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Unformatted text preview: 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. 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. Its transfer function is V/in. A linear velocity (in/s) drives the potentiometer to produce the feedback signal (V), not V/s. 4 Servo Analysis OpenLoop Gain defined by 11.23) ...(Eq .......... 1/s (V/in) ) (in/in ) /s)/mA ((in ) / ( H G G G GH 3 3 cyl svf a = = = V mA Servo Analysis Open loop gain is also referred to as the velocity constant. 11.24) Eq .........( H G G G GH k cyl svf a v = = 5 Servo Analysis Natural Frequency The analysis required to obtain the natural frequency of a servo cylinder connected to a load will be calculated. Model for this system shown in Fig 11.36 Servo Analysis 6 Servo Analysis The mass is held in position with a spring which suggests that the load has mass and elasticity. Stiffness describes the force required to produce a unit deflection. Compliance is the reciprocal of stiffness, thus the units are deflection per unit force, or in/lb f Servo Analysis The system (Fig 11.36) can be visualized as a mass held in position by a column of fluid. If compliance of this fluid is o , and the compliance of rod is m , then the total cylinder compliance is = o + m Generally, m << o , so the compliance of the oil is used as the cylinder compliance with negligible error. 7 Servo Analysis Hydraulic oil compresses when pressure is applied....
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This note was uploaded on 11/22/2011 for the course ABE 5152 taught by Professor Burks during the Fall '08 term at University of Florida.
 Fall '08
 Burks

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