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Unread 16-11-2001, 22:25
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Quote:
Originally posted by Jay Lundy
I'm only taking my first year of physics this year and I don't know much about motors but I'll give it a try.

First of all, I don't think the friction between the wheel and the ground has much to do with it. However, it becomes important when talking about traction, and at what point the wheels will start slipping. Friction is the only thing that allows for the transfer between rotational and translational movement.

Relating this to last year's game, the bridge was at an angle so N was less, so the force of friction was less, so the wheels slipped. Basically traction depends on the coeffecient of static friction between your wheels and the floor, your weight, and the angle of the bridge. If the force causing your wheels to rotate overcomes the coeffecient of static friction * N, then the wheels slip.

As for why the robot does not keep accelerating, I seem to remember seeing a chart that graphed torque vs. speed. As the speed got faster, the torque got lower. Eventually, there is no more torque and since T=Fd, there is no more force to cause the robot to accelerate.

Think of it like you trying to rotate something like a bike wheel. You can make it accelerate, but eventually the speed maxes out and you can't apply any force to make it go faster. I think the motor works similarly.

Basically, the only significant forces working against the robot is the friction in the wheel axle, and any other objects the robot may run in to. Although the coeffecient of friction in the ball bearing may seem small, it makes a difference when it has 130 lbs on it. Air resistance and the friction between the wheel and the floor are both very small and don't affect the robot much.

Of course, I may be wrong about any of this.

You are actually right in a lot of parts. I really said it wrong when I said the linear force on the wheel is limited by friction. What that really means is that the linear force coming from the wheel can only be so strong before it over come friction and start slipping...

And, you are right on with the situation on the bridge. When even a robot gets on the bridge, there's a certain force it need to push up the bridge.

HOWEVER, the speed torque curve is used to show the motor's responds to torque. When there's no load on the motor, it spin with a speed we call free speed. Then, as we put more and more load onto the motor, it spins slower and slower, until the motor stall. The amount of torque it takes to stall a motor is stall torque. With those two numbers, we can get a speed torque curve. So, the amount of torque didn't got less because motor speed is higher... but the other way around.

You see, when you mention about how rotating a free wheel, you are right that it can only go up to a certain speed. That happen because the force you use to spin the wheel is much less in the reference frame of the wheel. Imagine if you are using your hand to spin a bike wheel. When the bike wheel spin up to a certain speed, your hand simply isn't fast enough to catch up with the wheel and exert the same amount of force on the wheel.

So, when that happen to the drive train, as the robot goes faster and faster, the load on the motors is no longer as much as before, and therefore it should be spinning faster. So the RPM reflecting back to the gear driven by motor will catch up to the motor's RPM, and forces from the motor is only used to over come friction and other forces.

Hmm... Is this true? Is the speed of the motor maximum when robot is accelerated enough such that the momentum of the robot is carrying the robot forward and the load on the motors got much less than not zero speed?

Well, my question still remains. How do I calculate how strong the robot is pushing under different situation? How should we set up gear ratio when the force/load is changing all the time? And how do we calculate how much force it takes just to move the robot across the field, even when not going up the bridge? If the robot is dragging heavy objects such as two goals... how do we calculate the force it takes to push two goal + robot?
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