Inertial Navigation Systems — Page 235, Lesson 275

Right, let's pick this up with the accelerometer and the problem of gravity. We've got the accelerometer sitting on the gyro-stabilized platform, but what if it were hard-mounted directly to the aircraft structure? That's where the trouble starts. The issue is that the aircraft pitches and rolls. If the accelerometer is tilted along with the airframe, the pendulum inside it swings away from its null position. And what makes it swing? Gravity. The pendulum is a mass hanging in a fluid, so when the case tilts, gravity pulls that mass away from the centre. That movement is what the device senses as acceleration. So here's the cause-and-effect chain I want you to hold onto. The tilt causes the pendulum to move. That movement produces an erroneous acceleration signal. That false acceleration is then integrated to give an erroneous velocity, and that erroneous velocity is integrated again to give an erroneous distance travelled. So a simple pitch or roll angle creates a false acceleration problem, and the errors just build up through the integrations. The fix is straightforward: keep the accelerometer earth-horizontal. If the sensing element stays level, gravity never acts along its sensitive axis, so it never senses a component of gravity. It only senses the true horizontal accelerations of the aircraft as it travels across the earth's surface. That's the whole reason we need a platform at all. Now, let's talk about the gyroscope that makes this platform work. The INS uses an integrating gyroscope. That's a one-degree-of-freedom gyro, and the key difference from the rate gyroscope you're more familiar with is the restraint. A rate gyro uses a mechanical spring restraint. The integrating gyro uses viscous restraint — fluid, not a spring. Picture the construction. It's basically a can within a can. The outer can, which we call the frame, is filled with a viscous fluid. Inside that, the inner gimbal is pivoted about its vertical axis. The fluid supports the weight of the inner gimbal, and that's important because it reduces the bearing torques. Less friction in the bearings means the gyro behaves more ideally. So now we have the accelerometer and the gyro, and we mount them on a common gimbal assembly. That assembly is the platform. The platform is a mechanical device that lets the aircraft go through any attitude change while the innermost element — the bit the accelerometers are mounted on — stays earth-level. The gyros that stabilize the platform are also mounted on that innermost element. They feed signals to amplifiers and motors, and those motors drive the gimbals to keep the accelerometers level. Let me walk you through the closed loop, because this is the heart of the system. The gyro and accelerometer share a gimbal. If that gimbal tips off level, the gyro's spin axis stays fixed in space — that's the gyroscopic property. So the gyro's case moves off level relative to the spin axis. The amount the case has tipped is detected by the signal pick-off inside the gyro. That signal gets amplified and sent to a gimbal drive motor. The motor drives the gimbal back to level. And because the accelerometer is always kept level, it never senses a component of gravity — only the true horizontal accelerations. Now, in reality, we don't have just one of these. We have three integrating gyros mounted on the inertial platform, and their input axes are mutually perpendicular — at right angles to each other. And we have three gimbal motors, driving the platform gimbal rings about the pitch, roll, and vertical axes respectively. So the gyros sense any incipient displacement of the platform — any tendency to drift off level — and they activate the appropriate motors to provide the relative movement of the gimbal rings as the aircraft manoeuvres around the stable platform. So the whole picture is this: three gyros sense any tilt, three motors correct it, and the accelerometers stay level so they measure only true horizontal acceleration. That's the platform doing its job.