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Unread 01-09-2002, 07:50 PM
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4 Motor Drive Train! EEEEK!

I know some teams are attempting this, as they want to be able to pull these darn heavy goals. So I figured this thread my be of some use.

To those teams who have done 4 wheel drive in the past with drills and Fisher Price. Wanna post some designs?

Did you have the Drills and FPs in one transmission?
Did you gear down the Drills to 100rpm (FP's output rpm)?
Did you gear up the FPs from 100rpm to 300rpm (drill motor's rpm)?
Did you have problems with different RPMs between motors?

I figured I can easily gear down or up a motor to match a different one at no load. That's simple enough. But what happens when the load is increased on both motors? Will both motors spin at the same RPM still (will they decrease to the same RPM)? Or will one suddenly say, "Uhh, sorry big guy I cant handle all this torque, i'll just spin at 75rpm. I'm not a horse like you and can handle 100rpm at this torque level..."

Lastly, if a motor was running at 100rpm, and another was running at 110rpm -- and there's absolutely no other way to gear it down anymore mechanically, will this pose a problem? Should you adjust this electronically then?

-anton

ps. i wish i could borrow Joe J for a day. cough cough.
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Unread 01-09-2002, 08:19 PM
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Ok, I know I am NEW to this posting thing, but I do know a little bit.

Our team last year, attempted to do what you are talking about. I was told by my advisor what we did wrong, but I will let a senior member here tell you about that.
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Unread 01-09-2002, 08:52 PM
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umm... yeah, i need to know more details about what you guys did. if you can get him to share any problems you may have experience, i'm sure it will help all the newbies attempting this.

-anton
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Unread 01-09-2002, 08:59 PM
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Thumbs up

Im very curious about this myself. I know my old team had all kinds of motor tricks, but now that I'm on my own out here I need to learn for myself. It would be VERY convenient to link some chalupas to our drills...
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Unread 01-09-2002, 10:38 PM
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Our team had an idea to gear the motors electronically...

We would have a flywheel on the output axle of each motor .
On the outer edge of the flywheel, we would put that special reflective tabe at set degree (15 maybe). Then we would set up the optical sensor to count rotations. We would then make a control loop to compensate for differences in rotation....But since they banned using hte tape on your robot, we can't do it anymore

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Unread 01-09-2002, 10:49 PM
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You could use a pair of always-on LEDs mounted on the flywheels to build your timing light.
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Unread 01-09-2002, 11:17 PM
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Anton, check out this photo -

http://www.chiefdelphi.com/forums/pi...rrow=4&trows=3

You can see how they coupled the two motors together. I don't know they're exact ratio, but they told me that they tried to match the peak power curve for each motor. I think it's something close to 3:1, though, if not that exactly.

Nobody from Sachem actively posts on CD, but I am pretty sure at least one person reads it. Maybe they'd be willing to chime in and share.
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Unread 01-09-2002, 11:36 PM
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Last year--drill and F/P together

Last year we connected the drill and Fisher-Price motors end to end, ran the output of both through the drill gearbox, and drove our track drive from the output of the gearbox. While the no load rpm of the drill motor is higher than that of the F/P's, the difference is fairly small. We had them running at the same speed and it worked pretty well.

This year, I suspect that most of the four motor drive trains will use the drill motors and the new Chiphuas(sp).
There is a big rpm difference between the drill motors and the Chiphua's, ~20,000 vs ~5500 so the motors need to be geared separately to match rpm at the load point.
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Unread 01-10-2002, 03:11 PM
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trannys

Last year, we had the idea to mount the fisher price motor and drill motor onto the tranny and have them both spinning the second to final drive gear. However, we were told by our great tranny mentor that we couldn't put two motors on the same gear because they would burn each other up. I still doubt this claim.

However, why bother.

Three drill motors, three drive wheels. Other motors for spinning the three wheels. All (3) wheel drive, all wheel steer.

-smokescreen
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Unread 01-10-2002, 07:24 PM
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Gear them to match speed--run in the correct direction

The drill motors are about 20,000 rpm no load and the FP's are about 15,000. Ideally, these motors should probably be geared to have their rpm in that proportion. They are close enough, though, that we had no problem with them running the same rpm. Just make sure they are connected to run in the correct direction to not 'fight' each other.
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Unread 01-10-2002, 09:18 PM
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We have ahd this conversation before.

I will say it again: there is no magic in having 2 motors that are different inputting to the same gear.

The bottom line is that the speeds are required to be in some ratio that is fixed by the gearing. Assuming the same voltage to the motors, the motors then simply act like the SUM of the two motors.

The speeds are equal (or geared to a ratio) the torque out is like the sum of each motor at that speed.

One of the things that you have to keep in mind is that the speed torque curves are a line that extends into negative values.

If the speed of one motor is faster than the other can provide at the voltage provided, then it is providing negative torque -- that is, it is slowing down the other motor.

As to one motor burning up the other, this can happen if the speed are SO different that the driver motor is doing a lot of extra work to drive the other motor. In this case, the driven motor can also get hot because it is generating a lot of current that might be going back into the battery or it might be going to generate heat (more probable).

Joe J.
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Unread 01-15-2002, 11:47 PM
Jeremy Davis Jeremy Davis is offline
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60 Amp Breaker

To anyone who has attempted 4 motor drive trains in the past:
How much trouble have you had with tripping the 60 amp breaker with all four motors in use? It seems to me that that could occur fairly regularly given 4 powerful motors all drawing current at the same time.
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Unread 01-16-2002, 12:38 AM
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Almost never

We didn't find any problems with tripping the 60A breaker last year during normal operation on our 4 motor drive train.

The only times that it tripped was when we ran over the bridge too fast and landed with a thud. The breaker tripped due to the slam, not the overload.

Andy B.
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Unread 01-16-2002, 08:45 AM
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According to a post on another thread, at a load of 100 A, the breaker takes nearly two minutes to trip and takes 8 seconds at 200 A. We would need to have four motors with the 30 A breakers nearly tripping continuously for an entire match to trip the big breaker.
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Unread 01-16-2002, 09:41 AM
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Smile Drill motor and FP combo

Seems like a lot of people have used the drill motor and FP motor combo. We used it last year and had no problems. We went to 2 regionals, Nationals, 4 post season tournaments and many demos, ie lots of use. I think replaced only one FP. We use the FP on the back of the drill motor but not in line. We use a 12 tooth gear on the drill and a 16 tooth on the FP. Since the tail end of the drill motor shaft is very short and the shafts are so small in diameter it requires EB welding. Also need to make a motor housing that hold both motors parallel and at the proper center distance (we can adjust the center distance). Since both motors are on the input side of the drill gearbox even if one burns out torgue to just turn a motor is nothing so you can atleast still move. We had good speed and lots of power. In practices we could easily pull a loaded stretcher and push loaded load over the bridge at the same time. 2002, a drill motor/chalupa combo?

Jim from BUZZ team 175
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