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 (e) Feedback Systems Operation
 1)  Most of the servo-mechanism systems in the MEC have feedback systems to a comparative measurement device to null out the initial signal commands. Accurate operation of these feedback systems is necessary for the successful operation of the MEC. This operational description is directed to only the 2 feedback systems which have external components (VSV and VBV feedback cables). a) VSV Feedback System (Fig. 9)
 1.  The VSV feedback cable is a flexible cable which
_ connects the control VSV feedback system lever to the VSV actuator bellcrank at the VSV actuator on the left side of the engine core. Movement from this cable transmits the angular position of the VSVs to the MEC while the VSV actuator sets them. The position signal is compared through measurement linkage in the MEC with the position command transmitted by the 3D cam. When the position location and command are at the same time, the position signal (error signal) to the VSV pilot valve is discontinued (nulled) and the VSV actuators stop VSV movement.
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2. It is essential that an accurate transmission of
_ the stator vane position to the MEC be made for reliable operation of the engine. The accurate correlation between adjustments (rigging) of the cable for specified vane position and the MEC identification is very important. A tension load is applied to the VSV feedback cable during engine operation by a hydraulic load cylinder in the control that is coupled to the feedback linkage. This tension load must be simulated during adjustment rigging of the VSV feedback cable to the MEC (AMM 75-31-02/501).
 b) VBV Feedback System (Fig. 9)
 1.
The VBV feedback cable is a flexible cable which

_ connects the MEC VBV feedback system lever to the VBV actuation system at the master valve at the 9 o'clock position. VBV feedback cable movement transmits the position of the VBV's to the MEC while the VBV hydraulic motor and drive system operate them. The position signal is compared in the MEC through a comparative linkage system to the position command established in the MEC through linkage with the VSV feedback system. The VBV control valve discontinues operation of the hydraulic motor when the VBV feedback signal nulls the command input.

 2.
The VBV feedback system differs from the VSV

_ feedback system in that the feedback and command error measurement linkage system has a provision for repositioning the feedback linkage null position as directed by the VBV reset piston. This provides for actuation of the VBV's by other commands. Three commands to the VBV reset piston may do this: a command by the VBV reset valve when a rapid deceleration change in VSV positioning is sensed by the VBV reset valve, a command from the reverse signal valve when the reverser is actuated, and a command by the altitude bias valve.

 3.
Like the VSV feedback, it is important to

_ accurately adjust (rig) the VBV feedback cable to the MEC to assure the correct feedback inputs to the MEC. Also, like the VSV feedback system, the VBV feedback cable is maintained in tension during engine operation by a hydraulic load cylinder in the control that is coupled to the VBV feedback linkage. This tension load must be simulated during adjustment rigging of the feedback cable to the MEC (AMM 75-32-05/501).
 C. PMC (Fig. 10)
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CONTROL

A

REF WM 73-24-01, 73-24-11 Power Management Control Simplified Block Diagram Figure 10
 T81999
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