handbook of modern sensors

9.2 Tactile Sensors 329
(A)
(B)
Fig. 9.5. Tactile sensor with a piezoelectric film for detecting sliding forces: (A) cross-sectional
view; (B) typical response. (Adapted from Ref. [5].)
A piezoelectric tactile sensor can be fabricated with the PVDF film strips imbedded
in a rubber skin (Fig. 9.5A). This sensor is passive; that is, its output signal is
generated by the piezoelectric film without the need for an excitation signal. As a
result, it produces a response proportional to the rate of stress, rather than to the stress
magnitude. The design of this sensor is geared to robotic applications, where it is
desirable to sense sliding motions causing fast vibrations. The piezoelectric sensor is
directly interfaced with a rubber skin; thus, the electric signal produced by the strips
reflect movements of the elastic rubber which results from the friction forces.
The sensor is built on a rigid structure (a robot’s “finger”) which has a foamy,
compliant underlayer (1 mm thick), around which a silicon rubber “skin” is wrapped.
It is also possible to use a fluid underlayer for better smooth-surface tracking. Because
the sensing strips are located at some depth beneath the skin surface and because the
piezoelectric film responds differently in different directions, the signal magnitude
is not the same for movements in any direction. The sensor responds with a bipolar
signal (Fig. 9.5B) to surface discontinuity or bumps as low as 50 µm high.
The following are few more examples of sensors that use PVDF and copolymer
films [6]. Many tactile sensors are just sensitive conventional switches. However,
the reliability of conventional contact switches is reduced due to contaminates like
moisture and dust which foul the contact points.Apiezoelectric film offers exceptional
reliability, as it is a monolithic structure, not susceptible to this and other conventional
switch failure modes. One of the most challenging of all switch applications is found
in pinball machines. A pinball machine manufacturer uses a piezo film switch as a
replacement for the momentary rollover-type switch. The switch is constructed from a
laminated piezoelectric film on a spring steel beam, mounted as a cantilever to the end
of a circuit board (Fig. 9.6A). The "digital" piezoelectric film switch is connected to
a simple MOSFET circuit that consumes no power during the normally open state. In
response to a direct contact force, the film beam momentarily triggers the MOSFET.
This provides a momentary “high” state of the switch. The sensor does not exhibit the
corrosion, pitting, or bounce that are normally associated with contact switches. It can
survive in excess of 10 million cycles without failure. The simplicity of the design
makes it effective in applications which include counter switches for assembly lines