Biomedical Engineering Reference
In-Depth Information
FIGURE 10.15
A resistive strain gauge (unbounded-type) blood pressure transducer.
with special temperature compensation circuits are typically used in applications involving
strain gauge transducers.
Capacitive Transducers
The capacitance,
C
(in farad), between two equal-size parallel plates of cross-sectional
area,
A,
separated by a distance,
d,
is given by
A
d
C
¼
e
o
e
r
ð
10
:
12
Þ
10
12
F/m), and e
r
is the relative
dielectric constant of the insulating material placed between the two plates. The method
that is most commonly employed to measure displacement is to change the separation dis-
tance,
where e
o
is the dielectric constant of free space (8.85
between a fixed and a movable plate, as illustrated in Figure 10.16a. This arrange-
ment can be used to measure force, pressure, or acceleration. Alternatively, it is possible to
add a third plate and form a differential-type capacitance transducer (Figure 10.16b). In this
configuration, two of the plates are stationary, whereas the middle plate can be moved
freely relative to the position of the other plates, thus creating two variable-size capacitors.
d,
FIGURE 10.16
Capacitive displacement transducer: (a) single capacitance and (b) differential capacitance.
