Inside the accelerometer, we have a pendulum — Page 226, Lesson 272

Let me walk you through how an accelerometer actually works, because this is the heart of inertial navigation. Inside the accelerometer, we have a pendulum. When the aircraft accelerates, that pendulum swings away from its neutral position. Now, the key part is a pick-off device — and this device tells us how far the pendulum is off the null position. The null position is simply the pendulum's rest position, where it sits when there's no acceleration acting on it. Here's the clever part. The signal from this pick-off device is sent to an amplifier. Then, current from that amplifier is sent back into a torque motor, which is located inside the accelerometer itself. That torque motor generates a torque — a twisting force — which restores the pendulum back to the null position. So now we have a closed loop: the pendulum moves, the pick-off senses it, the amplifier sends current, the torque motor pushes the pendulum back to center. And here's the crucial relationship: the amount of current going into the torquer is a function of the acceleration the device is experiencing. In other words, the harder the acceleration, the more current is needed to hold the pendulum at null. So by measuring that current, we know the acceleration. Now, where do these accelerometers live? They're mounted on a platform. And there are two of them — one aligned in the north-south direction, and the other in the east-west direction. Often a third accelerometer is fitted to measure vertical acceleration. So you have three axes covered: north-south, east-west, and vertical. Once we have that acceleration, we integrate it — that's where the integrators come in. Integration of acceleration gives us velocity, and integrating velocity gives us distance. That's how the system builds up a picture of where the aircraft has moved from its starting point. Let me show you the arrangement — shows the direct drive shaft, and is the compass control panel you'd see in the cockpit. So to summarise the chain: acceleration moves the pendulum, the pick-off measures the displacement from null, the amplifier drives the torque motor to restore null, and the current in the torquer tells us the acceleration. Two accelerometers on the platform — north-south and east-west — with a third for vertical, and integrators turn that acceleration into velocity and then distance.