Pneumatics question: Can I have 2 solenoids going to the same piston?

Question is pretty straightforward as to what I want to do; is it possible to have two solenoids go to the same piston? What I want to achieve is to place a regulator/Needle Valve on one to limit the speed it extends/retracts with. They should not be fired at the same time. All I want to know is will this work? Thanks!

I believe this would violate R814 (Don’t connect solenoid outputs together).

Sorry, should have made this clearer; this is for an Offseason project - Not on the actual robot. Would connecting them be excessively dangerous/bad? I trust our software team to make sure they don’t both fire at the same time. Specifically I used the example of a piston - but this would rather be used for shooting T Shirts with different forces, so not a piston but still using a solenoid.

Oh, OK. I’ll take my yellow hat off, then. :slight_smile:

I’m not a mechanical expert, but I think the intent of the rule is to prevent you from double-powering pistons.

If you did this, I think you would want to put the secondary regulator upstream of the solenoid. You might also want to think about using a variable-flow/proportional solenoid instead.

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We built a T-shirt shooting tank robot last year. We ended up just using an analog pressure sensor and having the operator manually stop the compressor at the desired pressure.

We looked for an electronic pressure regulator, but couldn’t find anything viable. Then we tried a manual regulator and a motor to turn the dial. In the end, the regulator sets a safe max pressure, but the operator found it easier to just run manually.

You should be able to use flow regulators on the cylinder ports with a single solenoid to accomplish what you’ve described, and that would be FRC legal.
We’ve done it on intake actuation mostly, and usually asymmetric.

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The initial idea I had thought of when discussing the concept with one of our mentors was there are 2 ways (obviously more) to do this that we were looking at: Indirect and Direct. Indirect acting like artillery and shooting higher and thus having more variations in where it will land (best suitable for outdoor events like Football games). Direct would be more ‘tank’ like shooting just a little above where aiming (accounting for gravity). The direct would have a preset lower pressure to not nail anyone in the face.

To be honest I am not the brightest apple on the tree; what do you mean? The idea being we could vary it for two different ‘modes’.

I think the variable flow would be something to look into; I used a piston as an example but I really meant it would just shoot air to push the T Shirt forward. This would prevent the ability to double fire as air would just be dumped.

Oh, sorry, misunderstood extends/retracts.
So, you want a mode A where extend and retract are, say, speed/force = 1
and then a mode B where both extend and retract are speed/force = 0.5, correct?

I’d probably do that using a dual supply: one tank at one pressure and the other regulated down to a lower pressure, use one solenoid to switch between the two, and feed that switched supply to the solenoid you’re using to actuate the pneumatic cylinder.

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Yes I am pretty sure that is what we would want for example.

This is what you were talking about, right?

Correct, depending on how much air you’re moving the buffer tank at 30 psi may not even be necessary, but I’d include it if you have the space.

You can also use adjustable flow regulators on the two inputs to the solenoid shown if you’d like to variably limit the flow rate as well. Currently this just limits pressure, and the extension/retract speed will depend on loading and such.

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Perfect! Thanks! I will bring this up the next design meeting we hold!

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OK, the use case is an air cannon…

  • Do you have a slide that carries the shirts? Most t-shirt air cannons do not, or the slide is manually retracted, so that there is no need for pneumatic tube on that side.
  • To launch a t-shirt, you will need a significantly larger solenoid valve than FRC-legal. 3/4" diameter will work, but 1" is better. Of course, the plumbing should be at least as large. Be sure that you valve is rated for air - many valves of this size are rated for water but not air. The difference isn’t so much whether the valve will fail, but the consequences. Water at 60psi pours quickly out of a leak - air at 60psi essentially explodes because it is more compressible.* The same is true of the plumbing - use air-rated values, not water ratings. Also be sure your valve is of the solenoid type and normally closed, not the motor and valve type. Motor and valve will open too slowly to achieve a launch.
  • As to your original question, 3946 found that we could adjust launch range and net force by opening the valve for a shorter time interval. IIRC, we did long shots at 50-75ms, and shorter shots at 20-30ms valve time, so dynamic pressure/flow adjustments were not required. As usual with FRC, we built more than we needed, then tested and tuned empirically.

* The stored energy is \int P(v)dv. While the average value of P(v) isn’t all that different, \int dv definitely is. At 60psi (0.4MPa), air compresses by about 80%. At 4km depth (40MPa), water compresses by about 1.8%.

Team 3489 made a t-shirt launcher a few years ago. Instead of using large solenoid valves, we used small solenoid valves to actuate large pilot-operated valves.

Something like this:

We spent a tenth as much on something like this with twice the diameter:

Note - don’t try to drive this with a PCM/PCH, because this will use more than 100mA of current (If 2W means actuation power, I’d expect something on the order of 200-400mA). Use a light duty relay. And don’t keep it open for long periods of time, because it will get hot, but there’s probably no need for that in this use case.