Remote Indicating Magnetic Compass — Page 220, Lesson 259

Right, so we’ve got the flux valve detecting the Earth’s magnetic field, and we’ve got the gyro holding a heading. Now I want to walk you through how the system actually compares those two and corrects the gyro when they disagree. That’s the error signal comparison, and it’s done by something called rotor-stator comparison. Here’s the setup. On the gyro drive shaft, there’s a wound coil, and that coil is called the rotor. Around it, we have the stators, which generate an AC field. Now, the key principle is this: if the rotor coil is in line with that AC field generated by the stators, a secondary AC voltage is induced in the rotor. But if the rotor is at 90° to the AC field, no secondary voltage is induced at all. That position, where nothing is induced, is called the null position. And at any position other than the null, some secondary voltage is induced. So think about what that means. When the gyro shaft is perfectly aligned with the magnetic heading detected by the flux valve, the rotor sits at the null position, and there’s no error signal. The moment the gyro drifts or the heading changes, the rotor moves away from the null, and a secondary voltage is induced. That induced voltage is the error signal, and it gets passed to the precession amplifier. Now, in that amplifier, three things happen to that signal, and each one matters. First, it’s amplified. Why? Because the un-amplified error signal is not powerful enough to drive the precession motor. It’s too weak on its own. Second, it’s phase detected. This is the clever bit, and I want you to really get this. It’s important that the precession motor knows which way to turn. Suppose the gyro shaft is misaligned 2° clockwise from the null. The motor should rotate the shaft 2° anticlockwise to correct it. It should not rotate all the way round 358° clockwise to get back. If it did that, the system would go into continuous rotation, spinning forever. So the purpose of phase detection is to detect the sense of the error — in other words, which direction the error is in. Third, it’s rectified. Here’s why. The mechanism of the precession motor is an electromagnetic solenoid acting on a permanent magnet. And that requires DC. So the amplified AC is rectified to DC. Now, depending on the phase that was detected, that DC will either be in a positive or a negative direction. And that positive or negative direction is what turns the shaft either clockwise or anticlockwise — always taking the shortest route for error correction. So to pull that whole chain together: rotor detects the error, the signal is amplified because it’s too weak, phase detected so the motor knows which way to turn, and rectified to DC because the solenoid motor needs DC. And the result is the gyro is precessed back into alignment with the magnetic heading. Now, how do we actually read the heading? That brings us to the heading indicator. The heading indicator dial — which is the compass card — is directly driven by the shaft from the gyro. So as the gyro shaft turns, the compass card turns with it. The compass card rotates as heading changes, and you read the heading against the index line at the 12 o’clock position. That index line is called the lubber line. And on a typical heading indicator, you’ll also see a heading warning flag, which tells you the system isn’t reliable, and a heading selector with a heading bug, which you use to set a desired heading. But the reading itself — the actual heading — is where the compass card meets the lubber line at the top. So the whole loop is: flux valve senses the magnetic field, the rotor-stator comparison generates an error signal if the gyro disagrees, that signal gets amplified, phase detected, and rectified, the precession motor turns the gyro the shortest way back to the null, and the compass card, driven straight off the gyro shaft, shows you the corrected heading against the lubber line.