pic: Pseudo-Differential Swerve



BOM link
Onshape link

Specs:
19.44:1 reduction on 3.25" wheel for 13.66 ft/s free speed.
~6.66 lbs per module
Under $300 per module, discounting SRXes (under $500 with)
2.597in “tank wheelbase”

An idea I’ve had floating around for a while and finally got around to CADding. Two distinct powertrains, each motor feeding one wheel, produces a tiny differential drive that works as a differential swerve module.

Pros:
Mechanically simpler than true differential swerve (no triple-coaxial)
~7.25" diameter sweep and that’s it

Cons:
Can’t tune in the air
Tall (~12 inches)

RS7 encoders for speed control, TT-6127 for absolute position.
The other separate powertrain differential swerve I recall for sure is 4143’s module from 2016 offseason.

The detail design looks good, but the concept has prior art: https://www.chiefdelphi.com/media/photos/44055. The biggest “con” with this style is that a difference in traction between the wheels (such as when climbing an obstacle) would prevent the module from controlling its speed and heading.

The OP had cited that other diff swerve as the other one they’re aware of. I do believe that 323 had a similar motor power setup for a non-differential swerve, but can’t find screenshots at the moment.

Regardless, excellent modeling. It still doesn’t solve the incredibly complicated problem of controlling them though :stuck_out_tongue:

Which is why I linked that thread in the OP.

That is a good point about the difference in traction that I didn’t think of. That’s certainly a point to consider when picking the right design for the job. This year I probably wouldn’t have chosen this design due to the scale platform and the cable protectors, but with a flat field like Steamworks I think it would certainly be fine.

Edit: If anyone is looking for the original CAD link, I accidentally did some modifying on the linked version. Here’s a static link to the version discussed in this thread.

I’m sure that almost all designs in FRC have “prior art”. That being said, I like this implementation much more than previous ones.
What I like about this style of module is that it’s easy to understand and fairly simple to make. What I don’t like is the intrisically large packaging and the extra cost in gears. This design appears to only have 2 stages of reduction, so it’s actually probably saving money over a Versaplanetary-based design.
The slip ring will likely not be powerful to support both Talons + CAN, however, unless you put 30A breakers on the drive motors. Not terrible for 775s, but something to think about.
Another thing to take note of is the seemingly ~1200rpm turning speed. That’s also very hard to change without just making the wheels super far apart. One of those things that needs to be tested to be sure.

Turning speed seems to be the biggest challenge to differential swerve, and will need considerable testing (in addition to the current calculations) to find the max and ideal numbers for.

I think team 4143’s done that:

Wait, I just saw that the OP linked that, never mind :slight_smile:

Made one of these in meat space once…

Two stage reduction off the pair of 775pros for main drive. 3d printed pulleys as the second stage. CAN ran through one of the sets of wires in the slip ring. Power for the two 775s through the other two. They were on 30A breakers.

I moreso meant limiting the rotation in software to avoid spinning faster than the slipring is rated for.

Nothing special. The steering motors used MotionMagic, the max V was probably set below the max rate of the slip ring… I say probably because I certainly didn’t make any conscious efforts to limit it.

Not something you could do on this system easily though unless you were to cap the difference in velocity between the wheels?

This isn’t a “psuedo differential” - this is 100% a differential swerve, it just uses the ground / friction to transmit that differential force.

I’ve toyed with this concept a little bit as well, as it avoids the bevel gears altogether which is nice. The 4143 module that you linked to is the other one I’ve seen. So far it’s been really challenging to package it better than bevel gear differential swerves - and you can sorta see that here.

Did you guys have any issues sending the CAN through the slip ring? We have discussed it in the past but felt this would be a risk point. What slip ring did you use?

Might be more appropriate to call it a skid steer differential swerve?

(In b4 motor-in-wheel-skidsteer-differential-swerve)

There were no CAN errors in the logs from the slip rings. You’d have to ask John for the particular model, I don’t have it handy.

It’s on our list to test this more scientifically. We’ve got a couple of the rings and need to invest some time in a test rig. It seems to work well for the teams using it but I’m skeptical until it’s been tested and documented. It also necessitates a topology change to make it work.

The packaging is definitely what I dislike the most about this module. I’ve been running over this in my head for the past few days, and the only thing I’ve come up with that would remove a significant amount of height is to get rid of the unnecessarily long belt run. However this requires significantly changing the design, either moving everything below the bearing (which runs into significant cant forces) or putting the reduction into the plane of the bearing, requiring a significantly larger bearing.

If I have the time over the next few days I may explore the latter option a bit with a derivative of this design.

Unless the packaging gets a lot, lot better I don’t see any reason to run a traction-differential swerve, since gear based designs package much better and also can be built with no more beveling than a traditional swerve (@anand)

I thought the packaging was bad until I realized that you cut out the pivot gearbox altogether with this design- it just becomes a 16-style module with an extra 775.
At the very least, modules like 2767’s are much larger by comparison.

Just depends on if you need the height vs the width more. A lot of games I’d prefer to take up more space in and under the chassis plane than above it, but I suppose just as many games with tighter packaging down low exist. Just another design factor to think about.

I don’t want to suggest making the module ring wider, but I feel like if you did, then you could tuck the motors within the module so that they don’t have to clear the gears as much? The gears on each side don’t HAVE to be coaxial… Just something to think about.