Powering two compressors with one PCM
 

Hey guys!
I was wondering if it is possible to two compressors using only one PCM and if so, then how can it be done? We tried to solder the wires and power it but that seemed to cut the max PSI in half.
Thanks in advance!

Re: Powering two compressors with one PCM
 

If this is for the competition robot, you may want to start by reading R79 of the 2016 Manual (and take a guess that it's likely to be the same for next year). Given that rule, I would find it highly unlikely that the PCM is designed to handle more than one compressor. (Short version: No more than one compressor on the robot.)


If it's for a non-competition robot, carry on. But you may need to consider the power draw through the PCM of that second compressor.

Re: Powering two compressors with one PCM
 

Just hook the second compressor up to a spike relay (or, less ideally, any other speed controller if you don't have one) and toggle it in code using the pressure switch to turn it off.

As EricH mentioned though, not legal to have a second compressor on a competition bot.

Re: Powering two compressors with one PCM
 

Not for competition, you could get an automotive relay large enough to handle two compressors' maximum current. Then, wire the PCM compressor outputs to the coil on the relay, and power the compressors IN PARALLEL through the relay with a separate breaker/fuse. (If you wire them in series, you will get much worse performance!)

Edit:
Well, yes, that too. True story: Our rookie year, we wired the compressor directly through the pressure switch. Somehow the inspectors missed it, and we didn't realize it was illegal until I read the rules the next build season. As it was illegal and we won't do it again, I have never gone back to see what the current limit is on that pressure switch.

Re: Powering two compressors with one PCM
 

This is what we did for our T-Shirt Shooting robot.

Re: Powering two compressors with one PCM
 

Even simpler just wire a relay (or two) through the pressure switch and avoid a PCM altogether.

Re: Powering two compressors with one PCM
 

The issue that we are facing with the idea of just using a spike relay is the fact that the compressor class in wpilib only seems to be accepting the pcm as a parameter.
Could one of the teams who was able to use a relay share the code that they used for this situation?
Thanks in advance!

Re: Powering two compressors with one PCM
 

Older default code (2014 and before) would have some basic examples, though it should be pretty easy to program yourself.

There should be a way to read the pressure sensors state from the PCM in code, and if you have that, then all you have to do is use the output of the pressure state to toggle the spike on or off. The pressure sensor is a switch that activates at ~120psi and deactivates at ~ 100-110psi, so simply using it as a Boolean to toggle the compressor state should make it operate the same as a PCM controlled compressor would.

You can query the pressure switch and compressor status from the PCM and simply mimic the state of the PCM controlled compressor with the code on the bottom half of this page: https://wpilib.screenstepslive.com/s...anual-id=13810


The second compressor, for the purposes of code, isn't a compressor, it's a Spike-controlled motor. Just make sure that the spike turns on and off at the appropriate times via the methods mentioned above.

Re: Powering two compressors with one PCM
 

And don't try to put it in reverse! IIRC, we connected the black lead of our compressor directly back to the Power distribution board (through a splice), and just put the red lead to a spike output to ensure that we didn't apply reverse voltage. (I never found a rule AGAINST it.) While you can't use the other output on an FRC-legal robot (per 2016 R53, second sentence), for other projects, you can use that side to control something else, like a string of LEDs or other low-current-draw custom circuit.

Re: Powering two compressors with one PCM
 

So let me just get this clear? I just have to set the relay into forward mode in order to fill up the compressor?

Re: Powering two compressors with one PCM
 

Yes, presuming the red wire on the compressor is connected to M+ and the black wire on the compressor is connected to M- (or back to the black wire coming from the breaker panel).

Re: Powering two compressors with one PCM
 

To add to the above,
One and only one compressor on an FRC robot, please.
The PCM is not intended to handle the current of two compressors so a Spike or other relay is your only choice.
The pressure switch is a 3 amp device so it is not an alternative for powering a compressor. Even the smallest compressor teams will use, has a nearly 20 amp start and nearly 10 amps run current. The large Thomas type compressors have a 27 amp start and 12 amp run current.

Re: Powering two compressors with one PCM
 

You can ignore the PCM entirely and use a Y cable to control 2 spikes. Not sure if this would be legal in 2017, depends if the PCM is required. Either way you need to have the pressure switch in somehow.

On a spike powering a compressor, you are allowed to replace the yellow single blow 20A fuse with a resettable 20A fuse. I recommend doing this as sometimes the fuse can blow from inrush current, or inductive kickback when it turns off.