Page 399 - Mechanical Engineers' Handbook (Volume 2)

390 Basic Control Systems Design

Figure 8 Rotary potentiometer. 1 Figure 9 Linear variable differential trans-
former. 1

Displacement measurements can be used to obtain forces and accelerations. For exam-
ple, the displacement of a calibrated spring indicates the applied force. The accelerometer
is another example. Still another is the strain gage used for force measurement. It is based
on the fact that the resistance of a fine wire changes as it is stretched. The change in
resistance is detected by a circuit that can be calibrated to indicate the applied force. Sensors
utilizing piezoelectric elements are also available.
Velocity measurements in control systems are most commonly obtained with a tachom-
eter. This is essentially a dc generator (the reverse of a dc motor). The input is mechanical
(a velocity). The output is a generated voltage proportional to the velocity. Translational

Figure 10 Synchro transmitter control transformer. 1