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# Ch. 11b - Chapter 11 SERVO VALVES(contd Fluid Power...

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1 Chapter 11 SERVO VALVES (contd.) Fluid Power Circuits and Controls, John S.Cundiff, 2001 Servo Analysis The concept of gain has preciously been defined G = Output / Input Feedback principle : We sense the output, the error between input and feedback loop drives the system to the desired zero error condition, thus feedback helps control the system.

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2 Servo Analysis Feedback signal will be opposite in sign to the input signal. When correcting for drift in the output, we must move it back toward the set point. This correction is called negative feedback. Closed loop hydraulic systems are also called servo systems. Servo Analysis Feedback signal is typically a scaled DC voltage, which is proportional to the output signal. If the feedback signal is an AC sine wave it must be shifted in phase by 180 o from the input. When the amplitudes are equal, the two signals cancel each other, and the resulting error is zero.
3 Servo Analysis Block diagram of closed- loop system for servo cylinder. The servo valve transfer function is the flow transfer function, not the pressure transfer function. Input to the cylinder is a flow, in 3 /s, and the output is a linear velocity, in/s. Servo Analysis Transfer function is given by , G cyl = output = in/s = in = 1 = 1 input in 3 /s in 3 in 2 A where A = cylinder area (in 2 ). A typical feedback transducer is the potentiometer. It’s transfer function is V/in. A linear velocity (in/s) drives the potentiometer to produce the feedback signal (V), not V/s.

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