Compatibility with PLCs and PCs. Enfield Technologies often recommends using high-quality analog I/O modules with PLCs and digital-analog converters with computers. Analog control is recommended because it uses real-time control signals associated with voltage. Digital control is based on discrete time intervals, comparing and manipulating two digits (0 and 1) or combinations according to a set of instructions. Computers have high-speed processors but loop cycle times can often be longer than acceptable for discrete control. At the end-device level, real-time analog control provides quicker responses to command and feedback signals. Of course, a command signal source is still needed. While a simple potentiometer will do, PLCs and computers provide easily programmable logic to control the overall system. In this fashion, a hybrid system can be implemented to use the advantages of digital systems for logic control and analog systems for local device-level signal processing.

Cleanliness. Compressors generate heat and water and ingest dust. Typically, a drying and filtering system cools the compressed air and eliminates water and dirt. However, in most systems the air still contains some contaminants as it travels through the system, and galvanized piping that transports the air can let rust and scale enter the airstream.

Many solenoids are strong enough to shear most incidental dirt in the system, as is the linear force motor Enfield Technologies uses in place of solenoids. Particles can damage the spool or poppet, as well as coil windings, and thus impair proportionality. Therefore, in critical performance applications, a 5-m m prefilter and a 0.3-m m coalescing filter upstream of the valve inlet is recommended.

The same holds if the airstream contacts sensitive products such as food, pharmaceuticals, or semiconductors. However, with stronger forces on the moving parts of the valve, these fine filtration requirements are often overspecified. Valves routinely operate with little or no filtration for noncritical applications.

NOT JUST FOR MOTION

An example of a state-of-the-art servopneumatic control system is Enfield Technologies' LS System. It controls position, pressure, force, flow, or velocity in open and closed-loop applications. The system relies on a dedicated controller and a five-port, three-position, closedcenter directional valve.

The controller requires a minimum ±12 Vdc at 25 W or 1.0-A power source, and accepts a 4 to 20 mA, 0 to 10 Vdc, or ±10-Vdc command signal from sources such as PLCs and computers. It provides real-time command and feedback signal monitoring with a <100-m sec response time. Active valve-current feedback monitors the motor's position and immediately corrects for external disturbances at the valve before the system sees them.

Optional sensor signal conditioners amplify and linearize output for either two individual or one differential lowvoltagelevel pressure or force sensor. Differential pressure measurement provides an additional feedback path for applications requiring more stability under varying load conditions. A ramp conditioner modifies command-signal characteristics for opening and closing the servovalve. This helps tune the system to avoid overshoot, and is ideal for conserving energy by matching the servopneumatic control system to the mechanical system dynamics. The servopneumatic valve uses a directly coupled linear-force motor and lapped spool-and-sleeve construction for fast, smooth, quiet, stiction-free operation. The design provides infinitely variable and linear proportionality with almost zero hysteresis. Valve response time from closed-center to fully open in either direction is 2.5 msec, and it requires less than 1 A. Three valve sizes offer flow capacity to 46 scfm, and commonly control pneumatic actuators up to 5-in. bore.

It has also been used successfully for much larger systems, both with single and multiple valves commanded as a single valve, including a recent installation with a 12-in. bore cylinder variably positioning a 1.5-ton vertical load. The LS System is rated for clean, dry, nonlubricated air or inert gases from 28-in.-Hg vacuum to 150 psi, and ambient temperatures from 10 to 115°F.

While the system is designed for proportional pneumatic control, it can be used in speed applications with a total cycle time of 5 msec to open and close one port — four to seven times faster than typical solenoid-operated valves. Its trapezoidal command curve eliminates the end of stroke shock typically associated with high-speed, "bang-bang" applications.

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Enfield Technologies, (800) 504-3334, enfieldtech.com