CVT, which teams are planning on using them for next year?

[sanddrag]

I think you'd want current draw to abe a big factor in the auto "shifting" program. Also, it would have to be automated because I don't think a driver is skilled enough to control an infinitely variable transmission to the best of its potential. There is too much other stuff going on to think about exactly what torque/speed setting you need. Some drivers don't even make good use of a two speed.

[Jeff K.]

Well...maybe have it automated by using shaft encoders, but then if you got into a pushing match, and there was a problem and the CVT was still in high gear, maybe using the buttons on the controller, the driver could manually change the pulley diameters to low gear by pressing the buttons to slowly change the diameters. Or instead...maybe preset diameters that when the driver presses the left joystick button, it automatically goes to lowest gear for more torque, and then the right joystick button, middle gear for getting across the field more quickly.

For automating, you could probably have it that when it starts from 0 fps, the pulleys could automatically be at the smallest pulley to biggest pulley for most torque, and then gradually increase. Or something like that..

[mechanicalbrain]

i think it would be easiest to have the driver control it. instead of specific ratios a driver could push a button for more torque or one for more speed. i think my biggest fear would be the gears just slipping and a sliding.

[santosh]

^agreed. You get most of your power at a certain RPM. And yes if you do have a pushing match, you want to keep a very high RPM on a high torque setting.

[mechanicalbrain]

i wonder if you use a simpler design if you could make a mini disk CVT. as long as it stands up to the stress i think you can make them miniscule.

[John Wanninger]

Originally Posted by mechanicalbrain

i wonder if you use a simpler design if you could make a mini disk CVT. as long as it stands up to the stress i think you can make them miniscule.

Remember that torque x speed=transmitted power. On a cvt disk,( or even with gears or sprockets) the smaller the diameter, the greater the force required for a given torque. In other words if you make it small, it needs to spin really fast, or have really high contact force (or both). Even if the materials could withstand the contact forces, heat buildup due to inefficiency of non-point contact could be an issue and would be more troublesome as you go smaller as there is less surface area to dissipate the heat.

[mechanicalbrain]

ooh good point. darn physics.
actually my second fear is the heat build up. any suggestions on how to minimize it?

[Veselin Kolev]

The V-belt variable pulley size drive was exactly what I was hinting at. How it works.. simply, is that you take a standard v-belt pulley, cut it in half, and ride a belt over it. If you bring the half pulleys together, the belt rides higher on the pulleys. If you bring them apart, the belt rides lower. The problem is not how to get traction from the pulleys, thats the easy part. The hard part is how to reliably controll the distance between the pulleys. If that was figured out, CVT would be a joke for me. But untill then, I'm cadding new ideas.

[NoodleKnight]

lead screw and potentiometer, or a bunch of limit switches!

I have to agree with sanddrag, the CVT should be controlled via computer based system -- the driver won't easily be able to smoothly control the CVT's "shifting" as a computer-based system would, and by setting designated shift points for the driver to select, then you might as well have been better off building a shifting-gearbox with n-number of shiftpoints.
Manual override is always nice, yes, but only if the program dies or if the driver really needs to shift fast.
The CVT's shifting program should vary the transmissions reduction ratios on current draw (like sanddrag said) versus wheel speed. If the current draw is high and the wheels speed is low, the transmission should increase the reduction for more torque, etc...
And I don't really think that slipping in a CVT is a big problem, just build it around a high RPM/low torque area -- shouldn't having high RPM's help shifting anyways?

[santosh]

Originally Posted by NoodleKnight

lead screw and potentiometer, or a bunch of limit switches!

I have to agree with sanddrag, the CVT should be controlled via computer based system -- the driver won't easily be able to smoothly control the CVT's "shifting" as a computer-based system would, and by setting designated shift points for the driver to select, then you might as well have been better off building a shifting-gearbox with n-number of shiftpoints.
Manual override is always nice, yes, but only if the program dies or if the driver really needs to shift fast.
The CVT's shifting program should vary the transmissions reduction ratios on current draw (like sanddrag said) versus wheel speed. If the current draw is high and the wheels speed is low, the transmission should increase the reduction for more torque, etc...
And I don't really think that slipping in a CVT is a big problem, just build it around a high RPM/low torque area -- shouldn't having high RPM's help shifting anyways?

I think that slippage is a big issue. That is a big reason why teams are staying away from it. ABout that motors drawing amps thing, couldn't you have something on the controls that the drivr could see so that he doesnt draw too many amps.

I think however, that the driver should be able to control the positioning of the belt BUT, a program could change the position of the belts if the motor is about to kill itself from drawing too many amps. Am I making any snse. It would kinda be like a guided shifting systm howevr it would only shift for you if you were about to kill the system

[NoodleKnight]

Originally Posted by santosh

I think that slippage is a big issue. That is a big reason why teams are staying away from it. ABout that motors drawing amps thing, couldn't you have something on the controls that the drivr could see so that he doesnt draw too many amps.

I think however, that the driver should be able to control the positioning of the belt BUT, a program could change the position of the belts if the motor is about to kill itself from drawing too many amps. Am I making any snse. It would kinda be like a guided shifting systm howevr it would only shift for you if you were about to kill the system

Wouldn't placing the CVT in a high RPM, lower torque area fix that slipping problem? V-belts and pulleys, or friction plates & wheels would at least hold up in those conditions, or at least stick enough so that the wheels will break traction first, or am I missing something?

I get what you're saying, basically a fail-safe system that will stop the robot from dying. IMO, the point of a CVT is to eliminate the need for driver-interaction with the shifting mechanism and let the computer dynamically control the shifting-ability. But hey, don't let what I'm saying influence your designs, I'm just giving my viewpoint and searching for answers.

[santosh]

Originally Posted by NoodleKnight

I get what you're saying, basically a fail-safe system that will stop the robot from dying. IMO, the point of a CVT is to eliminate the need for driver-interaction with the shifting mechanism and let the computer dynamically control the shifting-ability. But hey, don't let what I'm saying influence your designs, I'm just giving my viewpoint and searching for answers.

You are correct about CVT needing less driver interaction, but it does also have a bunch of pros on its own. It servs a better than automatic transmissions becasue otherwise why would they put thm ther if there was already no driver interaction.Somewhere someone told me that ther was 25% better acceleration and you can build up pushing power faster. I do believe that a a program guiding the positioning of the belts would be easier to drive, but a manually controlled CVT would let you reap most of the benefits out of it. It is just like the choice, manual cars vs. automatic cars.

[John Wanninger]

Originally Posted by Veselin Kolev

...The hard part is how to reliably controll the distance between the pulleys. If that was figured out, CVT would be a joke for me. But untill then, I'm cadding new ideas.

I believe a spring on the output pulley forces those split sheaves together, which tensions the belt which spreads out the sheaves on the input shaft. The weights on the input shaft, through cam action, apply a counteracting force to push the sheaves together.
Someone correct me if I'm wrong here but I believe, additionally, the belt will tend to seek greater mechanical advantage by riding lower in the input shaft groove when it encounters a load. As the load increases, the ratio adjusts for greater mechanical advantage, and the input sheaves spread apart until reaching an equilibrium with the sheave "squeeze" force resulting from rotating weights. As the motor goes faster and/or the load decreases, the input sheaves are forced together, until there is a new equilibrium point, and of course, a new ratio.

[team222badbrad]

Hmm....

http://www.lovejoy-inc.com/variablespeeddrives.htm

Good thing I work at a machine shop!

The DoAll horizontal automatic band saw at work has one of those in it to change blade speeds...

Maybe some of you can find these useful!