In industrial control systems, an actuator is a hardware device that converts a controller cornmand signal into a change in a physical parameter. The change in the physical parameter is usually mechanical. such as position or velocity change. An actuator is a transducer.because it changes one type of physical quantity. say electric current, into another type of physical quantity, say rotational speed of an electric motor. The controller command signal is usually low level, and so an actuator may also include an amplifier to strengthen the ~ignal sufficiently 10 drive the actuator
A list of common actuators is presented in Table 5,4. Depending on the type of amplifier used. most actuators can he classified into one of three categories: (1) electrical,
(2) hydraulic, and (3) pneumatic. Electrical aCluators are most common; they include Ole and de motors of various kinds, stepper motors. and solenoids. Electrical actuators include both linear devices (output is linear displacement) and rotational devices (output is rotational displacement or velocity). Hydraulic actuators use hydraulic fluid to amplify the controller command signal. Thc available devices provide both linear and rotational motion.
Hydraulic actuators are often specified when large forces are required. Pneumatic octuators usc compressed air (typically "shop air" in the factory environment) as the driving power. Again. both linear and rotational pneumatic actuators are available. Because of the relatively 10••••airpressures. involved, these actuators are usually limited to relatively low force applications compared with hydraulic actuators.
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