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Unread 19-06-2003, 01:32
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Quote:
Originally posted by sanddrag
Of course, all of this performance couldn't have been because of the 4 motor drive, I mean c'mon.
It has a little to do with the 4 motor drive train of course . Just not as much as you think it is.

Consider the wheels for the moment. If you have frictionless wheels for the sake of the argument. Then you will have NO speed or pushing force at all, no matter how many motors you put on the drive system.

Take a look at the formula of friction for a moment:

F(s) = Mu(s)N where F(s) is how much friction force you can get out of your material, Mu(s) is the coefficient of static friction of your material, and N is how much weight you put on the material.

If you have lots of Mu(s), and no N, that means you have really good wheels with high friction, but no weight on the robot, which will yeild a low friction force. If you have lots of N, but no u(s), then that means you have a really heavy robot, but no friction on the wheels at all, which again yeild a low friction force.

Should be fairly easy to understand, right?

So, now that you know what wheel you use and how heavy your robot is limit how much force you can put on the wheels, let's look at the motors side. Every motor can output a certain amount of mechanical power, and mechanical power = force X velocity. You may set up the gear ratio to setup different combination of force and velocity, but at the end, you can only get so much power out of the motors. With more motors, you have more mechanical power. But that's just power.

Consider this simple example:

If you have 10 unit of mechanical power with one motor, you could setup a gear ratio such that you have 2 unit of velocity and 5 units of force, or 5 units of speed and 2 unit of force. If you have 2 motors, you have 20 unit of mechanical power, and you can setup a ratio and get 4 units of speed and 5 unit of force, or 10 units of speed and 2 unit of force.

In both cases, you can see how 1 motor and 2 motor setup could yeild the same amount of force. Only the one with more motors will go faster.

So, it's really comes down to how you setup your robot. If you have lots and lots of pushing force from the motors, but very crapy wheels, you won't be able to push anything around. If you have good traction wheels, with lots of pushing force, but only 1 motor, you can push everything around all right, just at 1 ft per minute.

In your case, when you have a strong AND fast robot, you can out push people because you can build up your momentum under a short distance. Otherwise, if you are in a deadlock with another robot with just as much pushing force but with only 2 motors instead of 4, and equal amount of traction, you won't be able to outpush it.

If you really want to learn all these, check out my notes in whitepaper. The title should be "WRRF motor selection lecture notes". Then you will understand in Engineering we don't do too much guess work .

http://www.chiefdelphi.com/forums/pa...gle&paperid=23
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Last edited by Ken Leung : 19-06-2003 at 05:25.
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