pic: Practice Mecanum Chassis

[Crazy Ivan]

Originally Posted by M. Krass

I was afraid that an active suspension would be necessary. While certainly not too hard to achieve, the added complexity and additional weight hurts the practicality of the design.

On your chassis, were the bolts that ran through the rubber cylinders riding in slots? It seems like they'd need to because the module pivots on the opposite end, but it's hard to see from the photo you posted -- which is otherwise enormously helpful. For as much as people are fascinated by these drives, I can't seem to find very many photos.

Could you describe a bit more about what you mean by 'aligning' the drivetrain?

The bolts ran in holes in the frame. As for aligning, you basically just have to make sure all your wheels start even to begin with. The first time we put the wheels on an pressed go, the robot started spinning in circles. We found it took a couple of tweaks and surprisingly high accuracy (we started to measure in centimeters ) to achieve a perfect result. But once it was set up, we were all set for 5 months of competition.

[yongkimleng]

Originally Posted by Andrew Blair

Perhaps an active suspension can be implemented without too much redesign.



When totally locked down with the spacers you've designed in, I believe that this could be strong enough to give you what you want. It appears you're using 1/8" or 3/16" stock- maybe an upgrade to 1/4" would give you all the strength you need there. I would think that if you got hit really hard, the ability of the mecanums to slide would keep you from bending the modules. The chain would still be an issue though perhaps.

Hmm i see an idler sprocket above the chain there. perhaps if it was on a spring-tensioned bar it can still hold the chain in place.. like of that in a bike.
then again, how much distance of movement in the suspension is required to keep all wheels on the ground?

[Madison]

Originally Posted by Andrew Blair

Perhaps an active suspension can be implemented without too much redesign.

After thinking some more about implementing a better suspension design, I think that I like your suggestion most. It's certainly simple and seems largely proven, especially after watching 40's robot move. My concern is that there may not be enough travel available with how I've interpretted your implementation, though. A similar rubber ring inserted into the U-channel with a bolt spanning through its center horizontally means that there's only about 1/2" of travel in each direction. It seems that this would be sufficient in all but the worst of circumstances, so I'm curious to learn more about what 40's experiences were regarding overall travel of the suspension.

I'll post another screenshot as soon as I clean up the new iteration a bit more.

[Crazy Ivan]

Originally Posted by M. Krass

After thinking some more about implementing a better suspension design, I think that I like your suggestion most. It's certainly simple and seems largely proven, especially after watching 40's robot move. My concern is that there may not be enough travel available with how I've interpretted your implementation, though. A similar rubber ring inserted into the U-channel with a bolt spanning through its center horizontally means that there's only about 1/2" of travel in each direction. It seems that this would be sufficient in all but the worst of circumstances, so I'm curious to learn more about what 40's experiences were regarding overall travel of the suspension.

I'll post another screenshot as soon as I clean up the new iteration a bit more.

From our experience, 1/2" should be more than sufficient for any playing surface out there. I would suggest putting the most amount of give in the system in the upward direction. So just having one 1" shock on the top would be enough.

[Madison]

So I've rebuilt things to include a pretty simple suspension, just as team 40 used. I've watched a bunch of video of their machine from Boston and I'm happy with what I see, so I see no use in reinventing the wheel. I also like how easy it ought to be to vary the diameter and type of material used for the suspension based on our experiences.

The yellow bolt represents the pivot point that will mount this assembly to the frame. The suspension piece will react against the channel present in the earlier iterations.

I'm now trying to adapt the transmission design to this self-contained module so that the chain remains in relatively constant tension during operation. It's not been fun; not because it's particularly difficult to accomplish, but because everything I've tried so far has been pretty ugly.

[Crazy Ivan]

I like the design, I cant wait to see the finished product!

Originally Posted by M. Krass

So I see no use in reinventing the wheel.

But isnt reinventing the wheel what Mecanum's all about?

[=Martin=Taylor=]

Are you sure bending the chain back and forth is a good idea? Especially when changing directions rapidly?

Wouldn't it be safer to somehow include your motors and idler sprockets in the wheel pods?

[Madison]

Originally Posted by Hachiban VIII

Wouldn't it be safer to somehow include your motors and idler sprockets in the wheel pods?

Yes, and that's what I'm working toward now. The slots in the plates show above will accept new transmission modules, thus connecting the transmissions to the same pivot, while still allowing easy adjustment of chain tension.

[Mike Nawrot]

Perhaps you can save some trouble by integrating the gearbox into the inner plate of the mecanum assy, and replace the sprocket with a gear, to eliminate the issue of tension all together. I don't know what capabilities your team has when it comes to milling out excess material in the gears, but if worst comes to worst, you can design a nice lightweight gear and outsource it to sdp-si. I'm not a fan of outsourcing in FIRST, but in the real world, outsourcing is useful, and if you don't have the machining capabilities, you have no other option. It's a very nicely designed module. Keep up the good work.

[Madison]

Originally Posted by Mike Nawrot

Perhaps you can save some trouble by integrating the gearbox into the inner plate of the mecanum assy, and replace the sprocket with a gear, to eliminate the issue of tension all together. I don't know what capabilities your team has when it comes to milling out excess material in the gears, but if worst comes to worst, you can design a nice lightweight gear and outsource it to sdp-si. I'm not a fan of outsourcing in FIRST, but in the real world, outsourcing is useful, and if you don't have the machining capabilities, you have no other option. It's a very nicely designed module. Keep up the good work.

I considered this and decided against it because I'd like some ability to play with and vary the final drivetrain ratio. Changing sprockets and adjusting tension allows this to happen pretty quickly and painlessly.

The latest iteration follows; and please note that I'm dissatisfied with how ugly the gearboxes are.

I neglected to color the pivot in this shot, but it's the long 3/8" bolt sticking out from near the middle of the image. The blue represents chain -- #25 this time around rather than #35 helps to achieve some tighter clearances.

The ratio as shown is 12:40 in the gears and 18:72 through the sprockets for an overall ratio of 13.32:1. I'm pretty confident this will be on par with or lighter than the kit gearbox arrangement that provides a similar 12.75:1 output.