There is another style of double solenoid valve that is spring centered. The center position can be either all ports blocked or the cylinder port open to exhaust. The all ports blocked is useful if you want to be able to stop the cylinder in a intermediate position. You do have to make provision to be able to dump the cylinder pressure with the one vent valve.
With the two position double solenoid valves, it is best keep one solenoid energized. Otherwise if the valve is bumped just right, it can shift by itself. The power draw the energized valve is minimal.
We tend to use single solenoid valves unless we have a reason to use a double. The advantage of the double is that the disabled state will not effect the position of the cylinder. There is a brief disable during the transition from autonomous to teleop where this can be important.
Do you have an example of manufacturer and part number of this type of valve that would be FRC-legal? I can search, but if you had one that you have been using, it would save some effort.
This is a closed center valve. It will more or less lock the cylinder mid travel. Air is compressible so the cylinder will deflect a bit. You will need to make provision to be able to release the trapped air by the rule required vent valve. You can do so by venting the cylinder to the pressure side through check valves.
The open center version will let the cylinder move as it want because both sides are vented to exhaust in the center position. It is rules legal because it does not trap air. Here is an example AVS-523E1-24D
It very definitely will deflect. 3946 tried something like this with two single solenoid valves for 2014 Aerial Assist to put the intake rollers in the desired position. Even with different pressures on either side (we had three regulators), it didn’t work very well; if the balls hadn’t been so compliant that year, it likely wouldn’t have worked at all.
Center closed valves - I thought those were NOT FIRST legal … because they trapped air.
Spcifically: “R86. Any pressure vent plug must be: A. Connected … to relieve all stored pressure …”
Since a center closed valve traps air, it doesn’t allow the manual vent valve to release “all” pressure. Hence, not legal.
Am I reading that corretly?
Yes you have to fully vent the pneumatic system with one vent valve. Simply installed a closed center valve (and two valves for that matter are not legal. To make closed center valve to be legal you have to make provisions to vent the downstream side of the valve when the high pressure side is vented. One way to do this is to use check valves on the downstream side so that it flows to pressure side system. The pressure on the downstream side will never higher than system pressure. When you vent system pressure, the down stream side also vents through the check valve. A little wordy but it works. If I get a roundtoit, I will make a picture of it.
I had a bit of extra time this morning, and I’ve been wanting a reason to practice using our pneumatic circuit drawing software, so I drew this up for fun:
I believe this would be a FIRST-legal circuit utilizing a closed-center, 3 position valve, though I welcome those who might know their way around pneumatic schematics better to critique it.
You’re probably right, I was focused on making sure the upstream air exhausted, but since I’ve shortened the exhaust path compared to the supply path the cylinder motion could end up more jerky than in your example circuit.
Once my office is open again I’ll have to see if I can build up a live demo.
The issue with that circuit is that if the cylinder was back-driven, it could overpressurize one side of the piston and consequently open up the check valve, sending air back to the line-pressure reservoir and allowing the piston to move (instead of holding firm against the back-drive pressure). To prevent that you’d need special check valves with more area on the tank-side than on the cylinder-side.
If I’m reading this right, isn’t that how our current system works? When you try to back drive a pneumatic cylinder now, if you exceed 60 psi the regulator vents.
That’s true, but when you stop pushing (against a 2-position valve) air flows back into the cylinder to push the piston back to its original position. With the 3 position valve in the center position that air leaves (through the check valves) and doesn’t come back.
Actually (actually this is getting kind of deep. Way deeper than your typical inspection.) The cylinder is part of the pneumatic system and needs to be protected by the one system pressure regulator at all times. So the check valves are necessary to be rule complaint.
Yes if you over pressure the cylinder it will relieve through the check valves. Yes when you do that, you could pull a vacuum on the other side of the cylinder. I suggest you do not burden your RI with those types of details.
After the cylinder has been over pressured it will likely be where you do not want it. I suggest you fix that in software. Engineering is always about details.
I think it’s fair to say we’re relatively deep into the weeds here (it’s probably worth splitting off into a separate topic since we’ve clearly strayed from the OP), especially since the majority of FRC applications of pneumatics don’t require intermediate positions, or anything other than a 5 port/2 position valve.
That being said, I don’t completely agree with your reading of the rules.
Yes, all working components must be downstream of the primary regulator, and yes the whole system must vent to atmosphere when the release valve is opened, but there aren’t any rules forbidding parts of the circuit from temporarily exceeding the 60 PSI working pressure limits. I think a reasonable reading of the rules allows for my circuit to be legal (regardless of its effectiveness), at least per the rules of the last few seasons.
And as someone who is not the strongest in software, I like mechanical solutions
This would be a question for Q&A or your LRI to settle. I am neither. But I would think R81 (2020 rules) would apply. The air in the cylinder fall into the class of “working pressure” in my mind.
R81. Working air pressure (air pressure used to actuate devices) on the ROBOT must be no greater
than 60 psi (~413 kPa) and must be provided through a single primary adjustable, relieving, pressure
regulator.
I did consider R81, and I agree that’s where things are a bit iffy.
If you consider “working air pressure” to be “the pressure of any air contained by the system downstream of the primary regulator”, then yes I agree with your interpretation. I’d probably even bet a dollar that’s the intent.
I do think “(air pressure used to actuate devices)” creates a bit of a gray area. If I’ve returned my closed center valve to mid position with the piston extended, I’m not providing any more air to actuate devices. But if an external force compresses your extended piston, this quantity of air between the valve and actuator is now at a higher pressure, without the supplying of additional air (or providing air at a pressure > 60 PSI).
Is this “lawyering” territory? It’s definitely closer than I’m fond of, but I think some small clarifications could eliminate any existing ambiguity.
The GDC has loosened the pneumatic rules considerably over the years. I think they err a little too much on the side off caution. [sarcastic mode] But the GDC has demonstrated several times that they don’t really care what I think. [/sarcastic mode.]
It would take a careful question to Q&A to get a real “official” answer.
