Fisher Price Motor Power

[RyanJK]

Hey all,

We're currently designing a mechanism to flip our robot back right side up in case we flip over during the match. We're thinking of using a fisher price motor outfitted with a fisher price gearbox to power this mechanism, which would be lifting an approximately 120 pound robot over.

I've done some calculations, and unless I made a mistake, it looks like this is theoretically possible. But, we've never used fisher price motors before.

Does anyone with more experience think this is possible, or practical?

Thanks for the help!

[Al Skierkiewicz]

Ryan,
So that your calculations work out, the actual weight you will be moving is closer to 151 pounds depending on the weight of your bumpers. 120 for the robot, 20 max for the bumpers, 11-12 for the battery and cables.

[RyanJK]

Thanks for the tip! I can't believe I didn't think of that! I'll have to redo my calculations and see if this is still possible...

[Dale]

The FP motors are amazingly powerful if geared appropriately. They can deliver almost as much power as a CIM. The main trick with them is under no circumstances should you let them stall or get close to it. They WILL smoke, unlike CIMs which can take all kinds of abuse. They will fry before the breaker can trip.

We've never smoked a CIM (though I suppose it's possible) but we have a whole bucket of burned up FPs!

[RyanJK]

I just redid my calculations, and being able to lift our robot (I'm assuming our robot is 150 pounds) with one Fisher Price robot is possible, but its going to be very tight.

From what you said Dale, I would be afraid that this would smoke out the motor, so I think we're going to be trying to lift our robot using 2 Fisher Price's.

Do you guys have any tips on how to make sure that our motors run simultaneously? In my expereience, with the CIMs at least, even though your programming is set to activate the motors at the same time, they still don't power simultaneously. With the design for our flipper, its really important to have the motors working simultaneously.

Thanks for all your help so far!

[Richard Wallace]

Originally Posted by RyanJK

... Do you guys have any tips on how to make sure that our motors run simultaneously? In my expereience, with the CIMs at least, even though your programming is set to activate the motors at the same time, they still don't power simultaneously. With the design for our flipper, its really important to have the motors working simultaneously.

Thanks for all your help so far!

To team-up two FP's mechanically you might try the new Double Doozy gearhead from AndyMark.

Each motor must have its own approved speed controller (see <R49> and <R55>) so getting them to work together electrically will be a matter for the programmers. This should not be difficult, unless the two motors (accidentally) get wired so that their torques oppose each other! If that happens you can fry one or both very quickly.

[RyanJK]

That sounds like a good idea! By any chance, do you know the torque ration of that gear box?

[Joe Johnson]

There is one question about motors that the answer is always YES:

Q: Can Motor X lift Load Y?
A:YES!

Here is a tougher question:

Q: Can Motor X lift Load Y in Time T?
A: Depends.

You can always gear the motor down enough to have enough force. You cannot get more power out of a motor than the peak power (for a given voltage).

As to letting out the magic smoke in a FP motor, this is quite easy to do, but it is also not too hard to avoid.

In my experience, if you design the system to load the FP motor at about 35-40% of stall torque you will

• have the motor running fast enough to keep good air flow on through the motor
• plus you are running near the peak power point of the motor
• plus you stay on the high efficiency side of peak power (this means you turn less electrical power into heat per per watt of mechanical power)
• finally if you need extra torque (e.g. because another robot is in the way and you have to "power through" them) you are moving the motor closer to its peak power point, not away from it.

Good luck.

Joe J.

[Al Skierkiewicz]

As a reminder, the FP motors have had a thermal protective device internal to the motor for the last year of two. It keeps the motor from smoking but it lets go when you need it the most if you haven't done your calculations correctly. Please be aware that covering the holes in the frame is a good way to find out how the thermal device works. It is in series with one of the brushes and you can see it using a bright light if you look.

[vamfun]

Originally Posted by Al Skierkiewicz

As a reminder, the FP motors have had a thermal protective device internal to the motor for the last year of two. It keeps the motor from smoking but it lets go when you need it the most if you haven't done your calculations correctly. Please be aware that covering the holes in the frame is a good way to find out how the thermal device works. It is in series with one of the brushes and you can see it using a bright light if you look.

Interesting Al... so we won't trash the motor if it overheats? That's somewhat comforting. I have been worried about stalling the front roller which is driven by a FP. We designed it to keep the friction loads under the torque limit but perhaps in some type of collision we could stall it.

Question to you and Joe J.: What continuous current do you think can we run if we are in stall without tripping the internal protective device? With cooling Joe says .35 to .4 of stall is ok so we are driving a max of .4*70*12= 336 watts.
Without the cooling how much do we have to derate the power?

We are driving it with a Victor but maybe we could switch to a JAG and use the current monitor on the CAN buss to compute the heat input to the motor with time and shut it down sensibly.

[Al Skierkiewicz]

Chris,
The protection is not terribly accurate. I can't give you specific trip points as there are too many variables. The heat of the motor, the motor case, the ambient air temp and if the trip has occurred once, all play into the trip point for thermal cutouts. From experience, the FP likes to be running fast. It has an internal fan of sorts that helps cool the armature. Running at lower RPM doesn't get much air moving inside, raising the temperature.

[Mike Betts]

Originally Posted by vamfun

Interesting ...Question to you and Joe J.: What continuous current do you think can we run if we are in stall without tripping the internal protective device?...

Chris,

While I am humbled among the likes of Al and the Good Doctor, I'll give you my 2 cents...

A motor at stall is delivering no mechanical power. All of the electrical power, even constant current, is converted to heat. Most external cooling that teams implement is over the case and not forced into the motor. The heat will build up in accordance with the thermal resistance of the internals of the motor.

This will likely happen very fast.

That little fan inside the FP is directly cooling the motor windings. If that little fan stops, my experience is that failure will occur very quickly.

In my opinion, the PTC added to the FP in recent years is to attempt to save a child's extremity or to mitigate an actual fire in the toy for which the motor was designed. It may not be fast enough to limit damage to the motor.

I would not design a system where the motor could stall.

JMHO,

Mike

[Joe Johnson]

Originally Posted by Mike Betts

<snip>

In my opinion, the PTC added to the FP in recent years is to attempt to save a child's extremity or to mitigate an actual fire in the toy for which the motor was designed. It may not be fast enough to limit damage to the motor.

I would not design a system where the motor could stall.

<snip>

PTCs (Postive Temperature Coefficient, a.k.a. resettable fuses) are used all the time in automotive (and other) applications and they can be designed to trip at basically what ever point the engineers pick. That point may be on millisecond before the motor ignites or it may be at 10% loading.

I suppose that FP has picked it to keep the motors alive and well not just safe from fires. I would guess that implies that the motors can probably go to stall for short periods of time without tripping (a few seconds I would guess) at least at room temp.

How long can you run at 40% of stall? Hard to say and I have no data since the PTCs were not in place when I last played FIRST. Mr. Betts is correct in that the fan will play a big (non-linear) role in determining when that PTC will trip.

I am betting that it will run forever at 40% -- note by the way that of the 336W of Electrical Power In (.4*70Amps*12Volts), you get 120W of Shaft Power Out (.4*.45N-m * .4*16,000(Rev/Min) (2 Pi Rad / 1 Rev) * (1 Min / 60 Sec) = 120W. The balance (210W) is turned into heat.

For your reference, a curling iron is about 10-20 W and a blow drier is about 1000-1500W. So... ...this motor is generating heat like 10 to 20 curling irons or 1/5th to 1/7th of a blow drier.

Either way it is a lot of heat! The only way that much heat is going to get out of that motor without a temperature rise that is going to trip that PTC is to have A LOT of airflow.

Bottom Line: Keep those motors turning!

Joe J.

[MrForbes]

Another point that you might consider is that gearboxes are not 100% efficient, and gearboxes that have a lot of reduction (like the FP or multistage planetary boxes, and worm gears especially) can have surprisingly low efficiency. If your design does not take the frictional loss of the gearbox into account, and you calculate that the motor(s) can just barely do the job, then in real life it probably won't work.

[Joe Johnson]

Originally Posted by squirrel

Another point that you might consider is that gearboxes are not 100% efficient, and gearboxes that have a lot of reduction (like the FP or multistage planetary boxes, and worm gears especially) can have surprisingly low efficiency. If your design does not take the frictional loss of the gearbox into account, and you calculate that the motor(s) can just barely do the job, then in real life it probably won't work.

Yes, efficiency is a huge factor and very important for sizing gearboxes and designing mechanisms.

I use the following for designing using torque*:

• Straight Spur Gear with good bearing condition 95%
• Straight Spur Gear with funky bearing conditions 90% (look at the final stage of the FP transmission for an example of a funky bearing condition)
• Planetary
  • Low ratio (<5:1) 85-90% (depends on how good the bearings are, the grade of the gears, the size of the planets w.r.t. their axles... things like that)
  • High ratio (>7:1) 50%
  • Very High Ratio (>20:1 stages) 15% <<these are great for speed reduction by division, but lousy for torque increases by multiplication
• Helical Gear, parallel axis 80%
• Helical Gear, cross axis see Worm Gear
• Worm Gear -- totally depends on lead angle
  • Best case 50-60% (high lead angles of 40 deg, good bearings, etc.),
  • Worst case 5-15% (lead angles of 10 deg, bad thrust management, etc.)
• Conical Gears / Bevel Gears depends on bearing arrangement and alignment 60-90%
• Chain 90% (assuming good alignment and tension)

This is PER STAGE.

Example: If you have a 4 stage 4:1 per stage spur gear gearbox with good bearings it would be .95^4 = 81% efficient. So... ...instead of getting a ratio of 256:1 your "effective ratio" (from a torque point of view) would be 207:1.

Continuing with the example, if you put a FP in with a stall torque of .45N-m then you would get 93N-m out of this gearbox, not 115N-m. Now suppose you are trying to lift your robot with this gearbox and you have the output connected to a .17m arm (and assume your robot weight is 600N, then you need 100N-m to lift your robot.

NOTE: You are not going to lift that robot, all you are going to do is turn a lot of electrons into heat.

Continuing, if you put a 3:1 chain stage between the arm and the gearbox, the effective ratio would be 560:1 (207X3*.9). You could put 250N-m of torque on your arm. Now your motor would be loaded at 40% of its stall during your lift (and the motor would be running at 60% of its free speed or the arm would be turning under load conditions at 12RPM = 16,000RPM *.6/(256*3)<<Note: Actual Ratio used for SPEED, Effective Ratio used for Torque).

Now you'd lift in a heartbeat (1/2 turn in 6 seconds -- well... ...kind of a LONG heartbeat ;-) and you have extra torque should another robot get in your way on the way up.

Life is good.... ...always.

Joe J.


*Some say I am too conservative but my experience with FIRST and with automotive actuators tells me that these numbers are not far from the right ones.