Control strategies in human pinch motion to detect the hardness of an object
Kinya Fujita*,** and Yasuhiro Oyama***Tokyo University of Agriculture and Technology
Koganei, 184-8588, Japanffijita@?c~tuat.a~jp**Iwate Universityoyama@cis.iwate-uaqjp
pinches object with equilibrium point tactics to perceive the hardABSTRACT

ness of the object.
1. INTRODUCI’ION

Human pinch is a motion to perceive an objects hardness aswell as a motion to hold an object. Numbers of studies have been
carried out on human pinch motion to understandhow human ap-“Precision grip” has been studied, which is a pinch motion

plies force to maintain an object against gravity using friction. How-

using thumb and index finger to hold a small object. Johansson et al

ever, no study has been performed to analyze the pinch motion for(1984) investigated the effect of the tactile information on precision

the hardness detection of an object. The understanding

grip [l]. Kinoshita et al (1997) studied the effect of the tangentialof the hard-

ness detectingpinch motion is expected to help developing artificial

torque [2]. The objective of these studies was to understandreality devices.

mechanism

In this study, the applied force, finger displacement

andtheof the precision grip to hold or pick up an object. The

main interest was to understand how human controls the grip force

contact area were measured in the pinch motions with five different

to obtain the adequate friction force for holding the object against

objects. The objects were transparent

gravity.

to allow the contact area

analysis, and with the same texture by putting polyvinyl film on

On the other hand, the pinch motion has another functional

them. Two of the objects were made of gelatin and the three wererole that is the detection of the hardness of an object. It is required

made of silicon rubber. Ten volunteers were required to answer theto display hardness by controlling the reflection force against finger

harder object from the two different objects presented in sequencepads in the field so-called virtual reality of tele-existence

without visual information.

Understanding

The task was repeated five times for[3,4].

the human pinch motion to detect the hardness iseach object in random sequence. The pinch motion for the first ob-

required to develop the hardness display devices for those applica-

ject was analyzed. The applied force, finger displacement

tions.

and con-tact area were normalized by each maximal value.

Few studies have been performed on the pinch motion with

The applied force to the harder object was higher as ex-

an elastic object. Mai (1991) reported the pinch force error while apected. The difference of the applied force between the hard and thesubject pinched two objects having different hardness with both

soft object was 84%. The difference of the displacement

hands [5]. Van Doren (1995) explained the mechanism of that error

was 91%.

In contrast to them, the difference of the contact area was 26%. The

using equilibrium-point

first finding in this study was that the contact area is almost constanties was to pinch an object at required force, not to perceive thedespite of the hardness difference in the pinch motion to perceive

hardness of an object.

the hardness.

model [6]. However, the task in these stud-

The hardness is perceived by sensing both the applied force

The next question was how human controls contact area

and the deformation of the object, while the deformation is sensed

constant without sensing the hardness in advance. The slopes of thefrom the change of the fingertip pressure and the finger span. Thatforce, the displacement

and the contact area were analyzed. The

means the hardness detection is an “active sensing” task which is

calculated values showed the same tendency as the peak value. It

based on the pinch motion. Therefore, it is important to understandsuggests that the control of the contact area is attained without feed-

the control strategy of the pinch motion, in order to clarify theback mechanism. The equilibrium point hypothesis was introduced

mechanism

into this experiment.

the analysis of the human pinch motion for hardness perception, theThe imaginary displacement

was calculatedof the hardness perception.

with the measured fingertip and the object hardness. The difference

hardness judgementof the imaginary displacement(1995) to evaluate the role of the tactile and kinesthetic

was 29%. It appeared that human

0-7803-5731-O/99/$10.00 01999 IEEEII -682

experiment

Regarding to the study on[7] was performed by Srinivasan
informa-Reference

Motor

Motor

Pinchforce

Control

Musclewithin the objects limitation, in order to detect the hardness withDeformat ion

accuracy. If the same force is applied to a soft object and a hard

Objectobject, the soft object might be broken or the hardness of the hard
Perceptual
Unitobject can not be exactly detected. However, human controls theHardness
Perception

force with the hardness of the object. The question was how humanmanages the pinch force with the objectf; hardness. Therefore, the(Informationabout

feedback mechanism was occupied in the initial model.

Deformation)

The question is what is the reference input as the informaFigure 1

Model of the hardness cognition in pinch motion.

tion about the object deformation.

Because the reference input isindependent from the object, the deformation information, which isequal to the reference input, will be a signal that is not affected bytion. No study has been performed on the pinch motion itself.

the hardness of the object. The experiments

This study was planed to reveal the control strategy in human pinch motion for hardness detection. The pinch force, finger
displacement

cuss the essential

were conducted to dis-in the hardness

perceiving

pinchmotion.

and fingertip contact area were measured during thehardness judgement tasks. This paper describes the “constant contact area hypothesis” arose from the experimental
scribes

informationthat experimental

results

3. MATERIALSAND METHOD

results, and de-by using “equilibriumpointmodel” proposed by Feldman [8].

Experimental system
An experimental

system shown in figure2 has been devel-

oped to measure the pinch force, the thumb displacement
2. MODEL OF HARDNESS COGNITION

and thecontact area during the pinch motion. The pinch force was detectedby the two strain gauges on the steel plate on which the specimen

Hardness can be defined as the ratio between the deforma-

was attached. The thumb displacement

was measured as the dis-tion and the applied force. It is required to sense both informations

placementin order to recognize the hardness. The force can be sensed as theimages of the contact area were recorded by a CCD camera and a“efferent copy” of the motor command in addition to the skin recep-

VCR. The recorded images were sampled as a 512 x 512 bit map

tor information. The deformation of the object can be perceived as

images using a video-input board. The contact areas were computed

the somatosensory

after the images were converted into binary images at every sample

informationfrom muscle afferents and as thetactile information from the skin receptors.

of the string which was glued on the thumbnail. Thetime. The resolution of the contact area in the utilized condition wasIn addition to these signal pathways, it is obviously neces-0.012 square-millimeters.

sary to control the pinch force, because the hardness detection is anThe experimental specimens were three silicon rubbers and“active sensing” task. A model of hardness cognition as shown in

two gelatins that are highly transparent. The specimens were cut infigure 1 was assumed. This model is based on the idea of feedbacka cylindrical form with 48mm diameter and 1Omm height. A vinylcontrol. It is desirable to apply higher force as much as possible

chloride film was placed on the specimens to unify the textures ofCCD CameraMirror(4Figure 2

Experimental@)tc)

system to measure the pinch force, thumb displacement and contact area during pinch motion. (a) side view, (b) topview, (c) set-up.

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the specimens. The finger span between the thumb and the index
finger was 60mm.

Results

The hardness of the specimens were defined as the ration of
the force and the displacement

while a 13mm diameter cylinder

The peak values of the pinch force, the calculated object

was pressed to the objects. The hardness of the specimens were 21,

displacement

100,690,2070,5280

were normalized by each maximum. The same trends are observed

g/mm.

and the contact area are shown in figure 4. The values

in the first pinch and the second pinch.
Experimental

conditions

The observed

Ten male volunteers, ages 21-24 years, were requested totrends

were reasonable

that the subject

pinched the harder object with higher pinch force but the object wasanswer the harder object from the sequentially presented two speci-

less deformed. The variation of the force, displacement

mens. In each trial, the subject was required to pinch the object

tact area were 91%, 84% and 26% of the each maximum in the firsttwice. One additional trial was allowed on the subject’s request.

pinch, 89%, 86% and 29% in the second pinch.

The all combinations of the hardness, including the same hardness,and the con-

The contact area less changed with the hardness of the ob-

were examined five times at random sequence. Therefore, the mea-ject. It was experimentally

surement was performed

100 times per subject. The subjects an-object until the contact area reaches particular value independent

swered the harder objects correctly in all trials. The visual informa-from the hardness of the object, while perceiving the hardness of antion was not provided. The following results and the discussions areobject.

demonstrated

that human pinches anabout the first specimen, because some subjects did not complete

The reference input of figure 1 appeared to be the antact

the pinch motion in the second specimen especially the hardness

area. If, the contact area is controlled with feedback control strat-

was obviously different.

Measured indexes

82 l5.
?0.8-An example of the measured pinch force, thumb displacement and the contact area-are shown in figure 3. The peak values
were calculated for both the first and the second pinch.
Because the thumb displacement
deformation and the objectdeformation,

First pinch

0 Displacement
0 Force-8
o %0.6Be.

includes both the thumbthe object deformation was

A Contact area
Second pinchgo.,ofcalculated from the object hardness property measured in advance

8 E0.2-and the applied force. The peak values were averaged for each768
Es0
bs:
Zespecimen.

.

n Displacement.

0 Force
A Contact area1.

1.

1

1

2

3Hardness(soft)

Figure4m 1.
4I5(Hard)

The pinch force, object displacement

and contact area

during the pinch of five objects. Values are normalized.J(4by each maximal values. The object displacementtime(sec.)

calculated from the precedently

wasmeasured hardness ofthe object and the measured pinch force.
c‘-

yO.82 (b)z 50.68 $04.
=o.

2%

Cl Displacement
0 Force
A Contact area.

go.2WE-$,80

Figure 3

The measured pinch force, thumb displacement’1*’-’
2

contact area.

5Figures

The slopes between 10% and 90% of the pinch force,of five objects.

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I4(soft)

Hardness

object displacement

and-1.
3

EG6%z ‘e(Hard)and contact area during the pinch

1al2 0.8'
P
B 0.6'.N
$0.4

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0.2
n

“looI

IO2 IO”

IO’

displacement

IO4

Hardness (g/mm)
Figure 6

The relationship between the hardness of the object andFigure7

Change of the object displacement

the applied pinch force.

describedby theequiriblium point model.

egy, there must be a difference in the transient response of the contact area, and the difference of the transient force must be less.
The slopes of the force, displacement

modeled as a spring. The muscle contractile force is indirectly defined as the force generated by the expanded spring. In this model,

and the contact area,

joint position is determined as the balance point of the antagonistic

detected in the first pinch motions are shown in figure 5. The trends

pair of springs. The joint position is controlled

are similar to the trends of the peaks. It appears that the pinch mo-natural length of a muscle by the central command. In other words,

tion is controlled without feedback of the perceived hardness. It wascentral nervous system controls displacement

suggested that the human pinches the object with open-loop control

That displacement

strategy while detecting the hardness of an object.

by changing thenot the muscle force.

is called imaginary displacement.

Van Doren extended

the equilibrium

point model to de-scribe the pinch of an elastic object. The mimetic diagram changing
Discussion

the position with the same imaginary displacement
figure 7. The imaginary displacement

Meaning of constant contact area

is described inchanges the finger position,

however the object deformation is different because the fingertip is

As shown in the previous section, human pinches an object

also deformed. Therefore, the imaginary displacement

is equal towith constant contact area while perceiving the hardness the object.

the sum of the finger deformation and the object deformation. The

Physiologically, the constant areais equivalent to the constant num-

imaginary displacementin the experiments

the measured hardnessof the objects and the finger. The finger

ber of the exciting skin receptors. Therefore,

the hardness of thewas calculated using

object can be perceived from only the receptor firing rate without

hardness was modeled using an exponential function as already re-counting in the number of the exiting receptor. Therefore, the hard-ported [9].

ness perception task is simplified

by controlling the contact areaconstant. It appears that the “constant contact area” is to reduce theThe calculated imaginary displacement
8. The measured finger displacement

is shown in figureis also shown because thecentral load in perception.
The relationship between the object hardness and the force
is represented

in figure 6. It is observed that the force is propor-tional to the logarithm of the hardness. The relation means that thereceptor firing rate represents

the logarithmic hardness of the ob-

0 Measured

ject. The human sensations such as auditory sensation are known tobe logarithmic. The logarithmic property of the hardness detection
appears to be natural.
As shown in the previous section, this logarithmic property
is supposedly attained by open-loop control strategy. The effect of
the finger mechanical property is discussed by applying the equilibrium point model proposed by Feldman [8], because the fingertip
hardness has been reported as exponential= 2.v,-02 lE
go.!i[9].
Figure 8Equilibrium point mtidel
The equilibrium point model is based on the idea that thecentral command shortens the natural length of the muscle that isII -685'.'*'*'.'
123Hardness(soft)

4(Ha?d)The calculated imaginary displacement

based on theequilibrium point model and the measured thumb displacement.

measured finger displacement

includes both the finger and the ob-ject deformation. The imaginary displacementagree with the measured finger displacement.
The measured
imaginary displacement

thumb displacement

References

should theoreticallyPIand the calculatedagreed well as shown. The object and thestant imaginary displacement

G.Westling: Roles of glabrous skin receptors

and sensorimotor

memory in automatic control of precisiongrip when lifting rougher or more slippery object, Exp. Brain.

thumb deformation appear to be properly calculated. The variationof the calculated imaginary displacement

RXJohansson,Res., 56-3,550-564(1984)

PI H.Kinoshita,

was 28.6%. The “con-control strategy” described the “con-L.Backstrom,

J.R.Flanagan,

Tangential torque effects on the control of grip forces whenstant contact area” well, because the variation of the imaginary dis-holding objects with a precision grip, J. Neurophysiol.,placement was considerably less than that of force (91%). The de-78-3,1619-1630(1997)

crease of the imaginary displacement with the increase of the hard-PI G.Burdea, D.Gomez, NLangrana,

ness comprises that there is some effect of “constant force strategy”

tual reality graphics simulationin addition to the imaginary displacement

R.S.Johansson:

tactics.

E.Roskos, P.Richard: Virwith force feedback,Int. J.

Comp. Simul., 5,287-303(1995)

141 K.Fujita: Development and evaluation of force feedback glove
Conclusions

for hardness display of virtual object, Trans. IEICE, J81-D-II-

The human pinch motion to perceive the hardness of an

10,2394-2400(1998)(in japanese)

151 N.Mai, PSchreiber,

J.Hermsdorfer:

Changes in perceived fin-object with unknown hardness was analyzed. It was revealed that 1)

ger force produced by muscular contractionshuman tends to pinch an object with constant contact area regard-and anisometricless the hardness of the object; 2) the “constant contact area” tactics

460(1991)

allows to perceive the hardness from only the steady state force asFl

equilibrium-point

appears to be attained by using “constant imaginary displacement”

492( 1995)

tactics. It is expected to develop a system which detects the pinch[71 M.A.Srinivasan,model,Exp. Brain

Res, 106-3, 488-

R.H.I.aMotte: Tactual discriminationness, J. Neurophysiol.,device.

under isometricExp. Brain. Res., 84-2, 453-C.L.Van Doren: Pinch force matching errors predicted by ana

the firing rate of the tactile receptors; 3) the “constant contact area”

force and controls the fingertip contact area as a hardness display

conditions,of soft-73-1, 88-lOl(1995)

PI A.G.Feldman: Once more on the equilibrium-point

hypothesis(i model) for motor control, J. Mot. Behav., 18,17-54(1986)[91 A.Z.Hajian, R.D.Howe: Identificationof the mechanicalim-pedance at the human finger tip, J. Biomech. Eng., 119-1,109114(1997)

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