Gregory's ServerThis Central Air Data Computer (CADC) was introduced in 1955. It computed airspeed, altitude, etc for fighter planes. But instead of a processor, it was an analog computer that used tiny gears for its computations. Let's look at how one of its modules works.🧵
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The CADC needs to know the temperature for its calculations. A platinum probe outside the plane measures temperature, producing a changing resistance. But the CADC needs to rotate gears. How does the CADC convert the resistance to a rotation? That's what I'll discuss today. The CADC looks like a random mass of gears, but it is constructed in a modular fashion. The temperature module is a wedge-shaped piece of gears and electronics that can be removed for service. It is independent of the rest of the CADC except for one gear that provides its output. A servo loop converts resistance to rotation. A motor rotates the gears while a feedback loop keeps it in the right spot. The amplifier generates an error signal to rotate the motor in the correct direction. The potentiometer indicates the current output. They didn't have op-amp chips back then, of course, so the amplifier consists of multiple boards of circuitry. The boards have a few germanium transistors (pre-silicon) but transformer-like "magnetic amplifiers" perform most of the amplification. To correct for errors, a correction factor is added, a function of rotation. This is implemented with a metal plate, warped into the correct shape with 20 tiny screws. A cam measures its position, which is added to the rotation with a differential gear mechanism. Just converting a temperature to rotation took a big wedge of gears. Although gear-based computation is bulky, an analog computer was the best solution at the time.  @kenshirriff use a vfd! They go up to 400hz, and probably cost $100. I use a vfd for... reasons... you can adjust, well, everything. I got a 3 phase output but turned out I only needed 1 phase  @kenshirriff  @kenshirriff "glad" to see 115V/400Hz was already in use back in the 50s... @kenshirriff looking forward to reading/seeing more about it once you have a chance to power it up! i enjoy seeing this sort of thing that fits neatly at the intersection of #avgeek, #vintagecomputing, and a whole ton of other topics that escape me atm. @kenshirriff Why that particular voltage and frequency? Did the creators learn all their EE through dance instead of words? @Urethramancer Apparently 400 Hz is popular for avionics rather than 60 Hz because the higher frequency means the transformers are much smaller.
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 @kenshirriff That thing looks like a nightmare to keep calibrated once the parts start to wear...  @kenshirriff What errors does that encompass? I would expect that this accounts for nonlinearity of the sensor, and that there's no need to account for errors in the amplifier itself (because we don't rely on ~any other properties than behaviour at input equal to 0). Are there other sources of systematic error I'm missing?  @kenshirriff can you do a post on magnetic amplifiers some time? I've never quite got how they work and you do explain things very well and very thoroughly.  @kenshirriff @kenshirriff I bet you could make a e6-b style circular slide rule using the intermediate products linearly on the rings? @kenshirriff If you need a mental break and enjoy old technology... I would recommend nerding out on this thread! Cool stuff! @kenshirriff you can't fool me Ken, this is clearly an industrial complex from some sci-fi tabletop skirmish game.  @kenshirriff before my eyes focused my brain was seriously forming the thought "wow that's a cool Lego set…"   @kenshirriff I'm really enjoying the videos on this. BTW as a (ex) glider pilot/nerd the pneumatic instruments, and analog to fairly sophisticated digital air data computers used in gliders can be fascinating. Total energy probes, variometers, netto variometers, digital total energy calculations, speed to fly computers, very accurate GPS computed winds, corrections for exotic effects etc.   "... The missile knows where it is because it knows where it isn't, by subtracting where it isn't from where it is…" Well now it all makes sense...  @kenshirriff awesome, thanks :) Also (imho ;)) highly entertaining on the subject, for it's crackly sound and distinct narration: such series of US-Navy training videos as i.e.:  |
Planes determine altitude and speed from air pressure readings. But near the speed of sound, things become very nonlinear. As fighter planes became supersonic in the 1950s, the CADC was built to compute these nonlinear functions using rotations of gears and cams.