Pnuematics Solenoids Common Ground

We are using a lot of SMC double solenoids and a PCM. To save wire, are our solenoids allowed to share a common ground wire, as long as it gets spliced into the appropriate leads on the PCM? We can’t find any relevant rules in the rule manual.

If the rules are unclear, it is better to ask this question on the official Q&A to get a ruling that you can point the inspectors to.

2019-R37 states you’re allowed to place them “one per channel”. Sharing a ground could be interpreted as having more than one solenoid connected to part of the same channel. Like @Peter_Johnson said, a Q&A is likely the quickest way to get an answer.

In my opinion, it just seems more correct to just add an additional PCM to your robot, which is completely legal.

We have a second PCM. The sharing the ground is for saving wire up a cable chain

I believe the basis behind the PCM solenoid limit is the tech specs of the PCM itself. It’s rated for a total of 500mA output across all solenoid outputs. It’s common for an individual solenoid to pull somewhere in the neighborhood of 50mA. With 8 channels, that makes sense. Letting teams double up would put a burden on both the teams and inspectors to ensure the electrical ratings were being followed - and probably lead to a significant number of overloaded PCMs.

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Doubling up just the common ground doesn’t change the total current. It only does that if you double up both wires.

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Disregarding legality.
If I understand correctly PCM has common positive and activates solenoids by modulating negative so you may need to use common positive and separate ground.

It is clearly legal to use one ground cable per solenoid… you can’t go wrong there.

In order for something to be illegal, there has to be a reason for it to be disallowed.

The most common reason is that it goes against one of the stated rules. I don’t see any rule that requires specific wiring of the ground wire of solenoid valves.

The next common reason would be that it is unsafe, or poor engineering practice. There are a few sources to consider here… the FIRST Pneumatics Manual, FRC Wiring Guide and PCM User manual come to mind. None of them (to my reading) state that you cannot do this. The fact that the PCM user manual states that current is limited by the total current across all ground lines suggests to me that the ground lines may be connected internally within the PCM. MAY be.

A rare reason would be that someone with specialized knowledge would be able to point out that this configuration was not considered by the authors of the manual, but would present a danger for X or Y reason.

I think you’re good on the first two tests… but the only way to be sure you won’t be caught on the third one would be to put up a Q&A. You risk getting the answer that while it’s not unsafe, FIRST expects one ground per solenoid in order to ease the inspection process, but whatever the ruling you’ll be able to ensure your robot is 100% legal before you get to your competition.

I’d suggest you wire one ground per solenoid… but don’t have a reason to require it. I’m looking forward to reading a Q&A response! Good question!

Jason

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Back in the day we used to do common grounds to actuate a double solenoid with one Spike relay.

And the common ground is still shown in the spike manual. https://content.vexrobotics.com/docs/spike-blue-guide-sep05.pdf

My team has posted this question to the QA at https://frc-qa.firstinspires.org/qa/318… Let’s see what FIRST says about the issue

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Looks OK, though (as @PeterK noted) you would be better off doing a common positive.

Q318 Sharing common grounds on solenoids

  • If we have multiple 12v double solenoids, could their signal lines share a common ground wire, as long as it is appropriately spliced into the PCM ports?

Answer

  • There are no rules prohibiting this; however, note that the PCM is low side switched so this configuration will likely not behave as you are expecting.

I just realized - this makes the PCM a valid way to control (short) 12V RGB LED strips. I had ruled this out recently because the strips usually have a common anode (positive) wire.