**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.

[HPA_Robotics_13]

I am confused. FIRST advises not to "stall" the FP motor. Does stalling mean having the motor stop at 0 rpm? If so, does this mean that one should not use the FPs to power an arm, in which case the motors will have to stop with the arm at a certain height with a load on the motor?

Also, I would really really like to know where "the torque curve" can be found.

I have never done any work with motors in past years, so any help would be great!

[Alex1072]

Originally Posted by HPA_Robotics_13

I am confused. FIRST advises not to "stall" the FP motor. Does stalling mean having the motor stop at 0 rpm? If so, does this mean that one should not use the FPs to power an arm, in which case the motors will have to stop with the arm at a certain height with a load on the motor?

Also, I would really really like to know where "the torque curve" can be found.

I have never done any work with motors in past years, so any help would be great!

Yes, using the FP to servo an arm would be a bad idea.

[HPA_Robotics_13]

Teams such as 254 (the Cheesy Poofs) used the FPs to power their arms last year. How did they make their setup work? I am very confused because I have heard many people, like Alex, say that it would be bad to use these motors for an arm mechanism, yet others talk about how it could work and how their team has done it.

The Cheesy Poofs beat my team in the quarterfinals at the Sacramento Regional last year, and I happen to know (I was the driver last year) that their arm worked phenomenally well.

Sometimes it's hard to tell who really knows what their talking about around here. No offense to you, Alex, I'm just confused.

[Alex1072]

Originally Posted by HPA_Robotics_13

Teams such as 254 (the Cheesy Poofs) used the FPs to power their arms last year. How did they make their setup work? I am very confused because I have heard many people, like Alex, say that it would be bad to use these motors for an arm mechanism, yet others talk about how it could work and how their team has done it.

The Cheesy Poofs beat my team in the quarterfinals at the Sacramento Regional last year, and I happen to know (I was the driver last year) that their arm worked phenomenally well.

Sometimes it's hard to tell who really knows what their talking about around here. No offense to you, Alex, I'm just confused.

None taken. I didn't mean to say that you can't use a FP motor to power an arm, I was just saying that it in general would be a bad idea to servo an arm with it (although I'm sure there is some clever ways to do this safely). We're using 2 FP motors this year to power our elevator, but we are making sure that they will never stall.

There are two factors that cause overheating when motors stall. First, the higher torque the motor provides, the more current it draws. This increases the heat output of motor's wiring. Second, motors usually cool themselves by spinning, so by stalling a motor you are also taking away the major form of heat dissapation.


Now, the FP motor given to us by FIRST was designed to run as 6V. When you double the voltage to 12V and stall it, you are actually more then doubling the heat output: Power = Voltage*Current. Stalling at 12V will draw twice as much current as stalling at 6V, so the current doubles as well. This means that the Power is actually increased by a factor of 4. Since the motor shaft is not spinning, no Work is being done on anything by the motor. That means all that 4xPower has to go directly into heat. *sizzle*, *melt*.


I don't know about stalling it when running at half (effective) voltage, but from what i'm understanding these motors are not of the highest quality, so even that tends to burn them out after a couple of seconds.

There is nothing wrong (I think) with stalling them at a low voltage, so maybe if you gear them down enough it would work (not sure about this)?

Hope that helps.