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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.
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.
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.
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.
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.
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?
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
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!
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)?
In terms of using the FP’s to servo an arm mechanism, there is a definite issue with using the motor itself to hold the arm in a position(stalling the motor and allowing heat build up). Some systems, however, use other mechanisms(worm gear, ratchet,…some crazy new idea) to limit the back-driving or at least the torque reqired by the motor to prevent it. This limits the heat generation in the motor as it requires less current to hold position. If you are careful you can come relatively close to balancing the heat transfer out of the motor with the heat generation.
Long story short, FP’s can power arm mechanisms very well but considerations must be made for the properties of the motor.
Regarding the FIRST email, the voltage limit will increase the chances of your FP’s living for obvious heat generation reasons. Some teams run at 12V safely, many don’t. If you are unsure and the FP’s are integral parts of your robot’s design, better to be safe then sorry.
BTW how about temp sensors on motors that control limiting voltage? (if you really want to squeeze all the power you can get with a smaller safety margin) You would need a reference temp for when you are close to burning out the motor but just at thought. Kinda like a smart heat sink/fan for your processor)
I am not an engineer, but I would like to suggest that teams be very careful that any arm being moved by the Fisher Price motors is not pushing on something immovable (like the ground). It is sometimes easy not to notice that the motors are straining if there is no motion. You need to alert your drivers to be very careful about that.
Also I suspect that some teams are trying to deal with large forces (such as a vision tetra on the end of a 10 foot arm) without gearing down enough. Calculate out how much force it takes to lift those tetras on a long arm.
So how is team 1072 making sure that the motor doesn’t stall? I suppose you have to have the torque from the arm when the arm is in the position you want, transferred to some other mechanism. With our arm design, and the way that team 833 has played FIRST in the past, I know that our arm will need to be up in the air many many times, for long periods of time.
So if you do a gear reduction from the motor to your arms pivot point this will reduce the strain on the motor at stall? is 7:1 enough? Also, if you put a heatsink on the motor along with a muffin fan, can you make up for the problem of heat enough that there shouldn’t be a problem or are there other issues than the heat?
What is the reason that this is not an issue for the other motors provided?
I assume that you mean 1:7 after the FP gearbox supplied in the kit. I can’t answer that conclusively without knowing the properties of your arm. Our team always calculates for worst case scenario, which would be in this case the arm being horizontal and fully extended with a full load (We would never do that, but the arm should withstand it very briefly without burning motors).
A heatsink and fan will cool the motor case but not the wire coils in the motor(at least not directly or fast enough). It will help but not as much as some think. Increased air flow through the motor along the surface of the coils is probably the best thing for this situation. Not sure what the best way to pull that off is. Any suggestions?