An actuator is normally controlled by the controller. The actuator, in turn, changes the output of an automated process. The actuator in an automated process may in actual fact be several actuators, each of which provides an output that drives another in the series of actuators.
Let’s consider the hydraulic actuator that controls the position of a load as demonstrated in the following figure:
The controller outputs a low current DC signal. The signal goes to the first stage of actuator, the amplifier, which outputs increased voltage and current. The large DC power supply and the amplifier are considered part of the actuator. The amplified DC causes the second stage of the actuator, the hydraulic servovalve, to open and allow hydraulic fluid flow; proportional to the DC it received. The servovalve, hydraulic fluid, pump, filter, receiving tank and supply lines are all components in the actuator. The fluid output from the valve drives the third actuator stage, the hydraulic cylinder, which moves the load.
The servovalve actually contains a complete closed loop position control system. The internal control system ensures that the valve opens proportional with the DC signal it receives. Therefore, this three-stage actuator which is a component of a closed loop control system includes another closed-loop control system.
Some actuators can only be turned ON or OFF for example, a heater fan helps to control the temperature when it is turned ON or OFF by a temperature control system. Pneumatic cylinders are normally either fully extended (“ON”) or fully retracted (“OFF”). Other actuators respond proportional with the signal they receive from a controller, a good example of such a kind of actuator is the variable speed motor. Hydraulic cylinders can be controlled so that they move to positions between fully extended and fully retracted.
Actuators can be employed to change variables as follows:
- Speed, acceleration or rate of flow of an object.
- Force or pressure acting an object.
- Presence or nearness of an object.
- Temperature of an object.
Also read:
- Electric Solenoid Actuator – Features & Operation
- Manipulating Devices & Actuators in Process Instrumentation
- Pneumatic Actuators
- Flow Control Valves
- Basics of a Control Loop
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