Conducting Rubber Force Sensor: Transient Characteristics and Radiation Heating Effect

doi:10.4236/jst.2013.33007

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Journal of Sensor Technology, 2013, 3, 36-41

http://dx.doi.org/10.4236/jst.2013.33007 Published Online September 2013 (http://www.scirp.org/journal/jst)

Conducting Rubber Force Sensor: Transient

Characteristics and Radiation Heating Effect

Masato Ohmukai1, Yasushi Kami1, Ken Ashida2

1Department of Electrical and Computer Engineering, Akashi College of Technology, Akashi, Japan

2Department of System Science, School of Engineering Science, Osaka University, Toyonaka, Japan

Email: [email protected]

Received May 20, 2013; revised June 20, 2013; accepted June 28, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Compression force sensors are indispensable to tactile sensors in humanoid robots. We are investigating the application

of low-cost electrically conducting rubber sheets to force sensors, of which the biggest problem is its poor reproducibil-

ity. We have found that the deposition of aluminum by a vacuum evaporation method shows such an excellent charac-

teristic that the sensor can be used in a wide range under 10.33 N/cm2. In this article, we investigated time response of

the sensors and also studied how the radiation heating during the vacuum evaporation process for Al deposition affected

their sensing property. We found that the radiation heating induces deterioration from the point of view of standard de-

viation of the output voltage of the sensors at a transient region. We convince that a low-temperature Al deposition

method should be developed to form electrodes on the electrical conducting rubber sensors.

Keywords: Conducting Rubber; Force Sensor; Electrode; Vacuum Deposition; Radiation Heating

1. Introduction

Along with the development of humanoid robots, force

sensors play a key role in controlling the actuation of robot

fingers [1]. In order to realize the elaborate function of

robot fingers, we need a sophisticated control system.

Even with a highly designed control system, a highly

reproducible force sensor is crucial for good control be-

cause the tactile information is used as control parame-

ters in dexterous manipulation of humanoid fingers [2-4].

The studies on tactile sensors have been fully reported

so far. The types of the sensors are ranged in a wide vari-

ety. A kind of them is typically based on a strain gauge

or a piezoresistive device. The resistance variation is

brought about by the applied strain in these devices. The

sensor arrays of this type have been reported nowadays

[5-7]. The arrays of micro sensors demonstrated the abil-

ity in position sensing. In some cases, micro-machine

technique was included to form this kind of sophisticated

sensors. Some researchers have studied three-dimension

sensor arrays that can detect shear forces [8]. Many other

sensors of different types have been reported widely

[9-29]. In order to miniature the sensing system, silicon-

based microelectromechanical system (MEMS) has been

challengingly applied [30,31] to piezoresistive [32-35] or

capacitive [36,37] type of tactile sensors.

We pay attention to a conductive rubber sheet that

varies electric resistance with compressive force. The

conductive rubber consists of an elastomer enriched with

conductive filler particles. The resistance of the rubber

sheet reduces when external compressive pressure is ap-

plied. The material shows isotropic conduction. The ad-

vantage of the rubber sheet as a force sensor is the low

cost of the flexible material as well as the large area sen-

sing ability. In order to deal successfully with the short of

reliability of the material, it still requires more improve-

ments in the fabrication of this sensor. The formation of

electrodes on the rubber surfaces comes into an issue at

first. Electrodes are a basic component of electric devices

but the issue has not been examined so far.

We investigated the rubber sensors with four kinds of

electrodes in our previous work: Al thin film, Al depos-

ited on the rubber sheet by a vacuum deposition method,

conducting adhesive Cu tape and silver paste spread on

the surfaces. The relationship of the resistance to applied

force has been studied and discussed from the point of

view of a force sensor in a robotic finger. Then we have

concluded that the Al deposition in vacuum is the most

preferable with regard to a force range and reproducibil-

ity [38]. However, we concerned the deterioration in-

duced by the radiation heating during the vacuum depo-

sition because the rubber sheet was installed very near to

M. OHMUKAI ET AL. 37

the evaporation boat, of which the temperature rose so

high as to melt Al.

In this article, we studied time response of the rubber

sheet sensors and discussed the standard deviation of the

transient output voltage. Further, we investigated the ra-

diation effect of the rubber sheet by performing a blank

test.

2. Experimental Details

We used commercially available electrically conducting

rubber sheets (30-mm square and 15-μm thick) in our ex-

periments. We prepared two kinds of metal contacts to

the rubber sheets: (Sample A) aluminum thin foil (12 μm

thick) was put on the surface and the perimeter was fixed

with adhesive tape and (Sample B) aluminum was depos-

ited on the surfaces by a vacuum evaporation method.

For a vacuum deposition of Al, a rubber sheet was in-

stalled at 13 cm from the evaporation boat where starting

material of Al (0.144 g) was filled. The boat was then

red-heated up gradually by electric current for 340 s. The

entire Al in the boat was evaporated and then deposited

on the surface of the rubber sheet. This process was re-

peated twice successively to form a 5.5-μm thick Al film.

The same film was also deposited on the other side

(Figure 1).

We prepared Sample C in the same way as Sample A.

Before attaching Al foil, the rubber experienced accept-

ing the radiation only from the empty boat in the vacuum

deposition chamber. This sample was used as a blank test

Figure 1. The outlook of the rubber sheet sensor with alu-

minum electrode deposited on the surface. On the deposited

aluminum, Al foil was attached as a conductor to a detect-

ing circuit.

of Sample A with a radiation effect during the deposi-

tion.

In order to apply the force uniformly in the surface

area, the sensor was put between two solid Al plates. A

calibrated weight of 0.5 or 1 kg was put on the plate at 0

s. Each weight corresponds to 0.54 and 1.1 N per 1 cm2,

respectively. The former value is in a typical force range

for normal finger manipulation that was reported to be

from 0.15 to 0.88 N [39,40].

We show in Figure 2 the detecting circuit of the sen-

sor. The r(t) denotes the resistance of the rubber sheet

sensor that has several mega ohm without any force onto

it and decreases down to the order of kilo ohm when the

sensor is pressed by a finger. A resistor (15 kΩ) and a

capacitor (4.7 μF) attached together in parallel was con-

nected in series to the sensor, as shown in the figure. The

output voltage V0(t) was taken across the added circuit

elements. The capacitor worked to reduce high frequency

noise. The V0(t) was measured every 5 ms until 1 s. This

1 s scan was repeated 100 times at each measurement.

3. Results and Discussion

We show the time response of the output voltage V0(t) in

case of Sample A in Figures 3 and 4, corresponding to

the weights of 0.5 and 1 kg, respectively. Transient re-

gion is extended to 0.4 s in Figure 3 while it is up to 0.6

s in Figure 4. The heavier weight demanded more time

to be settled to a final value. The final value is 2.61 and

3.39 V in Figures 3 and 4, respectively, but the repro-

ducibility is unfortunately poor. It is possibly derived

from the inherent property of the conducting rubber. The

relative error is 22.7% and 13.3%, respectively, so that it

is smaller when the weight is the heavier.

We have performed the same experiment on Sample B

and obtained the results shown in Figures 5 and 6. The

transient region is extended to 0.35 and 0.4 s, respec-

tively. The transient region is slightly reduced compared

with sample A. The feature is quite different from Fig-

ures 3 and 4 at two points on the whole. One point is the

r(t)

V0(t)

Figure 2. A detecting circuit for the rubber sensor r(t). Di-

rect current voltage of 5 V was applied across the terminals

(V in the figure), and the voltage of V0(t) was measured.

The R and C were 15 kΩ and 4.7 μF, respectively.

M. OHMUKAI ET AL.

0 0.2 0.4 0.6 0.8 1

t [sec]

V0(t) [V]

Figure 3. The time response of V0 when aluminum foil was

directly attached to the rubber sheet (Sample A). The

weight was 0.5 kg. The measurement was repeated 100

times.

0 0.2 0.4 0.6 0.8 1

t [sec]

V0(t) [V]

Figure 4. The time response of V0 when aluminum foil was

directly attached to the rubber sheet (Sample A). The

weight was 1 kg. The measurement was repeated 100 times.

0 0.2 0.4 0.6 0.8 1

t [sec]

0.5

0.4

0.3

0.2

0.1

V0(t) [V]

Figure 5. The time response of V0 when aluminum was de-

posited by a vacuum evaporation method on the rubber

sheet (Sample B). The weight was 0.5 kg. The measurement

was repeated 100 times.

0 0.2 0.4 0.6 0.8 1

t [sec]

0.5

0.4

0.3

0.2

0.1

V0(t) [V]

Figure 6. The time response of V0 when aluminum was de-

posited by a vacuum evaporation method on the rubber

sheet (Sample B). The weight was 1 kg. The measurement

was repeated 100 times.

output voltage is smaller by one order of magnitude. The

final values are 0.18 and 0.34 V, respectively. We con-

sider the smallness of the output voltage here. Al elec-

trodes were formed on the both side of the rubber and the

each electrode can firmly cling to the surface, which is

one of the advantages of vacuum deposition. Since the

sensor became rigid owing to the two pieces of metal

electrodes, the rubber sheet was possibly less deformed

by the same pressure for around 1 N/cm2. The difference

of final output value between the two samples is consis-

tent with the results reported in our previous results [38].

It should be noticed that the relative error for Sample B

was 59.5% and 43.2% in a final value, respectively. The

large error, which is far beyond twice of that of Sample

A, is actually annoying obstacle for a feedback control

system. The other point is seen in the convergence state

in the transition region. Sample A shows the gradual in-

crease with time even if there is a large dispersion, but

Sample B does not show any gradual increase but full

dispersion between zero and the maximum value at any

time in the transition region. The tendency is also based

on the rigid property of Al deposited rubber sheets.

We next consider the results in case of Sample C. The

obtained data are depicted in Figures 7 and 8, respec-

tively. Both the transient region and the magnitude of

0 0.2 0.4 0.6 0.8 1

t [sec]

V0(t) [V]

Figure 7. The time response of V0 when aluminum foil was

directly attached to the rubber sheet after the radiation

heating treatment as a blank test (Sample C). The weight

was 0.5 kg. The measurement was repeated 100 times.

0 0.2 0.4 0.6 0.8 1

t [sec]

V0(t) [V]

Figure 8. The time response of V0 when aluminum foil was

directly attached to the rubber sheet after the radiation

heating treatment as a blank test (Sample C). The weight

was 1 kg. The measurement was repeated 100 times.

M. OHMUKAI ET AL. 39

output voltage are similar to those of Sample A. We

might not notice the difference between Sample A and C

at a glance, but the relative error in the transient region is

slightly larger with a close eye. We then calculate the

standard deviation of each one hundred data under 0.2 s

(Figures 9 and 10) to investigate the relative error more

closely. Figure 9 gives the transient variation of the

standard deviation of the output voltage when the loaded

weight is 0.5 kg. Figure 9 shows an interesting fact that

the standard deviation noticeably reduces along with time

only in Sample A. Although the physical structure of

Sample C was the same as Sample A, the standard devia-

tion of Sample C behaves rather in the same way as

Sample B. It should be concluded that the tendency of no

reduction of the standard deviation derived from the ra-

diation heating effect during the deposition. It is because

the evaporation boat becomes red heated near the rubber

sheet during the deposition. Although we do not confirm

the surface temperature of the rubber sheet precisely, the

results of the blank test led us to the conclusion. Another

interesting point is that this conclusion is not the case

0.1 0.125 0.15 0.175 0.2

time [s]

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

standard deviation

Foil Heat Depo

Figure 9. The standard deviation of V0 when the weight was

0.5 kg as a function of time for the three kinds of samples.

Only Sample A shows the reduction of the standard devia-

tion with time.

0.1 0.125 0.15 0.175 0.2

time [s]

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

standard deviation

Foil Heat Depo

Figure 10. The standard deviation of V0 when the weight

was 1 kg as a function of time for the three kinds of samples.

All samples show the same characteristics.

when the loaded weight was 1 kg, the result of which is

shown in Figure 10. Then the effect of the radiation

heating only appears when the weight is small, in other

words, deformation of the rubber is not large. It should

be required to remove the radiation heating effect in or-

der to suppress the deterioration of the output voltage

fluctuation. A sputtering method may be a hopeful can-

didate for this problem.

4. Conclusion

We are studying the electro conducting rubber sheet to

apply it to a compression force sensor that can be realized

at formidably low cost. In this paper, two kinds of elec-

trode: Al foil and deposited Al by a vacuum evaporation

method, have been engaged. We found that the radiation

heating of the rubber sheet during the deposition is dete-

riorated in the point of view of standard deviation of the

output voltage under 0.2 s. It was clarified with the re-

sults including a blank test. However, this tendency was

not observed when the force was large. A low-tempera-

ture deposition method on the rubber sheet is expected to

improve the reproducibility of the output voltage.

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