Remote Indicating Magnetic Compass — Page 220, Lesson 259

Let's pick up right where the error signal gets turned into action. We've got the flux valve sensing the magnetic field, and the gyro holding a heading. Now, the whole point of this system is to compare those two and correct any difference. That comparison is done by something called rotor-stator comparison. Here's the setup. On the gyro's drive shaft, there's a wound coil. That coil is called the rotor. Around it, there are stators generating an alternating current, an AC, magnetic field. Now, here's the clever bit. If that rotor coil is perfectly in line with the AC field from the stators, a secondary AC voltage is induced in the rotor. But if the rotor is at 90 degrees to that field, no voltage is induced at all. That position, where nothing is induced, is called the null position. At any other angle, some secondary voltage gets induced. That induced voltage is our error signal. So, the size of that voltage tells us how far off we are, but that's not enough. That raw signal is too weak to drive the precession motor, so it gets amplified. But even amplified, it's not enough. The motor needs to know which way to turn. Imagine the gyro shaft is misaligned 2 degrees clockwise from the null. We want it to rotate 2 degrees anticlockwise to correct it, not spin all the way around 358 degrees clockwise. If it did that, the system would just go into continuous rotation. So the signal is phase detected — that's what tells the system the sense, or direction, of the error. Finally, the signal is rectified. The precession motor is an electromagnetic solenoid acting on a permanent magnet, and that mechanism needs direct current, DC. So the amplified AC is rectified to DC. Depending on the phase that was detected, that DC will be either positive or negative, which turns the shaft either clockwise or anticlockwise — always taking the shortest route to correct the error. Now, once the gyro is aligned and corrected, we need to read the heading. That's where the heading indicator comes in. The compass card, the dial you read, is directly driven by the shaft from the gyro. As the heading changes, the card rotates, and you read the heading against the lubber line — that's the index line at the 12 o'clock position. On a typical heading indicator, you'll also see the heading warning flag, the heading selector, and the heading bug, all working together on that instrument face.