In the DFplus valve, the movable coil connects to the spool. Energizing the coil moves the spool to a position determined by direction of current flow. Actual position is sent over a high-resolution feedback loop to the control electronics. When the signal switches off or in a power failure, a spring drives the valve to a defined position. This eliminates the active return spring and several bearings in the actuators.

The DFplus proportional directional valves from Parker Hannifin Corp., Cleveland, Ohio (, use voice-coil drive technology to give engineers performance they could once only get with more-expensive servovalves. In a nutshell, energizing a permanent-magnet cylinder drives a movable coil which, in turn, actuates the spool. As a result, the valves reach 400 Hz across their entire operating range and generate over 100 N of force. The valves will also operate without performance limits up to 350 bar. The spool mechanically connects to the drive for the best possible transmission of forces. The valves operate bidirectionally and use spool feedback to ensure precise, reproducible flow rates. All control electronics are contained in the valve. Typical applications where the Dfplus proportional valve can replace servovalves include fatigue testing, simulation, and hexapode platforms for automotive testing.