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(2)
Servo Loop
(a)
The magnetic heading signal from the magnetic field sensor is applied to the stator of the magnetic field sensor control transformer (CT) (Sheet 1, Fig. 5). The signal is coupled from the stator to the rotor when the rotor is at an off-null position. The coupled signal (heading error) is then applied through a l:l gain isolation amplifier to a demodulator-filter-modulator and an annunciator amplifier in the compass coupler. The demodulator-filter-modulator demodulates the 800-Hz signal, filters it, and remodulates it at 800 Hz. The purpose of the demodulator-filter-modulator is to reduce the effect of oscillations in the incoming signal which occur from vibration of the magnetic field sensor.
(b)
The heading error signal is then applied through the heading error amplifier (Sheet 2, Fig. 5) to the frequency control and limiter, the polarity detector, and the error detector. The frequency control and limiter develops a negative dc voltage proportional to the magnetic error and limits the level to 1.2 volts. The output is applied through a limiting resistor to the voltage-to-frequency converter to be formed into a series of pulses. The frequency of the output pulses is proportional to the voltage level of the input signal. The output is applied to the stepper motor control, which develops signals for the stepper motor to control its stepping rate and direction. As the stepper motor steps, its shaft mechanically positions the rotor of the control differential transmitter (CDX).
(c)
The INS platform heading signal is applied through the stator of the CDX to the stator of the servo CT and the INS heading data monitor (Sheet 2, Fig. 5). The platform signal is coupled from the stator of the servo CT to the rotor, where it is then applied to the servo-amplifier and to the servo CT null and continuity monitor. The platform signal from the servo-amplifier drives the servomotor which mechanically positions a rate generator and the rotors of the servo CT, magnetic field sensor CT, and seven synchro transmitters. The output of the rate generator is applied to the servo-amplifier to stabilize the system. The servomotor will continue to drive until the rotor of the servo CT is at a null position. If the system is not aligned to the aircraft magnetic heading, the CDX will be driven by the stepper motor. The rate at which the CDX is driven in a normal synchronization mode is approximately 1 to 2 degrees per minute. When the system servo loops are at a null, the angular position of the synchro transmitters is representative of the magnetic heading of the airplane.
(3)
Annunciator Operation
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(a) In the SLAVED mode the annunciator (sheet 1, figure 5) on the compass controller indicates the error between the sensed magnetic field of the magnetic field sensor and the heading output of the system. This is accomplished by amplifying the 800-Hz heading error signal from the isolation amplifier, demodulating it with an 800-Hz square wave from the 800-Hz reference square wave generator, and applying the output of the annunciator demodulator to the annunciator. If the magnetic heading is less than the servo loop heading output, the annunciator will be deflected to the right side of the center index mark.
(4)
Polarity Detection
(a) The direction in which the stepper motor steps is determined by polarity detection circuits (Sheet 2, Fig. 5). In the SLAVED mode the phase of the 800-Hz heading error signal is compared with the phase of the output from the 800-Hz reference square wave generator. The output of the polarity detector is a dc voltage, the amplitude of which is dependent on the phase of the error signal. In the stepper motor control, the output pulses from the voltage-to-frequency converter are combined with the output of the polarity detector to energize the four windings of the stepper motor in pairs. The sequence in which the windings are energized determines the direction in which the stepper motor steps, and the frequency at which the windings energize determines the rate at which the stepper motor steps. Each time the stepper motor steps, the output synchros rotate 1.3 minutes.
(5)
Fast Synchronization
(a) The system is locked into fast synchronization for 7 seconds when system power is applied, when system power is interrupted for more than 2 minutes, or when switching from one platform heading signal input to another as controlled by the power off and heading switching sensor. Fast synchronization is in effect until the magnetic error is reduced to less than 2 degrees, at which time the system automatically switches to the normal slaving rate of 1 to 2 degrees per minute. The system also will go into the fast synchronization mode when it is switched to SLAVED from SYNC, provided the platform heading valid signal is present and the magnetic error is greater than 2 degrees. Fast sync is controlled by the fast sync control circuit (Sheet 1, Fig. 5). If the system is in the SLAVED mode and the magnetic error is greater than 2 degrees, the system will go into fast synchronization upon receipt of the platform heading valid signal. Fast synchronization does not occur when switching from SYNC to SLAVED if a valid signal is not present at the INS valid sensor.
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本文链接地址：747飞机维护手册AMM CHAPTER 34 - NAVIGATION 第34章导航1(121)
