
General Forum Open Discussion of the VEX Robotics System that can be answered by anyone. VEX Robotics Engineers will not answer questions posted here; see Official VEX Technical Support below. 

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#11




Re: Shifting XDrive
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 Last edited by Previlant; 05102012 at 11:57 PM. 
#12




Re: Shifting XDrive
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 Xholonomic drive. 
#13




Re: Shifting XDrive
Quote:
On a slightly unrelated side note, 254 borrowed two 60T gears from us at Worlds and we never got them back. (it's okay!) 
#14




Re: Shifting XDrive
where's the continuous part?

#15




Re: Shifting XDrive
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 Xdrive Previn linked to, but I guess the cat is out of the bag now . 
#16




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.

#17




Re: Shifting XDrive
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.

#18




Re: Shifting XDrive
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. 
#19




Re: Shifting XDrive
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

#20




Re: Shifting XDrive
The math checks out. An Nmotor, Yratio "X" drive will move faster than an Nmotor, Yratio 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 Nmotor tank drive can be safely geared faster than the Nmotor "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. 