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Moving pneumatics: electronic motion controllers bring finesse of motion to cost-effective servo-pneumatic systems. (Design ideas: motion control).

Motion control can be a challenge, especially in industries such as food processing where hydraulic systems aren't always an option. Many motion control applications must avoid hydraulics, requiring designers to seek an alternative source for moving components, assemblies, or products. That alternative can be servo-pneumatics.

Servo-pneumatic systems can be used in applications that might benefit from the low equipment and maintenance costs, as well as the clean running and compressible nature, of pneumatic systems. They are enabled by "smart" electronic motion controllers and can offer an inexpensive alternative for certain motion control applications.

"Pneumatic systems rely on relatively low-cost air compressors and readily available `plant air' for compression," says Peter Nachtwey, president of Delta Computer Systems Inc. "Air's compressibility offers advantages in applications that can benefit from some `give' in the motion."

Nachtwey says air's compressibility, however, can yield drawbacks. It generally allows for less precise motion control than hydraulics. In addition, pneumatic systems typically support only two positions during operation: fully open or fully closed. These are often called bang-bang systems. "While inexpensive upfront, over time the shock and vibrations generated by these bang-bang systems can cause expensive equipment and material damage," he explains.

However, an electronic motion controller cap bring finesse of motion to servo-pneumatic systems. For instance, Nachtwey's company has developed the RMC100, a new electronic motion controller that supports advanced servo-pneumatic control systems. Such controllers are capable of infinitely variable midpoint cylinder positions. They use inputs from position and pressure sensors and sophisticated predictive control algorithms to determine the most appropriate cylinder midpoint position. Motion from this midpoint reduces the required stroke distance and speed, which allows for more manageable deceleration and reduced impact.

"Coordinated position and pressure control is key to providing tighter control and more flexibility than was previously possible with pneumatics," Nachtwey says. That's why the controllers employ multiple feedback loops.

For accurate pressure control, servo-pneumatic systems use inputs from pressure sensors on both the rod end and the closed end of the pneumatic cylinder to calculate the differential force. A magnetostrictive displacement transducer is often used for the position feedback loop.

"The two proportional integral and derivative feedback loops, using differential pressure and position from the transducers, provide much more precise pressure and position control than is typically achieved using pneumatics, allowing the controller to quickly and accurately respond to changing conditions by adjusting the drive output," says Nachtwey.

In addition to transducers and feedback loops, precision pneumatic control requires the use of a proportional servo valve rather than a pressure relief valve for the accurate control of air leaving and entering the cylinder.

"In typical pneumatic systems, pressure is built at a constant rate, and pressure relief valves release the air once the system reaches the desired pressure," explains Nachtwey. "The two end-points, then, are full pressure and released pressure, leading to the bang-bang operation of these systems. Although some pneumatic systems are capable of midpoint positioning, this position is generally not precise or variable."

With new electronic motion controllers, algorithms calculate how far to move the servo valve spool in order to meter air in and out of the cylinder. "This allows for accurate and variable open and closed positions, as well as infinitely variable midpoint positions," says Nachtwey. "The smooth motion that results can increase throughput and reduce equipment and material damage."

Motion controllers such as the RMC100 provide direct interface to the transducers and servo valves, without a separate interface module that can add time delays and system costs. These controllers also have standardized high-performance Ethernet and Profibus connections and can interface easily to other system computing elements.

More information is available by contacting Delta Computer Systems Inc., 11719 N.E. 95th St., Ste. D, Vancouver, WA 98682, calling (360) 254-8688, writing in 60 on our reader service card, or replying online at www.pddnet.com.
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Publication:Product Design & Development
Date:Oct 1, 2002
Words:638
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