Handling Objects of Unknown Characteristics

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HANDLING OBJECTS OF UNKNOWN CHARACTERISTICS P. Adl, Z. A. Memon and R. T. Rakowski ,\frwlIjar/IIr1l/g d l::l/gllltl'nng S'\"I/I'IIIS Dtpm/IIII'II/. Hml/tl

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Tasks such as grasping objects of unknown characteristic or handling items of varying batch sizes and compliance calls for intelligent manipulators. Excessive gripping forces could produce severe object damage. This paper outlines the details of a tactile sensor using magnetoresistive technology. The sensor not only has to make readings of normal, tangential and rotational forces but also be intergrated to a real time control system which copes with characteristic changes occuring within the body of the compliant object. Such a control strategy is described. Keywords. Robots; Sensors; Hierarchical intelligent control; Manufacturing processes ;Real time computer systems. INTRODUCTION

Simple and Multi-element devices may be manufactured economically and sensors may be operated in

No matter how robust and rigid a robot and how good its control system, a robot that is to be used in interaction with its environment needs sensory devices that can aid it in performing in these situations.Human beings achieve this through employing the visual and tactile sensation and a learning capability.

DC fields,

or AC fields of up to SMHz and

withstand working temperatures of greater than 200°C .The sensor characteristic is essentially a cosine function and hence nonlinear, however, the sensors may be operated in their quasi-linear region provided that they are not saturated.

MAGNETORESISTORS IN TACTILE SENSING Achieving human capabilities in terms of perception and flexibility are not yet realised in the

field of robotics,

Although machine vision is

well advanced the sense of touch remains a constant area of activity (Dario P., et aI, 1987). Since most tasks in the area of gripping and placing can be achieved by responding to contact and force, it is desirable to instrument the gripper with force and touch sensors (Harmon,L . , 1987).

A number of MR tactile skins have been proposed by Vranish (1984),Hackwood et al (1983) ,and Nelson et al (1985) ,The sensors proposed utilise varying techniques of signal aquisition and use DC, pulse magnetic fields or fixed magnetic dipoles mounted over a compliant medium. Tactile information is extracted in two distinct methods, i)Tactile imaging ii)Tactile force and shear

monitoring at the point of object/gripper interface.The sensors used in this work comprise the

This paper outlines a method of distinguishing between normal,

tangential,

and rotational force.

Control of the optimum grip force through the use

basic normal and shear force sensors , disscused later, and combination of the two provide infor-

mation to be used in a pick and place problem.

of these sensors in an adaptive real-time manner

ensures damage free handling of compliant objects

In order to translate physical movement due to

MAGNETORESISTIVE SENSORS Feromagnetic thin film magnetoresistive (MR) senSors are solid state magnetic sensors which can be employed in a wide variety of sensing and measuring applications. They have traditionally been used in read only heads for tape and credit card reading applications. In its simplest form the sensor operation relies on the fact that when a magnetic field which is orien~ed in the plane

of the HR element and perpendicular to its is placed near the sensor,

lengt~

a change in resis-

tivity (resistance) of the element occurs. magnitude of the change in

resista~ce

FORCE AND SLIP DETECTION

The

of the sen-

SOr is a function of the applied field amplitude.

force and slip into electronic signals using a magnetoresistive sensor it is necessary to introduce a magnetic field whose characteristics

change only through mechanical forces applied and this is achieved by introducing a compliant medium between the sensors and a magnetic field generated by a current carrying conductor direct-

ly above the sensor.

The basic force sensor comprises of a single ele-

ment

(Fig. 1) and any normal displacement in the

overlay conductor causes a change in the voltage

across the sensor due to the change of magnetic field strength at the sensor. Compression of the compliant medium will produce a change in the magnetic field intensity

~H,

and hence change in

64

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ffi

-- --

I~~ __

comPliant m e d m · u m

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conductor

_ NiFe element

...... ~ . ........ ..... '

.

:

......... ..

.

.uj!L._ ..~

substrate

COMPRESSION

:

~

-------,

. ...... .1 MR Element

Gold

Fig. 2. Serpentine-path Half - bridge Sensor

Xs

Fig. 1. Normal and Shear Force Detection

resistance o R. By using an AC magnetic field any external electrical noise can be filtered out

using band - pass filters (say, at 10kHz) which will improve the signal to noise - to-noise ratio and further improve the sensitivity. In future s e nsor systems cross-talk between normal force

sensor elements can be eliminated by applying different signal frequencies to adjacent conductors and using band-pass filters . Hysterisis is also

overcome by using AC fields by reversing the mag netic field to its initial state.

The slip sensor employs a similar principal but he re two adjacent elements are used as zero

centred potentiometers and any lateral displace ment of the conductor 5hows as a differential ch a nge ac r oss the sensors. The elements are set up in b ridge co n figuration. This reduces thermal drift and minimises the effect of common mode en viromental signals, such as the earths magnetic

field.

At present a pair of shear and force sensors is

used at each gripper finger. Since the spacial r e solution of the sensors is in the order of microns a nd g i ven t he wide bandwith of such sen sors complex arrays of such sensors seem quite

feasible. SENSOR CHARACTERISTICS AND PERFORMANCE

The graph of Figure 3 shows typical performance of slip sensors in separations of 0.2 up to 2mm. The slip sensor output remains independent of the g a p separ a tion for up to ± 2mm displacement. The graphs diverge after this point. This displace ment when applied t o rubber in shear enables a wide range o f f o rces to be detected .

Natural rubber is used as the compliant medium for its excellent mechanical characteristics and stability. In the compressive mode, the rubber response is linear up to 30% compression set and th e re are no problems with shear force detection

as long as the bonded faces of the rubber to glass substrate do not fail. To this end, the dynamic range of the sensors depends largely on the type of compliant medium used and a dynamic range of 20dB is achievable.

The overlay conductor is constructed by etching copper patterns on a My l ar backing. My l ar is us e d for its flexibility and compliance e nabling

a range of grasping actions to be detected. The field outside the flat conductors is governed by H = (tan - 1 (b + S/2h) - tan- l (b - S/2h)) I/2 where I = Current through conductor (A) S - Conductor width (m) h = sensor element separation (m) horizontal displacement of the conductor

b -

The magnitude o f H depends o n the size of the cur r e nt and width of the conductor directly so that the n a rrower the flat conductor,

the sharper t he

r e solution of the magnetic field and hence less The change in resistance due to MR property can be small compared with inductive coupling between

cross - talk between adjacent sens ors .

SENSOR

the sensor and overlay conductor. Such swamping of the MR sensor signal would produce a response

OUTPUT (.vl

which could be highly non - linea r. By folding the element path back onto itself (Fig. 2) the induc -

0.00

tive pick - up between adjacent elements is can -

' .00

c e ll e d out. Additional benefits of the serpentine con f iguration include

(i)

~

increase in el e ment

" .00

resistance all o ws higher bridge voltages t o be used improving sensitivity,

(ii)

current flow in

3.00

one sensor pad , produces some bias field for the adj a cent sensor pad ensuring operation in the linear region, (iii) the HR anis o tropic axis for

2.00

adj a cent pads are set at 90° forming a complemen t a ry pair e liminating the st e ady c omponent of mag -

1. 00

n e tic fi e ld and making it immune to th e t e mpe ra tu r e changes of thin f i lm resis ta nce ,

(iv)

the s e nsors offer a linearity of ± 1%, far supe r io r to the linearity of a single eleme nt.

(m

LATERAL DISPLACEMENT (mm) Fig. 3. Shea r Fo r ce Sensor Charac t eristic

Handling Ohjects of L'nknowlI Characteristics

65

The minimum detectable change in the magnetic 2 1 field is + 8 x 10- Am- when the skin is uncom-

The load-deflection curves for rubber in tension and compression are approximately linear for strains of the order of several percent. As these curves remain linear through the origin, the value of Young's modulus remains the same during compression and tension.The bulk modulus of rubber is many times greater than its Young's

pressed. The magnetic field at the sensors for = 20mA, S = SOOpm, h = 3mm is H = 60.65 Am-I. If the rubber were pressed to the threshold point the field strength would become: H = 60.73 Am-:, representing a new height of ho = 2.997mm well

modulus and this means that the rubber hardly

within the 20dB requirement for

changes its volume even under high loads, so that for most types of deformation there must be space into which the rubber can deform. The more restriction that is made on its freedom to expand the stiffer it will become, therefore allowance had to be made in the design of sensors to over come this problem. The coefficient of friction (p) for most dry rubber surfaces is generally unity, but for design purposes it is assumed that slip between object and sensor due to shear force will not occur if the ratio of maximum shear force to minimum compressive force is less than 0.3. If water (a lubricant for rubber) is present it will normally be squeezed out under

X mir..

SIGNAL ACQUISITION AND PROCESSING

Analog Informatlon

The two sources of force and shear information are processed independantly using conventional

signal aquisition techniques.

All the lead outs

from the edge of the substrates are screened to maximise the signal-to-noise ratio. The MR elements are powered using constant current sources and every sensor is activated using AC fields. In this way rows of sensory data can be processed using filters and differential amplifiers as

load.

close to the sensors as possible to eliminate ambient noise.

The dynamic range of the sensors , therefore, depends on the rubber compliant medium, the

strength of magnetic field and the output amplification of sensor signals.

Hierarchical Control

To build into the sensor a value for the mechani-

they carry out become more complex, to conserve valuable processing time of the controller and to

As robots gain more intelligence and the tasks cal force the simplified model of sensor geometry shown in Fig. 4 will be used. The displacement

keep functionality simple the need of a modular approach to design becomes more apparent. Tasks should be decomposed in an upside down approach and decisions should be made locally as far as

in the vertical direction is given by:

x=h-ho

x=HI!.3f(lRL-I/ f;!

t

+- (~2-1) +-!!:.]

1.tr

.".

16

possible with different levels interacting by

for which the symbols are as defined in Fig. 4,

simple commands providing natural break points

and Eo

for software and ease of debugging (Fig. Sa). The overall control strategy is based on the NBS Hierarchical Robot Control system (Barbera.,et al, 1982) modified to allow real-time adaptive gripping between sensor-gripper-object.

=

Young's modulus for the particular rub-

I = L(D-d)3/ 96 , A = L(D-d)/2.

ber, R = (D+d)/4,

Considering the case of no bending,

ber of IRHD

then for rub-

30:

x = (4.0473 x 10- 2) F

(mm)

From this expression for a maximum force of 50 Newtons,

x

=

high 151 .... 1 1

2.02 mm

Therefore,

cOl'!WI'\a.nda

for a dynamic range of 2 0 dB,

the mini-

mum detectable value X c,i e should be O. 0202mm which relates to a force of 0.49 Newtons. This relationship holds if the ratio of the outer

\

diameter 0 to the inner diameter d does not ex-

ceed 4.5.

proc •• sing

F

\

F

low

1.v.l

Fs (

Fn M

FS R~w

M

Fig. Sa. NBS

Fig. 4. Simplified Cross -sect ion of Compliant Medium

SENSORY OATA

M Co~trol

Model

H/\J COI"Y'IANO SIGN.

1'. ;\dl. I . ;\ . \lcl11o n a nd R. 1'. Rakowski

TABLE 1

Modified State Table Implementation J INP\lT COMMAND

l COMMAN D

!'RQCES SED

SENsaRV

MESS

STATUS

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M N W T M E R E A ~ R N N D F ~ E A E T S T ~ A T

LOWER LEVEL

t

i

I"IUNAl. STATUS

GRIPPER SENSOR FEEDBACK

OPEN !CLOSE

LEFT! RIGHT

UP! DOWN

otmvr

REGISTER

Fig. 5. Hierarchical Control for a Basic Adaptive Gripper Applicati on

System Realisation

The situation is somewhat simplified by the fact that the gripper controller is dealing with the lowest levels o f control in the "Control Decision Hierarchy " ,that is the joint and gripper coordination, up as far as the primitive cont rol level. The control block of Fig . 5 represents a plug compatible module communicating wi th the higher level control ler thr ough simpl e commands ,

issuing

status reports and in turn supplying commands for levels below. At every control level each input generates a definite ou tput or a comb inati on of

outputs acc ording to the information o rgan ised in the " State Table" at that level(Table 1). Hence each level cycles around INPUT PRE-PROCESSING;STATE TABLE IMPLEMENTATI ON;OUTPUT POST PROCESSING. The pre and p o st pr o cessing are only data preparation tasKS and the bulk o f processing is carried out by state-table implementation.

Hardware and Software Imp lementation To design the hardware in acc ordanc e with NBS strategy a multi processor approach was chosen with each processor dedicated to one contr o l task, such as gripper control , data aquisitaion, etc.. Fi gu re 6 illustrates the hardware implemen tati o n o f the Gri pper c on trol level. The state table f or ea ch l e v el is in the f o rm o f a 4kBytes multiport ed common memo ry board. To eliminate race conditions and preven~ loss o f data the communication bus was based on a 'First come - first serve ' princip le . Here, as so o ~ as on e CPU tries to talk to t he co~mon ~emory ~he o ~ h er pr oc ess o rs are blocked ou t i~m ediate :y. As a r es ult time wastage ass oci ated with sequential pol ling is avoided.

mach':' :-. e c ode

a~d

loaded

~

N E X A T T

f

t ~

A T U S

R E p

~ T

--- --NEXT ST AnJS

REQUEST R)R / SENSORY MES SAGE

Il~~~L I R~ ~

PROCEDURE CALL

I

~

SENSORY DATA REQUEST ..

NEXT COMMAND STATE. TA BLE

I SENSORY I REPORT flATUS I NEXT I ..f~T COM"'AND STATUS . , J

FOR NEXT

CO ...... JD fOR NEXT LEVEL OOWN

LEVEL UP

State Tables for Primitive level and Gripper Control level are given in Tables 2 and 3. To il lustrate implementation o f State Tables , execution of the OPEN command thr ough Primitive Control level(Table 2) is explained as follows: OPEN This command is issued by the higher level and initiates a check of the Internal status f o r the Pick and Place operation. If this is inc omp lete OPEN will return an appropriate error signal and the process is terminated , other wise the process will continue through the table until OPEN COMMAND is dec omposed to the lower level or if the gripper were open at the time the command was issued the OPEN CALL is cleared . Other commands from higher level follow a similar discription and every command initiates a chain of IF THEN ELSE procedures.Other commands at this level and at the Gripper Control level are executed in much the same way. Utility routines for various tasks of data preparation , pre - and post - processing and state table implementation were written using PL9 which is a 6809 devel opment tool and compiled into machine code. The use of this development system was purely due to availability and t o this end a ny other commercially available packages may have been used. Since every "exec uting owner" has a definit e procedure , modifications can be easily carried out and CPU EPROMs updated.

IVWLI'4!SI13

TO CP\J I

>-

0::

o w

" "o

TO CPU Z

Z

The Control software is based en ~he ~odified state table implemen t a tion ( ~ab:e :) at each level . Input cc~~a~ds are genera~ed at t~e highest level by the op era~ o r a~d passed d c w~ through to lower l evels . Every cc:":"'..'"":":and then starts a sequence of eve~~s ~ha t ~ake the for~ of procedures to which parame~ers are passed and r e ceived .Since the sensory pr oc esses are co~ ­ tinuous pr oc esses va ri o~s process :evels l end themselves ~aturally t o mod~lar desi gn . This provides for ease of ~odificaticn a~d ~ro~b l e shoo ting. Software is ~r i ~~en in a high l e v e l lan guage and c ompiled i n t o into onboard EP RC~S .

~

K

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0lJll'VT>

INP\1TS

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o

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TO

Fig . 6.

Hardware

Irr.plemen~ a~':' o~

Handling Objects of l"nkno\\n Characteristics

lUX .ve

ADAPTIVE GRIPPING The task for tactile sensors in handling objects of unknown characteristics is to determine an op timum grasp level without losing or damaging the object. Here the important criterion is the state of the interface between the gripper fingers and the object. There are three possibilities: whether the object is in contact with the fingers; it is gripped and or if the object is slipping. When a pick command is received, con trol is decomposed down to primary command level and subsequently to Gripper Controller. At this stage the slip and force informations are used to control the gripping process.

Adaptive control of objects of unknown weight and dimensions are achieved by aiming to attain an optimum gripping force from the Gripper Servo . To do this slip is translated as a judder or a rapidly changing shear signal due to an insufficient Grip Force. Normal force is then incremented until such time that slip is within ±5% of the previous cycle. Figure 7 shows the output from the gripper drive circuit as applied to a simple pick problem utilising a servo-driven parallel jaw gripper. The first part of the diagram shows a maximum negative voltage (maximum motor torque) during the 'open gripper' operation. The second part of the diagram shows the gripper operating in 'adaptive gripping' mode. Firstly the gripper closes with a maximum velocity . As soon as con tact is detected by the force sensors a short negative pulse stops the gripper motor dead. A minimum grip level is applied to ensure friction contact between the fingers and the object being picked up. The grip force level is then incremented during the lift operation until no change in slip signal is detected . This point would then indicate the optimum gripping force and the Lift/Place operation may then commence. The optimum force level proved somewhat difficult to achieve at first in that it caused chatter at the gripper fingers. This was overcome by introducing software hysterises at the servo-gripper driving module. Overall cycle time for gripping operatio n was O.4ms for normal force operation but introduc tion of shear raises this value to over lOms. Current work with single chip microprocessor s indicates that cycle times of 470ps are achievable .

CONCLUS IONS Work to date has indicated that Magnetoresistive technology can lead to devices s ufficiently robust to be used in a manufacturing e nvironment. Close packing of elements in arrays does not present a problem and the sensors may be employed in tailor-made applications s uch as pattern recognition, contour examination, gap detection and, of course, force and s l ip detection.

The use of hierarchical control strategy for adaptive gripping has shown much promise and current resear ch is exploring performance limits cf such a system. It is also possible cO modify the gripper control system to realise an adaptive asse~­ ly gripper based on shear and normal force signals recorded during peg-in-hole asserrbly.

6i

, ,,

"lfn FORCE L

Gripper Control Voltage

, 'OPTIMUM FORCE , 'LEVEL I

I

I

I

I

TIME

,,,

MAX -ve

,,,,,

,,

I.

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Spurious Response

Fig . 7.

I

I

TIME

,

,

Adaptive Gripping Profile

Furthermore, the software and hardware may be modified to include a bank o f data on handling objects through a learn mode so that objects may be identified according to tactile sensory patterns. ACKNOWLEDGEMENTS The authors are indebted to the U.K. Science and Engineering Research Council for the support of the work. REFERENCES Adl, P., and Rakowski, R.T. (1988), Tactile Sensor for Robot System. U.K. Patent Application, 88-20889.7. Barbera, A.J., Fitzgerald, M.L., and Albus, J. S., (1982) ,Concepts for a real-time sensory interactive control system architecture. Proceedings of the14th South Eastern Symposium on System Theory. Dario,P., et al, (1987), Multiple sensing fingetip for rob o tic active touch. Froc. of the3rd Int. Conf. on Advanced Robotics ICAR'87,IFS Publications, Springen Verlag. Hackwood,S.,Beni,G ., and Nelson,T.J. (1983), Torque sensitive tactile array for robotics, Proc.ROVISEC 3 . Harmon,L. (1987), Automated tactile sensing, Int. J. of Robotics Research, ~, NO. 2. Nels on,T . J.,Va n Dover,R .B .,Jin,S., Hackwood,S., Beni,G. (1985), Magnetoresistive tactile sensor for robots, Froc. of Int. Conf. on Materials in Computers, Robotics and Comun ication Idustry,Monterey, Canada. vranish, J .M(19841 ., Magnetoresistive skin for robots, Proc. ROVISEC 4.

68

P. Adl. Z. .-\. :'>! e mon and R. T. Rako\\'ski

TABLE 2

r"""
Primit i ve Control Level State Table

LPT ,

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, X

... HO

vu

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HO

,

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CXMUn

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x

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x

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INTe:INAL

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STATE FEeoe.a.CK FR0t-,4 LOW!R lEVe..S r. -

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.....

X

HO

X

Y!II

...

c.ou """"

DOWN

CN..l O"!N QRI""'"

OUTPUT

EXEOJTING OWNER

CNtf STAT!,

UII! P'IICT'lQH

X

uae

•

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wrniOO

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X

ore<

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X

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flOIt.,

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Gripper Control Level State Table

a.~ CCWr.4NC)

,

,

STA.TUS

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TABLE 3

5\.lI-

INTEANAl.

C
STATUS

... ..."'" HO

1

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HO

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.... ...."""'- """"'X

O"ef . . . . . P\,I..l.y

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OUTPUT ueCUTINO 0 _

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DON"T CAR! STATU