Octocanum Drive/ Mecanum Suspension Designs?

[Dunngeon]

Originally Posted by Tom Line

Be careful with this statement. Mecanum wheels absolutely requirement a certain amount of flex in a chassis system so that all 4 wheels maintain contact with the ground a majority of the time.

So either build a flexible chassis, or use suspension: one of the two.

A gyro can only do so much. With one wheel off the ground, you will run into a situation where the robot cannot correct itself and strafe at the same time. It will end up driving forward, backward, or sitting in place while the airborne wheel spins instead of sliding sideways like you want.

We ended up changing out our entire drivetrain to a slide drive at state champs. It ended up being lighter than mecanum and it always goes the direction you want it to.

Having one of four wheels airborne sounds like more of a manufacturing problem to me, rather than a mecanum specific issue. There's a crazy amount of teams using mecanum this year without suspension and very rigid (for FRC) frames, you don't NEED it. The bigger issue (for the vast majority of mecanum teams) is the center of gravity distribution and strafing, which can either be corrected by a gyro, encoders and some code or not using mecanum.

Edit: I should clarify, varying weight on mecanum wheels is one of the larger issues with mecanum. Having no contact with the ground for one of the four wheels isn't a problem that is specific to mecanum, it's a manufacturing issue.

[GeeTwo]

Originally Posted by Dunngeon

Having one of four wheels airborne sounds like more of a manufacturing problem to me, rather than a mecanum specific issue.

Yes, but it is often a manufacturing problem with the floor, rather than the robot. This year, as with many, it's a manufacturing design. There are these things called scoring platforms that disrupt the planarity of the field. Even in that lovely land of theory where everything works, this will result in one or more wheels becoming airborne when the robot meets one at an oblique angle.

[Dunngeon]

Originally Posted by GeeTwo

Yes, but it is often a manufacturing problem with the floor, rather than the robot. This year, as with many, it's a manufacturing design. There are these things called scoring platforms that disrupt the planarity of the field. Even in that lovely land of theory where everything works, this will result in one or more wheels becoming airborne when the robot meets one at an oblique angle.

Sure, the scoring platforms disrupt the planarity of the field, but when would you ever need to strafe ON the scoring platform? The primary use of holonomic drives have been in alignment to feeder station and landfill. Expecting mecanums to behave normally while going over or on the scoring platform isn't realistic, with or without suspension.

[GeeTwo]

Originally Posted by Dunngeon

Sure, the scoring platforms disrupt the planarity of the field, but when would you ever need to strafe ON the scoring platform? The primary use of holonomic drives have been in alignment to feeder station and landfill. Expecting mecanums to behave normally while going over or on the scoring platform isn't realistic, with or without suspension.

Even when you're driving forward, mecanum doesn't behave as you expect when a wheel is off the floor. The front of the robot will rotate towards the side of the airborne wheel. If it's a front wheel that comes off the carpet, this will tend to rotate the robot so that it is more nearly parallel with the edge you are climbing, usually making matters worse.

And I can certainly imagine wanting to strafe across the scoring platform to cap a stack that was already built, or to pack the stacks in close to each other to leave room for more.

[carpedav000]

I just thought of something and I was wandering if it had ever been done before. Has anyone done a octicanum-like drivetrain with slide drive that can switch to 6-wheel drop-center? It would definitely push it with weight, but it might make it more worthwhile to go into traction mode for prolonged periods of time.

[GeeTwo]

Originally Posted by carpedav000

I just thought of something and I was wandering if it had ever been done before. Has anyone done a octicanum-like drivetrain with slide drive that can switch to 6-wheel drop-center? It would definitely push it with weight, but it might make it more worthwhile to go into traction mode for prolonged periods of time.

You lost me at "octicanum[sic]-like drivetrain with slide drive". Octanum uses four mecanum and four traction wheels. Slide drive (aka H-drive) uses five omni wheels. 6-wheel drop-center uses six traction wheels. Would you mind backing up and taking another swing at this?

[Darkseer54]

Originally Posted by GeeTwo

You lost me at "octicanum[sic]-like drivetrain with slide drive". Octanum uses four mecanum and four traction wheels. Slide drive (aka H-drive) uses five omni wheels. 6-wheel drop-center uses six traction wheels. Would you mind backing up and taking another swing at this?

He is saying basically your idea for the sixwheel with drop down mec's, except instead of octocanum use nonadrive.

[carpedav000]

Originally Posted by Darkseer54

He is saying basically your idea for the sixwheel with drop down mec's, except instead of octocanum use nonadrive.

Yay, I didn't confuse everyone!

[carpedav000]

Originally Posted by GeeTwo

You lost me at "octicanum[sic]-like drivetrain with slide drive". Octanum uses four mecanum and four traction wheels. Slide drive (aka H-drive) uses five omni wheels. 6-wheel drop-center uses six traction wheels. Would you mind backing up and taking another swing at this?

NOTE: I'm saying "switch" because I haven't a clue what the technical term is

Okay, I said octicanum-like because it still involves the ability to "switch" drive trains. Basically, you have six modules with traction and omnis. Then you have a center omni hard mounted to the center in the normal H-drive way. When you "switch" to traction, the wheel in the center gets raised off the ground. When you "switch" back to omni, the traction wheels go back up and the center wheel gets lowered back down to the ground. Did I better your understanding of it?

[InFlight]

We've used the Vex Mecanum wheels with Andmark Nanoboxes with No problems at all this year. We have encoders on each gearbox (3D printed bracket), gyro correction, and a very stiff frame. Wheels are set up to drive on to the scoring platform, and strafe to pickup totes in the landfill.

The Nanoboxes have very limited clearance between the upper mounting bolts and the CIM motor. We had to machine down the mounting bolt head OD to make these gearbox noise free.

[Team3844]

Team 3844 has been playing around with some firestone air springs as actuators that double as suspension. We will have some to give out at Championships next week if you are interested.

[dougwilliams]

Originally Posted by Team3844

Team 3844 has been playing around with some firestone air springs as actuators that double as suspension. We will have some to give out at Championships next week if you are interested.

Unfortunately we won't be at Championships, but if you had a part number, pictures or CAD of your design we'd love to take a look.

[cad321]

Originally Posted by dougwilliams

Unfortunately we won't be at Championships, but if you had a part number, pictures or CAD of your design we'd love to take a look.

^Seconded

[Dunngeon]

Originally Posted by GeeTwo

Even when you're driving forward, mecanum doesn't behave as you expect when a wheel is off the floor. The front of the robot will rotate towards the side of the airborne wheel. If it's a front wheel that comes off the carpet, this will tend to rotate the robot so that it is more nearly parallel with the edge you are climbing, usually making matters worse.

And I can certainly imagine wanting to strafe across the scoring platform to cap a stack that was already built, or to pack the stacks in close to each other to leave room for more.

Please explain how so many robots with mecanum wheels are able to drive over the scoring platform perfectly fine at angles. It has quite a lot to do with the gyro/PID correction. (See videos of 1983, 3574 ect on PNW First youtube channel)

Also, if you're strafing ACROSS the scoring platform with your wheels on it how can you cap? The wheels will be where a stack is, unless you're like the High Tekerz and have you're wheels mounted perpendicular to the front of your robot w/ a cutout.



I'd also encourage teams to seriously consider just how much value holonomic motion is adding to your robot.

Ex: In the case of 4488 where everything is automated it is of high value to their game strategy (auto alignment to feeder station)
However, for a team with a built in ramp (2826) they easily align with the feeder station w/ a 6wd because they designed for imprecision, mecanum is of low value to them.

IN NO WAY am I saying that you should never use mecanum, but that you should honestly evaluate every option available to you in line with what Karthik outlines here. If, after honest analysis you believe that mecanum, octocanum or some other holonomic/holobrid is the best option; then go for it.

This section is a word of warning from a team that had a slide drive, and got rid of it because we weren't getting the value we expected out of it.

[GeeTwo]

Originally Posted by Dunngeon

Please explain how so many robots with mecanum wheels are able to drive over the scoring platform perfectly fine at angles. It has quite a lot to do with the gyro/PID correction.

Sounds like you covered the explanation pretty well. Corrections are always possible, sometimes within the time you have to design, build, and program them.

Originally Posted by Dunngeon

Also, if you're strafing ACROSS the scoring platform with your wheels on it how can you cap? The wheels will be where a stack is, unless you're like the High Tekerz and have you're wheels mounted perpendicular to the front of your robot w/ a cutout.

I was thinking more of aligning new stacks with old to make plenty of room. As we decided early not to climb on the scoring platforms (admittedly not our best decision ever), I haven't spent a whole lot of time thinking through these use cases.

Originally Posted by Dunngeon

I'd also encourage teams to seriously consider just how much value holonomic motion is adding to your robot.

Absolutely. We were planning for significant automation in alignment to totes that would have made slide drive essential. At one point, we had used all of our DIOs and had to use two off of the MXP, plus we were hoping to use a camera for left/right (strafe) alignment on totes. When we realized we were unable to program these cases robustly in time, most of the sensors came off.

Originally Posted by Dunngeon

This section is a word of warning from a team that had a slide drive, and got rid of it because we weren't getting the value we expected out of it.

Same here. At CMP, we'll be driving a 4 wheel omni, and have exactly 3 CIMs and no other actuators. By the time we made the "no strafe" decision, it was too late to go back to 6-wheel to see if it would work better for us, especially if we found out we'd have to switch back. We may very well have a 6-wheel drive for Red Stick Rumble. However, rather than shy away from holonomic, I'm working up some sensor-heavy programming exercises for the summer and fall sessions.

(On a couple of non-drive notes, we now have a better tote lifter that is much more likely to pass inspection than our original spring-cushioned "rake". Despite this, it still has the double end-lift as well as single side-lift capabilities, and is fully capable of doing either on the step as well as the floor. We have also practiced far more tote-flipping since Bayou than we did before. We're striving to make these "inaccessible" totes available to some major stacker 'bots that are running out of "easy" totes. Carson may prove to be the best division we could hope for! )