Servopneumatic valve offers high frequency response, low cost

Santa Clarita, CA--It's a classic engineering trade-off: Servopneumatics can push the performance of air-based systems beyond the "bang-bang" realm, but at a cost. And when the cost rises too high, the traditional value of pneumatics is lost.

Now, however, engineers from HR Textron say that users of pneumatics can have it both ways. Adapting an off-the-shelf brush-type motor and using a "rotary-to-linear" concept, they've created a low cost pneumatic servovalve that operates at much higher frequencies than comparably priced designs. The system, known as the Model 27N Rotary Direct Drive Servovalve, achieves an operating frequency of 90 Hz.

Up to now, servopneumatic valves have typically operated in the 20-30 Hz range. The reason: Most use linear solenoids or voice coil motors to drive the valve's spool. As a result, the force available to drive the spool is low, resulting in low frequency response.

The new system solves that problem, however, by using the rotary brush-type dc motor. The rotary motor yields the torque values and forces needed to drive the spool, and accomplishes it at a cost of just a few dollars per motor. That, in turn, enables Textron engineers to achieve the desired frequency response without high cost.

To obtain those advantages, however, Textron engineers first needed to deal with the inherent drawbacks of a brush-type motor design. Primary among those was the wound rotor configuration. Because all brush-type motors have a wound rotor, instead of a wound stator, they can't be easily connected to a controller. "The greatest challenge we faced was creating a reliable connection between the moving rotor and the fixed circuit board," notes Kim Coakley, engineering manager for HR Textron. "Since we were looking for a minimum life of 30 million cycles, just soldering wires to it would not work. They would simply break off."

Textron engineers considered solving the problem by switching to a brushless DC motor, but after studying the market, they found that brushless DC was far too expensive. "The brushless motors we looked at cost more than this entire valve," notes Bill Ratliff, the HR Textron project engineer credited with much of the development work on the Model 27N.

Ultimately, Textron engineers purchased a brush-type rotor and stator from a motor manufacturer, then employed their own control and connection techniques. To solve the connection problem, Ratliff used a flex circuit. The flex circuit, which consists of electrically conductive traces sandwiched between two layers of polyimide, slides over the top of the shaft, and connects the motor to the control board at the top of the valve. A special flex circuit retainer made from DuPont Delrin forces the contact pads onto the flex circuit. The retainer also holds the flex circuit connection in place when the motor operates.

During operation, the unit doesn't turn 360 degrees like a conventional rotary motor. Instead it rotates back and forth, moving just plus or minus 30 degrees. In that way, it imparts movement to the spool, which opens and closes the valve.

To accomplish that, however, the rotary motion of the motor must first be converted to the linear motion. Engineers achieved that by laser welding an eccentric element to the bottom of the motor shaft.

The resulting design provides higher frequency performance, which better enables the valve to deal with the issue of air's compressibility. That's important, because the non-linear behavior of air traditionally has been a problem for servovalves. "If you're going to compensate for the compressibility of air, you need quicker response," Coakley says. "The speed of this valve allows you to take advantage of more advanced algorithms and controllers."

By using a brush-type motor design, Textron engineers also kept the sales price of the valve to approximately $500. That makes it much less costly than hydraulic and electro-mechanical servo systems, while offering higher frequency response than other servopneumatic valves. "If you drive the price up too high, users will just choose a different method," Ratliff says. "But by keeping the cost of this valve down, pneumatics becomes a more viable option."

Additional details...Contact Kim Coakley, HR Textron, 25200 W. Canyon Rye Rd., Santa Clarita, CA; Tel: (661)702-5695; Fax: (661)702-5624; E-mail: kcoakley@ hrtextron.textron.com.