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Air jets imaging tactile sensing device for automation applications

R. Benhadj and B. Dawson

Robotica / Volume 13 / Issue 05 / September 1995, pp 521 529DOI: 10.1017/S0263574700018361, Published online: 09 March 2009

Link to this article: http://journals.cambridge.org/abstract_S0263574700018361

How to cite this article:R. Benhadj and B. Dawson (1995). Air jets imaging tactile sensing device for automation applications. Robotica, 13, pp 521529 doi:10.1017/S0263574700018361

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Robotica (1995) volume 13, pp 521-529. © 1995 Cambridge University Press

Air jets imaging tactile sensing device for automationapplicationsR. Benhadj and B. DawsonOn-Line Surveillance, Monitoring and Diagnostics Unit (OSMAD), School of Mechanical and Production Engineering,Kingston University, Roehampton Vale, Friars Avenue, London SW15 3DW (UK)

(Received in Final Form: October 4, 1994)

SUMMARYThis paper details the design principles of operation of apneumatic proximity-to-tactile sensing device for parthandling and recognition in a flexible manufacturingenvironment. The sensing device utilises a denselypacked line array of piezoresistive pressure sensors,providing continuous variable outputs. The sensing planeof the device incorporates a corresponding line array ofair jets which develop an air cushion when striking atarget of interest. The back pressure levels from these airjets form the basis for the task of target detection andrecognition.

KEYWORDS: Tactile sensing; Automation; Pressure sensors;Air jets imaging.

1. INTRODUCTIONIn developing a tactile sensing system, an overallstructure is usually adhered to which takes the followingform.

• A discretely distributed array of sensing sites isplanned;

• a corresponding number of signal conditioning circuitsare devised for the purpose of data acquisition;

• a medium or a membrane is used to transmit the effectof the desired stimulus to the sensing sites.

The sensing array consists of sensing elements that aretransducers which ideally respond to a form of stimulussuch as pressure,1"3 light,4-5 heat6-7 or displacement.8 Thesignal conditioning is used to relate the output record ofthe transducer (usually voltage or current) to themagnitude of the stimulus source. A medium ormembrane is usually used to transmit the effect of theexternal stimulus to the transducer, whilst in many casesprotecting the sensing elements from environmentalconditions. For instance, soft membranes such as rubberare used to prevent direct contact of a metallic objectwith a piezoresistive IC pressure sensor in order to avoiddamage to the sensor and short circuiting it. However,rubber allows direct transmission of pressure stimulus tothe IC pressure sensor.

In designing tactile sensing arrays, a number ofimportant factors must be considered. The mostimportant consideration is to ensure that an adequate

sensing resolution for a given application is planned for.The sensing resolution is further subdivided to spatialresolution (i.e. positioning) of sensing elements in thearray (on the surface of the sensing plane), and themeasurement sensitivity of the sensing elements at eachlocality. Tactile sensing is a relatively new technology,and has been mainly applied within the field of robotics.In this field, tactile sensors have been developed andused to emulate the human sense of touch. Therefore,the sensing resolution is often planned to closelyapproximate human mechanoreceptive capabilities.Complex configurations of tactile sensing, such as thoseof humans, are beyond the scope of the current state ofthe art. Most tactile sensing devices are devised torespond to the changes relating to a specific parameter ofinterest such as pressure or displacment. The sensingresolution along the sensing plane of approximately1 mm is found to be close to human touch reception.9 Interms of pressure stimulus, the most sensitive part of theskin is on the index finger, having a detection resolutionof 6 fim indentation or 6.8-10 grammes per centimetresquare for a comfortable touch.10

Other considerations include the choice of the sensingelement and the substrate or matrix that contains thesensing elements. With the transducer (i.e. sensingelement), one important consideration is to ensure that acontinuously variable output is achieved. This refers tothe sensor's capability to respond to changes in the valueof the applied stimulus in a continuous fashion. Themeasurement characteristics of the sensing elementsshould ideally be linear, hysteresis free and have a fastresponse time.1142 In reality, however, all sensingelements exhibit non-linear characteristics somewhere intheir working range. An important consideration istherefore the extent of the operating range for which alinear response to an external stimulus can be expected.The same consideration is also true of the effect ofhysteresis. For instance, with piezoelectric straingauges, the sensor response is quite non-linear and canbe affected by hysteresis when the device is supplied witha constant voltage. A constant current supply can beused with this device in order to ensure a more linearoutput response.

Other factors that might affect the performance oftactile sensors are fatigue of the membrane or substrate,drift in conductivity or saturation, noise and environ-

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522

mental conditions. Miniaturisation of sensing elementshas two main advantages. Firstly, it enhances the sensingresolution as denser arrays of these elements can beconstructed. Secondly, with smaller devices problems dueto non-linearity diminish. Miniature single crystal siliconsensing devices can now be machined, giving a robustmicromechanical structure.

In this paper, a novel design of a high resolutionpneumatic proximity-to-tactile sensing device is de-veloped based on a line array of back pressure air jetswhich monitor any change in the pressure of an aircushion. The air cushion pressure is generated by an airflow which strikes a target, and the air flow leaves an airchamber through an air nozzle. Each back pressure airjet is designed so that it is completely isolated from theair flow in the air chamber in order to avoid anyinterference between the air flow in the air chamber andthe back pressure air jet. The design of the highresolution proximity-to-tactile sensing device was de-veloped via a number of iterative designs to meet thedemands of a flexible, robust and cheap to manufacturesensing device.

2. PRINCIPLE OF DESIGN AND OPERATIONIn this paper the design principles of operation of a hardcompliant tactile sensing device is presented. The reasonfor opting for a hard compliant device is that it enablesrepetitive use without having to make contact betweenthe sensing plane and objects of interest, therebyeliminating problems such as wear and damage thatmight be caused to the sensing plane. In choosing a hardcompliant device, an active medium must be used totransmit the effect of the external stimulus to the sensingelement. A fluid or light are the obvious choices sincethey are cheap, wear resistant and easy to obtain. Dry airin particular is available in most industrial plants andlaboratories. It has no corrosive or contaminating actionas other fluids may have, and requires very little controlas is usually required when light is used as a transmittingmedium. Therefore, it was specified that the designwould be centred around the use of air as a transmittingmedium.

With air, two different approaches to measurementscan be adopted. Firstly, pressure can be utilised as aparameter which responds to the changes caused by theexternal stimulus. Secondly, flow rate or flow velocitymay be used. The former was selected as it wouldcorrespond more directly to pressure receptance incutaneous tactile sensing. In order to ensure sufficientsensing resolution, air flow should be concentrated as jetsand the pressure would normally be monitored fromindividual corresponding sensing elements. The con-figuration chosen was to measure the back pressure to anair jet striking a target. Each jet was then considered as asource of excitation (stimulus) that is monitored by acorresponding back pressure jet that leads to a sensingelement. In this way one may control the source ofstimulation (air jet pressure), monitor the feedback (backpressure jet) and ascertain information about the targetwhich affects the feedback pressure. An array of such

Tactile sensing

supply air jets and corresponding back pressure feedbackjets can provide quantitative information about theprofile of target surfaces.

2.1 Air flow in the tactile sensing blockTo assist in the design of the sensing device, anunderstanding of the theoretical behaviour of air flows inthe sensing device is helpful and is developed via thethree stages shown in Figure 1 and represented as FlowA, Flow B and Flow C. Flow A is the air flow leaving theair chamber of the sensing block through a nozzle. Theair flow is developed between the air chamber and thesensing plane. Flow B is the air flow between the sensingplane and the target, and Flow C is the air flow throughthe back pressure air jet.

A large volume air chamber is constructed in order tomaintain a constant pressure supply. The length of theback pressure air jet is kept to a minimum in order toreduce the pressure losses along the air jet length andalso to increase the volumetric flow accordingly. Thisalso ensures a sufficient pressure gradient between thejet's inlet and the atmosphere to sustain a high flow ratethrough the air chamber and the back pressure air jet.

The velocity distribution of the air which flows steadilyparallel to the axis in the annular space between twocoaxial cylinders of radii R and r, (i.e. Flow A) as shownin Figure 2, is given by the following equation (1).

Where /x is the air viscosity.

In r + a2) (1)

From the velocity distribution Vx(r) as shown in Figure 2,the two constant of integration A and B are determinedfrom the following boundary condition.

Vx = 0 at r = R and r = r{

From these boundary conditions, the constants ofintegration ax and a2 are determined as follows:

'-& ' " ' -(*)(2&3)

Therefore by substituting the two constant of integratingequations (2) and (3) in equation (1), the air jet velocitybecomes:

V^—^itdx (4)

The rate of volumetric flow is derived by doubleintegrating the velocity:

a-J/v.rdrdd (5)



Tactile sensing

AIR CHAMBER

523

TARGET

BACK PRESSURE AIR JET

Fig. 1. Air flow in the tactile sensor configuration.

AIR NOZZLE

SENSING PLANE

and hence the pressure drop along the air jet is given bythe following equation:

dp = -*(*)

(6)

From equations (1) and (2), the pressure drop (dp) canbe calculated and therefore the pressure (P2) of the airflow leaving the air chamber through the nozzle can beevaluated. The flow rate through the air chamber and thepressure entering the air chamber (P{) are set by aflowmeter and pressure regulator respectively.

When an air flow strikes a solid surface (i.e. a target ofinterest), it does not rebound from the surface as arubber ball would rebound. Instead, some of the air flowescapes on all sides of the target and an air cushion isformed in between the sensing plane and the target asillustrated in Figure 3(i), 3(ii) and 3(iii). The air flowleaves the air chamber with a velocity and pressure as

defined by equations (1) and (2). This creates a pressureat the surface of the target as an air cushion and this isassumed to be as follows:

-Pcushion = (Pressure leaving air chamber - Pressure loss)

The pressure .loss between the sensing plane and thetarget surface is related to the gap size (dx). Therefore,as the gap (dx) between the sensing plane (or the end ofthe air chamber) and the target surface reduces, thepressure loss decreases accordingly. It can therefore beinferred that as the gap (dx) reduces, the air cushionpressure tends to equal the pressure leaving the airchamber and therefore the pressure at the entrance ofthe back pressure air jet. Another important factor is theforce of the air flow that strikes the target surface. Thisforce should not exceed the force necessary to hold thetarget in equilibrium, the force normal to the targetsurface is determined as follows:

Fx = pVxQx (1)

where: p is the density of air.The air flow through a straight pipe of circular

cross-section with axial symmetry (in this case the back

AIR CHAMBERSACK PRESSURE AIR JET

BETWEEN THE AIR CHAMBERAND THE SENSING PLANE

Fig. 2. The air flow between the air chamber and the air jet nozzle.



524 Tactile sensing

AIR SUPPLY

AIR CUSHION

BACK PRESSURE

BACK PRESSUREAIR JET

Fig. 3(i). Flow distribution over a flat target.

pressure air jet) is similar to the preceding case asgoverned by Poiseuille's law applicable to blood flow innarrow vessels. Figure 4 illustrates the flow through aback pressure air jet where the volumetric flow rate Qx

through the entire cross-section is given by the followingequation:14

Qx = nR2Vav (8)

The average velocity in Hagen-Poiseuille flow,14 can beobtained as follows:

t = - ; V/W9 (9)

From equations (4) and (5), (dp) is the pressure changethrough the back pressure air jet (P2 - Pi). Pi is thepressure at the entrance in the air jet and isapproximately equal to the pressure of the air cushion. Pl

is the exit air pressure through the back pressure air jetwhich is monitored by the IC pressure sensor. If it isassumed that the back pressure air jet length (dx) is veryshort so that the pressure drop in the back pressure airjet tends to zero, then the input air jet pressure (or aircushion pressure) can be assumed to be equal to the exitpressure and therefore to the IC pressure sensor. Fromthis assumption, the flow rate Qx through the back

AIR SUPFUY

AIR CUSHION

BACK PRESSURE

•ACK PRESSUREAIR JET

AIR SUPPLY

Fig. 3(ii).. How distribution over a hemisphere.



Tactile sensing 525AIR SUPPLY

BACK PRESSURE


AIR CUSHION

AIR SUPPL.V

Fig. 3(iii). Flow distribution over a hemisphere.

pressure air jet can be evaluated from equations (4) and(5). Knowing the input flow rate through the airchamber, the flow rate and pressure loss between thesensing plane of the sensor block and the target can beapproximated.

Any decrease in the air jet length (dx) will result in adecrease of the pressure loss in the air jet as mentionedabove. Thus the shorter the air jet the less pressure dropis obtained in the air jet. This is one important designconsideration, while another consideration is thediameter of the back pressure air jet. In designing thetactile sensor, the diameter should be as small as possibleto enable a number of air jets to be compacted in a smallarea in order to achieve a high spatial resolution device.Equations (4) and (5) suggest that with a larger backpressure air jet diameter a lower pressure drop isachieved.

The theory outlined above proves useful in under-

standing the effect of parameters such as the dimensionsof the air chamber, the nozzle diameter and the lengthand diameter of the back pressure air jet. Theseparameters are very important in the tactile sensordesign. The derived equations are also helpful indetermining the amount of air supply one needs in orderto meet the design requirement. A low pressure supplywill limit the tactile sensor sensing sensitivity. Although ahigher pressure supply increases the tactile sensorsensitivity, higher pressure can destroy the IC pressuresensor element. It is therefore desirable to supply thedevice with an optimum pressure supply.

3. SIGNAL ACQUISITION AND PROCESSINGA simple electronic circuit is used to monitor the signalgenerated by the pressure sensor. A constant current of1 mA is used to energise the bridge circuit formed by the


FLOW THROUGH THEBACK PRESSURE AIR JET

Fig. 4. Air flow through a back pressure air jet.



526 Tactile sensing

four piezoresistive strain gauges. A high precisionpotentiometer is incorporated within the bridge circuitwhich allows the calibration of the output signal of thespan (bridge circuit output) and sets it to a zero readingat any back pressure. The analogue signal generated bythe bridge circuit (the sensor span) is then amplified by ahigh input impedance differential JFET type operationalamplifier with an output voltage gain of 17.8. Acombination of a diode and a zener diode are used at theoutput to prevent the signal from being dropped to avalue under 0 volt (or precisely negative signals) or tosignals exceeding 5 volts as the signals are subsequentlyfed to a digital circuit (A/D analogue to digitalconverter). The amplified analogue signal goes throughan analogue to digital converter before the signal isprocessed.

3.1 Sensing elementThe sensing element employed is a piezoresistive ICpressure sensor which uses a mechanical spring elementin the form of a diaphragm. The back pressures aredirected along the hollow tubes, applied to the springelements, and converted into mechanical strains which inturn provide continuous voltage outputs. The diaphragmis usually fabricated by employing a special anisotropicetching technique. This allows a number of diaphragmsto be produced on a thin silicon wafer. A pyrexconstraint plate is bonded to the silicon diaphragm platein order to isolate the sensing element from the packagestress. The silicon diaphragm is subjected to thedifferential pressure (Px - P2), where P2 is the pressure atwhich both plates were sealed together and corrected foroperating temperature. To measure the stress in theN-type silicon diaphragm, four P-type resistors (straingauges) are employed. These strain gauges are obtainedthrough a selective diffusion of boron into the silicondiaphragm, and the bonding between the strain gaugesand the diaphragm is achieved through the atomicstructure of silicon. This type of bonding eliminates theeffect of creep, which is the major source of instability inmetaliic or bonded types of strain gauge sensors.

Two of the strain gauges are located in an area ofcompression, while the other two are in an area oftension. Electrically they are interconnected into a fullyactive Wheatstone bridge configuration to maximise theoutput signal. This bridge provides an analogue outputsignal which is proportional to the input pressure. Thefull-scale span for this type of integrated sensor is100 mV. The particular type of IC pressure sensor usedin this sensing device has the following features:

• good solid state stability;• low noise susceptibility;• temperature compensation over the range of 0-50°C;• high resolution graded output in the range of 0-15 Psi

(gauge pressure) with an accuracy of 0.1%;• small weight, approximately 3 grammes.

Variable resistors in the conditioning circuit boardshould compensate or the IC pressure sensor by enablingit to work in the range of 0 to 50°C with a ±1% error in

the output signals (loaded and unloaded span). Thistemperature compensation is described.15

4. SENSING BLOCKThe sensing block is described in detail in 15 and isassembled to the signal conditionning circuit box via tworigid brackets on each side of the block and should bekept normal to the sensing plane of the sensing block.The piezoresistive pressure sensors are connected to thesides holes of the sensing block via flexible tubes.Flexible tubing is also used for the air supplied to thechamber. Each piezoresistive pressure sensor is con-nected to an individual variable resistor of an electricalcircuit and the output signals are transmitted to thecomputer interface through a 20 way ribbon plug asshown in Plates 1 and 2.

The prototype design of the proximity-to-tactilesensing block utilises the air jets in such a fashion thatthe tactile sensor has an acceptable spatial resolution (airjets spacing) and also a good sensitivity response. Theother major consideration is the ease and cost ofproducing the tactile sensing device. Furthermore, theposition of the air jets must be selected in such a way soas to ensure that the air supply jets do not impinge onthe sides of the column and cause turbulent conditions tooccur. The spatial resolution of this tactile devicedepends directly on the size of the back pressure air jets,as smaller the diameter of the back pressure air jets, thebetter the spatial resolution. The ideal tactile sensor asdescribed by Professor Harmon9 exhibits a spatialresolution of 1 mm and this emulates the human touchsensing. In this configuration, the size and the means ofmachining the air jets columns do not allow us toperform the ideal spatial resolution. In order to achieve areasonnable spatial resolution without affecting thesensitivity of the device a method of scanning the sameline of the target twice by sifting the sensing device byhalf of the its spatial resolution.li>16

The measurement resolution (or sensitivity) of thedevice was obtained by mounting the sensing block on avertical computer controlled milling machining centre. Aflat slab of material (a test piece) was held underneaththe spindle on the machine bed and in a pneumaticallyoperated jaw. The supply pressure and the volumetricflow rate were kept constant and the sensor block wasadvanced in the direction of the test piece (or target),reducing the gap in increments of 10 fim. When the gapwas less than 1 mm, a discernable change in the values ofthe IC pressure sensor output was observed for anyincrement of motion. The measurement resolution (i.e.sensitivity) was found to be 1 mV/fim. With the CNCmachine movement sensitivity of 10 /im, it is impossibleto fulfil the capabilities of the sensor. However, if aprecise automated platform is used, the device would becapable of discerning object features down to a micron.•Therefore, the tactile sensing device can be employed fordetection of anomalies such as cracks and burrs on objectsurfaces but not to ascertain surface textures.

The pneumatic proximity-to-tactile sensor monitors



Tactile sensing 527

Plate 1. Imaging tactile sensing device front view.

object profiles by measuring the gap between the sensingplane and the object surface. The sensor output is anarray of analogue signals from IC pressure sensors interms of mV. In using the tactile sensor, it is necessary tocalibrate it in order to obtain a relationship between mVoutput and the gap depth. The calibration curve varieswith the supply pressure and the volumetric flow rate.For a constant condition (i.e. constant supply pressureand volumetric flow rate), the calibration characteristiccurves of each individual sensing element of theproximity-to-tactile sensor device at different gap sizesbetween the sensing plane and the object to be sensedare fully described in reference.1S It should be noted thateach IC pressure sensor has its own calibration curve.Once a line array of mV values are converted to gapdepths using the appropriate calibration curves, theprofile of the object surface facing the sensing planewould be known.

5. TACTILE IMAGE RECONSTRUCTIONPlate 3 shows the recognition system setup which consistsof a very accurate X-Y-Z platform with a ±10/xmaccuracy for each axis. The positions of the three axesare monitored by a digital displays. The sensing device isfirmely fixed to Z axis and free to move in the X-Y-Zaxis. The sensing device is fed with air from a regulatedpressure supply line. Each pressure sensor circuit of thesensing device is supplied by ±15 Volts from a powersupply. The analogue signal from the pressure sensor areprocessed and amplified before being converted to digitalsignal by an A/D convenor. The digital signal from eachchannel of the convenor is interfaced an I/O interfaceboard. The signals are then accessed by a dataacquisition and processing algorithm.15'17

If we consider the idea of an image from the simplestpoint of view we could regard an image as atwo-dimensional function, where the value of the

Plate 2. Imaging tactile sensing device side view.



528 Tactile sensing

Plate 3. Image reconstruction set up.

function f(x, y) at spatial cordinates (x, y) in the X-Yplane defines a measure of the back pressure resultingfrom the proximity-to-tactile sensor. For our purposes,where ease of computer-based processing is a primaryrequirement, we shall be concerned with digital images,where we accept a suitable approximation of the functionf{x,y) in return for convenience in representation andsubsequent processing. A digital image is an image whichhas been approximated in two ways, corresponding tospatial and amplitude digitization.

Sometimes referred to as image sampling, this involvesrepresenting the national original continuous imagefunction as an array of specific samples at discrete pointswithin the two-dimensional frame of reference, asillustrated in above. In general, the spatially digitizedimage consists of (x, y) equally distributed samples,

where each discrete point in the array is identified as apicture element or taxel in this case.

Each taxel in the array has to encode the local imagevalue. An image intensity (back pressure reading) level isdesignated a grey level and the full range of intensitylevels available in a particular image is referred to as thegrey level. In this case, we are concentrating on binarizedimages, where the amplitude digitization occurs when thegrey level consists of just two possible levels (0 and 1), asthe resulting image consists of an array of points each ofwhich is either black or white. It is clear that binaryimages are simple to generate, store and manipulate,since each taxel is associated with a single bit ofinformation, provided that we retain enough informationfor subsequent processing requirements, there are many

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Tactile sensing 529

advantages in working with binary images where possiblein practical situations. Figure 5(i) and 5(ii) represent acomputer binary reconstruction of a cam at two differentorientations which comprises an array of 32 x 32 circularsensing sites, 1.5 mm diameter and 3 mm center to centerspacing, and it is approximated to human fingerresolution.

6. CONCLUSIONThe tactile sensing device presented in this paper canacquire object related information that correspondsclosely to the human sense of touch. The informationobtained is rather limited in diversity when comparedwith the human tactile capability. However, within itsnarrow band of cutaneous emulation the analysisperformed provides quantitative information that is notusually acquired by human touch reception. This sensingdevice provides information in a narrower bandwidththan that of a vision system. It acquires lower resolutionglobal information than vision. This makes tactile sensingmore useful for feature extraction (i.e. holes) and objectrecognition using predicates than for global imageprocessing. Therefore, it is concluded that the deviceshould acquire high resolution from predefined locationswithin an object view. In this mode tactile datainformation acquisition and processing can resolve inobject recognition in a faster cycle than with visualanalysis.

The other features of this tactile sensing device isfirstly less affected by environmental conditions such aslighting, smoke, or haze which affect visual processingand other tactile sensing. It does also not suffer fromproblems associated with most tactile sensors such aswear, fatigue, hysteresis and drift in conductivity becauseof its data acquisition from a "near-tactile" mode. Thedevised tactile sensor can, however, suffer from noiseand turbulent flow conditions. Because of its lowresolution, the problem of noise can be alleviated bysoftware compensation more readily than in the case ofvisual analysis. Air is used as a transmitting mediumbetween the effect of the external stimulus to the sensingelement, this medium is cheap, easy to control and moreavailable when compared to other transmitting mediasuch as a soft membrane, Xenon light, laser etc.

Examples of applications of such a device are:

• Objects indentification and orientation assessment17"19

• Gripping force monitoring• Object slip detection.

References1. P. Dario and D. De. Rossi, "Tactile Sensor and the

Gripping Challenge" IEEE Sped 46-52 (August, 1985).2. M.H. Raibert and J.E. Tanner, "A VLSI Tactile Array

Sensor" Proceedings of the I2th Int. Symp. IndustrialRobots and the 6th Int. Conf. Industrial Robot TechnologyParis (1982) pp. 417-425.

3. R. Benhadj, H. Rahnajet and M. Safa, "High ResolutionPneumatic Proximity Tactile Sensing Device" Int. J. AMT2(3), 59-72 (August, 1987).

4. K. Tanie, K. Komoriya, M. Kaneko, A. Fujikawa and S.Tachi, "A High Resolution Tactile Sensor" Proc. 4th Int.Conf. Robot Vision and Sensory Controls, London(October, 1984) pp. 251-260.

5. J.L. Shneiter and T.B. Sheridan, "An Optical TactileSensor for Manipulators" Robotics and Computer-Integrated Manufacturing 1(1), 65-71 (1984).

6. P. Dario, C. Domenici, R. Bardelli, D. De Rossi and P.C.Pinotti, "Piezoelectric Polymers: New Sensor Materials forRobotic Applications" Proc. 13th Int. Symp. IndustrialRobots (1983) pp. 14-34.

7. R.A. Russell, "A Thermal Sensor Array to Provide TactileFeedback for Robots" Int. J. of Robotics Research 4(3),35-39 (1985).

8. N. Sato, W.B. Heginbotham and A. Pugh, "A Method forThree Dimensional Part Identification by Tactile Transdu-cer", Proc. 7th Int. Symp. Industrial Robots (1987) pp.123-129.

9. L.D. Harmon, "Automated Tactile Sensing" Int. J.Robotics Research 1(2), 3-32 (1982).

10. Y. Zotterman (Ed.), Sensory Functions of the Skin inPrimates: (Pergamon Press, Oxford, 1976).

11. L. Harmon, "Touch-Sensing Technology" Proc. Robots ofthe 4th Int. Conf. (October, 80) pp. 375-390.

12. W.J. Dixon, M.B. Brown, L. Engelman, J.W. Frame, M.A.Hill, R.I. Jenrich and J.D. Roporek (Eds.), BMDPStatistical Software (University of California, Berkeley,California, 1983).

13. K.E. Pennywitt, "Robotic Tactile Sensing" Byte Magazine177-200 (January, 1986).

14. S.W. Ynan, Foundations of Fluid Mechanics, SI Unit (Ed.)(Prentice-Hall Int., London, 1970).

15. R. Benhadj, PhD Thesis (Kingston University, UK, July,1992).

16. R. Benhadj, B. Dawson and M. Safa, "Imaging PneumaticProximity-to-Tactile Sensing Device" Sensor Review 13(3),23-28 (August, 1993).

17. R. Benhadj, S. Sadeque and H. Rahnejat "A Knowledge-Based system for sensor interaction for Real-TimeComponent Control" Int. J. AMR 3(1), 77-102 (February.1988).

18. R. Benhadj, S. Sadeque and B. Dawson, "Tactile BinaryImaging Recognition Algorithm Using Geometrical Mo-ment Invariants" Sensor Review 13(4), 13-22 (November,1933).

19. R. Benhadj, S. Sadeque, B. Dawson and M. Safa,"Towards Unmanned Manufacture: Applications of anExpert System" CODEM 89 Int. Conf, Birmingham Poly.(September, 1989) pp. 206-209.


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