Drivetrain Help

[Ken Leung]

I actually dreamed about how to make this kind of CVT work last right, and it kept haunting me every time after I open my eyes in terror from the major confusions/problems I had to face every similar ¡§nightmare¡¨... Ever since then I wasn't able to get any decent amount of sleep... I wanted to figure this out so badly¡K

So I spent a lot of time thinking, and here is what I learned...

The way drill transmission switch gear is to slide one of the ring gears in one of the planetary gear stage to two positions: one where that ring is locked into place, and another where the ring spin along with the planetary gears...

So the speed of the ring gear determine how fast the over all transmission go, and thus the torque it can generate. If we are able to control the speed of that ring gear, then we can change the ratio of transmission on the fly...

There is a point when the ratio is just right such that the load on motor makes it most powerful/efficient, but it would be hard to reach that ratio just by running the ring at different speed. So, why do that? Why not gear the motor such that it let the motor to be more powerful/efficient when the ring gear is locked in place by the motor? And then spin the ring different direction to get more speed or torque?

For the drive train, given different situation there motor will require a different ratio to be most powerful/efficient...

Say free speeding, as the robot go faster and faster until motor get closer to free speed, and there's less and less load on the motor until the friction in the system is only load left. At that point the gear ratio can decrease so the wheels can go even faster... And that will get more power out of the motor (with bigger load and faster speed). And the high speed is the most important thing we can get out of this.

OR, if we are pushing against other robots. The drive train is often stalled because the opponents¡¦ robots have a similar ratio/traction. So, we increase the gear ratio until the motor stop stalling or opponent start moving. We won't be able to tell when we are getting the most power out of the motor, but at least we are getting something instead of just stalling motor. And the most important we get out of this is the torque...

I think a CVT will be beneficial for the robot, if we do it right. Say we pick a good ratio when we lock the ring gear in place such that the component is running a optimally (like a default ratio), then a CVT will be able to help you adjust the ratio at different time for different situation.

Well, about matching the speed torque curve of F-p with Drill's, I didn't really understand why at the beginning.

All I knew was that the ratio is changing as long as the ring gear is moving. It¡¦s like two racks with a pinion in between. When one of the rack is fixed, the gear move along with the other rack... When both rack move toward the same direction, the pinion travel along with them without spinning... Or when the two racks go opposite direction, the gear will be spinning but won't be moving in any direction... So, the rotation of the ring gear determines precisely how fast the output would rotate.

But that's an output shaft spinning with no load. When the output shaft is under load, the ring gear and the sun gear would react differently. So, a matching speed toque would help to get the reactions from ring gear and sun gear match up. It would require setting up a right gear ratio between F-p and ring gear, and the planetary gear ratio just right as well... And a lot more stuff you have to do to get it just right (which I can't think of)... But the main issue is to keep the two gears¡¦ reaction the same, so that the over all ratio stay the same. Unless the ratio increase with more load, which will be a good design because that setup automatically great more torque with more and more load onto it.

Hmm... This only raise more questions I have about this... Starting to involve physics and other science¡K But I will continue to study this until I find the right answer, or when someone posts it in the future¡K