Ed Howe
President Enfield Technologies
Trumbull, Conn.

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.