Ed Howe
President Enfield Technologies
Trumbull, Conn.
m sec. Servopneumatic, electromechanical, and proportional-hydraulic control systems all have a place in industrial-automation equipment, product testing, and animation. However, most applications do not need the precision of electromechanical systems or the high forces of hydraulics. Servopneumatics, which includes a controller, linear actuator, valve, and feedback sensors, can meet most requirements and offer substantial savings over the other systems. In addition, it eliminates potential contamination from hydraulic and lubricating oil in food processing and clean-room operations.
Electric drives can cost from several hundred to thousands of dollars, depending on motor size, sophistication of the controller, and nature of the mechanical components. Pneumatic systems are often half the cost. Hydraulic pump systems cost more than air compressors, and hydraulic components must be built to withstand higher pressures, so hydraulic drives can be several times more expensive than pneumatics. Maintenance costs of pneumatics are also lower, while life expectancy of all three systems is about the same.
Pneumatic systems control motion much like electric drives, starting and stopping anywhere within the actuator's range. They require a sophisticated controller and fast-acting proportional valve. The controller calculates a valve command based on setpoint and feedback signals to provide the necessary level and direction of airflow to the cylinder to execute the motion command. The overall effect is similar to an electric drive, though control scheme and components differ.
DESIGN ISSUES
Here's a look at some major considerations with servopneumatic systems. Compressibility. The "spongy" effect caused by air compressibility has long been an issue for system designers. However, today's controllers and multiple-feedback designs effectively monitor for the effects of compressibility and immediately make any needed adjustments. In some applications involving fragile products or humanlike motion, designers actually prefer to accentuate compressibility (typically by lowering the air pressure) because the compliance of compressed air provides the desired cushioning or soft-motion effect.
Stability. Pneumatic systems are perceived to be soft and unable to resist movements from external disturbances. In some cases this is true, but it is most often the fault of underdesigned systems. If the magnitude of external loads is known, air cylinders and valves can be sized to provide the necessary restoring force to overcome external forces. Stability can also be enhanced with control electronics. High speed or real-time controllers can continuously sense and quickly correct errors so that disturbances are well within acceptable levels and often unnoticeable.
Precision. Accuracy and repeatabilityof pneumatic systems is dictated by the response speed of the controlling electronics and valve, as well as response time of the feedback sensors, resolution, and linearity. If the electronics detect and resolve the commanded load status with high resolution, and valve response is fast enough, the actuator will stop at precisely the commanded position within a measurable and predictable error.
Error depends on stiction (static friction) in the load-support system, as well as acceleration and deceleration rates and the dynamic load. These factors apply to hydraulic and electric systems as well.
Load capacity. Like in any system, pneumatic components must be sized for the specified load and orientation. The power of pneumatics is often underappreciated. For instance, a 2-in.-diameter cylinder with 40-psi air can support 126 lbf; an 8-in. cylinder with the same pressure supports over 2,000 lbf. In most industrial applications, these are considered large loads. System pressures to 150 psi are commonly available, offering load capacities over 7,500 lbf for 8-in. bore cylinders.
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.
IN THE FIELD
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.
NOT JUST FOR MOTION
Servopneumatic systems are not only for motion control. They are also suited for many applications involving force, pressure, and flow control, and a variety of other more esoteric command objectives such as altering air flow for temperature control, or moving a screen to regulate light intensity, among others.
In processing and packaging machinery, for example, an application that can benefit from servopneumatics is dynamic rail guides. Such a system can eliminate the need for programming related to frequent and unexpected product changes (especially at contract warehouses) and can assist in rapid change-overs by reducing human errors and set-up time even for like-sized products.
Another servopneumatic application in processing and packaging machinery maintains constant pressure or flow rate in material feed tanks with constantly changing surface levels. This facilitates processing by providing a constant line speed on a manufacturing or packaging line.
Servopneumatics are also used to apply constant force while silk-screening or labeling curved container surfaces, as well as apply constant force when extruding or molding soft materials. In all of these cases, servopneumatics provides precise electronic control to traditional pneumatics by adding sensors to critical activities and closing the loop in an electronic controller — either at the system control level (PLC or computer) or the device level (valve controller). The opportunities to increase machine or system functionality, improve performance, and eliminate capital and operating costs in a variety of industries are limited only by the designer's creativity. Such improvements benefit not only machine builders, but their customers as well.
MAKE CONTACT
Enfield Technologies, (800) 504-3334, enfieldtech.com