Servo valve integration aided by brushless dc positioning

Mark Tandy -- Control Engineering, 9/1/1998

Some folks just can't leave servo valves alone. The latest approach combines electric servo motors' controlability with fluid power's muscle to create reliable, high-performance, proportional servo valves. Recent advances in magnetic materials, power electronics, and microprocessors are used to develop directly driven spool servo valves for numerous automation applications. Further combining electronics with the proportional servo valve gives users integrated motion control capability.

Not stopping there, HR Textron (Valencia, Calif.) recently introduced a series of servo valves incorporating dc brushless motors for spool positioning. In this series of rotary direct-drive valves—called R-DDV Servovalves—the spool is electrically positioned by the rotary action of a brushless dc (bldc) motor. Configured for accurate, limited-angle operation, the bldc motor incorporates a Hall effect device for internal closed-loop control and an "eccentric" on the output shaft (see illustration) for linear spool displacement. For overall system control, a precision feedback device is used. This differs from previous direct-drive servo valves, which typically use proportional solenoids as the drive mechanism.

Integrated electronics provide motor control, optimizing performance and linearity. The entire R-DDV Servovalve system achieves bandwidths surpassing current servo and proportional valve capabilities. Originally designed for U.S. Navy aircraft, HR Textron's servo valves now serve in other hydraulic and pneumatic system applications, including 6,000 R-DDV Servovalves operating in industrial settings.

Some advantages of this approach to proportional motion control in hydraulics and pneumatic systems include:

• High tolerance of system contamination;
• High bandwidth (up to 200 Hz);
• Reduced sensitivity to temperature, shock (60 g), and vibration (40 g);
• Near elimination of internal leakage; and
• Full performance independent of supply or differential pressure, or pressure variations.

This parade of improvements began in the 1950s when the electro-hydraulic servo valve (EHSV) was conceived to meet the U.S. aerospace program's need for a precise, hydraulic, modulating, four-way flow control valve. EHSV pioneers considered using an electromechanical device to position the valve spool, but were limited by available electric motor and control technology. They rejected this bulky method in favor of a device using hydraulic pilot pressure to position the spool. This is how the two-stage servo valve was born and evolved into several first-stage variations for spool positioning. One example is the dual-nozzle-flapper (see diagram).

In two-stage servo valve operation flow output is a function of spool position. The spool is moved by varying pressure in chambers at either end; movement continues until the displacement recenters the nozzles by means of a mechanical feedback spring that connects the spool to the flapper. This spring makes the servo valve a closed-loop device. Differential pressure is produced by varying current in the torque motor. This adjusts the flappers' position relative to a pair of nozzles.

Evolution of servo valves for industrial applications had to account for the different factory environment. New servo valve designs were required to eliminate the so-called "first-stage" pilot device with its small orifices, filtering, and complex assembly. The resulting "direct drive" initially used proportional solenoids for spool positioning, which increased reliability. The most recent designs use lower cost electric motor technologies for added performance and reliability.

For instance, HR Textron claims enhanced power efficiency for its R-DDV Servovalves and frequency response that exceeds comparable direct-drive and two-stage servo valves by 25-50% or more—independent of operating pressure—depending on valve size. R-DDV Servovalve construction and assembly is simpler, which HR Textron says further improves reliability for the end-user. Lab tests have exceeded 100 million cycles.

Because servo valves were developed for both hydraulic and pneumatic systems in low- and high-flow versions, they're typically used in wood processing and paper mills, robotic machining, metal forming, material handling, motion simulation, off-road vehicles, industrial turbine engines, and automatic braking systems. HR Textron expects use of these integrated motion control technologies to increase as industrial and other markets demand increasingly faster, more compact, electronically controlled solutions.

For more information, visit www.controleng.com/info.