Investigation of the Mechanical Reliability of a Velostat-Based Flexible Pressure Sensor

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Investigation of the Mechanical Reliability of a
Velostat-based Flexible Pressure Sensor
Anis Fatema, Ivin Kuriakose, Deeksha Devendra, Aftab M. Hussain∗,Member, IEEE
FleCS Lab, Center for VLSI and Embedded Systems Technology (CVEST),
International Institute of Information Technology, Hyderabad, India
∗Email: aftab.hussain@iiit.ac.in
Abstract—The technological advancements in healthcare mon-
itoring devices, automation, consumer electronics, and soft
robotics have resulted in extensive research in flexible pressure,
force, and tactile sensors. Piezoresistive sensors are the most
widely used flexible pressure sensors due to their low-cost
fabrication, high flexibility and simple data-acquisition circuits.
In this paper, we report the bending response of a velostat-
based flexible pressure sensor by examining its reliability when
subjected to repeated mechanical stress. The observed deviation
in output voltage was 0.95% for 15 mm, 0.95% for 20 mm,
0.97% for 25 mm, and 2.2% for 30 mm bending radii, for 150
bending cycles, with respect to the flat position. We present a
two-parameter (a,b) calibration for the pressure sensor with a
fixed bias resistance in the readout circuit. This model can be
used to further minimize the deviation due to bending cycles.
The results obtained from the experimental research have shown
a practical possibility of implementing velostat-based sensors for
both static and dynamic flexible systems.
Index Terms—Reliability; piezoresistive; pressure sensor; me-
chanical stress; velostat
I. INT ROD UC TI ON
Pressure sensors are commonly used for tracking and eval-
uating pressure in various applications which include but
are not limited to aerospace [1], [2], automobiles [3], [4],
biomedical [5]–[8], consumer and portable electronics [9]–
[13], environmental monitoring [14], [15], industrial [16], [17],
robotics [18], [19] and wearable electronics [20], [21]. Based
on the sensing principle employed, pressure sensors can be
categorised into capacitive [22], optical [23], piezoelectric
[24], [25] and piezoresistive pressure sensors [26]–[28].
Capacitive pressure sensors use a thin diaphragm as a
sensing element. They are most frequently selected for their
better tolerance to temperature drift, high scalibility for minia-
turization, low-power consumption, large dynamic range and
better sensitivity [22]. However, they require complex readout
circuitry and are better suited for measuring low pressures
[29]. Optical pressure sensors have great attributes such as
high sensitivity and outstanding immunity to interference.
However, their bulky system configurations hinder their ap-
plications in many areas where flexibility is a requirement
[30]. Piezoelectric sensor systems have advantage of dual
energy flow, i.e, they can be used as a sensing element
and also an energy generating element. They are generally
employed for measuring high dynamic pressures. However,
these sensors require sophisticated fabrication tools and a
complex electronic interface. Piezoresistive pressure sensors
use different types of materials and structures that can be used
in large array of applications. They have become a dominant
category in pressure sensing because of their facile fabrication,
low-cost, simple signal processing circuitry and standard data
acquisition process [26]. Flexible piezoresistive materials have
demonstrated greater advantages in easy deployment, which
is of particular importance for biomedical, soft robotics and
human-machine interacting systems [31]. This is because the
flexibility of any system can be determined by the material
used and the thickness in the direction normal to the axis of
flexing [32].
With the increase in demand for flexible pressure sensors, a
piezoresistive material named velostat has been extensively ex-
plored due to its flexibility which is well suited for mechatron-
ics and biomedical applications [33]. It is an elastic polymer
impregnated with carbon black to enable electrical conductiv-
ity. It is lightweight, heat sealable, flexible, and foldable. It can
be used in various applications due to its low cost and easy
handling process. Hopkins et. al used a velostat-based pressure
sensor for in socket pressure sensing. The system demonstrated
utility for assessing contact and movement patterns within a
prosthetic socket, potentially useful for improvement of socket
fit, in a low cost, low profile and adaptable format [34].
Athavale et. al custom designed a 4×2 configuration velostat-
based sensor array to assess electrode contact pressure during
in-vivo recordings in the gut. The designed array showed better
response and was more repeatable than a flexiforce A201
sensor [35]. In [33], [36], the sensitivity, repeatability and
hysteresis have been investigated when the sensor is placed
on a flat surface. In our previous work [26], we have reported
different static and dynamic characteristics of the sensor in
a 4×4 flexible pressure sensor array when placed on a flat
surface.
In this work, we present a systematic study to understand
the influence of repeated mechanical stress on the character-
istics of a velostat-based resistive pressure sensor. We present
a mathematical model to parameterize the reliability of a
piezoresistive pressure sensor. We have studied the effect
on the response of the sensor after being subjected to 150
bending cycles for up to 16 hours. This will help us understand
whether the sensor can be used on curved surfaces like tubes,
wearable hand gloves, textiles, human skin etc. and will enable
researchers to use these sensors with the awareness of its
advantages and limitations in different applications.
978-1-6654-4273-2/22/$31.00 ©2022 IEEE
2022 IEEE International Conference on Flexible and Printable Sensors and Systems (FLEPS) | 978-1-6654-4273-2/22/$31.00 ©2022 IEEE | DOI: 10.1109/FLEPS53764.2022.9781575
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Fig. 1. (a) Design of a sensor using a sheet of velostat sandwiched between
two sheets made of plastic with cross-bar copper electrodes, (b) Photograph
of a fabricated sensor showing its flexibility. (c) Experimental setup showing
measurement of force applied on pressure sensor with the help of a push-pull
force gauge, when placed on a curved surface.
II. METHODOLOGY
A. Sensor Design and Test Setup
The velostat-based pressure sensor was fabricated by sand-
wiching a velostat sheet between two plastic sheets along
with top and bottom layers of copper electrodes in a crossbar
architecture, as shown in Fig. 1a . The polymer sheet had a
thickness of 106±1µm. The plastic sheets had a thickness of
169±1µm resulting in total thickness of approximately 440
µm (measured using Mitutoyo micrometer). This made the
structure highly flexible. The sensor pixel dimensions were 10
mm ×10 mm. The working principle of the sensor pixel is
based on the variation of resistance of the polymer composite
because of applied strain. The sensor is connected to a bias
resistor, resulting in a voltage divider circuit. Hence, the output
voltage is given as
Vo=Vdd Rb
Rb+Rsensor (1)
where Rsensor is the sensor resistance, Rbis the bias
resistor and Vdd is the power supply voltage. This voltage
was obtained from the value read by the analog pin of the
microcontroller. When a force is applied on the sensor, its
resistance decreases, and the output voltage increases. We
selected the bias resistor as 1 kΩto obtain a certain range of
weights [26]. In order to determine the influence of pressure
on the velostat resistance, a setup with push-pull force gauge
(maximum load - 200 N) was designed as shown in Fig. 1c.
B. Calibration
Calibration is essential for any sensor to get precise, reliable
and reproducible results. Due to the non-uniform distribution
of carbon particles in velostat, the sensor response changes
after bending, hence, it is important to calibrate the sensor
for the bending radii of interest. The sensor resistance ap-
proximately follows an inverse exponential relationship with
respect to applied weight. Hence, the output voltage can be
written as
Vo=Rb
Rb+a
WbVdd (2)
where aand bare constants and Wis the applied weight.
To calibrate the sensors we need to find the two parameters a
and b, every sensor pixel in case of a large array. For sensor
systems that are subjected to static or dynamic flexing, it is
important to determine the variation of these parameters during
and after bending.
III. RES ULTS A ND DISCUSSION
In the first experiment, the response of the sensor was
recorded on a flat surface. The sensor responses were then
recorded on curved surfaces that with different bending radii
of 15 mm, 20 mm, 25 mm and 30 mm. 3D printed blocks were
designed to control the bending angle and bending cycles. The
Fig. 2. Output voltage for a sensor pixel for different applied loads, at various
bending radii.
Fig. 3. Variation of parameters aand bwith different bending radii.
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results from these measurements are presented in Fig. 2. We
can observe that as the bending radius increases, the output
voltage increases for the same weight because of the shape
deformation of velostat. Ideally, this is unacceptable because it
will make the sensor unreliable. However, the change in sensor
response with bending radius can be modelled as a change in
parameters aand bto predict the correct response for a given
bending radius. To examine the change in the output of the
sensor for a given bending radius, we found the values of aand
bwith bending Fig. 3. We need to account for this change if
we are using the sensor to sense pressure on a curved surface.
In the second experiment, we subjected the sensor to several
bending cycles at different bending radii and then tested the
response of the sensor by placing it on a flat surface under
constant load (0.5 kg). This experiment was performed to test
if the response of the sensor changes due to rolling, folding
or bending action, particularly in use cases where dynamic
flexing is required. The results are shown in Fig. 4a. We can
Fig. 4. Sensor response for various (a) bending cycles, and (b) bending times.
Load applied is 0.5 kg.
Fig. 5. Sensor response for repeated application of load for as-fabricated
sensor, and after 200 bending cycles.
notice from the graph that there is a very slight change in the
sensor response after bending it for many cycles. The deviation
in output voltage was found to be 0.95%, 0.95%, 0.97% and
2.2% for bending radii of 15 mm, 20 mm, 25 mm and 30
mm respectively, after bending for 150 cycles. In all cases,
the sensor output was measured in a flat position.
In the third experiment, we tested the dynamic response
of the sensor for various bending radii. The results shown
in Fig. 4b convey that there is a negligible change in the
output response of the sensor which shows the reliability of
the sensor. With respect to the output voltage at t= 0, there
is a deviation of 1.63%, 1.17%, 0.82%, 1.09% and 2.5% for
flat surface, 15 mm, 10 mm, 25 mm and 30 mm bending radii
respectively, after 16 hours of continuous operation.
In the final experiment, we tested the repeatability of the
sensor response without bending and after bending it for 200
cycles. The sensor response was measured for 50 loading
cycles, of which, results are shown for few cycles in Fig. 5.
Even after subjecting the sensor to 200 bending cycles, it
shows reproducible results for rise and fall times. Repeatability
is calculated as the difference in output produced when the
sensor is loaded with same weight for many cycles. The output
voltages for 0.5 kg load were found to be 3.54±0.06 V for
as-fabricated sensor, and 3.8±0.1 after 200 bending cycles,
while the rise and fall times were found to be 1.22 sand 1.34
sfor both cases.
IV. CONCLUSION
We explored the feasibility of employing a velostat-based
sensor on curved surfaces for measuring pressure. We have
presented the response of the sensor when used on curved
surfaces with different bending radii and also after bending
it for many cycles to test the mechanical characteristics. We
presented a mathematical model to parameterize the sensor
calibration curve. Experimental results show the possibility of
using it as a reliable device given rigorous calibration. It opens
a broad perspective of using velostat as a force sensor. The
sensor is highly scalable and can be made into a large matrix
array for various applications such as posture detection, po-
diatric pressure measurement system, gait recognition system,
body pressure measurement system, and so on.
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