Biomedical Engineering Reference
In-Depth Information
Figure 2.8
Piezoresistive sensor based on conductive elastomer using a separator [8]
Applied load
Carbon felt
Connecting
leads
Foil
electrodes
Figure 2.9
Schematic of a carbon felt tactile sensor [8]
touching fibers starts to increase and this leads to a further reduction in resistance. Carbon
fiber and carbon felt sensors are rugged and can be shaped. They can also withstand very
high temperatures and considerable overloads. One disadvantage of this sensor is that it
generates a great deal of electrical noise when the load is less than 10 g. However, these
sensors are very robust and well suited for sensing in very inhospitable environments.
2.8 Piezoelectric Sensors
A piezoelectric material is defined as one that either produces an electrical discharge
when subjected to a mechanical deformation or undergoes a mechanical deformation when
subjected to an electrical input. Polymeric materials that exhibit piezoelectric properties
are particularly suitable as tactile sensors.
PVDF is a classic example of a polymer that is now widely being tested for use in
tactile sensors. It is used generally in applications that require the highest purity, strength,
and resistance to solvents, acids, bases, and heat, and low smoke generation during a fire
event. In 1969, the strong piezoelectricity of PVDF was observed by Kawai [22]. The
piezoelectric coefficient (d
33
) of poled thin films of the material were reported to be as
large as 6 - 7 pC N
-1
which is 10 times larger than that observed in any other polymers.
