Servopneumatics Finding its Niche, Bit by Bit

Newton, MA—Makers of electric servo drives aren't glancing over their shoulders yet, but a new breed of smart air cylinders may be gaining on them.

Servopneumatic positioning systems, which use microcontrollers for intelligence and sensors for closed loop feedback, have begun making in-roads in robotic welding, packaging machinery, food processing, automotive assembly, amusement park animatronics, and even in biomedical testing of prosthetic joints, such as hips and knees.

The servopneumatic systems, which first reached the market in the early 1990s, are said to offer a less costly alternative to electric servo drives, especially where ultraprecise positioning capabilities are considered overkill. They also provide an alternative to hydraulic cylinders in situations where huge forces aren't needed, or where oil leakage is unacceptable.

"Servopneumatics is like hydraulics, only without the oil," says Ray Riedel, product sales manager for servovalves at Parker Hannifin Corp. (Elyria, OH), which entered the servopneumatic arena three years ago.

Makers of the air-based positioning systems say that the technology's acceptance was slow at first, mainly because many engineers felt more comfortable with electromechanical devices, such as motors and ball screws. "We had difficulty in marketing this technology to the motion control community at first," says Mark Tandy, business line manager for industrial products at HR Textron. "Even our best customers were initially skeptical. But most of them changed their minds when they saw what can be accomplished with it."

During the past two years, servopneumatic suppliers say the popularity of the technology has grown. "In the pneumatics business, it's coming on strong," notes Don Caputo, marketing manager at Parker Hannifin's Hydraulic Valve Division.

Computer-aided

That wasn't always the case, however. For decades, pneumatics were thought of as a so-called "bang-bang" technology, using mechanical stops to halt their motion. The idea of precisely controlling the position of such systems at resolutions down to a one-ten-thousandth of an inch was considered ludicrous.

Cheap Trick - EnduraTec's test systems keep costs down by employing servopneumatic systems instead of electric drives.

In the early 1990s, however, suppliers began employing microprocessors to deal with the inherent non-linearities caused by the compressibility of air. As a result, conventional pneumatic valves—which were typically all-the-way-open or all-the-way-shut—moved to the next level. Using software algorithms to deal with the non-linearity, valves were suddenly able to develop so-called proportional forces, enabling them to produce forces proportional to their openings.

Although acceptance was slow at first, engineers say they are now finding niche applications for the technology. The reason for their growing popularity is simple: Servopneumatic systems cost less than electric servos and big hydraulic valves.

"We're able to sell a pneumatic system for about one-third the price of a hydraulic system," notes Kent Vilendrer, chief technology officer and founder of EnduraTec (Minnetonka, MN), a maker of biomedical and material testers.

EnduraTec employs servopneumatic technology in systems that apply test forces to biomedical body parts, such as artificial hips, knees, fingers, and breast implants. The company has also sold its servopneumatic test machines to the state of Texas, which uses them to analyze asphalt samples.

With pneumatics, his company's testers can precisely position specimens, while reducing bulk and cost at the same time. For the systems, he employs servopneumatic valves from HR Textron (Santa Clarita, CA), which use a rotary brush-type dc motor to drive the spools. The design results in a frequency response of about 90 Hz and enables the actuator to produce the high forces that EnduraTec needs for its testing systems.

HR Textron is one of several companies that offer servopneumatic systems or components. Other key players include Parker Hannifin and Festo Corp. (Hauppauge, NY), a German-based firm that was almost alone in its dogged pursuit of servopneumatic technology until recently.

Festo is currently the only company that offers complete, off-the-shelf systems, including controllers, cylinders, proportional valves, and feedback devices. Parker Hannifin, meanwhile, stepped into the servopneumatics market more recently, offering a technology that it claims will close position loops far faster than competing systems can. To accomplish that, Parker's valves use a sandwiched pressure transducer module that provides closed loop feedback regarding the state of its pressure ports. As a result, the company's systems provide closed loop feedback from two sources: the valve's pressure ports and the actuator's position sensors.

"Because we close the loop in two ways, we can correct our position faster and hold it more accurately," Caputo says. Parker Hannifin's technology is currently being employed by an Ohio-based manufacturer of robotic adhesive-application machines for the auto industry. The machines employ servopneumatics for precise positioning of adhesives around vehicle deck lids and trunk lids.

Festo engineers say they've also seen increased use of the technology in "composite systems" that mix servopneumatics with electric servo drives in multi-axis motion applications. By mixing the two, they say, engineers can get the best of both worlds for their machinery.

Whether servopneumatics technology will grab a larger slice of the giant motion control pie is as yet unknown. At least one maker of the technology gave up on it during the 1990's; Mosier Industries, Inc. (Brookville, OH) marketed Servopnuematics before dropping the technology in the late 1990s.

Still, suppliers say they now expect the technology's niche to grow, mainly as a result of its cost advantages. In many cases, they say, a servopneumatic system can fulfill the needs of an application for a cost of $2,000 to $2,500, whereas an electric servo drive system might cost twice that much.

"We tell people that 80% of applications don't need the precision of an electric drive," concludes Sean

Tomlinson, a product management specialist for Festo Corp. "Why should engineers pay extra cost for precision they don't need?"