remote control air regulator

[rich2202]

Originally Posted by Kevin Sevcik

Lose power to your robot and your regulator stops adjusting. ... Lost control signal means it drops the regulated pressure to 0

It does not say what happens if it looses both Control Signal and Power. My guess is that if it looses Power, the "setting" of the regulator no longer changes.

So, what happens when the Main Breaker is hit? Which is lost first? Control Signal or Power? Let's say the Control Signal is lost first, then the Regulator becomes set to 0 PSI. That means it is trapping air. If you have the vent plug on the low pressure side, then the EPP4 will not let the high pressure air pass. If the vent plug is on the high pressure side, then you need to make sure it lets air pass in reverse.

[FrankJ]

The valve requires power is operate. As soon as it looses power all internal valves close. (per data sheet). So with a simultaneous loss of control signal and power, you might see it start to vent, but is would quickly fail closed. The actual pressure would depend a lot on the volume of the downstream side

Actually the data sheet shows that it is not relieving regulator so that specific model shouldn't be legal as a primary regulator. The relieving function requires active control of the regulator. The power failure mode (main switch off) would be to stop controlling presssure & not release down stream pressure when upstream is vented and vice versa preventing proper operation of a single point vent valve. (Maybe this would be a place for clever use of a check valve to allow the downstream to vent to the upstream side.) Another complication is the 24 VDC supply voltage. That would mean powering it through the PCM, which disables when robot is disabled or a custom circuit.

[Kevin Sevcik]

To clarify my statement for everyone, you have to really look at the diagram on how the regulator works. There's two servo controlled valves in the regulator. Those valves control pressure in a chamber above the plunger in the regulator. That chamber acts like the spring in a mechanical regulator. Here's what happens in the different situations, I think:

Power loss: Servo valves close and stop operating. Chamber maintains pressure. Regulator does the standard relieving regulator thing at last setpoint pressure.

Control signal loss: Per data sheet, exhausting servo valve opens, sets chamber pressure to 0 psi, which makes it like a fully closed mechanical regulator. 0 psi output.

Power loss and Control signal loss: Probably depends on the order and mechanics of those servo valves. I'd guess the likely outcome is a slight decrease in setpoint, and then it maintains that pressure. I guess if the command signal drifts up before power loss, maybe a slight increase in setpoint?

The latter would be the most concerning for me. I wouldn't want this as a main regulator in case a weird wiring fault, short-circuit, or something put a bad voltage on the control line. It looks like 5V is the midpoint in the 1-145 psi range, which would be ~70 psi, so a wiring short on your analog output would put you out of spec. A bad wiring short might put it at max range and dump 120 psi into your system. With mechanical regulators, inspectors can at least be fairly confident that once they're set, they're not going to randomly change. That's not really the case with an electronically controlled regulator.

[wireties]

Originally Posted by Kevin Sevcik

It looks like 5V is the midpoint in the 1-145 psi range, which would be ~70 psi, so a wiring short on your analog output would put you out of spec. A bad wiring short might put it at max range and dump 120 psi into your system. With mechanical regulators, inspectors can at least be fairly confident that once they're set, they're not going to randomly change. That's not really the case with an electronically controlled regulator.

The +5V can go through a simple resistor ladder. Unless we discount the laws of physics one can't get something greater than the 60PSI setpoint from a passive control circuit. And mechanical regulators move all the time, they are not impervious to the effects of vibration whereas the electrically controlled regulator basically is not.

I'm not a strong advocate here - just playing it out, exploring the tech. I posed a Q&A, we'll see what they say.

[Kevin Sevcik]

Originally Posted by wireties

The +5V can go through a simple resistor ladder. Unless we discount the laws of physics one can't get something greater than the 60PSI setpoint from a passive control circuit. And mechanical regulators move all the time, they are not impervious to the effects of vibration whereas the electrically controlled regulator basically is not.

I'm not a strong advocate here - just playing it out, exploring the tech. I posed a Q&A, we'll see what they say.

Yeah, except we all know how electronics go wrong on our robots. Swarf, plugging things in wrong, nicked wires. The main regulator is supposed to be a safety valve for the pneumatic system, like one of the breakers in the PDP. Once the wiring is verified, it takes a fair bit of ingenuity to bypass one of those breakers. Even more to bypass them all. With the electronic regulator, I'd be concerned that you're significantly lowering the ingenuity level to bypass the main regulator.

[Alan Anderson]

Originally Posted by wireties

The +5V can go through a simple resistor ladder. Unless we discount the laws of physics one can't get something greater than the 60PSI setpoint from a passive control circuit.

You can't exceed the desired voltage from a properly operating passive circuit. However, it's not a fail-safe. You can't ignore the potential loss of the ground connection going to the bottom of the resistor ladder, or the potential shorting of the "top" resistor.

[wireties]

Originally Posted by Alan Anderson

You can't exceed the desired voltage from a properly operating passive circuit. However, it's not a fail-safe. You can't ignore the potential loss of the ground connection going to the bottom of the resistor ladder, or the potential shorting of the "top" resistor.

I respectfully disagree. The safety of our robots depend on many properly operating passive circuits. And this particular passive circuit has little to no load. If it works at inspection, it is gonna work forever unless one physically compromises the components. This is true of many passive circuits in FRC robots.

Our resetting breakers are a more complicated passive circuit and we use those to protect many things on the robot. Heck, the power wires running to all our electronics are more complex "lumped" circuits. And an open ground connection results in no controlling voltage and the minimum output pressure. Still the worst fanciful failure imaginable gets you ~70PSI, not 120PSI, as the highest possible potential in the passive control circuit is 5VDC.

[Alan Anderson]

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.

Originally Posted by wireties

...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.

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.

[wireties]

Originally Posted by Alan Anderson

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.

How does a connection to the reference voltage fail other than by opening up? A zener clamping the voltage in the control circuit would be a feel good addition - can't hurt.

Originally Posted by Alan Anderson

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..

One can imagine wild failure modes with most parts of the fail-safe circuitry on the robot. I remind you the biggest fail-safe mechanism we have is implemented in software!

Recently I had the top come off breakers exposing the fuse. We can go back and forth all day and night about extraordinary failure modes. But one has to pick a prudent affordable safe approach.

Originally Posted by Alan Anderson

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.

I can indeed "invoke the laws of physics" and assert that the control circuit meets criteria for a fail-safe circuit in this application. I'm not ready to go there, just wanted some opinions. And I appreciate your opinion.

With respect the lack of specificity in the rules is the genesis of this thread. Where does it say the adjustment is mechanical? Did I miss that? There have been a few good pneumatic-related Q&As this season that resulted in corrections to the manual.