Re: pic: Butterfly Drive
 

You mean the strafing module, right?

We took inspiration from 148's strafing module (from their 2014 robot, Vader). The module itself spins on the axle that's powered by the CIM. So when the CIM is powered, the module spins down until a wheel contacts the wheel. The CIM will exert enough force to push down the module enough to have the ground exert a normal force on the wheel (which is what traction is dependent on) before the torque of the CIM spins the omniwheel.

At least that's how I understood it while designing this. We haven't actually done more serious math outside of a simple free body diagram. If someone from 148 or anyone with experience with this type of strafing module could explain more/clarify/prove me wrong, that would be awesome.

Re: pic: Butterfly Drive
 

Yep. That's what I'm talking about. The diameter of the wheel, clearance from the ground when horizontal, and gearing all play a role in how much normal force the wheel can exert on the ground.

We struggled with tuning the amount of force our strafing modules needed in 2015, but that was going over the bump + with and without totes, so it had to move more weight / accelerate more mass than just a robot and that weight varied quite a bit.

I looked at 148's design when thinking about ours, but we had one shot at getting it right (we swapped our drive from mechanum to this at state champs) so I ended up going with 2 strafe modules, held down with gas shocks counter balanced by surgical tubing so we could adjust the downward force.

The 148 solution is lighter and more elegant, but more difficult to get right.

Re: pic: Butterfly Drive
 

Hm... so I think the main issue revolves around when the tractions are down and the robot gets hit on the side, which might damage the modules, and the shaft especially over time. In addition, the way the piston is mounted probably also causes it to receive a lot of force on impact.

I might consider another design of the Butterfly, although I might move on to intakes or something other than drivetrains, because I've been doing that a lot. But anyways, here's what I was thinking about the new design:

• Module goes into a 3x2 boxtubing; there I can maybe make the modules almost as wide as the interior of the tube to have minimal flex and bending (this is my logic to this problem, but I might be wrong...)
• Pistons push down on the modules without any special mounting, similar to Neutrino's design
• Change in material of the module plates. Maybe something like polycarbonate? Not really sure on this one: I know polycarb is flexible, but to some extent

Re: pic: Butterfly Drive
 

My understanding of 148's strafing module design is that it works even better than that. The wheel in contact with the ground rotates in the opposite direction as the module itself tips, meaning that the wheel is basically driving itself into the ground. For an intuition of this, consider the boundary cases (module tilt angles close to 0 degrees and close to 90 degrees). While the omni wheel would just skip along the ground if it made contact while the module was barely tilted, at close to 90 degrees it would actually be able to lift the robot up, supporting most of its weight. Obviously one would want to design for a tilt angle somewhere between these extremes, but they illustrate what's going on very well. The great thing about this design is that the normal force on the strafing wheel varies drastically depending on what the robot is doing, so you only have high normal force on the strafing wheel when high torque is applied to it.

For those more knowledgeable than I: Is my understanding correct?

Edit: As far as I can tell, it would follow from this understanding that, in theory, a tilt angle from horizontal of greater than the inverse tangent of the wheel-to-ground coefficient of friction would guarantee that the wheel wouldn't slip (Well, until the point at which the linear relationship between torque and normal force ends because there simply isn't more weight available for the strafe wheel to support). However, in reality, friction is also a fickle beast and is highly dependent on the exact conditions of the interaction at any given time.

Re: pic: Butterfly Drive
 

Yeah you're right. I remember discussing this with someone when we watched the reveal video for the first time in 2014. But I completely forgot this time around, even while reviewing this CAD...

Re: pic: Butterfly Drive
 

Have you considered putting springs on the modules to hold them in place without air pressure?

There's nothing quite like losing air pressure in the middle of the match and being unable to drive.