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Unread 01-04-2016, 17:20
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Re: remote control air regulator

You're doing a reasonable job of considering some failure modes, but you seem to be dismissing what I think are the likely failures, and you definitely haven't addressed the worst case ones.

Quote:
Originally Posted by wireties View Post
...an open ground connection results in no controlling voltage and the minimum output pressure.
That assumes that the ground connection fails in the "right" place to remove it from the regulator as well as from the resistive divider. By proper design of the actual wire routing, that particular behavior on a loss of ground can be assured, but it requires more thought than just the obvious pair of resistors.

Quote:
Still the worst fanciful failure imaginable gets you ~70PSI, not 120PSI, as the highest possible potential in the passive control circuit is 5VDC.
Oh, I can imagine quite a bit more fancy than that. I don't even have to imagine battery voltage ending up on the analog signal -- I have actually seen it happen. A small addition to the divider circuit can address that, by putting a zener diode in parallel with the bottom resistor, but one must still recognize the potential for something like that to happen. Making a safety-critical part of the robot fail-safe takes real effort to get it right.

You can't just invoke the laws of physics and say nothing will go wrong. You have to see where the laws take you when your assumptions are violated. The robot rules regarding pneumatic systems are very specific, and they do a good job keeping pressures at the appropriate levels unless many things go wrong at once. I don't see an electronically-controlled primary regulator managing to fit in that framework, because it can let full pressure through with just a single failure.
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