LDM #281: Sperry Compass Computer MK9: teardown, reverse engineering and test

00:00 - Intro 00:47 - Electronic boards 02:59 - Electromechanical computing assembly 05:09 - How it works - Block diagram Detailed schematic diagrams with explanations: 09:35 - Slaving module 11:33 - 800Hz reference generator 15:17 - Amplifier module 17:16 - Gain Control Module 22:49 - Stepper Motor Control Board Test: 24:18 - Test Setup 25:14 - High Speed Mode - Gyro Heading change 26:20 - Flux Detector Heading Change 28:50 - Normal Mode Description from reverse engineering: This compass computer permits to correct the drift of a directional gyro by slaving at long term the output to a flux detector. The heading of the directional gyro is first regenerated inside the instrument using a classic servo control. A stepper motor permits to correct the gyro heading using a differential gear. The flux detector is connected to the stator of a transolver. One winding of the rotor is fed to a filter. The signal at 800Hz is synchronously rectified to give a DC error voltage. This error voltage is fed to an integrator whose output is fed to two comparators which deliver up and down pulses for the stepper motor. The output of the integrator is therefore at any time between these two thresholds, permitting a quasi-null voltage at the transolver output. This servo control loop is very slow, approximately 1°/min correction rate. A digital input signal permits a fast synchronization to the flux detector. The second winding of the rotor of the transolver is used for the measurement of the horizontal component of the earth magnetic field. As the first winding is used for the servo control its voltage is always closed to zero, therefore the voltage present across the second winding is maximum and proportional to the horizontal component of the magnetic field. This voltage is rectified and is used to increase the speed of the stepper motor when the horizontal component of the magnetic field is low. This avoids a too slow correction when the aircraft flies at high latitudes where the horizontal component of the earth magnetic field is low. A galvanically isolated AC input signal permits to inhibit the flux detector using a relay which can make a short-circuit to the rotor of the transolver. The threshold is adjustable internally. Maybe this input should be connected to the vertical gyro in order to inhibit the correction when the pitch or roll is too high, for which the flux detector doesn’t work properly.