Overview

Electric actuators are terminal execution units in industrial process control systems. They receive standard control signals from control systems (DCS, PLC, regulators) and convert electrical signals into mechanical displacement or torque to drive regulating equipment such as valves and dampers. By doing so, they adjust process parameters including medium flow rate, pressure and liquid level to realize automatic regulation and on-off control of production processes.

Compared with pneumatic actuators, electric actuators feature convenient power access, long-distance signal transmission and no requirement for an air source, which facilitates centralized control. Their drawbacks include high explosion-proof cost and limited overload capacity. They are widely applied in industries such as petrochemical engineering, power generation, water treatment and HVAC.

Basic Composition

Traditional split-type actuators consist of two main parts: a servo amplifier and an actuator body. Modern intelligent actuators adopt an integrated mechatronic structure, with core modules listed below:

1. Servo Motor: The power element that converts electric energy into rotating mechanical energy, including AC servo motors, DC servo motors and stepper motors. The motor outputs high rotating speed with low torque.
2. Reduction Transmission Mechanism: Adopts worm and worm gear, gear, ball screw and other structures to reduce rotating speed and amplify output torque or thrust. It also converts the rotary motion of the motor into required linear or angular motion.
3. Position Feedback Unit (Position Transmitter): Potentiometers, Hall sensors or encoders collect the actual position of the output shaft and convert it into 4~20mA or digital signals fed back to the control loop to form closed-loop control.
4. Control Circuit Board (Servo Controller): Performs signal comparison, amplification and logical calculation. It receives external setpoint signals, compares them with valve position feedback signals, and outputs driving commands to control the forward and reverse rotation of the motor.
5. Protection Mechanism: Includes torque protection, travel limit, overheat protection and manual operating mechanism. When the output torque exceeds the limit or the actuator reaches the fully open/fully closed position, the motor power supply is cut off to prevent damage to valves and actuators. Manual operation is available upon power failure.

For routine inspection and upkeep, see our guide to electric actuator maintenance.

Core Working Principle (Closed-loop Regulation)

Regulating electric actuators are typical negative-feedback closed-loop control systems. The standard input signal is generally a 4~20mA current signal (digital bus signals such as Modbus and HART are also supported).

1. Signal Comparison: The setpoint signal sent by the control system (representing the target valve position) is transmitted to the servo controller and compared with the actual valve position signal collected by the position feedback unit to calculate deviation.
2. Motor Driving: If the setpoint signal does not match the feedback signal, deviation is generated. The amplified deviation signal drives the servo motor to rotate forward or reverse; the greater the deviation, the more the actuator moves.

• Setpoint > Feedback: Motor rotates toward the opening direction;
• Setpoint < Feedback: Motor rotates toward the closing direction;
• Setpoint = Feedback: Deviation equals zero, the motor stops, and the valve position remains stable at the set value.
  3. Mechanical Conversion: The high-speed rotation of the motor is decelerated and torque-amplified by the reducer, driving the output shaft to generate linear displacement or rotation angle.
  4. Position Feedback Update: While the output shaft moves, it drives the position sensor to update the feedback signal in real time. Continuous comparison with the setpoint signal is carried out until the deviation is eliminated, and the actuator maintains its position.

Simplified principle of on-off electric actuators: It only receives on-off signals and drives the motor to run to fully open or fully closed limit for shutdown, without continuous regulation of intermediate positions.

Three Main Types and Motion Principles

According to the output motion form, electric actuators are divided into linear stroke, part-turn (angular stroke) and multi-turn types. Their control logic is consistent, differing only in mechanical conversion structures.

1. Linear Stroke Electric Actuator: The motor rotation is converted into linear reciprocating thrust via a screw mechanism. Its output parameters are thrust (N/kN) and stroke (mm). It drives linear movement of valve plugs of single-seat regulating valves, sleeve valves, angle valves and other regulating valves for continuous flow regulation.
2. Part-turn (Angular Stroke) Electric Actuator. It outputs rotational torque within 0~90° without multi-turn rotation. The motor and reduction gearbox output rotary torque to drive butterfly valves, ball valves and air dampers. Suitable for on-off or 90° angular regulation applications.
3. Multi-turn Electric Actuator: The output shaft needs multiple rotations to complete opening or closing and outputs torque. It is used for gate valves and globe valves, whose valve stems need lifting and lowering to open or close through multi-turn rotation. Mostly applied to on-off valves of large-diameter pipelines.

Features of Intelligent Electric Actuators

New-generation intelligent electric actuators are embedded with microprocessors and no longer rely on external servo amplifiers:

• Support digital communication for remote reading of valve position, torque and fault information;
• Automatic calibration of stroke, zero point and full scale;
• Built-in fault diagnosis; loss-of-signal protection (hold position, fully open or fully closed optional);
• Software-configurable torque protection and travel limit; local operation panel for man-machine interaction.

Principle of Protection Mechanisms

6. Travel Limit: Mechanical or electronic limit cuts off motor power supply at preset fully open and fully closed positions to prevent overtravel.
7. Torque Protection: Torque sensors monitor the output load. In case of valve sticking or locked rotor leading to excessive torque, power supply is cut off to protect the motor and valve stem. It is the most critical overload protection for valves.
8. Motor Overheat Protection: Built-in temperature switch shuts down the motor when the winding overheats to avoid burnout.
9. Manual Clutch Mechanism: When power fails, the handwheel is engaged manually and disengaged from the motor gearbox for on-site manual valve operation.

Application Summary

Essentially, an electric actuator is a device that converts electrical signals into mechanical energy and achieves accurate position control relying on closed-loop negative feedback. In an automatic control system, the controller is responsible for “calculation”, while the electric actuator performs “execution.” It adjusts valve opening to stabilize process parameters.

With the popularization of fieldbus and intelligent technology, intelligent electric actuators can perform basic regulation as well as equipment condition monitoring, becoming important field instruments for industrial digitalization.