Automating Manufacturing Systems - Process Control and ...

discrete sensors - 4.21
electrode
metal
electrode
dielectric
electrode
electrode
Figure 4.24
Dielectrics and Metals Increase the Capacitance
The sensors are normally made with rings (not plates) in the configuration shown
in Figure 4.25. In the figure the two inner metal rings are the capacitor electrodes, but a
third outer ring is added to compensate for variations. Without the compensator ring the
sensor would be very sensitive to dirt, oil and other contaminants that might stick to the
sensor.
electrode
compensating
electrode
Note: the compensating electrode is used for
negative feedback to make the sensor
more resistant to variations, such as contaminations
on the face of the sensor.
Figure 4.25
Electrode Arrangement for Capacitive Sensors
A table of dielectric properties is given in Figure 4.26. This table can be used for
estimating the relative size and sensitivity of sensors. Also, consider a case where a pipe
would carry different fluids. If their dielectric constants are not very close, a second sensor
may be desired for the second fluid.