Dual versaplanteary's with different reductions question

[marccenter]

Dear CD,

On our intake mechanism this year we have found dual purposes. One function operates best at a 10:1 reduction for increased speed and the other at more like 100:1 for increased torque. Tomorrow we will be adding two separate motors and it appears there may be two options available.

1) Make both versaplanetary gear reductions something in the middle, right now 40:1, and compromise a bit on both functions. This is the planned route.

2) What would happen if we have one versaplantary gear reduction be 10:1 and the other 100:1? They are chain driven to the same shaft so I was wondering what issues would I have if I only enabled the 10:1 motor/gearbox when speed is desired and only enabled the 100:1 motor/gearbox when torque is desired? Obviously the speed would decrease somewhat at the 10:1 condition because the other motor/gearbox is along for the ride and similarly the torque would be decreased from the 100:1 motor/gearbox. Would the negative tradeoffs be worse than option 1?

Best regards,

[kitare102]

I would't be worried about the 100:1 fighting the 10:1. Just moving a 10:1 with your hand should show you that there isn't all that much to fight against.

However, I'd be worried about driving the 10:1 at any reasonable speed, because you'll have to turn the motor (unpowered) on the 100:1 stage 10x as fast as the motor (powered) on the 10:1 stage. If the final output is only spinning at 500RPM driven by the 10:1 stage, the motor on the 10:1 is spinning at 5000RPM, and the motor on the 100:1 stage will need to go 500*100=50000RPM.

I'm not familiar with the internals of the motors, but I don't think any motors in FRC would have bearings rated for 50000RPM. I believe the fastest free speed of any FRC legal motor is ~18000RPM, and I'd be surprised if any motor would be designed to handle a substantial amount past it's own top speed.

I'm not an electrically oriented guy, but another problem with driving a motor that fast is that when being driven, motors act (in some form or another) as generators. This means spinning the motor shaft with another power source will generate back voltage that I'm not sure the system is able to handle.

Sorry for the wall of text, this is actually a very interesting question.

[chadr03]

I don't think you would like the results.

If you back drive a motor it becomes a generator and feeds power back into the system. With a 100:1 reducton gearbox if it is backdriven makes it a 100:1 increaser so you would have a generator moving pretty quick. This is putting power on the wrong end of your speed controllers and can damage them.

Also have you ever tried to backdrive a 100:1 recuction. It takes quite a bit of torque even if it is not trying to generate electricity.

I think your best best is to find a middle of the road solution or try some shifting/clutch mechanism that will allow your mechanism to work without having to backdrive the unused motor.

[pmangels17]

You could explore options of driving them both with one-way bearings. That might do the trick, since they wouldn't backdrive each other. This could be an issue if you ever wanted to run your intake in reverse, but it could be something to consider if you are only driving it in one direction. Or you could have each motor spin a different shaft/roller/wheel, and depending on the geometry of everything it could work out.

[GeeTwo]

Did you consider gearing fast, then using an encoder and driving the motor at two different speeds? You'd need to verify that you can get enough torque at both speeds to do your job, of course. It would require more motor than a neat shifter would, but would be much simpler and less expensive to build.