IEEE Robotics & Automation Magazine - December 2022 - 118

connection circuit achieves a Wheatstone bridge, with each
piezoresistor on one arm.
It can be proved that this bridge structure increases the
sensitivity of the pressure-sensitive detector by about two
orders of magnitude, without affecting the detector's natural
frequency. Then, when the whisker shaft comes in contact
with an object and is deformed, the position and
orientation of the center
connector will shift and
rotate, which will generate
strain in the slender
beams. Because of the
strain, the piezoresistors
on the four beams will
have changes in their
resistance, which produces
electric signals in the
Wheatstone bridge circuit
for further processing.
obtain metal wires that connect the components to form a
microelectromechanical system circuit. Finally, after the
silicon dioxide passivation layer is removed, the cantilever
is etched in the device layer.
As for the whisker shaft, stiffness and toughness are two
It is commonly argued
that one key unresolved
issue for whisker sensors
is the low scope of
reconstruction.
Fabrication of the Whisker Sensor
The four-beam structure of the whisker sensor is fabricated
on a silicon substrate by using SOI technology [9]. The
manufacturing uses the following procedures, illustrated
in Figure 2. First, plasma chemical vapor deposition is
used to deposit a silicon dioxide layer on the device layer
to obtain a mask for the doping process. Second, through
the mask, a window pattern is etched using lithography
on the oxide layer, and boron is implanted to form piezoresistors.
The next step is to form an insulating dielectric
layer of the metal wire through the silicon nitride passivation
layer, which is deposited by low-pressure chemical
vapor deposition. Then, the metal layer deposited on the
top is patterned and etched using photolithography to
aspects used to evaluate material suitability. Various materials
were tested with the base of the whisker sensor. When
rigid materials are used, severe damage to the suspended
four-beam structure is likely to happen owing to the lack of
cushioning when the whisker is deflected. If materials that
are generally flexible are used, elastic deformation may
occur, storing the energy generated by contact with an
object and delaying or stopping the reaction at the base of
the whisker. Moreover, an excessively slender structure prevents
deflections from being transmitted to the whisker's
base, making the base unable to shift the position of the center
connector or generate a signal when the whisker tip is in
contact with an object.
After experimenting with various materials, nylon was
finally adopted because of its relatively high stiffness and
higher energy conductivity. When the whisker length and
scanning speed are fixed and the same object is touched
at a certain distance, the sensor's output signal is larger
with a nylon whisker, which increases the sensitivity. The
main parameters of the nylon-based whisker sensor are
given in Table 1.
(a)
(b)
Modeling of the Whisker Sensor
To assess the functional properties of the whisker sensor and
evaluate its texture discrimination and contour reconstruction
performance, we carried out a modeling analysis. To
enable the robotic rat to distinguish four different textures,
we identified four features in the sensor signal that can
express the different characteristics of the textures. By
extracting these features from the original signal and applying
a feature-based SVM algorithm, four texture classifiers
were established for the corresponding four textures so that
the robotic rat's ability to discriminate textures could be
realized. Additionally, using the Wheatstone bridge and EBB
analysis, we calculated the position of contact of the whisker
shaft and object from the sensor's output voltage signal. By
recording the centroid trajectory of the robotic rat, the contour
of the scanned object can be reconstructed. The specific
methods are described in the following sections.
(c)
(d)
(e)
Silicon
SiO2
Si3N4
Au
Ti
B
Figure 2. The beam structure fabrication steps. (a) The oxidation
masking. (b) The boron ion implantation. (c) The formation of
the insulating dielectric layer. (d) The deposition of metal. (e)
The etching to form a beam structure. Au: gold; SiO2: silicon
dioxide; Ti: titanium; Si3N4: silicon nitride; B: boron.
118 * IEEE ROBOTICS & AUTOMATION MAGAZINE * DECEMBER 2022
Table 1. The whisker sensor parameters.
Symbol
Quantity
L
D
W
S
R
Whisker length
Whisker diameter
Weight
Value
50 mm
180 μm
2.5 g
Printed circuit board size U12.5 * H 3.2 mm
Resolution
10 mV
IEEE Robotics & Automation Magazine - December 2022

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