Biomedical Engineering Reference
In-Depth Information
film has been used as the basis for the design of a matrix of high spatial resolution tactile
sensors, the same crosstalk problems have often been reported [5]. Another problem with
a matrix array of PVDF sensing elements is its bulk, due to the fact that a coaxial feed
is required for each element, and the potential for instability. These factors combine to
make it unsuitable for most robotics applications, so researchers are now concentrating
on developing tactile sensors with a minimal number of sensing elements.
In the following sections, various types of tactile sensors will be discussed, together
with their Principle of operation and applications.
2.2 Capacitive Sensors
Capacitive sensors make use of the change in capacitance between two electrodes. For
example, when pressure is applied, the membrane electrode deflects and changes the
gap, and therefore the capacitance, between the two electrodes in direct proportion to the
pressure exerted. Dhuler et al . [7] designed a silicon-based capacitive pressure sensor, in
which the electrodes were made of a planar comb structure. The sensor element comprised
two parts: first, a movable elastic structure that transforms a force into a displacement,
and second, a transformation unit consisting of electrodes that transform the displacement
into a measurable change in capacitance. By measuring the capacitance change on both
sides, high linearity and sensitivity were obtained. Compared to piezoresistive sensors,
capacitive sensors have better long-term stability, higher sensitivity, and no hysteresis.
However, capacitive pressure sensors require more complex signal processing and are
also more costly to produce.
2.3 Conductive Elastomer Sensors
Pliable materials that possess defined force - resistance characteristics have received a
lot of attention in tactile sensor research. The basic principle of conductive elastomer
sensors lies in measuring the resistance of a conductive elastomer (or foam) between two
points. The majority of sensors use an elastomer that consists of a carbon-doped rubber.
Figure 2.3 shows the schematic of a conductive elastomer sensor.
In the sensor shown in Figure 2.3, the deformation due to an applied force causes a
change in density and, subsequently, a change in the resistance of the elastomer.
Applied force
Increase in particle density
R2
R1
Figure 2.3
Schematic of a conductive elastomer sensor
 
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