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Autopilot servo motors

Historically they were always brush motors - presumably because that's the obvious and simple choice, and there is no way for such a motor to go haywire and suddenly run at 100000rpm or whatever.

But they always wear out, sometimes a lot sooner than later

The King servos are a bad example because they contain two brush motors: the real motor, and a tachometer which uses metal (not carbon) brushes. If the main motor wears out, you lose the servo. If the tachometer wears out, the servo amplifier will destroy the main motor because it thinks it is underperforming so it drives it with an aggressive waveform and blows it up.

I asked Garmin at a recent show if they use brushless motors. The man might have made this up on the spot but he said Yes. Interesting!

One issue is that the King servos are rated down to -55C. This makes it very hard to do a brushless design because none of the control chips (either for 3-phase brushless motors, or for stepper motors) are rated down to -55C. One would have to do a totally discrete-component design, though there are very rare microcontrollers which will go down to -55C.

What is the reason for -55C? Surely this will never be encountered in an unpressurised aircraft, where the servo is mounted, and in a pressurised one the servos are normally under the floor i.e. in the cabin.

Administrator
Shoreham EGKA, United Kingdom

none of the control chips are rated down to -55C

Or you evaluate the control chips down to -55C. I suspect most chips still work at -55C, just nobody needs this so no manufacturer does the work characterizing them to this temperature

LSZK, Switzerland

Yes that will work, but you can't do that while pretending to be doing "proper design".

The manufacturers will guarantee data sheet operation only in the specified temp range.

I would think that putting every one into a -55C freezer would take too much messing around. You have to pack the item into a sealed bag with silica gel, otherwise the condensation (as the temp drops and meets the air dewpoint) covers any PCBs with liquid water and shorts everything out. Or you buy a very expensive environmental testing chamber which dries the air...

Administrator
Shoreham EGKA, United Kingdom

Putting the stuff in a special freezer/oven is exactly what they do to overrule the manufacturers' limits. I've seen how they do it at an avionics development shop in Germany.

You have a big job doing that with a processor because any tiny part of it could stop working at a given temp outside the data sheet spec.

This is an old tactic for breaking secure chips. You freeze them (or undervoltage them) until the crypto bit stops working, and you read the data out

It's easy to get everything down to -40C. That is "industrial/automotive". -55C used to be common in the 1970s but I believe the military discovered they were being ripped off by 10x to 100x for that extra bit.

Administrator
Shoreham EGKA, United Kingdom

Yes that will work, but you can't do that while pretending to be doing "proper design".

Why should this be improper? Why does it matter who pays the salary of the guy doing the evaluation/characterization?

Or you buy a very expensive environmental testing chamber which dries the air...

Well by aerospace standards these are cheap 8-)

Everything else doesn't work, even such a temperature forcer is a bit problematic at -55C, even the tiniest air leak tends to suck in moisture which quickly causes ice buildup. You want to test the circuit biased, that means you need to get wires into and out of the chamber.

I believe the military discovered they were being ripped off by 10x to 100x for that extra bit.

Well if they buy devices in pathetic quantities and expect huge amounts of quality paperwork, someone has to pay for this. They can't expect commercial devices to subsidise their obsession with paper!

Same thing now with automotive. They always think they get commercial pricing and lots of papers for free.

LSZK, Switzerland

You have a big job doing that with a processor because any tiny part of it could stop working at a given temp outside the data sheet spec.

Sorry to break the news, but no sufficiently modern processor (or other device) has 100% test coverage at speed.

If a design breaks at -55C, it was marginal at -40C already, the difference being only 7%.

Most problems at low temperature are mechanical, for example different dimension changes of different packaging materials and the PCB, etc.

LSZK, Switzerland

Processors will be tested like any other complex logic i.e. with test vectors which are designed to exercise every part of the chip. I used to do this on ASIC designs.

Obviously you can't test a 1 million gate chip with every logic combination.

I don't know what chip makers do (because they obviously don't test at -40C and +85C) but they probably run the vectors with a certain speed margin. In lower temps, chips get faster so you are "allright" testing at +20C - so long as you didn't engage in certain "interesting" logic design practices which make stuff like self modifying code look entirely respectable

Administrator
Shoreham EGKA, United Kingdom

because they obviously don't test at -40C and +85C

Huh? Not every chip is necessarily tested at other than room temperature, but the design is certainly evaluated at these temperatures.

The problem at low temperature is hold violation, and no, you don't need interesting logic design practices to run into this problem if you don't care.

LSZK, Switzerland

We have produced Automotive electronics and 100% full range temperature tested them, for -40 to +85. This was for a Japanese car company and volume was in the region of 80,000pcs/year.The test kit was not unusual or particularly expensive.

The same kit could do -55C

Darley Moor, Gamston (UK)
14 Posts
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