31-08-2006 20:00

Billfred

Re: pic: Practice Mecanum Chassis

Whoa, nice work.

Are y'all planning on using a belt drive for the wheels, or is that just a placeholder?

31-08-2006 20:02

Madison

Re: pic: Practice Mecanum Chassis

It just means that I'm too lazy to model roller chain.

31-08-2006 20:05

Billfred

Re: pic: Practice Mecanum Chassis

I guessed so (the would-be pulleys didn't look like pulleys).

Can you post a close-up of the transmission? Unless I'm a victim of forced perspective, it looks like a right compact number.

31-08-2006 20:36

=Martin=Taylor=

Re: pic: Practice Mecanum Chassis

What is the purpose of that third sprocket? Sensor perhaps?


Why exactly are you using an AM mecanum anyways? Didn't you make some really neat mecanums earlier this year?

31-08-2006 20:37

SamC

Re: pic: Practice Mecanum Chassis

What program is that modeled in?

31-08-2006 20:44

Madison

Re: pic: Practice Mecanum Chassis

Yes, as soon as I finish modeling the rest of their parts. They're missing a bearing, a bushing and encoders. There's not much to look at, really. It's a two-stage reduction based on some of the parts provided in the kit transmissions.

The idler will drive encoders for position sensing. They also increase chain wrap.

I'm using AM's wheels because the wheels I developed do not belong to me. They were also far more expensive to manufacture.

Solidworks 2005

31-08-2006 21:31

Jeff 801

Re: pic: Practice Mecanum Chassis

what motors are you going to use?

31-08-2006 22:30

Bill_Hancoc

Re: pic: Practice Mecanum Chassis

Wow that is some amazing modeling...what was that done with....looks very cool and promising

31-08-2006 22:33

Joe_Widen

Re: pic: Practice Mecanum Chassis

Almost looks real. Great CAD. Good luck with your mecanums.

31-08-2006 22:56

DB_UPS

Re: pic: Practice Mecanum Chassis

wow ... thats really good!!

[edit]
Are they the AndyMark Mecanum wheels?
[/edit]

31-08-2006 23:27

Greg Needel

Re: pic: Practice Mecanum Chassis

From picture description: "This is a look at what FRC488 is planning to prototype this Fall thanks to AndyMark's new mecanum wheels."

01-09-2006 00:57

Lil' Lavery

Re: pic: Practice Mecanum Chassis

What's the purpose of the large plates on either side of the wheel? Why not a much smaller system to mount the wheels and/or bearings?

01-09-2006 09:29

Crazy Ivan

Re: pic: Practice Mecanum Chassis

Wow, great work. Thats an impressive model. I have one question though, do you plan on putting a suspension on you chassis? I can tell you from our experience with mecanum that constant contact is important. Our team, 40, used rubber bumpers on our wheel pods to keep contact. Also at nationals we saw another team (I'm terrible at remembering team numbers) who used small pnumatic pistons for a suspension.

Looks wicked cool though, Good luck!

01-09-2006 10:27

Alex Cormier

Re: pic: Practice Mecanum Chassis

i am guessing protection to the wheels and stability?

01-09-2006 13:12

Madison

Re: pic: Practice Mecanum Chassis

Each of the four wheels will be driven by its own, small Chiaphua motor. I'm presuming the kit will remain unchanged in that respect next season.


They're so large so as to protect the wheels from impact some. Last season, our omniwheels were unprotected and took a few good hits from robots with higher chassis rails, causing them to bend. We were always able to bend them back, but they were also a lot cheaper and easier to reproduce so we had plenty of spares. They also look pretty cool.

Thanks. Do you have any photos of the arrangement you used? I've currently arranged for the large wheel guards that hold the axles to use vertical slots so that we can adjust their position relative to the frame. This would allow us to raise or lower the wheels individually to ensure contact in case the frame warps or bends, but it wouldn't do this dynamically during a match. How important do you think it is that the wheels sit on an active suspension?

The design I've made previous to this included a more substantial, pivoted subchassis for the two rear wheels that would dynamically stabilize things, but it seemed a bit like overkill. That's shown below:
It's an older model, so there're no chains on this one. They didn't disappear or anything.

01-09-2006 13:49

yongkimleng

Re: pic: Practice Mecanum Chassis

wow that looks cool! im also doing a vex assembly in solidworks.. but doubt it can turn out so nice...
where u ge the macanum wheel part frm? wonder anyone has omniwheel part for me to import :S

01-09-2006 14:52

artdutra04

Re: pic: Practice Mecanum Chassis

You can download the STEP model from this page: http://www.andymark.biz/mecanum.htm

01-09-2006 17:48

Crazy Ivan

Re: pic: Practice Mecanum Chassis

The slots are a good feature, but I would say that an active suspension is almost without a doubt nessisary. Some sort of ridged adjustment only works if the playing surface is compleatly level and smooth (unlike the floor of the Georgia dome that is full of warps). And even if all the wheels apear to be touching, just a small amount of slip with one wheel can cause you drive train to go funky. Even with our active suspension, it took a good hour to align and even then it only strafed strait in only the right hand direction, with a small arc to the left.

Unfortunatly we don't have any pictures of the wheel pods up close, but I'll give my best try to describing them. With both systems I mentioned before, one end of the wheel assembly must be on some sort of hinge or pivot (we did this with a removable steel pin). The other end of the wheel assembly was held by screws going through the ridge part of our frame, and threaded into the wheel assembly with cylinders of gum rubber inbetween the frame and wheel assembly. A similar system was used, only with 3 inch pistons at 45 degree angles.

The other method of suspention you can use (as I belive you alluded to earlyer) is the system team 190 used in 2005, where the wheels were ridged, but it was there frame that actually had one big pivot in the middle.

Hope this helps!

01-09-2006 18:08

Lil' Lavery

Re: pic: Practice Mecanum Chassis

116 found in 2005 that contact was incredibly important with our holonomic drive (I'd assume it would be the same with a mecanum). Even the very small elevation change of the plywood triangles in front of the loading stations caused problems, as one wheel would lose contact with the ground. (If we drove "North" corner first. Once the North corner was raised happened, either the "East" or "West" wheel would lose contact, as the robot had a slight tip to one side to rebalance itself, which was problematic, as the East and West motors are needed for movement on the North/South axis). We had a system where we could adjust the wheels relative to the frame, but it wasn't active, and it couldn't be done on-field. So, even the slightest imperfections can cause problems if you don't have some sort of suspension system.

01-09-2006 18:23

sanddrag

Re: pic: Practice Mecanum Chassis

It seems like you could make a flexible enough frame to avoid having to do suspension.

01-09-2006 19:28

Astronouth7303

Re: pic: Practice Mecanum Chassis

Also, when team 190 did this in 2005, they ended up spliting their chasis in 2, so that the back two wheels swiveled, somewhat similar to your design.

I didn't have the opportunity to actually see them in action, but they did describe it well.

01-09-2006 20:55

Andrew Blair

Re: pic: Practice Mecanum Chassis

868 used a pnuematic suspension on their mechanum drive, for the purpose of lowering their entire base, not just keeping contact- to my knowledge. But I'd imagine their design might help you with your design problem if somebody's got a good picture.

02-09-2006 13:43

Crazy Ivan

Re: pic: Practice Mecanum Chassis

Huzzah! I found a picture of our suspension. The black plastic circles were eventually replaced with solid rubber. Its a little hard to make out but the hinge is near the vertical bar on the left side of the image.

02-09-2006 13:58

Billfred

Re: pic: Practice Mecanum Chassis

Interesting.

I assume the rubber circles were attached somehow--does the movement of the module affect how the robot drives?

02-09-2006 14:07

Crazy Ivan

Re: pic: Practice Mecanum Chassis

The rubber circles/rubber cylinders/shocks were held by a 2 inch bolt that went through the frame and screwed into the top of the wheel pod assembly. The modules movement actually is surprisingly good at keeping the wheels in contact with the ground. Once it was aligned, it worked quite well on many different surfaces.

02-09-2006 15:12

Mike Nawrot

Re: pic: Practice Mecanum Chassis

Awesome design. Nice and simple. There are a few improvements I see possible. Using 25 chain instead of what appears to be 35 will save some weight, because it will allow you to achieve a smaller pitch diameter on the drive sprocket, which will allow you to use a small sprocket on the wheels to maintain the same ratio, resulting in less material in sprockets and a shorter length of chain. Also, the rollers on you mecanums are all aligned in the same direction, and last I checked, they should oppose eachother to actually achieve omnidirectional motion. That's just a small detail though. Also, I'm a bit concerned about the chain on the rear wheels, since when the wheel assembly pivots, the chain will be forced to twist in a bit of an unnatural fashion. Other than that, the design is amazing in the lines of simplicity. And I like the green AM mecanums

[edit] I just realized that AM doesn't have the .step for both left and right wheels, so ignore my comment about the direction of the rollers.

02-09-2006 20:51

Madison

Re: pic: Practice Mecanum Chassis

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?

We have a lot of material for #35 chain on hand already, including a pretty good variety of sprockets and a few dozen feet of chain. It's also really easy to track down aluminum sprockets for this pitch from sources like IFI or AndyMark and they both tend to have much shorter lead times than places like Stock Drive Products. I'm okay with taking on some extra weight here if it means fewer headaches.

Yep. I'm just too lazy to create a mirror of the wheel.

Me too! I rationalized it by saying, "that's what prototypes are for," but since it seems like an active suspension will be very important to making this work correctly, I'll likely correct this in the next iteration. I could include it among the pivoting subframe, but then that starts to become very large and it's hard to mount additional mechanisms to it. I was hoping the chain would have enough play in it, since I don't imagine the subframe will raise or lower more than about 1/4".

Thanks for the continued discussion everyone.

03-09-2006 02:40

yongkimleng

Re: pic: Practice Mecanum Chassis

man this is getting exciting, I've never experienced mecanums before.

any way to flip the step import in solidworks?

As for the suspension, usually how much vertical movement would be enough to keep the wheels on the ground? Looks like the rubber is sufficient, but that depends on the overall weight of ur final bot right?

I was thinking of a pneumatic suspension where the pressure applied on each wheel could be adjusted as required (maybe not on-the-fly). anyone done that before?

03-09-2006 08:16

Andrew Blair

Re: pic: Practice Mecanum Chassis

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.

03-09-2006 09:27

Crazy Ivan

Re: pic: Practice Mecanum Chassis

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.

04-09-2006 03:13

yongkimleng

Re: pic: Practice Mecanum Chassis

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?

04-09-2006 13:38

Madison

Re: pic: Practice Mecanum Chassis

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.

04-09-2006 15:50

Crazy Ivan

Re: pic: Practice Mecanum Chassis

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.

06-09-2006 21:03

Madison

Re: pic: Practice Mecanum Chassis

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.

06-09-2006 21:15

Crazy Ivan

Re: pic: Practice Mecanum Chassis

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

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

07-09-2006 02:07

=Martin=Taylor=

Re: pic: Practice Mecanum Chassis

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?

07-09-2006 02:13

Madison

Re: pic: Practice Mecanum Chassis

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.

07-09-2006 12:25

Mike Nawrot

Re: pic: Practice Mecanum Chassis

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.

10-09-2006 20:17

Madison

Re: pic: Practice Mecanum Chassis

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.