Machine Design

Closed-Loop Pneumatic Control In A Compact Package

A newly developed pneumatic-control system provides an alternative to electrical or electromechanical motion control. The Pneumatic Control System (PCS) is a compact electronic controller with pneumatic valves that accurately positions an actuator rod or rotating shaft and holds it in position even when subject to external force. The product is said to produce the same accuracy as more-expensive electrical servomechanisms and temperamental electro-mechanical ball-screw actuators. The system accurately stops and holds the rod in any position and is designed to control both linear and rotary pneumatic position-feedback actuators.

The PCS accepts a 0 to 10-Vdc, 0 to 20-mA, or 4 to 20-mA analog command signal, which is used as a reference to move a load and hold its position. For example, if an application has a stroke of 10 in., a 1-V change in command voltage moves the cylinder 1 in. Similarly, a change in command signal of 0.005 V produces a position change of 0.005 in. For rotary applications, the convention is similar. If the application has a rotation of 180°, a 1-V change in command signal produces 18° of rotation.

The system compares the position feedback signal from the actuator to the command signal. All valves close when the difference between the command and feedback signal — called the error term is zero. This traps air on both sides of the actuator piston and holds the rod in position. If an external force attempts to move the rod, the system reacts by increasing the reaction force. Likewise, if the command signal changes, the system responds to equalize the feedback and command signals.

The manufacturer, Bimba, recommends PCS systems for applications that require flexibility or control. Specific applications include flow gates that control the rate of product flow to down-stream processes, robotic-arm end effectors, and exhaust dampers in process control systems.

Repeatability of the system in linear applications can reach ±0.004 in. at an average velocity of 6.50 ips while carrying loads of 90 lb. In rotary applications repeatability is ±0.15° at an average rotational velocity of 15°/sec.

This information supplied by Bimba Manufacturing Co., Monee, Ill.

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