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
Received May 20, 2013; revised June 20, 2013; accepted June 28, 2013
Copyright © 2013 Masato Ohmukai et al. This is an open access article distributed under the Creative Commons Attribution License,
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
C
opyright © 2013 SciRes. JST

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
C
r(t)
V0(t)
R
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. 38
0 0.2 0.4 0.6 0.8 1
t [sec]
4
3
2
1
0
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]
4
3
2
1
0
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
0
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
0
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]
4
3
2
1
0
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]
4
3
2
1
0
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
0
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
0
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.
REFERENCES
[1] M. H. Lee and H. R. Nicholls, “Review Article Tactile
Sensing for Mechatronics—A State of the Art Survey,”
Mechatronics, Vol. 9, No. 1, 1999, pp. 1-31.
doi:10.1016/S0957-4158(98)00045-2
[2] Z. Li, P. Hsu and S. Sastry, “Grasping and Coordinated
Manipulation by a Multifingered Robot Hand,” The In-
ternational Journal of Robotics Research, Vol. 8, No. 4,
1989, pp. 33-50. doi:10.1177/027836498900800402
[3] A. D. Berger and P. K. Khosla, “Using Tactile Data for
Real-Time Feedback,” The International Journal of Ro-
botics Research, Vol. 10, No. 2, 1991, pp. 88-102.
doi:10.1177/027836499101000202
[4] P. A. Schmidt, E. Mael and R. P. Wurtz, “A Sensor for
Dynamic Tactile Information with Applications in Hu-
man-Robot Interaction & Object Exploration,” Robotics
and Autonomous Systems, Vol. 54, No. 12, 2006, pp.
1005-1014. doi:10.1016/j.robot.2006.05.013
[5] K. Kim, K. R. Lee, W. H. Kim, K. Park, T. Kim, J.-S.
Kim and J. J. Pak, “Polymer-Based Flexible Tactile Sen-
sor up to 32 × 32 Arrays Integrated with Interconnection
Terminals,” Sensors and Actuators A: Physical, Vol. 156,
No. 2, 2009, pp. 284-291. doi:10.1016/j.sna.2009.08.015
[6] J. Engel, J. Chen and C. Liu, “Development of Polyimide
Flexible Tactile Sensor Skin,” Journal of Micromechan-
ics and Microengineering, Vol. 13, No. 3, 2003, pp. 359-
366. doi:10.1088/0960-1317/13/3/302
[7] Y. Zhang, “Sensitivity Enhancement of a Micro-Scale
Biomimetic Tactile Sensor with Epidermal Ridges,” Jour-
nal of Micromechanics and Microengineering, Vol. 20,
No. 8, 2010, Article ID: 085012.
doi:10.1088/0960-1317/20/8/085012

M. OHMUKAI ET AL. 40
[8] W.-C. Choi, “Polymer Micromachined Flexible Tactile
Sensor for Three-Axial Toads Detection,” Transactions
on Electrical and Electronic Materials, Vol. 11, No. 3,
2010, pp. 130-133. doi:10.4313/TEEM.2010.11.3.130
[9] K. Noda, K. Hoshino, K. Matsumoto and I. Shimoyama,
“A Shear Stress Sensor for Tactile Sensing with the Pie-
zoresistive Cantilever Standing in Elastic Material,” Sen-
sors and Actuators A: Physical, Vol. 127, No. 2, 2006, pp.
295-301. doi:10.1016/j.sna.2005.09.023
[10] L. Beccai, S. Rocdella, L. Ascari, P. Valdastri, A. Sieber,
M. Carrozza and P. Dario, “Development and Experi-
mental Analysis of a Soft Compliant Tactile Microsensor
for Anthropomorphic Artificial Hand,” IEEE/ASME
Transactions on Mechatronics, Vol. 13, No. 2, 2008, pp.
158-168. doi:10.1109/TMECH.2008.918483
[11] H. Lee, J. Chung, S. Chang and E. Yoon, “Normal and
Shear Force Measurement Using a Flexible Polymer Tac-
tile Sensor with Embedded Multiple Capacitors,” Journal
of Microelectromechanical Systems, Vol. 17, No. 4, 2008,
pp. 934-942. doi:10.1109/JMEMS.2008.921727
[12] S. Miyazaki and A. Ishida, “Capacitive Transducer for
Continuous Measurement of Vertical Foot Force,” Medi-
cal and Biological Engineering and Computing, Vol. 22,
No. 4, 1984, pp. 309-316. doi:10.1007/BF02442098
[13] Y. Hasegawa, M. Shikida, D. Ogura, Y. Suzuki, and K.
Sato, “Fabrication of a Wearable Fabric Tactile Sensor
Produced by Artificial Hollow Fiber,” Journal of Micro-
mechanics and Microengineering, Vol. 18, No. 8, 2008,
Article ID: 085014. doi:10.1088/0960-1317/18/8/085014
[14] J.-S. Heo, J.-H. Chung and J.-J. Lee, “Tactile Sensor Ar-
rays Using Fiber Bragg Grating Sensors,” Sensors and
Actuators A: Physical, Vol. 126, No. 2, 2006, pp. 312-327.
doi:10.1016/j.sna.2005.10.048
[15] E. Cheung and V. Lumelsky, “A Sensitive Skin System
for Motion Control of Robot Arm Manipulators,” Robot-
ics and Autonomous Systems, Vol. 10, No. 1, 1992, pp.
9-32. doi:10.1016/0921-8890(92)90012-N
[16] E. S. Kolesar, R. R. Reston, D. G. Ford and R. C. Fitch,
“Multiplexed Piezoelectric Polymer Tactile Sensor,” Jour-
nal of Robotic Systems, Vol. 9, No. 1, 1992, pp. 37-63.
doi:10.1002/rob.4620090104
[17] J. Dargahi, M. Parameswaran and S. Payandeh, “A Mi-
cromachined Piezoelectric Tactile Sensor for an Endo-
scopic Grasper—Theory, Fabrication and Experiments,”
Journal of Microelectromechanical Systems, Vol. 9, No.
3, 2000, pp. 329-335. doi:10.1109/84.870059
[18] J. R. Flanagan and A. M. Wing, “Modulation of Grip
Force with Load Force during Point-to-Point Arm Move-
ments,” Experimental Brain Research, Vol. 95, No. 1,
1993, pp. 131-143. doi:10.1007/BF00229662
[19] P. Dario and D. de Rossi, “Tactile Sensors and Gripping
Challenge,” IEEE Spectrum, Vol. 22, No. 8, 1985, pp.
46-52. doi:10.1109/MSPEC.1985.6370785
[20] N. Wettels, V. Santos, R. Johansson and G. Loeb, “Bio-
mimetic Tactile Sensor Array,” Advanced Robotics, Vol.
22, No. 8, 2008, pp. 829-849.
doi:10.1163/156855308X314533
[21] I. Manunza and A. Bonfiglio, “Pressure Sensing Using a
Completely Flexible Organic Transistor,” Biosensors and
Bioelectronics, Vol. 22, No. 12, 2007, pp. 2775-2779.
doi:10.1016/j.bios.2007.01.021
[22] T. Sekitani and T. Someya, “Stretchable, Large-Area Or-
ganic Electronics,” Advanced Materials, Vol. 22, No. 10,
2010, pp. 2228-2246. doi:10.1002/adma.200904054
[23] D. Bloor, K. Donnelly, P. J. Hands, P. Laughlin and D.
Lussey, “A Metal-Polymer Composite with Unusual Pro-
perties,” Journal of Physics D: Applied Physics, Vol. 38,
No. 16, 2005, pp. 2851-2860.
doi:10.1088/0022-3727/38/16/018
[24] V. Maheshwari and R. F. Saraf, “High-Resolution Thin-
Film Device to Sense Texture by Touch,” Science, Vol.
312, No. 5779, 2006, pp. 1501-1504.
doi:10.1126/science.1126216
[25] S. Ando and H. Shinoda, “Ultrasonic Emission Tactile
Sensing,” IEEE Control Systems, Vol. 15, No. 1, 1995, pp.
61-69. doi:10.1109/37.341866
[26] R. S. Dahiya, M. Valle and L. Lorenzelli, “Spice Model
of Lossy Piezoelectric Polymers,” IEEE Transactions on
Ultrasonics, Ferroelectrics and Frequency Control, Vol.
56, No. 2, 2009, pp. 387-396.
doi:10.1109/TUFFC.2009.1048
[27] G. M. Krishna and K. Rajanna, “Tactile Sensor Based on
Piezoelectric Resonance,” IEEE Sensors Journal, Vol. 4,
No. 5, 2004, pp. 691-697.
doi:10.1109/JSEN.2004.833505
[28] T. J. Nelson, R. B. V. Dover, S. Jin, S. Hackwood and G.
Beni, “Shear-Sensitive Magnetroresistive Robotic Tactile
Sensor,” IEEE Transactions on Magnetics, Vol. 22, No. 5,
1986, pp. 394-396. doi:10.1109/TMAG.1986.1064386
[29] Z. Wen, Y. Wu, Z. Zhang, S. Xu, S. Huang and Y. Li,
“Development of an Integrated Vacuum Microelectronic
Tactile Sensor Array,” Sensors and Actuators A: Physical,
Vol. 103, No. 3, 2003, pp. 301-306.
doi:10.1016/S0924-4247(02)00392-8
[30] D. J. Beebe, A. S. Hsieh, D. D. Denton and R. G. Radwin,
“A Silicon Force Sensor for Robotics and Medicine,”
Sensors and Actuators A: Physical, Vol. 50, No.1-2, 1995,
pp. 55-65. doi:10.1016/0924-4247(96)80085-9
[31] M. R. Wolffenbuttel and P. P. L. Regtien, “Polysilicon
Bridges for the Realization of Tactile Sensors,” Sensors
and Actuators A: Physical, Vol. 26, No. 1-3, 1991, pp.
257-264. doi:10.1016/0924-4247(91)87002-K
[32] S. Sugiyama, K. Kawahata, M. Yoneda and I. Igarashi,
“Tactile Image Detection Using a 1k-Element Silicon
Pressure Sensor Array,” Sensors and Actuators A: Physi-
cal, Vol. 22, No. 1-3, 1990, pp. 397-400.
[33] L. Liu, X. Zheng and L. Zhijian, “An Array Tactile Sen-
sor with Piezoresistive Single-Crystal Silicon Diaphragm,”
Sensors and Actuators A: Physical, Vol. 35, No. 3, 1993,
pp. 193-196. doi:10.1016/0924-4247(93)80151-6
[34] B. J. Kane, M. R. Cutkosky and G. T. A. Kovacs, “A
Traction Stress Sensor Array for Use in High-Resolution
Robotic Tactile Imaging,” Journal of Microelectrome-
chanical Systems, Vol. 9, No. 4, 2000, pp. 425-434.
doi:10.1109/84.896763
[35] H. Takao, K. Sawada and M. Ishida, “Monolithic Silicon

M. OHMUKAI ET AL. 41
Smart Tactile Image Sensor with Integrated Strain Sensor
Array on Pneumatically Swollen Single-Diaphragm Struc-
ture,” IEEE Transactions on Electron Devices, Vol. 53,
No. 5, 2006, pp. 1250-1259.
doi:10.1109/TED.2006.872698
[36] Z. Chu, P. M. Saoor and S. Middelhoek, “Silicon Three-
Axial Tactile Sensor,” Sensors and Actuators A: Physical,
Vol. 54, No. 1-3, 1996, pp. 505-510.
doi:10.1016/S0924-4247(95)01190-0
[37] M. Leineweber, G. Pelz, M. Schmidt, H. Kappert and G.
Zimmer, “New Tactile Sensor Chip with Silicone Rubber
Cover,” Sensors and Actuators A: Physical, Vol. 84, No.
3, 2000, pp. 236-245.
doi:10.1016/S0924-4247(00)00310-1
[38] M. Ohmukai, Y. Kami and R. Matsuura, “Electrode for
Force Sensor of Conductive Rubber,” Journal of Sensor
Technology, Vol. 2, No. 3, 2012, p. 127.
[39] R. H. LaMotte and M. A. Srinivasan, “Tactile Discrimi-
nation of Shape: Responses of Slowly Adapting Mech-
anoreceptive Afferents to a Step Stroked across the Mon-
key Fingerpad,” The Journal of Neuroscience, Vol. 7, No.
6, 1987, pp. 1655-1671.
[40] R. H. LaMotte and M. A. Srinivasan, “Tactile Discrimi-
nation of Shape: Responses of Rapidly Adapting Mech-
anoreceptive Afferents to a Step Stroked across the Mon-
key Fingerpad,” The Journal of Neuroscience, Vol. 7, No.
6, 1987, pp. 1672-1681.