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running two 12V motors on one analog stick 
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Rookie

Joined: Wed Nov 12, 2008 10:47 pm
Posts: 8
Post running two 12V motors on one analog stick
We are wondering, since it is't possible to synch 12V motors, we're wondering if it is possible to run two 12V motors off one analog stick. If it is, we would appreciate an attached example program.


Wed Jan 21, 2009 5:27 pm
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Joined: Thu Oct 09, 2008 7:58 pm
Posts: 79
Post Re: running two 12V motors on one analog stick
Yes it is possible.

Where motorA and motorB are your two DC motors and ur using joystick 1 and y1.

motor[motorA] = joystick.joy1_y1;
motor[motorB] = joystick.joy1_y1;


Wed Jan 21, 2009 8:39 pm
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Joined: Sun Nov 16, 2008 3:07 pm
Posts: 45
Post Re: running two 12V motors on one analog stick
You can also wire both motors to the same speed controller if you want them to do more or less the same thing.
(there are some problems with this - for example, you only get one encoder feedback in this case, so the commanded PID output will be appropriate for that motor, and the other will just run open loop on that same voltage. In our bot, we do that with the two motors on a side, and it mostly works pretty good except when the wheel with the encoder gets popped up in the air, which causes the PID loop to cut back on the voltage, since it is turning really easy, which causes the other motor on that side to also not have much voltage, and the net effect is sort of like anti-traction-control. It would be better if the 2 motors were mechanically geared together (or geared together in software as you wished in the first place). There are also limits to how much current a single speed controller can supply. I don't know how much - I haven't seen it leak smoke with 2 of the Tetrix motors connected (although we did let a little of the diagnostic smell out of a motor by stalling the one without the encoder and not the one with the encoder. I'm not an electrical engineer, so I don't fully understand the workings of the magic smoke, but I think when the un-encodered wheel stalled, the back EMF in that motor went to zero, leaving only the small resistance of the motor winding to slow down the current from the speed controller. This diverts all the current from the motor with the encoder into the stalled one, which causes the encodered wheel to slow down, which causes the PID algorithm to increase the voltage, which causes more current to go through both motors, but the un-encodered one still wasn't turning so most of this additional current went through that one instead of the other one, so the PID algorithm increased the voltage even more.... hey. what is that smell? Why aren't there some current limiting devices like FETs or breakers or Fuses in this kit?)).

If that ramble wasn't more than you asked for, then I can't imagine what it would take. :-)


Wed Jan 21, 2009 10:51 pm
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