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# Ch. 11a - Chapter 11 SERVO VALVES Fluid Power Circuits and...

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1 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.

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2 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.
3 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 fc , adds to other pressure drop so that the relief valve pressure is: P tot = P DCV + P cyl + P fc

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4 Pressure compensation in the flow control valve maintains a constant P fc ,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 decreases due to increased internal leakage. Leakage between pressure line and the return line in DCV increases. Pump and valve leakage reduces cylinder speed. The DCV and flow control valve can be replaced with a servo valve while keeping the rest of circuit the same except for an added pressure line filter.
5 Total pressure at the relief valve for a given load with filter is P tot1 = P sv1 + P cyl1 + P f Assume the corresponding flow across the relief valve is Q r1 . If cylinder load increases, pressure drop across cylinder becomes P cyl2. Total pressure at relief valve becomes P tot2 = P sv1 + P cyl2 + P f Flow across relief valve increases to Q r2 . Thus motor slows down. If pressure drop across servo valve changed to P tot2 = P sv2 + P cyl2 + P f = P tot1

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6 Flow across relief valve returns to Q r1 , and the speed of cylinder is back to it’s original value. In many applications, a servo valve is just a means of programming an orifice in the circuit. Programmable Orifice Servo valve with line-to-line lap condition. Similar to 4-way DCV, the valve has four ports: pressure, return, Port A, and Port B. When spool is shifted, two orifices are created.