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

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?

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

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

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

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.

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

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.

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

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!

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

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.

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

I get what you're saying, but the information you speak of, the 25% better acceleration -- doesn't that information come from the automotive industry?
Internal combustion engines have a torque/HP curve, and the CVT chooses to vary its reduction ratios within that curve to get the best efficiency, where as with gears the driver has to go in and out of that curve (or the peak of the curve) while accelerating.
Electric motors have a linear torque line, where their peak torque is at stall.

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

Cars and robots are two different things. On a car you know when the optimal shift point is. You're also only concentrating on one thing--staying between the white/yellow lines.

With a FIRST robot you're doing a multitude of things, and anything that allows the driver to have one less thing to think about is going to give you an advantage. It's like having a 4 speed transmission--does the driver have time to think about what speed he needs to be in, how much current he's drawing, etc, or do you just make an algorithm that shifts automatically to always keep the drive in the gear that gives optimum performance for a given situation?

If you think it'd be hard on a driver to find the right gear instantaneously with a 4 speed, think about how much thinking it'd require with an infinite number of gears.

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

hehe you should see DARPAs grand challenge! :D also that should show you why its not wise to have too much computer controlled things.

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

Sure it's wise. We just haven't figured out quite how to do it. That's the whole point of the competition. The future is in autonomy.

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

right but their are going to be conditions where the computer will be wrong and you want more or less torque. atleast have it so you can disable it and manually control it if neaded.

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

You're right Mechanicalbrain. Sometimes, it would be nice to have a computer control it, but then for driving, it's not that hard, so a driver can think about other things too. I was on the drive team this year and I had also been to go in and out of turbo very easily without really having to think about it and concentrating on it.
There are also some flaws if you have it controlled automatically. Sometimes, you may need to quickly switch to a higher torque setting while the robot is about to go into a pushing match, and if you hit the other robot at high speeds, maybe the sensor might get damaged and so the gearing has the chance of staying in the same position and it will not change your gearing for when you go to a stop and chances are you will be pushed around, while a driver that can manually override over it can start gearing it up so that you have more torque going into a pushing match so that you can possibly win. Something like this actually happened on our robot this year. We had a limit switch to detect when the arm is too high, so the motor powering the spool that pulls the cable stops. During one match, it had gotten damaged and we were not able to lift up our arm at all during the whole match just because of that. But also Cory, how you said the future is autonomy, robots aren't perfect yet. It would be nice to have it controlled automatically, so you are correct, but at times, a manual would be better.

And if the problem comes to it that a driver starts getting nervous because he forgets how to control it, he should've maybe practiced somewhat before to get the feel of which buttons or however layout you're going to use, and also the layout of the control board plays a part into it. The driver should pick out the buttons for which function on it so that it is easier for him to drive.

That's just my two cents and my opinion. Just so ya..

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

I don't think anyone here is against a manual option (redundancy is always good). You're not going to use the system's full potential if you don't setup some sort of computer control though.

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

mechanicalbrain that's what I've been saying. You basically let the computer handle the CVT's shifting, but in case of a system failure or emergency, you'd have the option of a manual override.

Stonefan5271138, sure using your robots "turbo" option was relatively easy -- team 100 had it on previous robots as well -- due to the fact that you only needed to press a button, just two variables, on or off. Your brain can probably subconciously detect when you need that extra power boost or when you don't. With a two speed transmission it's almost the same, except instead of having a program-limited speed, it's now mechanically limited.
Like what Cory said before, once you start having more than two speeds, like a 4 speed transmission, it gets a lot harder for the driver to keep his/her attention on the competition while trying to get the right gear ratios. Sure with driver practice, one could almost master the system, but for some teams, practice time is cut short due to some unexpected situation.
Now, with a infinite amount of gears, getting the right ratio's will be an even bigger challenge. Sure you could just designate set ratios to n-number of buttons, but that pretty much defeats the purpose of a CVT with an endless amount of reduction ratio's, unless you plan on pushing many buttons for many different gear ratios, which would be kind of funny. (You could have your second driver do it, haha)
So, I'll say it again, CVT's should be autonomously controlled. Set it and forget it -- if the shifting program has a flaw, have a programmer fix it. Design the hardware to last for at least one more competition than your team registered for. And I never (at least didn't mean to) mentioned anything about ruling-out a manual override option. its always good to have a manual override -- It's why the field has a big red button to stop your robot in case something bad happens (although I heard on the 2005 field it didn't exactaly work).

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

I see what you're saying NoodleKnight, about how a driver has more difficulty and how it's harder for like, 4 speeds and how it may seem impossible for a CVT by just pressing buttons. A 4 speed shifting gearbox has 4 buttons for the different speeds. For a CVT, really, you should just need two buttons to control the shifting of it. One to gear it up gradually, and the other to gear it down. You could probably just use the buttons on the joystick to do that.
For a Variable Pulley Diameter System, you could design it in a way that the two pulleys change diameters together, so that when one pulley diameter increases, the other will decrease accordingly. This part could be done by programming, so you are right that it can be done automatically and it would be easier for a computer to control it, but it could also be done mechanically. For the changing of the gear ratios can be controlled by simply pressing the left button on the joystick or the right button on the joystick to change it. If it's that simple, I really don't see much of a need for the programming to control the gear ratios.
So overall, computer control in a Variable Diameter Pulley is a good thing to control the 2 pulleys accordingly, but it is not needed if a driver could do it with just pressing one of buttons that he can easily reach and get accustomed to, probably in just a matter of hours.

But then for this type of CVT or another type, how would you program it to get the maximum efficiency out of it?

...That's just my two cents.