Actuator

What is an actuator?

An actuator is a device that converts a control signal and an energy source (electricity, compressed air or hydraulic oil) into physical motion or force in order to act on a process or machine. Actuators carry out the actions in an automated system: opening a valve, extending a cylinder, running a motor or closing a gripper. Where a sensor measures, an actuator intervenes; together they form the field components beneath every PLC or DCS.

🧠 What exactly does an actuator do?

An actuator is the final element in a control system or control loop. It always works in three steps:

1. Receive — the actuator gets a control signal from a PLC, DCS, HMI or controller. This can be a digital on/off signal, an analogue 4-20 mA signal or a message over a fieldbus.
2. Convert — using its own energy source, the actuator turns that signal into linear motion (push, pull, lift) or rotary motion (quarter-turn, multi-turn or continuous rotation).
3. Execute — the motion drives a valve, pump, conveyor, robot arm or other mechanism and so changes the state of the process.

An important distinction: the control signal itself carries almost no power. A 20 mA signal can move a valve weighing hundreds of kilograms because the actuator draws its force from compressed air, oil or mains power. A relay or motor starter often sits between the controller and the actuator to switch that heavier power.

🔧 What types of actuators are there?

Actuators are usually classified by their energy source and by the motion they produce.

A control valve is in fact a combination: a valve body with a (usually pneumatic) actuator and a valve positioner. In the process industry it is by far the most common actuator. Pneumatics dominates where motion must be fast, simple and explosion-safe; Hydraulics where very high forces are needed; electric actuators where precision, energy efficiency and diagnostics are decisive.

📡 How is an actuator controlled?

The way an actuator is controlled determines how much information you get back from it.

Valve positioners deserve special attention. A positioner compares the requested signal with the actual position of the valve stem and sends just enough air to the actuator until the two match. Modern digital positioners also record friction, stroke and response time, making them an important data source for predictive maintenance. For faults on the current loop, NAMUR Recommendation NE 43 applies: a current of 3.6 mA or lower, or 21 mA or higher, indicates a fault.

Mechanical interfaces are standardised too. ISO 5211 defines the flange between a valve and a quarter-turn actuator (ISO 5210 covers multi-turn), and the NAMUR interface to VDI/VDE 3845 defines how solenoid valves, positioners and limit switch boxes mount on the actuator. As a result, valve, actuator and accessories can come from different suppliers.

🔄 What is the difference between a sensor and an actuator?

In a closed control loop they work together. The sensor measures the process value, a PID controller compares it with the setpoint and the actuator applies the correction, for example opening a valve further when the temperature is too low. Through feedback the controller immediately sees the effect. A slow or sticking actuator is one of the most common causes of a poorly performing control loop, even when the controller itself is perfectly tuned.

🏭 Where are actuators used?

• Process industry — control valves, isolation valves and pumps in chemicals, refining and pharma, often in ATEX zones
• Water and energy — electric valve actuators in pumping stations, treatment plants and locks, often operated remotely
• Machine building — pneumatic cylinders, grippers and servo axes in packaging and assembly lines
• Robotics — servomotors for precise, programmed motion
• Building management — damper and valve actuators for ventilation, heating and sun shading
• Automotive — electronic throttle valves and windscreen wipers in the vehicle, and robot cells in the factory

🧭 How do you choose the right actuator?

In practice you work through these steps when selecting an actuator:

1. Required force or torque — determine the maximum torque or thrust including a safety margin, for example 25 to 50% above the valve’s breakaway torque.
2. Motion and stroke — linear or rotary, quarter-turn or multi-turn, and how fast the motion must be.
3. Duty type — for electric valve actuators, EN ISO 22153 (the successor to EN 15714-2) distinguishes class A (on/off), B (occasional positioning), C (modulating) and D (continuous modulating).
4. Safe position — what must the actuator do when power or signal is lost? This drives the choice between spring-return and double-acting designs.
5. Environment — temperature, dust, moisture (IP rating) and explosion risk. In ATEX zones the requirements of Directive 2014/34/EU and IEC 60079 apply.
6. Signal and integration — does the actuator fit the existing I/O, fieldbus or IO-Link master, and what diagnostics does it provide?
7. Maintenance and service life — consider spare parts, manual override and the availability of diagnostic data.

🔐 What does fail-safe mean for actuators?

Every actuator needs a predictable safe position for when the control signal, air supply or power is lost.

• Fail-closed (FC) — a spring closes the valve, for example on the fuel supply to a burner.
• Fail-open (FO) — a spring opens the valve, for example on cooling water or pressure relief.
• Fail-in-place or fail-last (FL) — the actuator stays in its last position, common for electric actuators without a spring or battery.

In a SIS, the actuator is the final element of the safety function. Studies in the process industry show that valves and their actuators often account for half or more of all failures in safety instrumented functions. That is why IEC 61511 and SIL calculations require periodic proof tests, often supplemented by a partial stroke test in which the valve moves a short distance without disturbing the process.

For machinery, the focus is on stopping drives safely. IEC 60204-1 defines stop category 0 as the immediate removal of power to the machine actuators; the Safe Torque Off (STO) function from IEC 61800-5-2 implements this within the drive itself. An emergency stop often acts on this function through a safety relay or safety PLC. From 20 January 2027 the Machinery Regulation (EU) 2023/1230 applies, which also sets explicit requirements for protecting control systems against tampering.

🛡️ Why do actuators matter for OT security?

An actuator is the point where a digital attack turns into physical consequences. Stuxnet demonstrated this by manipulating the speed of frequency drives powering centrifuges while operators saw normal values. Smart actuators with HART, IO-Link or Ethernet are also network devices in their own right, with firmware and configuration. Include them in your asset inventory, restrict parameter changes and place them in the correct zone according to IEC 62443. Monitoring through a separate, read-only channel, as in NAMUR Open Architecture, makes diagnostics usable without opening write access to the field.

❓ Frequently asked questions

What is the difference between an actuator and a motor?

An electric motor converts electrical energy into rotation, whereas an actuator is any device that produces physical motion in response to a control signal. An electric motor can therefore be an actuator, but an actuator can also be pneumatic or hydraulic. An electric valve actuator combines a motor with a gearbox, limit switches and control electronics into a single actuator.

Is a solenoid valve an actuator?

A solenoid valve is an actuator: a solenoid converts an electrical signal into a short linear stroke that opens or closes a valve. In practice a solenoid valve often pilots a larger pneumatic actuator, such as a cylinder or quarter-turn actuator. The solenoid valve is then the link between a digital output and the compressed air.

What does a valve positioner do?

A valve positioner makes sure a control valve takes up exactly the position requested by the 4-20 mA signal. The positioner measures the actual valve position and adjusts the air supply to the actuator until the deviation is zero. Digital positioners also provide diagnostics on friction and wear via HART or a fieldbus.

When should you choose pneumatic and when electric?

A pneumatic actuator is inexpensive, fast, robust, easy to make fail-safe with a spring and suitable for hazardous areas. An electric actuator is more precise, more energy-efficient because no compressed air is needed, and provides richer diagnostics. Where compressed air is already available, the process industry usually chooses pneumatic; in new installations without an air network, the share of electric actuators is growing.

What is the fail-safe position of an actuator?

The fail-safe position of an actuator is the position it automatically takes up when power or the control signal is lost, such as fail-closed or fail-open. Usually a spring or energy store ensures the actuator reaches that position without external power. The choice follows from the risk analysis, for example a HAZOP.

How are actuators connected to a PLC?

Actuators are connected to digital or analogue outputs of a PLC, or via a fieldbus, Industrial Ethernet or IO-Link. Simple on/off actuators use a 24 V output, control valves a 4-20 mA output and servo drives usually ProfiNET or EtherNet/IP. Large motors are switched through a contactor or variable frequency drive rather than directly by the PLC.

📌 In summary

An actuator is the “muscle” of an automation system: it uses electricity, compressed air or oil to turn a control signal into physical motion that changes the process. The right choice depends on force, motion, duty type, safe position and environment, and modern actuators with HART, IO-Link or Ethernet deliver valuable diagnostics but also call for OT security.