Fisher Price Motor Power

[Joe Johnson]

Originally Posted by vamfun

Yes, we are all humbled

I think from the discussion, that the FP motor will trip the PTC if it is stalled, although, I have yet to see any test data. I would propose a test where the motor is stalled and voltage increased until the PTC trips to see what the max continuous current is (at least for the lab conditions) , except I don't want to risk a motor for the test. My hair dryer has tripped a few times when the dust builds up over the fan exhaust, but it has always recovered. I would hope the FP would recover too. Has anyone actually tripped an PTC yet?

I really am not sure how to design a fixed roller that sucks up a ball to ensure a motor will not stall. It would seem that this requires limiting the normal force on the roller which is difficult when a ball is squished between a wall and a robot. Even with a frictionless back bar, it seems the ball can deform and push hard against the roller. If the roller is at all sticky getting a normal force equal to the output torque/radius doesn't seem too difficult.
We are using a FP with a banebot 16:1, a 1.6 in dia roller. This can deliver about 30 lb of tangential force to the ball and getting a normal force 30/u_roller seems plausible for any reasonable u_roller.

Squirrel and others seem to be able to spin the ball on the rug which implies u_roller>u_rug. Using a slippery roller can keep from stalling probably , but degrades ball magnet performance.

So, we limit the normal force on the roller and actually shut down the motor when the normal force lifts up the roller. I still would be very happy to know if we do stall that the motor is not damaged.

If you can use a Jaguar and use CAN, you can easily know the current. There are other ways to measure or infer current (sensors, shut resistors, measure speed & calculate, etc.). Once you have current you can easily build a "how close to tripping the FP bi-metal breaker am I?" model. It will not be too hard. If you are using Labview, I think it may be trivial.

It won't perfect but FIRST almost never requires perfection. Build a simple predictor, tune in some parameters and viola! You can know if your rollers are on the edge of a nervous breakdown ;-)

Cheers,
Joe J.