Machine Design

Pneumatic Controls

Simple control tasks can be candidates for pneumatic, electropneumatic, and hybrid pneumatic controls. Purely pneumatic controls must be used in certain cases, as in the explosive environment found in paint-spraying operations or chemical plants. When safety is not the prime concern, a pneumatic circuit is generally more economical than electronic control when handling up to about 50 I/Os, or 12 to 20 sequencing steps. This assumes that pneumatic power components, such as cylinders or rotary actuators, are being controlled. If electrical power components, such as dc motors, are used, electronic control is probably better, even in simple applications.

In cases where either pneumatic or electronic control can be used, trade-offs between solenoids and air-piloted valves may become important. Solenoid valves are more expensive than similarly sized air-piloted valves in many cases.

Also, air-piloted valves may respond more quickly than solenoids. A common misconception is that because an electrical signal travels faster than a pneumatic one, overall response is quicker. Actually, response depends on the element being driven. In high-speed applications, air-piloted valves are often a better choice.

Hybrid controls take advantage of both electronic and pneumatic controls. An electric-to-pneumatic interface circuit allows PLCs to control air-operated components. This enables pneumatic power components to take advantage of PLC capabilities such as reprogramming. Air-piloted hydraulic power valves provide hybrid control of hydraulic cylinders.

TAGS: Controllers
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