High-performance valves are usually classified as either servo or proportional, a distinction that gives an indication of expected performance. Unfortunately, this classification tends to generalize and blur the true differences between various valve styles. Selection depends on the application, and each valve has merit when it comes to controlling pressure or flow.to designate a proportional valve schematically, the familiar digital directional control valve symbol in the illustration above  is augmented with two straight exterior lines outside the valve envelope that run parallel to the valves long axis. these additional lines indicate that the spool has the ability to assume and maintain any intermediate position over the full range of spool travel.

Traditionally, the term servovalve describes valves that use closed-loop control. They monitor and feed back the main-stage spool position to a pilot stage or driver either mechanically or electronically. Proportional valves, on the other hand, move the main-stage spool in direct proportion to a command signal, but they usually do not have any means of automatic error correction (feedback) within the valve.

Confusion often arises when a valve's construction resembles a proportional valve, but the presence of a spool position feedback sensor (usually an LVDT) boosts its performance to that rivalling a servovalve. This reinforces the concept that designers and suppliers should use common terminology and focus on the performance reuirements of the particular application at hand.

Typically, proportional valves use one or two proportional solenoids to move the spool by driving it against a set of balanced springs. The resultant spool displacement is proportional to the current driving the solenoids. The springs also center the main stage spool. Repeatability of the main-stage spool position is a function of the springs' symmetry and ability of the design to minimize nonlinear effects of spring hysteresis, friction, and machining tolerance variations.

Servovalves

The term servovalve traditionally leads engineers to think of mechanical feedback valves, where a spring element (feedback wire) connects a torque motor to the main-stage spool. Spool displacement causes the wire to impart a torque onto the pilot-stage motor. The spool will hold position when torque from the feedback wire's deflection equals the torque from an electromagnetic field induced by the current through the motor coil.fig. 1. first-stage configurations for nozzle flapper and jet-pipe valves. These two-stage valves contain a pilot stage or torque motor, and a main or second stage. Sometimes the main stage is referred to as the power stage. These valves can be separated primarily into two types, nozzle flapper and jet pipe, Figure 1.

The electromagnetic circuit of a nozzle flapper or jet-pipe torque motor is essentially the same. The differences between the two lie in the hydraulic bridge design. A hydraulic bridge controls the pilot flow which, in turn, controls the main-stage spool movement. In a nozzle flapper, the torque produced on the armature by the magnetic field moves the flapper toward either nozzle depending on command-signal polarity. Flapper displacement induces a pressure imbalance on the spool ends which moves the spool. In a jet pipe, the armature movement deflects the jet pipe and asymmetrically imparts fluid between the spool ends through the jet receiver. This pressure imbalance remains until the feedback wire returns the jet pipe or flapper to neutral.

Historically, jet pipe and nozzle-flapper servovalves have competed for similar applications that require high dynamics. Typically, better first-stage dynamics gives the nozzle flapper better overall response, whereas improved pressure recovery of the jet/receiver bridge design gives the jet-pipe motors higher spool driving forces (chip-shearing capability). Both valves require low command currents and therefore offer a large mechanical advantage. Motor current for these style valves is typically less than 50.0 mA. Note that these servovalves are also proportional valves, because spool displacement and flow are directly proportional to the input command.