Thread: HELP!
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Unread 17-03-2008, 22:04
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Re: HELP!

Quote:
Originally Posted by bobdahaxor View Post
I have some clarification questions and then an answer based on my assumptions (questions).

1. Can you turn the robot well when you are already moving?

Better, but not amazingly

2. Does the problem only happen when you are sitting still and you want to turn without moving forward first?

more or less yes

3. do you have the wheels on one side go in reverse when turning or are you trying to turn with just one drive wheel? (see our experience below for why I ask this question).

one is in reverse and one is going forwards
Thanks for the clarifying questions; they help provide some clues as to what is actually going on.

One of the issues with a 2WD system is the issue that was pointed out by dtengineering in his post -- you want as much of the weight to be on the driven wheels (in your case, the IFI traction wheels) as possible, with as little weight as possible on the undriven wheels (the omni wheels).

When the robot fails to "spin in place" when you drive one wheel forwards and one in reverse, which of the following happens? (1) The traction wheels spin on the carpet, with one going forwards and one going backwards, or (2) the motors are stalled out, trying to rotate, but nothing is moving at all, the wheels aren't even spinning.

If the problem is case (1) with the spinning wheels, you should be able to resolve the problem by moving heavy components from the omni-wheel end of the robot to the traction-wheel end of the robot. In particular, the compressor and battery are two excellent candidates for this. I suspect that this scenario is the problem that you are experiencing. Any weight you can move from over the omniwheels to over the traction wheels helps in two ways: (a) by reducing the resistance to turning since there is less weight on the undriven wheels and (b) increasing the traction available to the drive wheels by increasing the weight on those wheels. If the wheels are spinning due to a lack of traction, no gear ratio changes will be able to fix the problem.

The other possibility is case (2) with the traction wheels being stalled out. I would be amazed if this is the problem, as there is actually quite a lot of torque available in your transmission setup. If this is happening, it is almost surely due to your omni wheels not sliding sideways readily, either because they are somehow getting caught on the carpet, or the rollers are not spinning easily. Another possible root cause if you are seeing this problem would be binding of the transmission mechanism.

As for the single CIM drive mechanism potentially having insufficient power, our team's hurdling base used a single CIM drive on each side, with a final ratio of about 19:1, but using 12.5" wheels, giving almost exactly the same peak travel speed (and thus the same torque, since we are using the same motors) as your configuration. We had no troubles turning, but we had the weight of the robot very nearly centered over the drive wheels. This configuration still had excellent acceleration, even though it wouldn't win many pushing matches.

I believe that moving weight from the rear (the omni end) of the robot to the front (the traction wheel end) will probably solve your turning problem. However, it will also introduce another problem that your drivers will need to deal with... When the weight is largely over the front driven wheels and you then are traveling forward at full speed and suddenly apply full reverse power, the robot will have a big tendency to tip over forwards. Just something to watch out for...

Please let us all know what you end up doing to fix the problem.
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Last edited by Ken Streeter : 17-03-2008 at 22:08.