Figure 21.11 is a simplified cutaway view of a stepper-motor-driven servovalve controlling a hydraulic motor. As it receives current pulses, the stepper motor turns in increments of a revolution. Stepper motors may require anywhere from 100 to 500 pulses per revolution. A stepper-motor drive is reliable and repeatable, and produces high torque.

Figure 21.11











This type servovalve is more dirt tolerant than other designs, does not require specific electronics, does not need feedback transducers, and is easy to troubleshoot. It may be used as a stand-alone valve for acceleration and/or deceleration circuits or to control flow -- with or without feedback. Like other servovalves, it has little or no land overlap. It has a precisely fitted spool to reduce leakage. There are no control orifices to plug, so fluid cleanliness is not as important as with a standard servovalve.

Feedback to a stepper-motor-driven servovalve is mechanical and internal, similar to the rudder control in Figures 21.4, 21.5, and 21.6. This means that when the motor meets resistance it cannot overcome, it will stall. When the motor stalls, there is no external feedback to show it has not made its predetermined position. Adding a limit switch or other external means helps this problem, but now the circuit resembles a standard on/off solenoid-valve setup.

The response of a stepper-motor driven servovalve is a little better than the best proportional valves, but not equal to top-of-the-line servovalves.

In the cutaway view, a stepper motor drives a threaded shaft in a threaded spool. The spool can move in and out, but it cannot rotate unless feedback from the rotating hydraulic motor turns it. Electric pulses to the stepper motor turn the screw in the spool, making the spool shift. Spool shift ports fluid to the hydraulic motor, making it turn. When the hydraulic motor starts to rotate, it turns the spool. The mechanical linkage turns the spool in reverse of the stepper motor, shifting the spool to stop hydraulic motor rotation. When the stepper motor turns, the hydraulic motor rotates. The faster the stepper motor receives pulses, the faster the hydraulic motor turns. When the stepper motor stops turning and shifting the spool, the hydraulic motor continues to rotate until it brings the spool back to its center position. Reversing rotation of the stepper motor reverses all the actions above -- including hydraulic motor's rotation direction.

From the above explanation it is obvious that pulsing the stepper motor a certain number of times at a given rate, turns the hydraulic motor a certain number of revolutions at a preset speed. If external forces try to move the hydraulic motor from its off position, spool shift — caused by feedback rotation — ports oil to offset these forces.