Two types of actuators are common: pneumatic actuators and electric actuators.
Pneumatic actuators utilize an air signal from an external control device to create a control action via a solenoid. These are commonly available in two main forms: piston actuators and diaphragm actuators.
Piston actuators - Piston actuators are generally used where the stroke of a diaphragm actuator would be too short or the thrust is too small. The compressed air is applied to a solid piston contained within a solid cylinder.
Piston actuators can be single acting or double acting, can withstand higher input pressures, and can offer smaller cylinder volumes which can act at high speed.
Diaphragm actuators - Diaphragm actuators have compressed air applied to a flexible membrane called the diaphragm. These types of actuators are single acting, in that air is only supplied to one side of the diaphragm, and they can be either direct acting (spring-to-retract) or reverse acting (spring-to-extend).
Their range of applications is enormous. For example, the smallest can deliver a few inchpounds of torque where the largest are capable of producing in excess of a million inchpounds of torque.
Electric actuators are motor driven devices that utilize an electrical input signal to generate a motor shaft rotation. This rotation is, in turn, translated by the unit’s linkage into a linear motion,which drives the valve stem and plug assembly for flow modulation.
In case of electric signal failure, these actuators can be specified to fail in the stem-out, stem-in, or last position. Commonly used motors for electric actuators include steppers and servos.
A step motor uses gears with increments in the range of 5,000 to 10,000 at 90 degree rotation for accurate positioning at lower speeds. The disadvantage is that steppers may lose synchronization with the controller when employed in an open loop without an encoder or if they are undersized for an application.
Servos, by definition, are closed loop and provide superior performance at high speeds, but at a higher cost. High precision screws and anti-backlash mechanics provide accuracies to ten-thousandths of an inch. Standard precisions with standard components range from a few hundredths to a few thousandths of an inch.
Brush DC motors and AC motors are sometimes used with limit switches when positioning accuracy is less critical. The motor is connected to a gear or thread that creates thrust to move the valve.
To protect the valve the torque sensing mechanism of the actuator turns off the electric motor when a safe torque level is exceeded. Position switches are utilized to indicate the open and closed position of the valve. Typically a declutching mechanism and hand wheel are included so that the valve can be operated manually should a power failure occur.