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

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.

If you would like to stop receiving these messages, please see the bottom of this message for instructions.
__________________________________________________ ____

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!

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

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.

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

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.

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

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.

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

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.

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

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.

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

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?

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

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.

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

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

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

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

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

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.

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

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!

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

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

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

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.

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

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.

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

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)

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

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.

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

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?

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

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?

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

Do a search for posts by 254 members... they explained in great detail how they made their arm work. Im pretty sure it was Jay Lundy.

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

Well we've finished our arm driven by two FP's and now I'm feeling nervous. We've built a new gearbox for a total reduction of 1700:1 and it works great and only draws 15 A (each) when picking up a tetra at the end of its 5' arm. However, that's without modifying the program to limit it to 6 V. I wonder if it will still work?? Theoretically 15 Amps at 12 volts should translate to 30 Amps at 6 Volts, but unfortunately heating follows I^2*R which means we'll end up with more heat loss :( This sucks!

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

Jizvonius, how would holding the arm at it's full position - say, at a 40-50 degree angle up - be much different than holding it straight out? I understand that the torque will be a little bit less because of the angle, and the center of mass is closer to the pivot, but is it that big of a difference on a 5-foot arm?

From what I can tell it is a very sketchy thing to use the fisher-price motors to power an arm that needs to able to stop at various angles to retrieve, manipulate, and place tetras from loading stations and onto different height goals.

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

I can't say that the difference will be that big in a 5 foot arm. However, some teams have arms much larger than that.

The statements that i made earlier were general. They were based on the fact that I've seen many an arm work well with FPs powering joints. Holding position solely with the FP is indeed sketchy. I was just saying that with a little ingenuity some of the load can be taken off the FP to make a reliable arm pivot.

Some ideas that i had for limiting backdriving of the pivot to save FPs.

-generic worm gear system. (lead angle about 20 degrees saves some efficiency but increases backdriving)

-Servo/piston controlled gear brake near motor side of gearbox

-piston FP combo with piston regulated to lower pressure(helps with movement and holding position)

Feel free to call me crazy. It's happened before. Just back it up with something.

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

We're not using it for rotation, so our problem is much simpler. Since the new specs of the van door came out, we redesigned our elevator to be powered by 2 fisher prices. Basically we're using two of them at a reletivly low spool diameter to make SURE that it won't stall. I think our grabber would actually break before it stalls.

EDIT: I just remembered that one solution we came up with in our brainstorming (at one point were were thinking of doing a rotating arm), we were going to extend the bar past the pivot point and attach a cable to the back end, and a point equally far away from the pivot at the front of the arm. A cable would go down to a spool, wrap around it a bunch of times, and then go back up the other attachment point. That may pose entanglement issues though. Just a though.

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

Probably too late to pull it off, but if you used the DeWalt transmission for the FP, you could leave in the anti-backdrive pins and just not run the motor when you don't need to.

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

What would any of you say about having a reduction on your system so great that you would just rely on the torque it takes to backdrive the motor being greater than the force of the tetra and your arm? This way you do not stall the motor, and it holds position.

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

Back on topic - I.E. the unavailability of the kit FP.

They promised more information to follow, yet none has come this way.

All who plan to use - or misuse - the FP motor had better e-mail frcteams@usfirst.org and Sharon Gele <sgele@usfirst.org> in particular and implore them to let us use last year's motor as well. That would solve the availability problem and the 6v motor in a 12v system problems as well.

I do not understand why they have not done so already. :(

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

Some people asked about the 2004 254 robot. Yes, we did use The FP motors last year to power our arm, and no we are not using them for our arm this year. It was our original intent to use them, but we have changed our design, based on the FIRST blast information. We will be using the FP motors as supplied by first in our drive drain, but will use in tandem with other motors.

We are planning to use them through a unmodified PWM - We have no plans at this time to regulate the voltage down to six volts before sending it to the PWM, because 1) we don't know if that's legal (would require addition of some inexpensive electronics -a couple of high amperage solid state voltage
regulators) 2) We don't know if the PWMs would work correctly, if only 6 volts is being fed into them( I don't see why they wouldn't, but I am not a EE!). Our plan is to run the FPs and replace or remove them if they fail. We did have to fine tune our gearing so that the other motor overdrives the FP and not the other way around. We are estimating the FPs are contributing < 25% of the power to our drive.

To everyone struggling with this - good luck. Overcoming limitations in your
supplied equipment is part of the challenge.

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

Jack,

Please relax. I assure you that FIRST (and others) are doing everything they can to get the situation resolved. It is the number one priority for the engineering department at FIRST. Fisher-Price and Mabuchi are also scrambling to get a solution. Things are not always as simple as they seem.

-Paul

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

Thanks Paul.

I have one question. Everyone says that the FPs will inevitable fail. Besides JVN's beating, has anyone actually had this year's motor fail yet? If so, can we hear about the situation it was used in and the failure that occurred?

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

If designed in correctly, the FPs should not fail.

What has been discussed is that if you stall an FP at 12V it will fail very quickly. Stalling an FP at 6V should be OK for quite a while. Operation at high voltage and high torque (low speed) is problematic but you have an increased chance of failure the higher in voltage/torque you go.

At issue is that these are self cooling motors (you can see the fan in the end). If you put enough torque to slow the fan (motor) above 6V, you are at risk.

Regards,

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

Everyone should read this before continuing discussion here.

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

Zan,

IMHO, how to address the design issue of a 6V FP is still relevant.

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

Last year First included a current sensor as part of the kit. If any teams still have the current sensor, they could use it to monitor and control the current to this years Fisher price motor and gear box. It could be used as a fixed point limit or with a little more programing starting curves could be implemented.

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

hmmm... back to the drawing board, and digging around in previous year's junk.. i think were gonna attempt to put in the old motors.. what's everyone else planning on doing this late in the game??

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

While the individual components would qualify, I'm not sure that last year's printed circuit board is available to all teams... If not, it would be illegal...

[edit] Not really illegal, but you would have to cost account a custom PCB... Not trivial... [/edit]

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

If teams have the parts, perfboard and kludge it. There are also 1 chip solutions from digikey.

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

This post may be helpfull for people trying to account for the cost: http://www.chiefdelphi.com/forums/sh...7&postcount=42

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

Another option for teams electing to run the 6 volt motor is to limit the peak voltage going into the PWM to avoid over heating the motor. I tested running a Blue Victor using a 6 volt supply and it appeared to run well. I am not an EE, so I can't say if this is safe for the PWM - I would continue to run the PWM fan on a separate 12v circuit. I also am not sure if this violates some other safety/construction rule. So I'd appreciate it if an EE and someone from the rules committee would chime in. I checked on Digikey, and there are many solid state voltage regulators at 5, 6, 7 volts and up to 20 amps. There is a cheap hobbiest trick that uses 2 diodes to float the center tap of a 5 volt regulator. This should result in a 6.2 volt output on the regulated supply.

Even the 12v FP motors and transmissions tend to heat up in competition - so I would consider using a fan to cool them. A simple trick is to position the PWM near the motor so that air is always blowing over the motor.

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

Alan,

Just limit the PWM signal to a 50% duty cycle (verify with a good analog meter like a Simpson) and you do the same thing (and stay within the rules to boot).