The actuator plays the role of limbs in an automatic control system: it receives the controller signal and changes the manipulated variable so the process runs as required. An actuator comprises an actuating mechanism and a regulating mechanism — the former produces thrust or displacement from the control signal, the latter alters energy or material flow accordingly, most commonly a control valve. On site the actuator handles the process medium directly; wrong selection or use makes automatic control difficult, so selection, use and commissioning matter greatly.
By energy source actuators divide into pneumatic, electric and hydraulic — each with its own strengths and applications. As technology advances, actuators are steadily becoming intelligent; smart valve positioners for pneumatic actuators and all-electric actuators have accelerated that trend, opening new ground for actuator applications.
Hydraulic actuators deliver the greatest thrust and are usually mechatronic units, but are bulky and thus little used today. Their thrust is irreplaceable in some large installations — the Three Gorges ship locks use hydraulic actuators.
In pneumatic actuators the actuating and regulating mechanisms form one unit. The actuator is either diaphragm type (Fig. 1) or piston type (Fig. 2). Pistons have long stroke for high-thrust duties, driving the stem directly or via worm gearing; diaphragms have short stroke and drive the stem directly. Direct-acting types push the stem down as signal pressure rises; reverse-acting push it up. Output displacement is proportional to input pressure — the higher the pressure the larger the travel, and the stem rests where pressure balances the spring. Pneumatic actuators come in quarter-turn and linear-stroke versions.
As pneumatic actuation grew more intelligent, the smart valve positioner became an essential companion: a highly integrated microcontroller replaces force balance with electrical (digital) balance, converts electric commands into pneumatic positioning increments, drives the actuator from digital open/stop/close signals, and reads valve position via a high-accuracy sensor performing the electric-to-pneumatic conversion. Smart positioners raise output force and speed, positioning accuracy (resolution down to ±0.05%), overcome stem friction and position precisely. Such ever-smarter accessories make the pneumatic actuator smarter too.
With simple structure, large thrust, smooth reliable action and intrinsic safety/explosion protection, pneumatic actuators are widely used in safety-critical processes such as chemicals and refining.
Here actuating and regulating mechanisms are separate. The actuator comes in quarter-turn and linear types, receives DC signals from the controller and converts them into angular or linear displacement to operate valves, dampers and the like (construction in Fig. 3).
Electric actuators are weaker on explosion protection, their motors are not very fast, and the motor is easily damaged when travel is blocked or the stem jams. Despite continuous improvement and growing application, overall they remain less widespread than pneumatic types.
With automation, electronics and computing advances, more and more electric actuators are becoming intelligent. A new product designed by Shanghai Automation Instrumentation on imported technology adopts dual CPUs, electronic torque limiting, electronic travel limiting, digital position transmitter, automatic phase discrimination, infrared remote commissioning, fault diagnosis and multiple automatic protection switchovers — raising stability, reliability and accuracy. It has passed explosion-protection, RF interference, ESD and fast-transient-burst tests: intrinsic error below ±1.5%, hysteresis 1%, enclosure IP65, ambient -25℃ to 70℃ — internationally advanced figures.
The intelligent electric actuator is a new class of final control element: it changes the control-valve stem position directly from the electric control signal. As demands on accuracy and dynamics rise, the electric actuator offers the fastest response, making control more reasonable, convenient and economical — and is winning users. It also spares the air supply equipment and piping a pneumatic installation requires.
With science and technology advancing, we have reason to believe future actuators will be as intelligent as the rest of the instrument family.
Editor's note: Figs. 1–3 (diaphragm/piston actuators and electric actuator construction) were not preserved in the web version.
