Chapter 7—AUXILIARY MACHINERYmost commonly used when fine control is desiredis the servovalve (servocontrol). Servocontrol maybe defined as a control actuated by a feedbacksystem which compares the output signal with theinput or reference signal and makes correctionsto reduce the differences. The feedback signal maybe provided by fluid pressure, mechanical linkage,electrical signal, or a combination of the three.One type of hydraulic servovalve is illustratedin figure 7-4. The valve is controlled by twosolenoids through an amplifier which energizeseither the right or left solenoid, depending on theinput signal. The valve shown in figure 7-5 hasthe right solenoid energized; this causes the reedto block the right nozzle and causes a pressureincrease in chamber A. The increased pressurecauses the spool valve to start sliding to the left.As the spool valve moves, it uncovers the highpressure line to chamber D (right side of thepiston) and the return line from the left side ofthe piston through chamber E (nonpressure sideof the hydraulic system). The synchromotor isgeared to the actuator shaft. As the actuatormoves to the left, the synchromotor rotates andproduces a feedback signal to the amplifier. Whenthe feedback signal and the input signal arematched, the solenoid is deenergized and themagnetic reed returns to the neutral position. WithFigure 7-5.—Servovalve—actuated.7-21
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