handbook of modern sensors

9.2 Tactile Sensors 331
Fig. 9.7. Piezoelectric film respiration sensor.
sensitive, faithfully duplicating the effort of the drummer or pianist. In electronic
pianos, the piezoelectric film switches respond with a dynamic range and time constant
that is remarkably similar to a piano key stroke.
Textile plants require the continuous monitoring of often thousands of lines of
thread for breakage. An undetected break event can require that a large volume of
material be discarded, as the labor costs to recover the material exceed the manufacturing
cost. Drop switches, where switch contact closure occurs when the thread
breaks, are very unreliable. Lint fouls the contact points, resulting in no output signal.
A piezoelectric film vibration sensor, mounted to a thin steel beam, monitors the
acoustic signal caused by the abrasion of the thread running across the beam, analogous
to a violin string (Fig. 9.6C). The absence of the vibration instantly triggers the
machinery to stop.
Figure 9.7 shows a PVDF film tactile sensor for detecting the breathing rate of a
sleeping child, where minute movements of the body resulted from respiration had to
be monitored in order to detect cessation of breathing [7]. The sensor was placed under
the mattress in a crib. The body of a normally breathing child slightly shifts with each
inhale and exhale due to a moving diaphragm. This results in a displacement of the
body’s center of gravity which is detected by the sensor. The sensor consists of three
layers, where the PVDF film is laminated between a backing material (e.g., silicone
rubber) and a pushing layer. The pushing layer is fabricated of a plastic film (i.e.,
Mylar), whose side facing the PVDF film is preformed to have a corrugated surface.
Under the variable force, the PVDF film is variably stressed by the groves of the
pusher. This results in the generation by the film of electric charge. The charge flows
out of the film through a current-to-voltage (I/V ) converter which produces variable
output voltage. The amplitude of that voltage within certain limits is proportional to
the applied force.
Another type of tactile sensor is a piezoresistive sensor. It can be fabricated by
using materials whose electrical resistance is a function of strain. The sensor incorporates
a force-sensitive resistor (FSR) whose resistance varies with applied pressure [8].
Such materials are conductive elastomers or pressure-sensitive inks. A conductive