An actuator is a device that uses a form of power to convert a control signal into mechanical motion. From electric door locks in automobiles, to ailerons on aircraft, actuators are all around us. Industrial plants use actuators to operate valves, dampers, fluid couplings, and other devices used in industrial process control. The industrial actuator can use air, hydraulic fluid, or electricity for motive power. These are referred to as pneumatic, electro-hydraulic, or electric actuators.
Pneumatic actuators can utilize a cylinder, diaphragm, or rotary vane design. Compressed air acts upon a piston or paddle to move it in one direction. A counterforce, consisting of a spring or a second volume of compressed air, is applied to the opposite side of the piston or paddle to reverse or maintain position. For industrial control, a pneumatic actuator requires positioning electronics, and an instrument air system. Of the three types of actuators, pneumatics tend to have the lowest initial price. However, operating costs are high due to the need to generate clean, dry, compressed air. Maintenance requirements are also the highest of the three actuator types.
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Electro-hydraulic actuators operate a piston with pressurized oil. A motorized pump sends fluid from a reservoir through a control valve(s) to opposite sides of a cylinder. The high power density of this system allows for high thrust, and fast operating speeds. Electro-hydraulics tend to have a high initial price. Routine maintenance involves periodic replacement of seals, o-rings, etc. Since the oil used in hydraulic equipment can pose a fire hazard, this type of actuator may not be suitable for some industrial environments.
Electric actuators utilize an electric motor and gear reduction to produce a force or torque. A variety of technologies can be used for an electric actuator design. Motors may use AC or DC power, and an asynchronous (squirrel cage) or synchronous design. Gearing systems can include worm, spur, scotch yoke, among others. Gear lubrication may consist of an oil filled gear box, or heavy duty grease applied to the gear surfaces. Various accessories are often available to supervise and report on the actuator’s status and operating conditions. There is a wide variety of electric actuators on the market with different technologies, prices, performance, and quality.
Because actuators are closely integrated with driven elements like valves, and dampers, they are often referred to as assemblies. Examples include motor operated valves (MOV), air operated valves (AOV), and damper drives. Frequently, the actuator may be assumed to come with the equipment package. Examples include control valve, or air damper. In practice, actuators are the critical link between the control system and the final element. Its performance can significantly impact many aspects of an industrial plant’s operations and products. Care should be taken to ensure the correct actuator technology and type is selected.
When selecting an actuator, the actuator should have performance characteristics that will enable a control system to perform as designed.
The key actuator performance characteristics are as follows:
- Precise, repeatable positioning typically better than 0.15% of span.
- The ability to start and stop instantaneously without dead time or position overshoot.
- Continuous duty rating without limitations on the number of starts per minute.
- Perform consistently and unaffected by load.
- Rugged industrial design capable of operating in difficult environments without an effect on performance.
- Minimal periodic maintenance required.
An actuator designed with these characteristics provides two extremely important advantages:
- An ability to follow the demand signal from the controller precisely and instantly. This ensures that the actuator responds exactly as directed by the controller. Thus, the actuator is not the limiting factor in the control loop and the controller can function to its optimal levels.
- A high degree of maintenance-free reliability. An actuator designed to function as outlined above by default is more rugged than typical actuators. By design, then, it is capable of a much higher degree of reliability.
Beck electric actuators provide the necessary control and reliability that is required for many damper applications. The Group 11 actuator product line is often found on low to medium torque applications (20 lb-ft to 5,200 lb-ft). Group 22 actuators are utilized for high torque applications (3,000 lb-ft to 8,000 lb-ft), and Group 31 compact rotary actuators are utilized on low torque applications (15 lb-ft to 30 lb-ft).