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A rotary actuator is a device that produces rotary motion from some energy source. The simplest actuator of this type uses linear motion in a single direction to produce the rotation. Electrical rotary actuators use a source of electrical energy such as an electric motor. This type of rotary actuator is the most common, although rotary actuators might also use hydraulic energy from pneumatic tubes or mechanical energy from springs.
An electric rotary actuator can produce continuous rotation when its power source is a typical electric motor. It might also produce fixed movement to a specific position when its power source is a stepper motor or servomotor. A torque motor is similar to a stepper motor, except that it produces a specific torque. The torque from the motor causes a specific rotation unless some opposing torque balances it.
A stepper motor is a type of electric motor that moves a fixed distance for each application of power. It can produce continuous movement by rotating at a fixed speed, and it can move to a specific angular position. A rotary actuator can use one or more datum sensors with any position encoder, allowing a stepper motor to move to any angular position.
A servomotor, or servo for short, is a device that consists of several specific components. It requires a motor that usually is electric, although servos also can use hydraulic motors. Servos need a gearing system to convert the high rotational rate of the motor to a lower rotation rate that produces higher torque. A servo also requires a position encoder.
The servo’s position encoder allows the servo’s control system to determine the angular position of the electric rotary actuator. The position encoder sends a signal to the control system, indicating the desired angular position. The controller subtracts the encoder’s current position from the desired position to obtain an error code. This error code allows the controller to activate the motor, causing it to rotate until the motor is in the correct position. An electric rotary actuator that uses a servo is common in radio-controlled models.
A more recent type of electric rotary actuator makes use of memory wire to achieve a very low weight. The wire receives an electrical current that heats the wire above a specific temperature, causing the wire to change shape. This change in shape applies a torque to the drive shaft, causing it to rotate. The wire cools when it no longer receives current, causing it to assume its previous shape. This removes the torque from the drive shaft.