pic: 4143 differential swerve concept

[Chris is me]

I really like this concept. Maybe it's not the most practical thing in the world, but not "wasting" any motor power on steering and being able to just have a big ol' dead axle tube to pivot with is pretty elegant.

Are both Colsons cantilevered?

Since you already have to heavily modify the Nanos, I think you could get a degree more compact with a custom two stage gearbox. Saves you some weight and packages a bit better. I guess the space savings doesn't help much since the CIMs are sticking out past the gearboxes anyway, but hey, more room for encoders.

[Nuttyman54]

First thought: This is going to be impossible to make a module drive straight. A very small speed differential between the two wheels will make this turn for any reasonable driving speed gearing.

Second thought: Making a module point in the desired direction requires running the differential for a short (possibly VERY short, depending on the gearing) period of time and then stopping. This seems like it would be highly inaccurate.

Third thought: Trying to make 3 or 4 modules all point in roughly the same direction this way without mechanically connecting them seems impossible, especially given the update rates of our control system. To make it controllable for steering, you would need to gear way too low to be usable as a drivetrain.

This was a good thought exercise.

[GeeTwo]

Increasing the spacing between the wheels should improve steering stability. Perhaps if they were outboard of the gearboxes things would improve enough to become usable. With a bit more adjustment, this could also be used to lower the CoG of the module even farther, though with an increase in MoI around the serve axis and a need for more real estate for the module.

Originally Posted by carpedav000

If you didn't wamt steering motors on the modules, couldn't you just put a gearbox in the middle and belt-drive it to each module? Saves motors and weight.

If I understand correctly, that would be crab drive, not swerve. Crab allows you to translate anywhere, but not rotate the robot effectively.

[Chak]

Cool concept. Can the problems with steering-too-fast be somewhat alleviated by putting the wheels farther apart? Then the module gets super wide, but what if you take two of these super wide modules and build a "skateboard swerve"?

Let the thought exercise continue!

[Chris is me]

Originally Posted by Nuttyman54

First thought: This is going to be impossible to make a module drive straight. A very small speed differential between the two wheels will make this turn for any reasonable driving speed gearing.

I think this depends on how low-friction the bearing setup for the steering is. Going with a more high-friction solution (i.e. a big hunk of close fit Delrin) could provide enough resistance to prevent slight differences in speed from spinning the module. Combine that with some closed loop velocity control between the two wheels, and I think it's a solvable problem. Not saying it isn't more trouble than it's worth, though.

[RyanShoff]

There would be not a differential, just two cantilevered wheels. All the comments so far are spot on. A higher friction pivot bearing would help. You would probably want 3 encoders, but maybe just one would work. The control loops are real interesting. I was thinking you could have one module with minicims to get around the cim motor limit.

One idea no one talked about is putting something like a caster angle into the modules. You could shift the wheels back slightly from the centerline of the pivot. The advantage being that they might track straight easier. The disadvantage being reversing direction would need a 180 degree twist.

This was mostly a modeling exercise, but with some development, it could be a viable drivetrain.

Step files coming soon.

[JesseK]

Originally Posted by Nuttyman54

First thought: ....

Do your thoughts on overall plausibility change if a non-standard wheel configuration (e.g. 3 pods in a triangle) is used?

[cbale2000]

Interesting design, one thought though:

Assuming FIRST keeps the same rules that they have the past few years, in all likelihood there will be a limit of 4-6 CIMs on a robot which would make this design as shown illegal assuming 4 wheel swerve. That said, you could just swap them out for MiniCIMs.