**FIRST EMAIL**/Fisher-Price Motor Information

[sanddrag]

Hello. This is an important announcement for all FIRST Robotics Competition teams. Please share this information with the rest of your team in a timely manner. If you think that other members of your team would also like to receive these announcements, please encourage them to sign up at http://listserv.leapit.com/cgi-bin/l...join=frcpublic.

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Greeting Teams:

Caution is advised regarding the Fisher-Price motor. Because of the apparent short supply of Fisher-Price motors (made by Mabuchi), FIRST advises that teams try very hard to protect the Fisher-Price motor from being burned up or fried.

FIRST advises that:
You consult the torque curve for the motor and do not operate beyond the peak power point
You not "stall" the motor, which will subject the motor to very high current
You limit the voltage to the motor (via programming) to 6V
You use a 20A Snap Action breaker
We do not guarantee that this will protect your motor from damage, but it may help you.

FIRST is working hard on replacement motors. More information to follow soon.

Go Teams!

[ahecht]

Does the advice aboout programming the motor for only 6V seem like a bad idea to anyone else? I would think that the motor would be much more likely to stall at a lower voltage.

[Joe Ross]

Originally Posted by ahecht

Does the advice aboout programming the motor for only 6V seem like a bad idea to anyone else? I would think that the motor would be much more likely to stall at a lower voltage.

Yes, the motor would stall at half the stall torque. However, when it stalled it would draw half the stall current as at 12v and most importantly, 1/4 the power.

[Jack Jones]

Well - thanks, but no thanks. I certainly hope that the follow-ups will really addess the issue. That is, at 6v the motor is almost useless - at 12v the 13 tooth tranny it blows the final stage through the side of the case.

That warning has nothing to do with the fact that the kit 13 tooth w/ Mabuchi motor has been discontinued by FP - for obvious good reason.

[Kevin Sevcik]

Umm... Has anyone else noticed that it's technically impossible to limit the motor to 6V in programming? I mean... We're talking PWMs here. It's all 12V with varying duty cycle. And at 50% duty cycle, I don't think the motor would magically smooth the pulses out into a 6V signal.

I realize that 50% duty cycle would probably have mostly the same effect as running at 6V, but there's an important distinction there and a statement like this could serve to easily confuse rookies that are still grasping the concept of PWM.

All that said, Team 57 is still going to be mighty careful with our FP. I also forsee a large market for them on CD-Swap, for those teams that are not making use of them.

[Alan Anderson]

Originally Posted by Kevin Sevcik

Umm... Has anyone else noticed that it's technically impossible to limit the motor to 6V in programming? I mean... We're talking PWMs here. It's all 12V with varying duty cycle. And at 50% duty cycle, I don't think the motor would magically smooth the pulses out into a 6V signal.

In effect, that's exactly what motors do when driven with pulse-width modulated power. The frequency is high enough, and the inductance of the motor is high enough, and the resistance of the motor is high enough...it makes a dandy low-pass filter.

Even if it isn't really smoothing the pulses completely, the thermal mass of the windings is great enough to make the average power more important than the peak power.

[jgannon]

I'm guessing that FIRST is erring on the side of caution on this update, which is understandable. However, all of our torque and speed calculations assumed that we would be running at 12V. In past years, what programming techniques have teams used to get as much power as possible out of the Fisher-Price motor without releasing the magic smoke? Maximum PWM value? Feedback analysis?

[Cory]

In the past the FP's we've received have been designed to run at 12 volts, so this problem wasn't nearly as significant.

[jgannon]

Originally Posted by Cory

In the past the FP's we've received have been designed to run at 12 volts, so this problem wasn't nearly as significant.

Are you saying that this year's FP is not identical to last year's?

[Cory]

This year's FP is much more powerful when run at 12v that last year's. It's max power is higher than the CIM's, albeit you'd melt it. Last year's FP's were significantly less powerful

[jgannon]

Originally Posted by Cory

This year's FP is much more powerful when run at 12v that last year's. It's max power is higher than the CIM's, albeit you'd melt it. Last year's FP's were significantly less powerful

Hm... I hadn't realized that, and neither had anyone else here, apparently. I just thought we were going to have a bunch of spares. Thanks.