Shifting X-Drive

[Previlant]

Also if anyone's interested, if you make the x drive past 45 degrees you would get more speed in one direction and more torque in the other direction, essentially making it like a shifting drive train without actually having shifting mechanisms.
http://www.youtube.com/watch?v=QLMWJQC71LA&feature=plcp

[dontworryaboutit]

Indeed a cool way to make an effective CVT (continuously variable transmission). But, I'm a little wary of that design because of the pivot point. Can it support the full weight of the robot?

Other than that though, very nice use of the speed advantage of an X-holonomic drive.

[bobsalive]

Originally Posted by dontworryaboutit

Indeed a cool way to make an effective CVT (continuously variable transmission). But, I'm a little wary of that design because of the pivot point. Can it support the full weight of the robot?

We've got most of an arm/lift on that base right now, and it seems okay, although we haven't made it pick up a gazillion stacks yet. With our testing so far it seems that it should work, although that might change.

On a slightly unrelated side note, 254 borrowed two 60T gears from us at Worlds and we never got them back. (it's okay!)

[vdsa]

Originally Posted by dontworryaboutit

Indeed a cool way to make an effective CVT (continuously variable transmission).

where's the continuous part?

[Vex Mundi]

Originally Posted by vdsa

where's the continuous part?

If you use a continuous actuator instead of the pneumatic cylinder then you can get the full range of ratios 1 - ∞, not just 1 and 1.4.

We were hoping to be the first team on the forums to point that out, in our writeup of the offset X-drive Previn linked to, but I guess the cat is out of the bag now .

[mr. giggums]

How would you calculate speed and torque if the offset is x degrees apart? I tried to figure this out but I don't know enough about vectors.

[Vex Mundi]

Originally Posted by mr. giggums

How would you calculate speed and torque if the offset is x degrees apart? I tried to figure this out but I don't know enough about vectors.

I have a draft document on this that I'll try to upload tonight. I'll see if I can get our webmaster (bobsalive) to add it as a page on our website tomorrow.

[dontworryaboutit]

If the offset is X degrees from the wheel being pointed straight ahead, the force component straight ahead is given by cos(X) times the force when X = 0. The maximum speed is given by 1/cos(X) = sec(X) times the maximum speed when X = 0. You can verify this result by multiplying the speed by the force:
cos(X) * 1/cos(X) = 1
which says that you are not losing any energy as you rotate the wheels. You are losing a lot of energy to friction as X --> 90 degrees because the rollers are turning very fast.

[Vex Mundi]

Yes, dontworryaboutit is right (as usual). Here is an in depth explanation if you are interested: https://docs.google.com/open?id=0B_I...0Q2aDBXeXlKb1U

[JoeG]

The math checks out. An N-motor, Y-ratio "X" drive will move faster than an N-motor, Y-ratio tank drive. However, this is a little bit misleading. For a single speed system, you loose signifigant amounts of torque in the "X" drive. Therefore, the N-motor tank drive can be safely geared faster than the N-motor "X" drive, cancelling out the difference if we're talking about two separate robots.

Cool shifting concept though! Reminds me of this. I wonder if a similar linkage system could be applied to reduce the number of cylinders/air consumption required.