SERVO VALVES University of Florida（伺服阀佛罗里达大学）.pdfVIP

Chapter 11 SERVO VALVES Fluid Power Circuits and Controls, John S.Cundiff, 2001 Servo valves were developed to facilitate the adjustment of fluid flow based on the changes in the load motion. 1 Typical Open-Loop System Goal is to maintain constant speed as the magnitude of the load changes. As load increases Torque on hydraulic motor increases, pressure then increases and leakage in the motor increases. Pressure increase is immediately “seen” at the pump and where leakage increases and flow decreases for each revolution. Increased pressure causes pump input torque to increase, which causes the electric motor to slow down reducing the pump output even more, and thus reduces motor speed even more. 2 Closed loop system Speed feedback signal causes the pump displacement to increase until motor speed equals desired speed. Within operating range, a load increase does not reduce motor speed. Meter-out circuit. When extending, cylinder speed is controlled by flow from rod-end through control valve. Pressure drop ∆P , adds to fc other pressure drop so that the relief valve pressure is: ∆P = ∆P + ∆P + ∆P tot DCV cyl fc 3 Pressure compensation in the flow control valve maintains a constant ∆Pfc ,by adjusting the size of the orifice as flow changes. The flow control valve is also temperature compensated, adjusting the orifice based on fluid temperature. As load pressure goes up, pump flow dec