development of a dexterous gripper for nuclear application

8/14/2019 Development of a Dexterous Gripper for Nuclear Application

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Proceedings of the 1997 EFlEIntemational Conferenceon Robotics and AutomationAlbuquerque, New M exico - April 1997

Development of a Dextrous Gripper for Nuclear ApplicationsA.Dut t a,G.R.Muzumdar,V.T.Shirwalkar K.Jayarajan,D Venkatesh and M.S .Ramakumar

Division of Remote Handling & RoboticsBhabha Atomic Research CentreMumbai 400085 India

AbstractTh e need for dextrous manipulation arises when thephysical dimensions and mechanical properties of th ematerials to be handled may not be known precisely.

For such a pplications the gripper should be able to con-trol the position of i t s f inger or the force which it exertson the object, and also be able to detect slip and takecorrective action. Th is paper describes certain aspectsof design involved and experiments carried out using adeztrous gripper which is being developed for nuclearapplications.1 Introduction

In the past two decades researchers in the area ofdextrous manipulation have at tempted to emulate hu-man dexterity in Robots and Telemanipulators. How-ever progress in these attempts have been rather slow,as pointed out by Howe et al. [l] ,which is mainly dueto a limited understanding of the physical aspects ofsensory and control interactions during grasping andmanipulation of an object. An examination of howhumans grasp an object will reveal that we first seethe object and estimate how much force may be re-quired to lift it by using knowledge based on previousexperience. We then apply a grasping force which isslightly higher than the force required to lift the ob-ject , continuously monitoring the process to see if th eobject is slipping. If slip is detected we increase th egrasping force. The grasping process can be dividedinto a number of steps such as positioning the fingersnear the object, sensing when contact has been madeand then controlling the forces that occur while ma-nipulating the object.

In th e nuclear industry, Robots and Telemanipula-tors are used for separating humans from hazardousenvironments. The need for dextrous manipulationin the nuclear industry is even more pronounced dueto the fact that improved dexterity enhances safetyin the material handling and manipulation aspects ofhighly radioactive materials with unknown mechanicalproperties and physical dimensions. In such cases the

robot may have to handle objects which may be hardor fragile, making sure that it does not damage theobject nor does it allow the object t o slip out of thegriper. To accomplish this t he gripper should be ableto control t he position of it s finger or th e force which itexerts on the object, and a t the same time it should beable to detect incipient slip and take corrective action.For this application a Hybrid position/force controlgripper with slip sensing ability is being developed.This paper essentially describes certain design aspectsinvolved and experiments carried out using this grip-per. A few research issues which still remain to beaddressed are also identified.

Figure I : Photograph of Gripper2 Background

An examination of the human hand would showtha t the skin on the surface of our hand is compliantlysupported by the soft pulp inside. Whitney [a ] haspointed out tha t, active or passive compliance is a wayof assuring stable force control, although a t the costof poor dynamic performance and positional accuracy.Cutkosky et al . [3] and Shimoga et al. [4] have carriedout various experiments to ascertain the best skin ma-terial for the contact area of a robots hand. According

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to them polyurethane rubber and other types of foamrubber or sponge are best suited from the view pointof friction, compliance and other desired mechanicalproperties. Akella and Cutkosky [5] have carried outexperiments on finger tips which are not only soft butalso actively controllable. Although rubber seems tobe the best choice, its susceptibility to rapid disinte-gration when exposed to nuclear radiation makes itunsuitable for use in a radioactive environment. Di-rect use of rubber is further limited in handling sharpobjects and corrosive materials.

Human fingers are equipped with two types of sen-sors, one for estimating the total resultant force ex-erted by the finger and the other, in the form of smallnips on the surface of the skin, for dynamically sensingincipient slip. Recent studies on slip sensing [7,8] areof particular relevance to this paper. These works il-lustrate how contact and incipient slip signals may bedetected and processed for atta ining fine manipulationcapabilities. An important point in choosing the forceand slip sensors is tha t the working of both the sensorsshould be mutually exclusive. Various types of tactilesensors have been reviewed in [6].

Various researchers [1 ,2 ,8 ,11,13] have pointed outthe special control problems associated with dextrousmanipulation. One of the major problems is that thesta te of the system keeps changing as contact is madeor broken, or other dynamic effects like rolling or slid-ing take place. There is still no definite answer as towhat is the best strategy to achieve stable responsiveforce control in a dynamic environment. Various ap-proaches include Stiffness control (see Salisbury [9]),Impedance control (see Hogan [ lo] ) and Hybrid posi-tion/force control (see Raibert and Craig [l l]). n thepast few years most researchers have either stressed onthe pre or post contact phase of the grasping problem.Akella and Cutkosky [5] and Paul [13] have pointedout that the middle phase which deals with the prob-lems encountered when switching over from positioncontrol to force control is still poorly understood.3 The Gripper3.1 Mechanical Characteristics

In designing an industrial gripper for nuclear appli-cations apart from the usual mechanical and controlaspects we also have to consider the effect of radia-tion and corrosive agents on the gripper ma teri al. Thegripper being developed, as shown in Fig.l, can han-dle a weight of 2.5 kg. It consists of two fingers whichare actuated by a parallel-link mechanism which isdriven by a 12 V permanent magnet DC motor. Apre-calibrated strain gauge of 350 ohm resistance andgauge factor 2 , is used as the force sensor. The force

r-----l f l Strain

Figure 2: Schematic of the Gripper

sensor is mounted on a cantilever type of link whichis so placed that it measures only the resultant forceexerted by the finger on the object irrespective of theposition of the object between the fingers. Maximumopening of the gripper is 80" A 10 turn poten-tiometer is used for obtaining position information. Asmentioned earlier rubber cannot be extensively usedbut at the same time some sort of compliance has tobe provided on the finger. To overcome this problemwe have used a thin stainless steel sheet as the maingripping surface on the finger and it is supported bytwo small pieces of rubber to provide compliance. Toincrease the coefficient of friction between the objectand the finger, small diamond shaped protrusions havebeen machined on the steel sheet. These protrusionsalso aid in providing slip information.

The slip sensor used is a piezo electric crystal. Thissensor works essentially like the "pick-up head" of agramophone record player. It is fitted in between thefinger and the support steel sheet as shown in Fig. 2.The slip sensor signal is analyzed to determine whencontact has been made and to subsequently verify ifthe object is slipping. The gripper is actuated by a DCMotor via a wire rope. Thi s was found to be suitableas the gripper may be mounted on different robots ortelemanipulators in which case the distance betweenthe gripper and motor may have to be varried. How-ever the distance between the motor and the grippershould be as short as possible to reduce the effects ofelasticity and friction. In the test setup the wire rope

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length is about 10 cms.As shown in Fig.2 the relation between the force ex-erted by the finger and the wire rope tension is givenby:

Signalconditioning

where:T = wire rope tensionF = force exerted by the finger on the object6 = angle between two links.p = opening of the gripper

- buffer

For a practical application we first have to decide uponthe size of the object (value of p ) and then the force tobe applied (value of F ) by the finger. Once the valueof p and F are fixed the wire rope tension required iscalculated automatically from equation 1 and 2.3.2 Control system3.2.1 Signal processingThe stra in gauge bridge is excited by a precise con-stant voltage reference source. The analogue outputfrom the strain gauge bridge is amplified by using aninstrumentation amplifier with a low drift and highCMRR, which is connected to the gripper controllerthrough a 12 bit analogue to digital converter.

The vibrations caused by slip are reflected aschanges in charge generated in the piezo electric crys-tal. The current generated due to this charge is firstconverted to voltage by an I /V converter and then itis amplified by an instrument amplifier. This outputis then compared with a preset threshold value andfurther processed to produce TTL level pulses beforegiving to the microcontroller. If the TTL output is1 it indicates that slip is occurring, and if it is 0 itindicates that the object is not slipping. Based on thisresult t he slip compensation algorithm in the micro-controller either augments the force being applied bya preset value or lets it remain a t the existing value.

+ADC HAmplified sensorCPU I8096

The gripper opening position is sensed by the multi-turn potentiometer. The position information is suit-ably amplified and processed to match with full scalerange of a 10 bit Analogue to Digital converter inbuiltto the gripper controller.3.2.2 ControllerThe Hybrid position/force control system is an archi-tectural concept to operate within the constrains of

I d

1-+conditioning Signal

Figure 3: Block diagram of control system

the task i.e. position control and force control. Themain function of the Hybrid Control system is to de-cide at which point the controller should switch fromposition control to force control. The controller usedfor implementing the Hybrid Position/Force controlis a 8096 microcontroller operating at 10 MHz. Theslip compensation algorithm is also implemented bythe same microcontroller. The microcontroller essen-tially maintains the feedback control loop in positionor force mode of the gripper by appropriately actu-ating the DC motor through a Digital to Analogueconverter and power amplifier. All the algorithms arewritten in assembly language for fast processing.4 Experiments and Results4.1 Experimental Procedure

The gripper was fitted on a platform as shown inFig. 1 and a few tasks were performed. The chosentask was a simple gripping operation of applying apredetermined force on an object and verifying if it isslipping.The task was divided into the following steps:

0 Close the fingers using position control.e Switch over to force control and apply a predet-

ermined force once contact is made.0 Monitor the slip sensor to determine if slip is

occurring, and if so, increase the gripping forceby a preset value.The results obtained are discussed in the next section.

4.2 ResultsThe first plot (A) in Fig.4 shows the position of the

finger with respect to time. Shortly after the gripper

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( A )Finger position vs Time

-v

3 6 9 12t ime(sec)0)

TTL output

Force vs Time ( w i t h o u t sl ip)

v

% 2 L1n ;z 2 )0L I04 I

3 6 9 12t ime(sec)

Slip signal vs T i m e

I i m . . I

3 6 9 12t ime( sec)( D lForce vs Time (wi th sl ip)

41z l

Figure 4: Experimental results

is actuated the gripper finger reaches its desired posi-tion, grips the object and remains steady thereafter.It may be noted tha t the interactions that occur whenthe finger first makes contact with the object, such assmall vibrations, are not reflected in this plot. Thismay be because of the compliance in the system dueto which the vibrations are damped out before theyreach the potentiometer.

The second plot (B) in Fig.4 reflects force exertedby the finger on the object with respect to time. Inthis case no slip occurs so the gripping force remainssteady.

The third plot (C ) shows how the slip signal is re-flected with respect to time as the object slips throughthe fingers. As soon as the slip signal exceeds thethreshold value, point " A , the force being exertedby the finger is augmented by a preset value. Thethreshold value has to be very carefully set after ex-perimenting with various types of objects.

The fourth plot (D ) in Fig 4. indicates how thegripping force is augmented if slip is detected. Atpoint "A" the controller understands that the objectis slipping and it automatically augments the forcebeing applied. It is important to note that the forcebeing applied should be lncremented by a value suchthat it is just able to hold the object without damagingit .5 Conclusion and Scope

In this paper we have discussed about the design,development and experimentation of a dextrous grip-per. The results obtained verify tha t prevention of slipis possible by controlling the force being applied by thegripper. However, the force response of the gripper isaffected by the elasticity and friction of the wire ropedrive. To increase the response of the gripper, it maybe directly actuated by a DC motor,

There is still further scope for improvement of thedextrous gripper. The following issues still remain tobe addressed :e Controlling the rate at which the force is increasedor decreased as the gripper holds or releases an object.e Considering the exact dynamic effects which occuras the finger makes contact with the object.

References[l ] Robert D. Howe, Nicolas Popp, Prasad Akella,

Imin Kao and Mark R . Cutkosky. "Grasping, ma-nipulation and control with tactile sensing." Pro-ceedings of th e IEEE International Conference onRobot ics and Automat ion, pp. 1258-1263, Cincin-nati , Ohio, May 13-18, 1990.

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[a ] D. E. Whitney. Historical prespective and st ateof the art in robot force control. Intl. Journal ofRobotzcs Research, vol. 6, no. 1, pp. 3-14, 1987.

[3] M. R. Cutkosky, J . M. Jourdain and P. K . Wright.Skin materials for robotic fingers.Proceedzngs ofthe IEE E Internatzonal Conference on Robotzcs andAutomataon, pp. 1649-1654, Raleigh, NC, March 31-April 3, 1987.

[4] A. A. Shimoga and A. A. Goldenberg. Soft ma-terials for robotic fingers.Proceedzngs of th e IEEEInternatzonal Conference o n Robotacs and Automa-tzon, pp. 1300-1306, Nice, France, May 12-14, 1992.

[5] Prasad N . Akella and Mark R. Cutkosky. Con-tact transition control with semiactive soft finger-tips. IEEE transactzons on Robotacs and Automa-tzon, vol 11, No. 6, pp. 859- 867, 1995.

[6] H. R. Nicholls and M. H. Lee. A survey of robottactile sensing technology. Internatzonal Journal ofRobotacs Research, vol. 8, no. 3, pp. 3-30, June,1989.

[7 ] Marc R. Tremblay, Mark R. Cutkosky. Estimat-ing friction using incipent slip sensing during a ma-nipulation task. Proceedzngs of t he IEEE Interna-taonal Conference o n Robotzcs and Automa taon, pp .429-434, Atlanta, Georgia, May 2- 6, 1993.

[8] Jae S. Son, Eduardo A. Monteverde, and RobertD. Howe. A tactile sensor for localizing transientevents in manipulation. Proceedings of the IEEEInternatzonal Conference on Robotzcs and Automa-tzon, pp. 471-476, San Diego, California, May 8-13,1994.

[9] J .K . Salisbury. Kinematic and force analysis of ar-ticulated hands. In Robot hands and the mechanicsof manipulation, MIT Press, Cambridge, MA. 1985.

[ lo] N . Hogan. Stable executions of contact task us-ing impedance control.Proceedings of the I E E E In-ternational Conference on Robotics and Automa-t ion, pp. 1047-1054, Raleigh, NC, March 31-April3, 1987.

[ l l]M . H. Raibert and J . J . Craig. Hybrid posi-tionfforce control of manipulator. Transact ions ofA S M E ,Journal of Dynamzc Sys t ems, Measurementand Control, vol. 102, pp. 275-282, 1981.

Automat ion.pp 396-401 March 31-Apr il3, Raleigh,NC, 1987.

[13] Richard P. Paul. Problems and research issuesassociated with the Hybrid control of force and dis-placement.Proceedings of the Internatzonal Con-ference on Robotics and Automatzon, pp. 1966-1971,Raleigh, NC, March 31-April 3 , 1987.

[14] D . T. Pham and W . B. Heginbotham. RoboticGrippers. IFS(Pub1ications) Ltd., UK, 1986.

[12] Thea Iberall. The nature of human prehension:Three dextrous hands in one.Proceedings of theIEEE International Conference on Robotics a n d

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development of a dexterous gripper for nuclear application

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