We left off with the pendulum sitting on its null position — Page 226, Lesson 272

I want to walk you through the heart of how an accelerometer actually works, because this is the sensor that tells the inertial navigation system how it's being pushed around. We left off with the pendulum sitting on its null position. Now, the key idea is that we need a device that tells us how far that pendulum has been deflected off null. That's the pick-off device. It senses the displacement of the pendulum from its centered, null position and produces a signal proportional to that displacement. That signal goes to an amplifier. The amplifier boosts it, and the amplified current is then fed back into a torque motor that's built right into the accelerometer. This torque motor generates a torque that pushes the pendulum back toward the null position. So we have a closed loop: the pendulum moves off null, the pick-off senses it, the amplifier drives the torquer, and the torquer restores the pendulum to null. Here's the crucial relationship: the amount of current flowing into the torquer is a function of the acceleration the device is experiencing. Think about that. The harder the platform is accelerated, the more the pendulum wants to swing off null, the more current the torquer needs to hold it back. So by measuring that torquer current, we directly measure the acceleration. That's the whole trick of the force-balance accelerometer. Now, how do these get installed? The accelerometers are mounted on a platform, and there are two of them for horizontal sensing. One is 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 orthogonal axes covered: north-south, east-west, and up-down. Let me make sure the loop is crystal clear, because this is the operating principle you'll be examined on. The pendulum is the sensing element. The pick-off device measures its deflection from null. The amplifier takes that small signal and makes it strong enough to drive the torque motor. The torque motor applies a restoring torque to bring the pendulum back to null. And the current in that torquer is your acceleration readout. Every part of that chain matters, and the current-to-acceleration relationship is the output you'll use downstream for integration into velocity and position.