All About Pneumatic Actuators - Types, Applications and Uses
Pneumatic actuators use air power to create rotational and linear motion. In a broad sense, a cylinder is a pneumatic actuator. For the purposes of this article, pneumatic actuators designed to generate rotary motion for operating valves, automated manufacturing, clamping work, etc., are the main focus. Their use in linear applications such as valve operation and positioning is also discussed. Generally, cylinders are not included.
Pneumatic Rotary Actuator
Pneumatic rotary actuators rely on various mechanisms to generate rotary motion. Two common styles use pistons or diaphragms. In one approach, the piston moves the rack through the pinion, which rotates to create a limited range of rotation at the output shaft. Another style uses a Scotch yoke, a well-known kinematic link that produces up to 90°of rotation on the output shaft. The third method does not use pistons or diaphragms but instead uses one or two vanes that are pressurized to create rotational motion directly within the circular housing.
The rack and pinion type uses at least one, and sometimes two, or four cylinders to drive the rack through the pinion. The pinion rotates in response to drive the output shaft. Although modulation is possible, the rack and pinion actuator will continue to rotate the pinion until it reaches the end of its stroke. In many cases, the piston in the cylinder will act on large coil springs that allow the valve to return to a safe position during power interruptions. Diaphragms are sometimes used in place of pistons, but the principle of operation remains the same. The number of revolutions that a rack and pinion actuator can make is limited only by the length of the rack.
Fork actuators are typically limited to a 90°rotation range and can see applications in angle rotary valves. Pneumatic vane actuators can generate more than 360°of rotation.
Pneumatic actuators are used in conjunction with on/off valves such as ball valves. They can also be applied to control valves. To regulate fluid flow, the control valve must be in the neutral position. Modulating pneumatic actuators can be used for this task. Typically, an industry-standard air pressure signal of 3-15 psi is sent to an adjustment positioner on the valve, which adjusts the stem (in the case of a 1/4 turn valve) anywhere between 0 and 90°based on the downstream flow measurement. Electro-pneumatic positioners use electrical signals to do the same thing.
Motion control applications using pneumatic rotary actuators are typically rack and pinion or vane types. Typically, a double-acting rack and pinion arrangement is used. Multi-position actuators with three, four, or five stops are often used for sequential assembly operations. Rotary actuators can also be used for indexing, stepping, and pick-and-place movements.
Pneumatic Linear Actuator
Pneumatic linear actuators are used for vertical valves to directly operate gates, globe valves, etc. Two types are commonly used diaphragm and piston. Diaphragm styles are popular because their wide surface area can generate enormous forces at modest air pressures. The diaphragm is a rubber membrane whose edges are sealed against the housing of the actuator. Air pressure moves the diaphragm up or down against spring pressure, depending on whether the actuator is designed to fail open or fail close. The stroke length is usually shorter than that of a piston valve, whose stroke depends only on the length of the cylinder, not the amount of stretch the diaphragm can withstand.
Piston actuators can be sized to produce the appropriate drive force based on the pressure of the available air, and can be manufactured as double-acting and spring return. Some linear actuators use the familiar air spring in place of the diaphragm.
Since globe valves are common in control applications, modulating linear actuators can be used. Pneumatic regulating valves are particularly effective because their speed can be adjusted by throttling the airflow. As a result, a pneumatic actuator can accelerate quickly toward the set point, but slow down when the set point is reached, eliminating overshoot. Unlike electric actuators, pneumatic actuators can operate continuously.
Combined Rotary/linear Actuator
The ability to provide linear travel and limited rotation is convenient in certain applications, such as workholding. The gripper can be raised and swung from the workpiece to allow it to be removed, then pressurized and re-engaged after positioning a new workpiece.
Pneumatic actuators are a proven method of obtaining rotational and linear motion. For valve actuation, whether on/off or control, air provides a reliable, safe, and economical method. Deducting the cost of supplying the compressed air itself, air actuators are generally more cost-effective than electric actuators, up to a certain diameter valve. As valves became larger, the economy began to favor electric actuators.
Electric valve actuators are used more and more frequently. Nonetheless, pneumatic actuators have their supporters because they are simple, robust, and easy to fail-safe. They can move faster than electric actuators and are not affected by the duty cycle. Efforts are underway to make traditional pneumatic and hydraulic actuators more compatible with distributed control systems.
Rotary actuators are used in many motion control systems to operate, for example, pick and place handlers or grippers. Linear cylinders have long been a staple of automation and remain so despite the influx of advanced stepper and servo motor-driven electric actuators.
While their primary application is point-to-point motion, feedback-equipped cylinders are enabling servo-pneumatic actuators to handle positioning applications that are too fine for regular cylinders but too coarse for electric actuators. This device can provide high forces in a small package and can operate continuously without generating heat. Part of their development was figuring out a control algorithm that would account for the compressibility of the air, which always makes the cylinder better for point-to-point motion.
This article briefly discusses pneumatic actuators, their types, and their main applications. If you want to buy pneumatic actuators, please contact us.
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