Octocanum Drive/ Mecanum Suspension Designs?

[dougwilliams]

As an off season project for our team we are looking at creating an Octocanum drivebase. We have experience with our own custom chassis, and have been using Mecanum drive for the last many years.

We have seen a few designs around (1086 / Blue Cheese, 3847 / Spectrum, 488 / Xbot, and 1540 / Flaming Chickens).

From my research, I can only find one example of an Octocanum that provides suspension when in the Mecanum mode (team 1540). It seems like with this drivebase we wouldn't want to miss that opportunity. Does anyone have any experience with that? And can anyone point me to any CAD designs around that are set up like that?

[Electronica1]

Originally Posted by dougwilliams

As an off season project for our team we are looking at creating an Octocanum drivebase. We have experience with our own custom chassis, and have been using Mecanum drive for the last many years.

We have seen a few designs around (1086 / Blue Cheese, 3847 / Spectrum, 488 / Xbot, and 1540 / Flaming Chickens).

From my research, I can only find one example of an Octocanum that provides suspension when in the Mecanum mode (team 1540). It seems like with this drivebase we wouldn't want to miss that opportunity. Does anyone have any experience with that? And can anyone point me to any CAD designs around that are set up like that?

Mecanum wheels apply force to the side when in use, so it is generally safer to be mounted to the frame rather than on a module. Plus, you tend to spend most of your time on mecanum, only using the traction wheels when you are interacting with the opposing alliance. In the event you are low on pressure, you can still use your main drive mode without issue.

[dougwilliams]

Originally Posted by Electronica1

Mecanum wheels apply force to the side when in use, so it is generally safer to be mounted to the frame rather than on a module. Plus, you tend to spend most of your time on mecanum, only using the traction wheels when you are interacting with the opposing alliance. In the event you are low on pressure, you can still use your main drive mode without issue.

Agreed on the point of spending most of our time on mecanum. I guess the trade off you made on your chassis was that suspension for the mecanum was not worth as much as the possibility of losing pressure and having the system fall back to the traction wheels?

[cad321]

2386 used mecanum for the first time this year and I too was concerned with suspension/ full wheel contact. In order to help keep our masts from swaying at the front of the robot, we used tensioned steel cables going from the front upper most point to the opposite rear low point. Although this did help with the sway of our masts, it also worked as a phenomenal way of ensuring all wheels were in contact with the ground. if the rear wheels off the ground, we loosened the cable lowering the wheels and vice-versa.

Should we ever go mecanum again, we will most certainly use this method of ensuring the wheels all touch the floor. To make it work you just need a rigid, tall super structure and build some flex into your chassis. Would highly recommend this method to others as it is both simple and easy to execute.

[Dunngeon]

Alternatively, you could use a gyro to correct for the drift/inconsistencies and not have a suspension system. I know many teams that have mecanum this year who have no suspension, such as 1983, 2990 ect.

[Tom Line]

Originally Posted by Dunngeon

Alternatively, you could use a gyro to correct for the drift/inconsistencies and not have a suspension system. I know many teams that have mecanum this year who have no suspension, such as 1983, 2990 ect.

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.

[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.

[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.

[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.

[dougwilliams]

Originally Posted by Tom Line

Be careful with this statement....

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

A gyro can only do so much... .

Snipped quotes above and referencing everyone else's subsequent quotes as well here:

I don't have any hard measured evidence, but it certainly seems like the drive-ability gets worse over time. I believe this is due to slowly degrading the chassis alignment over time from hard use (maybe due to chassis warping from hard bumps, post-competition demos where someone runs it into a wall, etc).

We did our first gyro corrected drive system this year, and it makes a difference, but cant account for everything. Now in our post-season, if we look at the wheels when strafing one always seems to be slightly off from the others; maybe it's not resting on the ground with the same force as the others.

That's what led me to inquire about the utility of the pneumatic suspension benefits of the octocanum drive. I agree we want mecanum to be our primary, higher speed drive, but its seems if all the parts are right there because of the octocanum, it would be a shame not to find a way to add the suspension to the mecanum mode.

But, there's trade-offs with every engineering decision, and we will have to decide if the potential for a loss of pressure and reverting to traction, or some other octocanum failure is too big a risk to take. Or find a solution that doesn't suffer from that drawback.