pic: FRC488's Octocanum Ver 2.0

[BJC]

Comments/ questions are bolded

Originally Posted by Madison

We pivot the traction wheels down for a few reasons. First, by putting them nearer to the center of the machine, it makes turning with four high-traction wheels a bit easier to accomplish. We also gear down further to that wheel set, so it makes sense for it to be at the end of the gear train.

I was always under the impression that the traction wheels were suppose to be on the outside to aid in pushing/ not being pushed; that not being able to turn was desired because you are only ever going straight when pushing? Also, if the mecanums are on the inside won't you rock up onto the mecanums when pushing someone? Additionally, are you concerned with your wheel pods being bent in when your traction wheels are down and someone T-bones you going 14+fps?

The gear ratios are 8.45:1 at the mecanum wheels -- that's one of the options for the Toughbox Nano -- and about 25:1 at the traction wheels. The traction wheel ratio makes things pretty slow, but it's easy to swap in sprockets to speed things up as required.

Any concerns about the gearbox ripping itself apart when you are switching wheels at full speed and both wheels are momentarily touching the ground or do the mecanums slip enough for this to not be a problem?

The kit frame is 1.25" thick and the output of Toughbox is 1.5" from its edge, so when using a 6" mecanum wheel, we'd end up with just .25" of ground clearance if we built things the other way around.

How high do the inner wheels pivot off of the ground, and how dropped are the traction wheels when they are down?

The subframe, right now, is 1x1x.125" angle bolted to the perimeter of a .125" perforated PVC sheet and hung from the Toughbox Nano housings. It is intended to hold electronics and would need to be enhanced if it were going to see more substantial loading.



This is based on AndyMark 6" mecanum wheels -- http://www.andymark.com/product-p/am-0136.htm

We used this last season and I'm happy with the build quality and performance. They're heavy, but they're not as heavy as the 8" set we originally played with years ago and we're willing to deal with the weight and cost in place of building our own.

Our mecanum implementation worked. I didn't program it, so I can't speak too much to what made it work, but our programming team did a fantastic job there. We had accurate, fast, field-oriented drive that allowed us to fully use the movement capabilities of the mecanum wheel set. In my experience, most teams fail to achieve the level of control we managed.

Overall, very cool. I'm a fan of the ability to quickly strafe in any direction, to hold your ground when pushing, and with proper programming basically what amounts to a shifting drivetrain. Looks like you have a good thing going here.

[Jonathan Norris]

Any thought into making the Mecanum wheels the pivoting ones?? Our team last year developed some CAD prototypes for this combination, and eventually didn't implement it due to the added complexity. However, we did use mecanum last year, and didn't have a great experience with it. I think one of the biggest problems we saw (other then malfunctioning Jaguars...), was that when all four wheels were not on the ground (due to an un-even playing field, and seams in the carpet) the controllability of the system declined, and even worse the power of the drive system declined. Some of this we could correct with programing, but loss in power and acceleration was really noticeable. The loss in acceleration is I believe the biggest downside of using mecanum, the reality I saw was that our robot was just far slower in accelerating, changing direction, and stopping (decelleration) then high traction based drive systems.

I was wondering if you pivoted around the traction wheels instead of the Mecanum wheels in the octocanum, if you would see any advantages because the pistons could act as a suspension system. I've been told that mecanum drive systems perform better with a suspension system, I would be interested in hearing from teams that have used suspension in their mecanum drives. But for me I would need to see a big improvement in acceleration/deceleration to advocate for using mecanum again.

[Ether]

Originally Posted by Jonathan Norris

The loss in acceleration is I believe the biggest downside of using mecanum, the reality I saw was that our robot was just far slower in accelerating, changing direction, and stopping (decelleration) then high traction based drive systems.

What do you think would cause a mecanum to stop unacceptably slowly?

Are you saying it slides across the carpet when you try to stop quickly?

Do you have any video showing this?

[Dad1279]

Originally Posted by Jonathan Norris

....
I was wondering if you pivoted around the traction wheels instead of the Mecanum wheels in the octocanum, if you would see any advantages because the pistons could act as a suspension system. I've been told that mecanum drive systems perform better with a suspension system, I would be interested in hearing from teams that have used suspension in their mecanum drives. But for me I would need to see a big improvement in acceleration/deceleration to advocate for using mecanum again.

That is what we did. Not necessarily for the suspension, but we wanted the traction wheels to be the most outward wheels for stability, and on the the fixed axles for strength. Also, if there was an pneumatic failure, we wanted the default wheels to be the traction wheels.

Not shown in the sketch below, the pistons were vertical, one on each side, and they pushed down where the front and rear pivot arms met in the center of the robot. We used the 8" Andymark wheels, not by preference, but budget. We had a new, unused set on the shelf as spares from the prior year. Also not shown below, the transmissions and outer axle bearing were rigidly mounted to chassis.

Yes, 8 motors on drivetrain, 1 CIM and 1 CIM-U-LATOR/775 per wheel.



While I personally preferred the field-orientated driving, unfortunately (for me) mentors don't drive. Our driver preferred traditional controls, so that his orientation didn't change when swapping between traction and mecanums.

[Joe Ross]

Originally Posted by Dad1279

While I personally preferred the field-orientated driving, unfortunately (for me) mentors don't drive. Our driver preferred traditional controls, so that his orientation didn't change when swapping between traction and mecanums.

You could do something like 451's SOAD - Semi Omni Arcade Drive.