(d) VOR Capture Submode 1) When the capture submode is initiated by LBS drop-out, the radio beam is engaged and used to control the roll axis. The modulated radio beam deviation signal is amplified by a gain programmer which is in the maximum gain condition at this time, and applied to summing point 6 past de-energized switch RS-14 of figure 6. Because there are no other signals present at summing point 6 at this time, the amplified radio beam deviation signal is fed, essentially unaltered, to an amplifier limiter. The amplifier limiter amplifies the signal and the amplified signal is applied to summing point 2.
2) Course error signals from the heading course error synchro in the CDI are fed through a bus synchronizer. The bus synchronizer rejects extraneous quadrature components of the heading course error signal and synchronizes the signal with the autopilot 400-cps reference. The output from the synchronizer is fed to summing point 12 past de-energized switches RS-19 and RS-20. Heading course error signals from summing point 12 are amplified and fed to summing point 2. At summing point 2, the heading course error signals are summed with radio beam deviation signals. Radio beam deviation signals alone would result in the generation of a bank command which would turn the airplane toward the selected VOR radial, while course error signals alone would result in a bank command which would turn the
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500 VOR Mode Switching Logic Diagram
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airplane toward the course selected on the CDI. The result of summing the radio beam deviation and heading course error signals is the production of a continuously decreasing bank command which will cause the airplane to approach the selected VOR radial in an exponentially decreasing turn. The signal resulting from the summation at summing point 2 is fed to summing point 7 through bank angle limiter which establishes a bank angle command limit of 25 degrees for this submode. The output signal from the bank angle limiter is summed with the resolver sine output signal at summing point 7. Because this signal is proportional to bank angle, the output from the bank angle limiter is established as a bank angle command. Output signals from summing point 7 are fed to summing point 8 through the bank rate limiter which establishes its inherent roll rate command limit of 4 degrees per second during this submode. Bank angle command signals are summed with tachometer feedback signals at summing point 8 and the resulting signals are applied to the motor amplifier. The motor amplifier drives the roll computer whenever there is an output signal from summing point 8. When the roll computer is driven, the CT develops an error signal. The CT error signal results in aileron displacement as described in roll attitude hold mode. This aileron displacement produces banked turns in the direction required to decrease the CT output toward a null. Unless the VOR mode is disengaged, the roll channel operates in the capture submode from the time the submode is initiated until the conditions are satisfied for the VOR on-course submode.
(e) VOR On-Course Submode
1) The roll control channel makes an automatic transition from the capture submode to the VOR on-course submode when the airplane bank is less than 6 degrees and the heading course error is less than 15 degrees (See figure 10.) Bank angles less than 6 degrees are sensed by the roll erection cutout switch in the vertical gyro switch supplies a signal to the roll channel interlock circuits. Heading course error is detected by the rate sensor portion of the over-station rate sensor which receives heading course error signals past switch RS-21, summing point 11, and an amplifier. (See figure 6.) When the heading course error signal decreases to a level corresponding to a heading error of less than 15 degrees, the rate/heading sensor supplies the interlock circuits with a signal which initiates the on-course submode when the bank angle is simultaneously less than 6 degrees. (See figure 10.)
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2) The on-course submode operates essentially the same as the capture submode except that radio beam deviation signals are applied to summing point 5 past deenergized switch RS-13 of figure 6. Heading course error signals are also applied to summing point 5 but past deenergized switch RS-24. The radio beam deviation and heading course error signals are summed and applied to the heading/beam integrator which is released from the reset state during this submode. The heading/beam integrator effectively increases the long-term gain of the radio beam deviation signals and provides long-term washout of the heading course error signals into summing point 2, which the heading/beam integrator feeds. The increase in long-term (low frequency) radio beam deviation gain provides for crosswind drift compensation with minimum standoff error. Output signals from the heading/beam integrator are summed with radio beam deviation and heading course error signals, and the resulting signal is applied to summing point 7 through the bank angle limiter. The bank angle limiter establishes a bank angle command limit of 8 degrees for this submode. Output signals from the limiter are summed with RS signals at summing point 7. The output signals from summing point 7, which will be present whenever the bank angle associated with the RS rotor position fails to correspond to the bank angle command, are fed to summing point 8 through the bank rate limiter. In this submode, bank rate limiter operates at the lowest limit level (normally 4 degrees per second); however, the roll rate command limit is reduced further to 1.3 degrees per second by the application of additional tachometer feedback to summing point 8 past deenergized switch RS-4. The tachometer feedback signals are summed with outputs from the bank rate limiter and signals resulting from the summation are applied to the motor amplifier. The roll computer will provide an output signal when it receives an error signal from summing point 7. When the roll computer is driven, the CT develops an error signal. This error signal results in aileron displacements as described in roll attitude hold mode. Aileron displacements produce banked turns in the direction required to reduce the output from summing point 2 and the CT error signal to a null. The roll channel continues to operate in the on-course submode until either the over-station submode is initiated or the VOR mode is disengaged.
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