Some actuators must move at a precise speed, stop at a close-tolerance position, or produce a very accurate force to perform the work for which they were designed. With the proper input signals and feedback devices, proportional or servovalves can make an actuator perform any or all these functions flawlessly.
Rolling mills turn out sheet consistently to a tolerance of ±0.0005 in. at sheet speeds of 2000 to 5000 feet per minute. Hydraulic cylinders controlled by servovalves maintain the proper force and position the rolls precisely from feedback signals sent by sensors that measure metal thickness, cylinder force, and position. Airline pilots train in simulators moved by hydraulic cylinders so precisely that the pilots get the feel of landing gear raising and locking in position. Even entertainment rides use servovalves to make passengers think they are in 20-ft waves when they are actually in an enclosed articulated room in a shopping mall.
For less precise movement, there are proportional valves that mimic the output of servovalves but respond more slowly. They are less expensive than servovalves and more contamination tolerant, so they have replaced cam valves and other mechanical devices used to get smooth motions.
Hydraulic proportional directional control valvesThe symbol and cutaway view in Figure 12-1 represent a direct-acting proportional valve that handles flows as high as 10 to 30 gpm in D03- or D05-size valve interfaces. Proportional valves use the same interface standards as NFPA and ISO directional valves so they can be installed in a circuit without having to change the piping.

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