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Originally Posted by Rob Colatutto
There was no option to slow down, the robot slowed itself down when the additional load of the goals was added to the robot. The drills couldn't fully handle the extra 362 pounds and so the motor slowed down and then the Atwood picked up for it, drawing less current than running the Atwood at a slower rpm. We also had a 'software shifter' which took the joystick feedback and divided it by 1.5 so the operator could switch the speed down when latching onto goals.
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Matt and Rob are both on the same page with this one.
Basically it is possible to design a gearbox to emphasize different motors depending on the torque-load on the gearbox (which determines motor performance... speed, efficiency, current draw, etc).
This means: your gearbox is designed with 2 motors. When the robot is moving across the field unimpeded, the torque load on the motors is such that one motor is carrying most of the load, and the other is somewhat dragging within the gearbox. When the robot is under a high-load condition (like towing goals, or pushing against another robot), there is more torque loading on the motors, and now the other motor's output is greater.
I don't like this method.
What we use on 229, (and many other teams have used, with great success) is simply matching the free speeds of the motors. This means, that if the robot is opperating under a no-load condition, both motors will be spinning the same speed. Will this ever happen: No! But... it works out pretty well. Essentially you get a nice balance of torque output from each motor at all different load conditions. It's not perfect, but it works well.
We also couple this with a 2-speed shifter. This allows the designer to provide 2 different torque output conditions, depending on torque loading. (Need to tow a goal? Just downshift!)
*shrug* This is all a matter of preference. There has actually been quite a bit of debate on this subject within the past year (Tytus' differential combiner concept). I think for now we'll stick with matching free-speed, simply because it's easy, and it works well enough for our applications.
Good luck,
John
PS - As always, for more information about the theory and physics involved in this stuff, look for other posts on this forum (There are plenty!), contact your friendly neighborhood FIRST mentor/enginerd. (Or drop me an IM.)