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

[Jizvonius]

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)

[DougHogg]

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.

[HPA_Robotics_13]

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?

[Jizvonius]

Originally Posted by HPA_Robotics_13

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?

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?

[Cory]

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.

[Doug G]

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!

[HPA_Robotics_13]

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.

[Jizvonius]

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.

[Alex1072]

Originally Posted by HPA_Robotics_13

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?

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.

[Kevin Sevcik]

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.

[HPA_Robotics_13]

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.

[Jack Jones]

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.

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.

[Paul Copioli]

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

[sanddrag]

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?