentioned and combines them with a vehicle suspension model, which captures such factors as weight transfer, to calculate decel and stopping distance.
Actual vehicle data is taken to be able to input wheel brake specific torque. Additional information about the base brake system is then fed into the model to calculate a parameter called apply system gain. This variable is defined as the pedal ratio divided by master cylinder bore area.

A 3-dimensional plot of apply system gain is shown in Figure 13. The larger the gain, the larger the mechanical advantage in transferring energy from the driver’s foot to the wheel brake, and the larger the pressure which can be applied for a given input force. The tradeoff is pedal travel. It is also increased in proportion to mechanical gain, which ultimately limits the amount of gain for the entire system. An output plot for a typical midsize vehicle is shown in Figure 14. In this single graph, the failsafe performance of the selected system may be analyzed for deceleration and pedal travel at both LLVW and GVW conditions.

When evaluating the question of 2-wheel vs. 4-wheel braking, this program was used to evaluate deceleration capabilities of the same vehicle but with either one or both axles active in braking. The estimates for a specific North American light duty truck are shown in Table 2.
In this particular case, there was a 32% increase in estimated vehicle deceleration at LLVW and a 37% increase in estimated vehicle deceleration at GVW. It may also be observed higher apply system mechanical gains are required for 2-wheel brakes while trying to achieve equivalent vehicle braking forces. This can create some practical problems. Pedal ratios can become very large which dictate specially designed pedals to maintain a minimum master cylinder push rod arc length, or conversely, master cylinder bores become very small (i.e. less than ø 19.0) with long travel requirements.

As previously noted, not all classes of vehicles necessarily show significant improvement in moving from 2-wheel to 4-wheel backup braking mode. However, designing for only 2-wheel failsafe may exclude optimization on some types of  vehicles that can benefit from 4-wheel braking. The range of improvement from the limited numbers of vehicles, which have thus far been analyzed, has run from a few percent to over 30%. In each case, cost vs. performance trade-off should be evaluated when selecting the final design since additional hydraulic components may be required for 4-wheel backup.

PEDAL FEEL EMULATOR LOCKOUT

The graph in Figure 16 shows a typical, customer requested force-displacement curve required for the emulator.
Figure 17 shows a typical hardware arrangement to meet the pedal feel requirements. This unit consists of a master cylinder with emulator piston and spring assembly. As the driver’s foot applies the brake pedal, an input push rod displaces the primary master cylinder piston, while at the same time the isolation valves in the HCU are commanded to close. This blocks both primary and secondary master cylinder outlet ports. The secondary piston becomes locked in place due to the trapped fluid. The fluid contained by the primary piston is displaced into the drill path, which leads to the emulator assembly. As pressure continues to build, the spring begins to deform under the load from the hydraulic pressure acting on the surface of the piston. This causes additional displacement that allows the brake pedal to move in proportion to the force exerted by the driver. The force vs. travel characteristics can be “tuned” to customer directives. If the vehicle is required to stop in failed system, the isolation valves remain open so that fluid is permitted to flow to both the wheel brakes and the emulator.

Since emulator piston deflection occurs at relatively low pressures, the compliance of the wheel brakes and emulator are additive, which results in additional pedal travel. Thus the driver’s total available input energy for failsafe braking is reduced by the additional emulator displacement which does not contribute to vehicle braking.

One solution is to incorporate a solenoid valve lockout device in-between the emulator and master cylinder. Whenever a failure is detected, the ECU would causethe valve to close to prevent the unwanted displacement. A second, more cost-effective approach to consider is to incorporate a seal bypass arrangement in the main bore as shown in Figure 18. In normal boosted operation, as previously noted, the primary and secondary outlets are both blocked by the isolation valves. Since the secondary piston is held rigidly in the bore, the fluid from the primary piston is permitted to flow around the by-pass lip seal as shown in Figure 19 and into the drilled path for the emulator.

As before, in failsafe braking, the isolation valves open and both primary and secondary piston circuits are now directly connected to the wheel brakes. As the secondary piston can now move forward, the lip seal re-enters the main bore and keeps the higher pressure primary circuit from losing any additional fluid to the emulator. It should be noted, however, with any types of lockout mechanism that they are ineffective should there be an “in-stop” failure. Once the PFE displacement is utilized, that amount of pedal travel is lost until the subsequent stop. Thus, worst case conditions must always be considered in the final design process assuming no lockout present.

CONCLUSION

Similar to the days of early ABS introduction, multiple EHB hydraulic design configurations have emerged. From the mid 80’s through the latter part of the 1990’s
numerous ABS configurations ranging from hydraulically boosted open systems, to four valve flow control designs, to modulators based upon ball screws and electric motors came to market before the 8-valve, closed recirculation system became the de facto standard. As with any new technology, there are concerns and tradeoffs to be dealt with. In the case of the electro-hydraulic brake they center around increased electrical and mechanical complexity, failsafe braking performance, accumulator safety, and 2-wheel versus 4-wheel backup modes. Each of these concerns has been answered by prudent designs and incorporation of new component technologies. The configuration adopted in Delphi’s EHB development  has included use of four-wheel failsafe with individual isolation pistons and utilization of mechanical pedal feel lockout. This particular design allows system flexibility, inherent accumulator precharge isolation, and the ability to tune for optimum failed system stopping performance for all vehicle classes.
 Ultimately, no matter which final configuration is selected for a specific vehicle platform, it will have to undergo the rigors of full brake system validation. A carefully de-signed and implemented EHB system holds the promise of enabling the new brake-by-wire features while still reliably performing the everyday task of stopping the vehicle.