A pressure-regulating servovalve (also called a pressure servoregulator) is a device that converts an electrical input signal, usually a control current, into a hydraulic output signal in the form of a servo-controlled pressure in an output circuit.
One of the widest applications of pressure-regulating servovalves in aviation lies in controlling the pressure in aircraft hydraulic brake circuits. Three-way pressure-regulating servovalves are used in which a slider connects an output port alternately to a feed port or to a return port according to whether the pressure at the output port (also called the output pressure) is below or above a required pressure. The slider is moved by the pressures in two pilot chambers fed with pressurized fluid from a pilot unit and acting on the slider in opposite directions and by the output pressure which feeds an output chamber connected to the output port. The pilot unit is an electro-hydraulic amplifier comprising an electric torque motor driving a moving mechanical member in proportion to the torque motor control current. The mechanical member is often a vane disposed between two nozzles facing in opposite directions, but it can comprise any other system which by moving creates two pressures such that the difference between them is proportional to the torque motor control current. The resulting pressures constitute the pressures in each of the pilot chambers of the slider and are called the pilot pressures. To regulate the pressure to the required value (called the servo-control pressure) the torque motor control current is varied to vary the slider pilot pressures, the resultant force of which opposes the resultant force of the output pressure. If the pressure in the output chamber is too low the slider moves to connect the output port to the feed port in order to increase the output pressure; if the output pressure is too high the slider moves to connect the output port to the return port, so that the-output pressure falls. The slider returns to an equilibrium position, i.e. isolates the output port, as soon as the output pressure reaches the servo-control pressure.
In such pressure-regulating servovalves the output pressure is applied directly to the slider. The slider responds immediately to any sudden changes in this pressure, with the result that the response time of the system is good but pressure control can be unstable.
The distance of the servovalve from the unit whose pressure it regulates requires a large volume of fluid in the pipes, leading to resonant frequencies which are low, sometimes as low as a few Hz. It is then necessary to increase the inherent damping properties of the servovalve without compromising its response time.
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