The Pressure Altimeter — Page 63, Lesson 70

Let’s start with the servo-assisted altimeter, because this is where the pressure altimeter gets its real precision. I want you to picture the basic pressure altimeter first — the one where the capsules move a pointer directly through a mechanical linkage. That works, but it has a weakness, and that weakness is exactly what the servo-assisted altimeter is built to fix. Here’s the problem. At high altitude, the change in pressure for a given change in height is much smaller than it is at low altitude. Think about that for a moment. Near the ground, a small climb gives you a big pressure drop. Up high, the air is thin, so the same climb gives you only a tiny pressure change. Now, the capsules in the altimeter respond to pressure. So at high altitude, for a given height change, the capsule movement is relatively small. And if the capsules are driving the pointer through a mechanical linkage, that small movement has to push against friction in the linkage. Friction that is proportionally much more significant when the movement is tiny. The result is bigger errors and more lag — the pointer lags behind the true altitude. So the solution is servo-assistance. The idea is that the capsules don’t drive the pointer directly. Instead, they just sense the pressure, and a motor does the heavy lifting. This gives the altimeter two big advantages: an increased operating range, and improved accuracy, particularly at high levels. Now let me walk you through the principle of the servo altimeter, because it’s a neat chain of events. The small movements of the capsules are detected by a very sensitive electromagnetic pick-off. That pick-off produces an electric current. That current is amplified, and the amplified current drives a motor. And that motor rotates the counters and the pointer. So the capsules only have to move a tiny amount — the motor provides the power to move the indicators. Let’s look at the schematic in Figure 5.7, the Servo Assisted Altimeter Schematic. I want you to see the components in the chain. We have the capsules, which are connected to static pressure. We have the electromagnetic pick-off, which is the heart of the sensing. Then we have an amplifier, labelled AMP. Then a servo motor and gearbox. Then a worm drive mechanism. And then the indicators — the counters and pointer. There’s also a cam mechanism and an AC exciter in there, and I’ll explain those in a moment. Now, the pick-off itself. This is the clever part. It’s an E bar and an I bar. The E bar has three legs — a middle leg and two outer legs, labelled ‘A’ and ‘B’. AC — alternating current — is fed to the middle leg of the E bar. That sets up alternating magnetic fields in the outer legs, ‘A’ and ‘B’. Here’s the key detail. The coils on these two outer legs are wound 180° out of phase. So when the exciter induces a current in each leg, those currents are equal in strength but opposite in phase. Now, the I bar sits near the legs of the E bar. When the I bar is equidistant from the legs — that is, when no pressure change acts on the capsules — the currents in legs A and B cancel each other out. Equal and opposite, so they sum to zero. No net signal. Now, when pressure changes, the capsules expand or contract. That moves the I bar on its pivot. The I bar closes the gap at one end of the E bar and opens it at the other. So now the I bar is no longer equidistant. The magnetic coupling is stronger on one side and weaker on the other. The currents no longer cancel. You get a net signal — a current that represents the pressure change. That signal goes to the amplifier, which drives the servo motor, which turns the counters and pointer through the worm drive. So the whole system is a closed loop. The capsules sense the pressure, the pick-off converts that tiny movement into an electrical signal, the amplifier boosts it, the motor does the work, and the indicators show the result. The cam mechanism and the worm drive are part of the mechanical transmission that converts the motor’s rotation into the movement of the counters and pointer. One more thing to note. The excerpt mentions that at least one counter-pointer altimeter driven directly by pressure capsules has been produced, but most instruments of this type are servo-assisted. So the servo-assisted version is the standard. The direct-drive version exists but is the exception. Let me show you the schematic so you can see how all these components connect. So to sum up the whole chain: static pressure enters the capsules. The capsules move the I bar. The I bar unbalances the magnetic fields in the E bar. That produces a current. The amplifier boosts it. The servo motor turns. The worm drive and cam mechanism move the counters and pointer. And the result is an accurate altitude reading, even at high altitude where the capsule movement is tiny and friction would otherwise cause errors and lag.