Sensors and servocontrols

The components making up closed- and open-loop servosystems for process control are examined, with particular emphasis on those involving electronics. Distinctions are made between active and passive, analog and digital and direct and remote sensors, and the characterization of sensors according to measurement range, sensitivity, fidelity, linearity, resolving power, signal-to-noise ratio, response time and influencing factors is considered. The operating principles of linear variable differential transformers, inductosyns, capacimetric sensors, fiber optical proximity sensors, optical coders, laser Doppler interferometers and television cameras as linear or angular displacement or velocity sensors are presented in detail, and considerations influencing the choice of a specific type of sensor are indicated. Attention is then given to the characteristics of analog and digital comparators, and to the power components of process control systems, including thyristors, torque motors, and stepping motors.