A pneumatic actuator converts energy in the form of compressed air into motion. Different manufacturers offer pneumatic actuators in varying forms, some converting the compressed air energy into linear motion, and some to rotary motion.
There are several basic types of pneumatic actuator: spring/diaphragm, piston, and rotary vane.
- The spring/diaphragm style pneumatic actuator requires compressed air to push a diaphragm against a plate which is opposed by a spring. When the pressure is reduced, the spring will retract the diaphragm. By varing the pressure, position is achieved. This type of actuator can “fail-open” or “fail-closed” when air pressure is lost with the spring returning the actuator to the rest position.
- The piston style pneumatic actuator utilizes a piston inside a cylinder. Movement of the piston is caused by applying more or less force on one side of the piston. Double-acting piston style pneumatic actuators have air pressure applied to both sides of the piston while single-acting piston style pneumatic actuators use a spring on one side and modulate pressure to the other side. The linear motion of the piston can be used directly for linear motion actuation or it can be converted to rotary motion using a rack and pinion or similar mechanical arrangement. Piston actuators may be identified with a cylinder diameter and stroke length. Larger cylinders are capable of exerting more force.
- The rotary vane style pneumatic actuator works similar to the piston style pneumatic actuator having two pressurized chambers. Instead of a cylinder form factor, the housing is shaped like a pie wedge. A paddle with an output shaft separates the two chambers. Varying the differential across the paddle moves the paddle and output shaft accordingly through its 90 degrees of travel.
Use of a pneumatic actuator with modern controls typically requires an analog to pressure converter for delivering a control signal setpoint and some type of positioner that translates the physical position of the actuator into an analog or digital position feedback signal.
Some characteristics of pneumatic actuators are that they are capable of modulating process control and can handle high torque loads, but since the power source is a compressible gas, they are prone to stick/slip response, limit cycling, and tend to have performance inconsistency. Performance degradation occurs quickly, particularly when the high level of maintenance requirements are not kept up with.
Pneumatic actuators tend to have a relatively low purchase price, but life-cycle costs incurred from frequent maintenance, costs associated with management of compressed air systems, ancillary equipment costs such as positioners and A/D converters, along with opportunity costs lost in poor process control tend to make pneumatics more costly to operate in the long-term.