Coaxial Swerve Derivation with Paired Modules

[Tom Line]

Our team is looking at different swerve variations and considering trying one in the off season.

Ether was nice enough to do the derivations for equations and wheel speeds and post them in an excel format for unicorn drives, and straight up "crab" where all the modules are linked is simple to program.

Has anyone performed a similar variation for a paired-module swerve where 2 have 2 sets of modules, when each set has linked steering and drive?

To more explicit, picture 4 modules where the left side modules are turned and driven together, and the right side modules are turned and driven together.

[Ty Tremblay]

190 did this in 2009. It's basically tank that can strafe and gave us pretty good mobility for the motor choices we had that year.

Nowadays, with the amount of motors you can pick from, the benefits of full swerve outweigh the drawbacks.

[Dave McLaughlin]

1983 also did this in 2009. Ty is right, with the amount of similar motors a full swerve would be the way to go if you are confident you can build it and control it.

[efoote868]

Originally Posted by Ty Tremblay

Nowadays, with the amount of motors you can pick from, the benefits of full swerve outweigh the drawbacks.

I'm going to respectfully disagree. I've seen veteran teams sit idle for matches because of their swerve drives, for trivial reasons (e.g. the wheels weren't properly aligned at the start of the match).

At any rate, every game deserves thorough evaluation on the pros and cons of each drive train, because ultimately propellers will rule when it's a water game.

[pwnageNick]

Originally Posted by efoote868

I'm going to respectfully disagree. I've seen veteran teams sit idle for matches because of their swerve drives, for trivial reasons (e.g. the wheels weren't properly aligned at the start of the match).

I think his point was that if you have the capability to build a functional swerve drive, would there be an advantage do pairing modules together in pairs, rather than all together, or all independent, and how to implement this. In the high majority of cases I doubt running modules in pairs instead of independently would cause a team's robot to sit idle in a match. That would be more due to the overall fact that the team wasn't able to get swerve running properly in general.

I cannot speak to what the pros and cons of running them in pairs would be. We ran all 4 of our modules independently this year. Off the top of my head the only advantage I can see to pairing instead of all independent is using two less motors, but at this point with the number of motors available and the weight that teams have been able to get their swerve drives down to, that doesn't seem like a huge plus.

-Nick

[AdamHeard]

I agree with others.

Full independant is mechanically easier and it lets your 4 corners exist in isolation from each other.

Also, it's trivially heavier as you're not linking power transmission over long distance, so you eliminate all that weight (but do add two motors at .5 lbs each + gearing).

[Clem1640]

Some benefits of pairing swerve modules would be:
* Ability to drive (2) swerves with (3) CIMS
* Easier to add gear shift

But, what kind of drive action/control are you seeking: Crab or Snake? Either could be accomplished through pairing, but not both together.

For example of crab & snake, see:
http://www.chiefdelphi.com/media/papers/2400
http://www.chiefdelphi.com/media/papers/2401

[Craig Roys]

Originally Posted by Clem1640

Some benefits of pairing swerve modules would be:
* Ability to drive (2) swerves with (3) CIMS
* Easier to add gear shift

But, what kind of drive action/control are you seeking: Crab or Snake? Either could be accomplished through pairing, but not both together.

For example of crab & snake, see:
http://www.chiefdelphi.com/media/papers/2400
http://www.chiefdelphi.com/media/papers/2401

This is one of the reasons we were considering looking at two pairs...we wanted to see about getting 6 CIMS to power the 4 wheels.

[Tyler2517]

Originally Posted by Craig Roys

This is one of the reasons we were considering looking at two pairs...we wanted to see about getting 6 CIMS to power the 4 wheels.

Why do you want 6 cims for a swerve drive? It seems a bit over kill when a swerve is best at out maneuvering not out pushing/running.
You could try a 3 wheel 6 cim having the best out of both worlds.

[Chris is me]

Originally Posted by Tyler2517

Why do you want 6 cims for a swerve drive? It seems a bit over kill when a swerve is best at out maneuvering not out pushing/running.
You could try a 3 wheel 6 cim having the best out of both worlds.

6 CIM drives really aren't about pushing harder; they don't, at least not with shifters. They're about accelerating better when geared for a high speed at full weight. A swerve that accelerates more slowly than the standard tank drive would be less agile - this would be bad for outmanuvering.

[asid61]

6 cims is not for pushing, because the 120a breaker basically limits your torque, even if it delays a few seconds. Your wheels will lift before that happens at lower speeds (~7fps IIRC). It does, however, give you much better acceleration.
The main problem with a 6 cim swerve is that you lose all your cims that could go to a manipulator. Plus, you either have to do a central gearbox or go with a 6-module swerve.
Using centralized gearboxes saves weight (chain weighs very little as a matter of fact) but at the cost of not being able to do complex maneuvers. For programming purposes, a simple crab would work better in my opinion, but mechanically it will be hell to line them up due to chain spacing.

The chain for crabs also has to be designed around for anything that goes above the chassis. The high number of chains also can lead to faster breaks, so using #35 chain could be an advantage maintanence wise.
Chain weight is 0.087lbs for #25, 1.8lbs for #35 per foot.

One way to line up swerve modules is to stretch a piece of surgical tubing between the end of one wheel's axle to the end of another and tie it off there. That keeps both wheel facing forwards. Then you calibrate at 0, and remove the tubing.

[Ether]

Originally Posted by asid61

6 cims is not for pushing, because the 120a breaker basically limits your torque... It does, however, give you much better acceleration.

How do you get "much better acceleration" if you don't have more torque?

[pwnageNick]

Originally Posted by Ether

How do you get "much better acceleration" if you don't have more torque?

This. Without more torque, then there is no reason why your wheel spin-up would be decreased, thus helping your acceleration. What's your reasoning behnd saying you do not get more torque?

Originally Posted by asid61

The main problem with a 6 cim swerve is that you lose all your cims that could go to a manipulator.

I think some teams on Einstein this season would have disagreed with you.

-Nick

[Aren Siekmeier]

Originally Posted by Craig Roys

This is one of the reasons we were considering looking at two pairs...we wanted to see about getting 6 CIMS to power the 4 wheels.

It's been a while since I've done the math, perhaps those with more recent, published models can confirm. This is all based off my intuition.

Since you have a 4 wheel drive, put 2 CIMs on modules in opposite corners, with 1 CIM in each of the other corners. The power you're supplying to your wheels should still be balanced on either side, regardless of the direction of movement which is defining said "sides" at any given time.

Adding in arbitrary rotation on top of translation complicates things a bit, since now the power available to pull off the rotation will depend on how far each "unit" of power is from the center of rotation. In particular, if you're spinning about one of your modules, if it's a 1 CIM module, you have 5 CIMs available to do the maneuver, while if it's a 2 CIM module, you only have 4 across the other 3 modules. If you spin about a point away from all the modules, you have all 6 CIMs available. A 4 CIM, 4 wheel swerve also fails to have this type of symmetry, but to a lesser extent.

In any case, a good speed control loop will ensure you still get the desired motion, but the max acceleration and force with which you can perform a maneuver may vary because of the loss of symmetry. Since a good swerve will require extensive off season testing anyway, I'd recommend trying this out so you don't have to try transferring power between modules, and seeing if the performance is acceptable.

Or go 3 wheel

[efoote868]

Originally Posted by pwnageNick

I think his point was that if you have the capability to build a functional swerve drive, would there be an advantage do pairing modules together in pairs, rather than all together, or all independent, and how to implement this.

Yep, read that in a different context. Thanks for shining a different light on it.