What Is an Actuator? How It Works, Types, and Examples

Energy is converted into a controlled mechanical response, whether that be motion, position, force, or torque, through the use of actuators. Sensor systems do the observational part of the control system; the controller determines what action to take based on sensor input; the driver or valve controls the energy for an actuator to act on an actual physical load.

• An actuator converts available energy into controlled mechanical action.
• Energy sources and output motion have their own classification systems.
• Electric, pneumatic, and hydraulic actuators vary mainly in force output, controllability, infrastructure, and maintenance needs.
• Selection of an actuator requires consideration of the load, travel distance, speed, duty cycle, control method, mounting conditions, environment, and required failure behavior.

What Is an Actuator in Simple Terms?

In non-technical language, an actuator is a device within the control system that provides action. The actuator will take the usable energy supplied by the system and create a physical action; this may involve limited distance travelled by the actuator but can include rotation, elongation of rods, movement of valves, clamping, and holding.

The term actuator represents a function rather than a form of product. While some products may contain components for sensing, control, switching, and actuation, the functions of those components remain separate and distinct.

Where Does an Actuator Sit in a Control System?

In the entire control chain, the actuator occupies a location between the regulated energy and the load. Both sensing and actuation are considered distinct functions in the context of NIST’s article on smart sensors and actuators, even if both are physically contained within the same assembly.

The report from the sensor about the state of the automated gate, a request for movement from the controller, regulation of current by the driver, and movement of the load by the actuator represent four main interactions used to automate a gate. The four interactions of sensor, controller, driver, and actuator can allow a feedback loop back to the controller to correct the requested action or validate it.

How Does an Actuator Turn a Command Into Motion?

The conversion of available energy into mechanical motion through actuation can occur in different ways, depending on the actuator type. Electric, pneumatic, and hydraulic actuators use different conversion or transmission mechanisms and can have very different characteristics that affect precision and speed of operation. Each type of actuation also has unique failure modes.

Actuation is accomplished in four main steps:

1. The controller logic issues a command.
2. The driver or valve meters available energy.
3. Actuation produces force or torque on the load.
4. Depending on the feedback device(s) engaged, the feedback devices may measure the final results.

Families implement the conversion differently. Festo’s actuator and drive overview covers electric mechanisms that use motors with belts or spindles, plus direct rotary drives; a Festo pneumatic cylinder moves a piston with compressed air, while a Parker hydraulic rotary actuator uses fluid pressure to turn a shaft.

Small commands can direct larger electrical power stages, air valves, or hydraulic valves; signals specify action, while energy paths make it possible.

What Are the Main Types of Actuators?

While there is no universally accepted count of types of actuators, there are classifications for actuators based on their energy source, conversion principles, and mechanical outputs. Keep the energy family and motion family completely separate, as they are two distinct properties of an actuator.

The various families of energy sources and conversions include:

• Electric (electromechanical conversion).
• Pneumatic (compressed gas).
• Hydraulic (pressurized liquid).
• Mechanical (levers, cams, springs, or other direct or stored mechanical input).
• Specialized designs (thermal, magnetic, piezoelectric, etc.).

The different forms of mechanical outputs include:

• Linear motion (movement along a path).
• Rotary motion (shaft rotation or turning).
• Limited-angle or oscillating motion (defined arc).
• Other forms of mechanical output (bending or shape change).

Each actuator can be classified according to the two separate axes of the energy and mechanical output families: pneumatic cylinders are pneumatic and linear, hydraulic rotary actuators are hydraulic and rotary, and electric screw actuators are electric and linear.

How Do Electric, Pneumatic, and Hydraulic Actuators Differ?

The primary difference between hydraulic, pneumatic, and electric actuators is the energy family associated with the actuator. The energy family determines the controls, infrastructure, and failure modes for the actuator. Speed, precision, and force will vary depending on the design or application of the actuator; therefore, the application characteristics of the respective actuator families should be compared based on system conditions, such as load, rather than universal rankings.

One example is the simple hydraulic formula (Force = Pressure × Piston Area). If 2,000 psi were applied to a 3.0-inch bore piston, this piston’s full area would equal approximately 7.07 in²—thereby producing an ‘ideal’ extension force (without accounting for frictional losses, seal drag, etc.) of approximately 14,100 lbf. The rod-side retract force would be less due to the reduction in effective area caused by the presence of the rod.

Is a Motor, Solenoid, or Cylinder an Actuator?

Yes, all three (motors, solenoids, cylinders) can be classed as actuators.

For example, in some of the basic switch documentation provided by OMRON, an actuator is defined as the lever or the push button that transmits the external force to the switching mechanism; this definition identifies a switch operating member rather than describing an actuator as an industrial assembly designed to convert a controlled energy path into mechanical load motion.

Where Are Actuators Used?

Robots have their joints designed primarily to provide torque and speed, minimize backlash, and provide feedback to the controller. Clamps and gates have their own set of functional parameters, but they still operate under the same basic principles as robotic actuators (force, stroke length, and cycle rate). Valves can be designed to produce either torque or thrust, both of which can be modulated, and many also have a defined failure position.

When Does an Actuator Need Feedback?

Feedback is not required for all actuators. Feedback becomes important when the control system requires knowledge of the actual output of an actuator rather than just sending commands to it.

In an open-loop controller, the position of the actuator is not continuously measured in order to command its operation; whereas within a closed-loop controller, the actual position of the actuator is regularly measured by a sensor and the output is compared to that of the command to identify an error between the two and create corrective action. There are several different types of actuator feedback such as potentiometers, Hall sensors, encoders, and endpoint signals; however, the specific type of feedback provided by each actuator may vary by model.

Does an Actuator Use AC or DC?

An electric actuator may have either an AC or DC power supply; pneumatic and hydraulic actuators are not classified using AC or DC, but electric actuators are specified by model and their specifications will outline the voltage, current rating, interfacing requirements, and types of feedback for that actuator. Thomson provides an overview of its Electric Actuators that includes both AC and DC options.

Before determining an actuator’s power, controls, or wiring, it is essential to identify its family.

What Parts Does an Actuator Have?

An actuator consists of various components depending upon the family and physical design:

• An electric linear actuator may contain multiple component parts including a motor, gears, screw, bearings, extension tube, housing, electrical limit switch, and feedback device, if installed.
• A pneumatic cylinder would contain a series of ports, a cylinder body, piston, cylinder seals, and rod; a valve and air preparation unit would not be classified as parts of the actuator, but rather as parts of the pneumatic actuator system.
• A solenoid actuator consists of at least one coil, magnetic path, armature or plunger, and may additionally include a return element and electrical connection.
• A hydraulic rotary actuator consists of an actuator housing with multiple passageways for the fluid, multiple surfaces that act under hydraulic pressure, seals between the surfaces, and the actuator’s output shaft or linkage.

General diagrams that show common actuator parts become accurate only after naming the actuator family. The components of a solenoid actuator cannot represent an actuator of a different type, such as hydraulic, pneumatic, or thermal. Just using a category name or label does not give the complete picture of the actuator’s terminals, maximum allowable voltage, driver topology, type of valve plumbing, feedback protocol, or limitations on the environment where the actuator would be used.

How Do You Choose an Actuator?

Selection should begin with the mechanical result required at the load, then work backward through motion, energy, control, and safety constraints.

The distinction between static (i.e., not moving) and dynamic loads (while moving) means that they cannot be used interchangeably; static load defines capacity when the actuator is at rest vs. dynamic load defines capacity when it is in motion. Always refer to the manufacturer’s definition and ratings for a specific model rather than trying to use a definition from another manufacturer.

Precision requirements are determined in specific numerical units; “accurate” represents an ambiguous phrase to describe precision. Thomson’s precision linear actuators list repeatability figures for these types of ball-screw mechanisms at or around ±0.05 mm, which is much tighter than what a simple end-to-end pneumatic or hydraulic cylinder is built to achieve. The application must indicate the actual repeatability required and be compared against the actual repeatability for each type of actuator family.

The duty cycle is model- and application-specific, but can be calculated. For example, if the manufacturer defines a 10-minute cycle, then a 20% duty cycle would equate to 2 minutes ON and 8 minutes OFF during the 10-minute cycle. The cycle may change depending on the load, ambient temperature, voltage, or the test conditions established by the manufacturer.

Mounting geometry is sometimes more important than the load capacity. In a hinged mechanism, the angle of the actuator changes throughout the stroke; thus, the amount of force needed from the actuator may be much greater than what is indicated by only the weight of the object being lifted.

Failure behavior should be intentional. In valve applications where there may be a spring-return actuator, when actuating energy is lost, the actuator moves the valve to a predetermined position; on the other hand, fail-open, fail-closed, or fail-in-place behavior must match the process hazard as well as the system design.

How Do You Test an Actuator Safely?

The absence of motion does not indicate actuator failure; the actuator may still function correctly if the controller logic does not send commands to the actuator, if the regulator does not supply energy to the actuator, the mechanism binds, or the feedback indicates an unreasonable state. Verify the commands sent to the actuator, the energy path, the actuator mechanism, the load, and the feedback circuits separately before replacing the actuator.

Refer to the specific manual from the manufacturer of your actuator to determine your next action in a safe manner. The Thomson Electrak HD Installation and Operation Manual addresses an electric product and its exact configuration; the Festo Safety Engineering Guidelines reference pneumatic and electric hazards; while Parker’s Helac L40 Service and Repair Manual addresses a hydraulic rotary family.

Isolate the appropriate energy source, secure the load, and adhere to the exact manual. Hazardous testing or repair should only be conducted by qualified personnel.

For electric linear actuators, repeatedly experiencing a hard stall can cause mechanical stress, heating of the motor, and decreased actuator life. Depending upon the model, limit switches, current sensing, or controller limits may be used to mitigate sustained over-travel or stalling of the actuator.