=Paper= {{Paper |id=Vol-1324/paper_11 |storemode=property |title=Study of Static Tactile Detection Threshold via Pneumatically Driven Polydimethylsiloxane Membrane |pdfUrl=https://ceur-ws.org/Vol-1324/paper_11.pdf |volume=Vol-1324 |dblpUrl=https://dblp.org/rec/conf/tabletop/KunchornsupLBFP14 }} ==Study of Static Tactile Detection Threshold via Pneumatically Driven Polydimethylsiloxane Membrane== https://ceur-ws.org/Vol-1324/paper_11.pdf

Study of Static Tactile Detection Threshold via
Pneumatically Driven Polydimethylsiloxane Membrane
Wissawin KUNCHORNSUP Fabrizio LEO Franco BERTORA
Despina FRAGOULI Simona PETRONI Luca BRAYDA
Fondazione Istituto Italiano di Tecnologia, Genoa, Italy
luca.brayda@iit.it

ABSTRACT Soft actuation
In this work we investigate how the tactile detection Novel approaches [2,3] try to utilize advances in MEMS
thresholds of air-driven actuators are influenced by design technology, to increase the power-to-weight ratio and create
parameters as membrane thickness and diameter, when the portable solutions. A growing interest is in the fabrication
finger is in contact with the deformable circular elastic of soft actuators, i.e. actuators having lightweight mass, low
membranes. We report lower thresholds with decreasing damping coefficients and low stiffness [4]. They do have
membrane thickness and increasing diameters. These results the big advantage of avoiding dot-by-dot assembly of the
can help in the design of soft actuators for pin-arrays used actuators, as a large area full of actuators can be produced
as graphical tactile displays for visually impaired people. at once using batch-processing technologies. Thus, the cost
is, ideally, mainly defined by the number of required
General Terms
processing steps, regardless of the number of pins/actuators
Tactile Displays, Pin arrays, Psychophysics, Pneumatics.
realized on the given surface.
ACM Classification Keywords Most of the MEMS-based soft actuation mechanisms have
been applied to Braille geometries, mainly because Braille
H.5.2. Information interfaces and presentation: User dots are small, but very few of the considered architectures
Interfaces are purely designed for tactile graphics, without necessarily
providing text [5].
Beyond Braille
INTRODUCTION
In general, the tactile display market was static in the recent When dealing with perception of tactile graphics, as
years and no breakthrough is being observed. However, this compared to Braille reading, the objective shifts from
topic attracts a lot of attention and many research centers understanding information coded as single, separated small
and companies are dealing with large area tactile displays to dots to picturing a sequence of symbols forming
make the digital world more accessible to the visually geometrical primitives. The specifications of Braille can
impaired community or to provide eyes-free computer- therefore be relaxed: on the one hand, graphical raised line
human interfaces. Information is generally organized as drawings can be perceived as low as 200 µm [6], while
small pin-shaped actuators, named taxels as they can be Braille requires dot elevation to be in the 600-900 µm
considered the tactile equivalent of the pixel [1] range; on the other hand, larger dots are generally perceived
Applications span from text reading to tactile graphics. better than small dots [7]. Specifically, large stroke seem
Besides Braille bars, no portable device of reasonable cost not be a requirement when dots are larger than 2mm [8]. In
exists, which have an area large enough to display graphical line with that, response times in shape recognition using
information, for example maps and scientific content. some three-dimensional tactile displays were inversely
proportional to the object size. In addition, the third
In fact, commercially available systems use technologies dimension was not reputed necessary compared to a purely
which have very high costs per pin, due to the large number bistable pin [5].
of individually assembled components. Cost linearly
increases with the number of pins and grows exponentially Therefore, the question is if one can build controllable
when organizing pin in two-dimensional arrays. actuators able to deliver sufficient force, enough stroke,
without increasing too much the dot diameter, which would
Permission to make digital or hard copies of all or part of this work for limit the resolution and as a consequence the number and
personal or classroom use is granted without fee provided that copies are
type of displayed information.
not made or distributed for profit or commercial advantage and that
copies bear this notice and the full citation on the first page. To copy Pneumatically-driven actuators [9, 10, 11] are ideal
otherwise, or republish, to post on servers or to redistribute to lists, candidates to study such sufficient conditions, as they
requires prior specific permission and/or a fee.
TacTT2014, November 16th, 2014, Dresden, Germany. provide a wide range of low-to-high controllable forces.
Copyright is held by the owner/author(s).
Doh E., et al. in 2011 fabricated 3-axis tactile display We also considered different thicknesses of the contact
actuator using PDMS pneumatic balloons for a robot membrane, namely 200, 600, 1000 and 2000 µm. Our goal
assisted surgery system. The configuration could stimulate was to derive minimal physical requirements, in terms of
the finger with normal and shear forces, producing force per area unit, so that a single dot is statistically
displacements of 1.5 mm in each direction [12]. distinguished from an otherwise smooth, non-rigid surface.
We investigated possible dependencies from taxel diameter
NI card PC+MATLAB
and membrane thickness.
Amplifier

DC
finger-like
power
supply

Force
meter Tactile unit Movable station with
Figure 2. The model to convert displacements into contact
micro-scale
areas.

EXPERIMENTAL SETUPS
The first experimental setup is aimed at measuring physical
characteristics of taxels having variable diameter and
contact membrane thickness. The second setup was aimed
at estimating tactile detection thresholds when using the
taxels of the first setup.
b) c) Materials
Ecoflex®50, Smooth-On, a Polydimethylsiloxane (PDMS)
Figure 1. (a) Force-displacement setup (b) Single tactile composite, was used as the membrane in contact with the
unit with small and big cubes (c) Membrane inflated via finger. Elastosil 43, WACKER, an adhesive, was used to
micro-pump. stick the PDMS membrane on top of the support, made of
rapid prototyping material Verowhite 835. Rapid
prototyping technology was used to fabricate taxels shaped
They can also very well mimic soft actuation systems, such
as cuboids of variable size. Pneumatic energy was output
as dielectric elastomers and shape memory alloys, for which
from a piezolelectric micro-blower from Murata, able to
the pressure below the actuator is constant, i.e. does not
reach pressures higher than 2kPa.
increase when being pressed by the finger. Constant
pressure also allows to mimic latching properties i.e. the Tactile Unit Fabrication
capability of the system to withstand the finger force with Ecoflex®50 is two-component based PDMS, composed of
minimal energy consumption. A and B; the ratio of mixing (A:B) is 1:1. The mixture was
In addition, the kind and thickness of the membrane in spin-cast by 300 rpm for 30 s to create homogeneous
contact with the finger is generally taken as a fixed surface on TEFLON petri-dish. The spin-cast mixture was
parameter. Therefore it is generally unknown if varying treated in vacuum to avoid air bubble when it solidifies
thickness also varies perceived information. before curing overnight at 80 ºC. When membrane was
ready, it was adhered on the Elastosil coated substrate,
In this preliminary study we investigate static tactile which was printed in two configuration: a first setup
thresholds of single air-driven taxels, when the dot diameter involved small cubes 19 x 21 x 12 mm , a second setup
is larger than Braille specifications, namely 4mm and involved bigger cubes 30 x 33 x 32 mm (see Figure 1b and
10mm. The first value corresponds to the inter-dot distance 1c). Then it was cured overnight at 50 ºC to obtain fully
known to be optimal for roughness perception [8], the cured membrane and stable properties. The 3D-printed
second is close to the two-point discrimination thresholds component, with the membrane on top, was wedged on the
on the palm. nozzle of the piezoelectric micro-blower. In the second
setup (big cubes), a further 3D-printed component
TacTT’14, November 16–19, 2014, Dresden, Germany.
Copyright is held by the owner/author(s).
surrounded the micro-blower to minimize accidental motion deformation behaved as an inflating sphere of radius r given
of the nozzle. by:

(1)

where r = radius of curvature; δ2mN = displacement at 1or 2
mN (for the DMA and the force meter respectively) of
preload wrt the reference surface; w = width of the hole
(the taxel diameter).
When the finger or force meter exerted a force (red arrow)
on the "on" taxel, the elasticity of PDMS allows the probe
to flatten the sphere up to a distance δ from the
background.
The flattened area is the probe-taxel contact area is given
by:
Figure 3. Psychophysical setup: the taxel fixed on a
metal plate. ( ) [( (( ) ) ] (2)
and the estimated pressure is obtained by dividing the
measured force by A(δ).

Force-displacement setup
The force-displacement setup is shown in Figure 1a. The
Psychophysical Setup
tactile unit was inflated by the micro-pump. To produce the
In a first psychophysical experiment, thought as a pilot
desired pressure, the micro-blower was voltage-driven at
study, we used the small cubes, to be held between the
resonance by a sinusoidal wave, with fixed frequency 43
thumb and the middle finger. The index finger had to touch
kHz and amplitude proportional to the desired pressure. We
the PDMS membrane. In a second experiment (see Figure
calibrated the voltage-pressure relationship to compensate
3), participants had their dominant hand resting on an
for different micro-pumps, which exhibited repeteable but
Aluminium plate, where the setup with big cubes was
slighlty different characteristic functions. The micro-pump
mounted and fixed.
was supplied via DC power supply (TTi, Ex354 Tv, Triple
Power Supply 300 W), and controlled by a MATLAB script Nineteen participants (thirteen women) had a mean age of
(R2012b, 32 bit) through a National Instrument card (NI 33 years (range 26-41). For each participant, 5 min practice
USB 6211, 16 inputs, 16 bit, 250 kS/s, Multifunction I/O), preceded threshold estimation. Then, they underwent at
and a custom-built amplifier. least 6 tactile detection threshold estimation blocks to
define for each participant their detection threshold (target
Preliminary measures were obtained with a DMA (Dynamic level of 75% correct) using the method of constant stimuli.
Mechanical Analyzer) to obtain force-displacement curves Each threshold estimation block comprised 30 trials in
when force exceeded 1 mN (the DMA precision). which tactile stimuli with different pressure levels were
Subsequent displacement measures were obtained with a presented pseudorandomly either in a first or in a second
moveable micro-scale station (THORLABS) and a force interval lasting 3.5 seconds. Each pressure level was
meter (Mecmesin, BFG 10 N). Specifically, a zero reached as described in the force-displacement setup.
displacement is equivalent to a PDMS surface with 2 mN
(the force-meter precision) of contact force with the force First interval was defined by a single-beep sound. Second
meter probe. The microblower was controlled to provide interval was signaled by a double-beep sound. Participants
variable pressure values. For each pressure value, were instructed to touch the tactile unit with the index
displacement was decreased up to zero with the micro-scale finger of their dominant hand each time they heard the
while measuring blocking forces. sound and to verbally report whether they perceived a
displacement of the membrane in the first or the second
Since no control on the actual pressure at the membrane interval (two-interval forced choice detection task; 2IFC).
was available (only the output from the micropump nozzle Control condition was a zero displacement interval.
was known) we compensated possible air leakages out of
the small/big cubes by normalizing pressure values fed to To estimate the threshold for 75% correct detection, we
the psychophysical setup. used the psignifit toolbox (psignifit.sourceforge.net) version
3.0 for Matlab, which implemented the maximum-
The true pressure values were estimated with the model likelihood method described in [13] for Weibull curve
depicted in Figure 2: we assumed the membrane fitting.
When the estimated pressure of different setups resulted measures ANOVA with membrane thickness (200, 600,
different, we normalized the thresholds for the relative 1000 and 2000 µm) as within subjects factor. The ANOVA
efficiency. Unless otherwise stated all the reported statistics showed a main effect of thickness [F(3,27) = 19.11; p =
are one-tailed t-tests.
A

B
Figure 4 shows preliminary force-displacement curves
with a taxel diameter of 10mm and variable thicknesses
of the PDMS membrane.

RESULTS
Force-displacement Measures
As an example, Figure 4 shows preliminary force-
displacement relationships, using a 10mm diameter taxel
and variable membrane thicknesses. As expected, the
blocking force increases as displacement decreases. Thinner
membranes seem to transmit more force on equal Figure 5. Force vs displacement (A) and equivalent
displacements, or alternatively seem to be deflected more force vs area (B) for two setups. The relative efficiency
on equal forces. Qualitatively, thicker membranes also look is estimated with the flattened sphere model.
stiffer. This is expected to have consequences at
psychophysical level. .000001]. Follow-up one-tailed t-tests showed that
Figure 5(A), instead, shows the force-displacement curves detection thresholds significantly increase with membrane
obtained with two independent setups: a small and a big thickness (threshold for 200 µm: 0.05 kPa ± 0.01 SE; for
cube topped by a 200 µm membrane. The same was done 600 µm: 0.07 kPa ± 0.01; for 1000 µm: 0.1 kPa ± 0.01; for
with bigger cubes (not shown). The lowest forces (this time 2000 µm: 0.46 kPa ± 0.09; all p values < .03; see Figure 6).
measured with the force meter) are limited to 2 mN. Figure
Secondly, we analyzed results obtained with big cubes in
4 also shows that at the considered pressure (0.32 kPa), the
which we manipulated target diameter and membrane
displacement values are above known stroke thresholds (i.e.
thickness. We first compared tactile detection threshold
200 µm for rigid mechanical couplings). The flattened-
performance when touching stimuli with different diameters
sphere model allows to derive Figure 5(B), which shows the
after pooling over for membrane thicknesses. Threshold
force-area relationship.
was significantly higher for 4 mm (5.36 kPa ± 0.85)
Since a bias between the setups is apparent, the derivative compared to 10 mm target (0.10 kPa ± 0.01, p < .0000001).
of the force-area curves is taken between 0 and 10 mN to Then, we considered separately different membrane
normalize subsequent psychophysical results. For example, thicknesses. Detection thresholds are still significantly
the setup with the small cube, which appears more efficient, higher for 4 mm compared to 10 mm targets (for 200 µm
exhibits an overestimated pressure of a factor 1.72. thickness: 4.17 vs. 0.07 kPa; p = .002; for 1000 µm
thickess: 6.16 vs. 0.12 kPa; p = .00006; see also Figure 7).
Psychophysical Results
On the contrary, detection threshold did not differ with
We first compared tactile detection thresholds using small
different membrane thicknesses (threshold for 200 µm
cubes with different membrane thicknesses. 75% correct
thickness: 1.89 kPa ± 0.86, threshold for 1000 µm
detection levels (kPa) were entered into a repeated
(10mm)

(4mm)

Figure 6. Tactile detection thresholds (75% correct
detection level) for different membrane thicknesses
using small cubes.

thickness: 2.90 kPa ± 1, Mann-Whitney U = 40.5; p = .23).
However, when considering separately 10 mm and 4 mm
diameter stimuli a different perceptual profile emerges.
While the difference between 200 and 1000 µm thickness
for the 4 mm diameter is still not significant (threshold for
200 µm thickness: 4.17 kPa ± 1.16, threshold for 1000 µm
thickness: 6.16 kPa ± 1.14; p = .13; see Figure 7b), when Figure 7. (A) Tactile detection thresholds (75% correct
considering 10 mm diameter stimuli, detection threshold is detection level) for different membrane thicknesses and
significantly lower for 200 µm compared to 1000 µm 10 mm stimuli diameter using big cubes (B) Tactile
thickness (threshold for 200 µm: 0.07 kPa ± 0.008, detection thresholds for different membrane thicknesses
threshold for 1000 µm: 0.12 kPa ± 0.01; p = .047; see and 4 mm stimuli diameter using big cubes.
Figure 7a).

A trend suggests that the perception of thicker membranes
DISCUSSION is more problematic with small diameters as well, even if
In our first experiment we checked for possible influences more experiments are needed to confirm this aspect.
of variable membrane thickness on tactile detection One could argue that to better understand what is the main
thresholds of a taxel with fixed diameter. Subjects detected factor underlying perceptual thresholds a constant
more easily thin membranes. This means that mechanical diameter/thickness ratio should be considered (i.e. the
energy is transferred more efficiently from an air chamber larger taxel is a "zoomed" version of the smaller one). Yet,
to the finger mechanoreceptors with as little transducing had we done so, to keep such ratio constant we would have
material as possible. considered thicknesses of 500um and 2000um for the larger
Force-displacement relationship indicates that part of the taxels, for which, approximately, the perception thresholds
energy is dissipated in the elastic deformation of thicker of the first might have been three times higher (while with
membranes. Increasing thickness by a factor of 10 increased 4mm taxels it increases by 50% only). The higher
thresholds by a factor of 5. Thin membranes seem therefore sensitivity with larger taxels can be possibly explained by a
to be preferable when building such actuation systems. combination of two other factors: a larger area of
mechanoreceptors (mainly SA1) and a different finger/taxel
In our second experiment we checked for possible mechanical coupling.
influences of taxel diameter on tactile detection thresholds,
with varying thicknesses. When small-diameter taxels were In fact, qualitative observations from our subjects report
"on" they were less distinguishable from their "off" state as that the hole of the 3d-printed taxel (i.e. the contours of the
opposed to large-diameter taxels. Reducing diameter by a free-standing membrane) was considered a cue to
factor of 2.5 increased thresholds by a factor of between 50 distinguish "on" from "off" taxels, meaning that the
(using thicket membranes) and 60 (using thinner underlying structure and geometry of the rigid support can
membranes). be very important.
The differences between our setups, which we attempted to surface. Somatosensory & Motor Research,, 1, 1 1983), 21-
compensate, may be due to air leakage, or to elastic 31.
properties of the Ecoflex membranes [14], which varied [8] Blake, D. T., Hsiao, S. S. and Johnson, K. O. Neural
across time. Further data collections, averaging force- Coding Mechanisms in Tactile Pattern Recognition: The
displacement curves across different taxels, will be Relative Contributions of Slowly and Rapidly Adapting
necessary to clarify this aspect. Mechanoreceptors to Perceived Roughness. J. Neurosci.
This study is important for designing perceivable taxels Methods, 17, 19 1997), 7480-7489.
with technologies even beyond air-driven setups, i.e. [9] Yobas, L., Durand, D. M., Skebe, G. G., Lisy, F. J. and
whenever a taxel is powered without latching mechanisms Huff, M. A. A novel integrable microvalve for refreshable
and whenever pressure under a soft actuator can be braille display system. Journal of Microelectromechanical
assumed constant. Our results can also be useful for the Systems, 12, 3 2003), 252-263.
design of static soft taxels, which have very different [10] Wu, X., Kim, S. H., Zhu, H., Ji, C. H., and Allen, M.
(generally more stringent) requirements as opposed to G. 2012. A refreshable Braille cell based on pneumatic
vibrating taxels. We emphasize that static taxels better microbubble actuators. Journal of microelectromechanical
approximate paper-based tactile maps, currently the system (2012), 21, 4.
standard for visually impaired subjects. [11] Li, M., Luo, S., Nanayakkara, T, Seneviratne, L. D. ,
Dasgupta P. , and Althoefer, K. “Multi-fingered haptic
Overall, larger taxels with thinner membranes looked palpation using pneumatic feedback actuators,” Sensors and
perceivable with less energy. However, this implies Actuators A: Physical, vol. 218, pp. 132–141, Oct. 2014.
decreasing the resolution of any pin-array display. Trading [12] Doh, E., Lee, H., Park, J., Yn, K.S. 2011. Three-axis
off energetic requirements with resolution seems therefore a tactile display using PDMA pneumatic actuator for robot-
necessary step in the design of graphical pin-array tactile assisted surgery. In Proceeding of the Transducers’ 11 (The
displays. Beijing, China, June 5-9, 2011). 2418-2421.
ACKNOWLEDGMENTS [13] Wichmann F. A. , Hill, N. J. “The psychometric
This research is partially supported by the EU FP7 STREP function: I. Fitting, sampling, and goodness of fit,”
project BLINDPAD (Personal Assistive Device for BLIND Perception & psychophysics, vol. 63, no. 8, pp. 1293–1313,
and visually impaired people), under grant 611621 and 2001.
partially by the Fondazione Istituto Italiano di Tecnologia. [14] Roth, C. B. and Dutcher, J. R.. Glass transition and
The authors would like to thank Diego Torazza and Claudio chain mobility in thin polymer films. Journal of
Lorini for support about electromechanics of the setup. Electroanalytical Chemistry. 584 (2005) 13-22.

REFERENCES

[1] Hammond, F. L., Kramer, R. K., Qian, W., Howe, R. D.
and Wood, R. J. Soft tactile sensor arrays for
micromanipulation. City, 2012.
[2] Shikida M., I. T., Ukai S., Miyaji T., Sato K.
Fabrication of a bubble-driven arrayed actuator for a tactile
display. Journal of Micromechanics and Microengineering,
18, 6 2008), 065012.
[3] Camargo, C. J., Campanella, H., Marshall, J. E., Torras,
N., Zinoviev, K., Terentjev, E. M. and Esteve, J. Batch
fabrication of optical actuators using nanotube–elastomer
composites towards refreshable Braille displays. J.
Micromech. Microeng., 22, 7 2012), 075009.
[4] Asaka, K. Soft Actuators - Materials, Modeling,
Applications, and Future Perspectives. Springer, 2014.
[5] Shinohara, M., Shimizu, Y. and Mochizuki, A. Three-
dimensional tactile display for the blind. Rehabilitation
Engineering, IEEE Transactions on, 6, 3 1998), 249-256.
[6] Jehoel, S., Dinar, S., McCallum, D., Rowell, J. and S.,
U. A scientific approach to tactile map desing: minimum
elevation of tactile map symbols. City, 2005.
[7] Johansson, R. S. and LaMotte, R. H. Tactile detection
thresholds for a single asperity on an otherwise smooth