Mecanum Revelations for Rich & Duane
I'm finally slapping together a quickie bot using my Vex wheels. I always wondered about whether there's an optimum chassis aspect ratio (wheelbase vs width, x pattern, square, rectangular). I asked a few builders and the general consenus was no. I started to roughly copy Rich's very cool-looking Expo bot's proportions. But when I got just the front 2 wheels on the chassis, I rolled it around on the floor and it pivots perfectly around a point on the chassis, pretty far forward of about where Rich's rear axle centerline would be. That is, if I move my rear "axle" somewhat forward of what looks proper, then the robot will be able to pivot perfectly (no wheel scrubbing) around the middle of the rear axle just by driving the two rear wheels in opposite directions, with the front wheels not powered. And vice versa. It will make the chassis look wide and quite frankly, a bit goofy, but something tells me that's the way to go for optimum maneuverability with the weight evenly distributed between all 4 wheels. I know Rich & Duane have built their mecanum bots. Any feedback, guys?
Maybe the answer is online, I just haven't found it yet.
I'll post a pic tomorrow.
Maybe the answer is online, I just haven't found it yet.
I'll post a pic tomorrow.
Comments
As far as the length vs width - I think that a wheelbase equal to the wheel track would be best for ease in calculating the maths involved in programming precise movements, but esthetically I don't like it. If the distances are known then you can account for it. I can't think of any reason why there would be a benefit to a particular aspect ratio that is unique to the mecanum wheel.
edited to remove stupid theory and conceal the fact that I don't think good sometimes
http://www.rapidonline.com/Education/Mecanum-Wheel-100mm-4-inch-pack-of-4-70-6234
And this working robot is even more ridiculous: http://robomatter.com/Shop-By-Robot/VEX-Robots/Hardware/Mechanics/Mecanum-Wheel-4-4-pack
For a sanity check regarding wheel orientation and aspect ratio just look at the Airtrax forklifts.
I don't want you to think I'm ignoring this thread, I just don't have anything useful to add. (After looking at my robot, I did have some thoughts.)
But, I wont let that stop me from replying.
I purchased four of these motors with encoders during Pololu's Black Friday sale. I concidered using them with the Mecanum wheels but decided to use them on a different project.
I agree with Rich; you probably have your wheels mounted reversed from the "usual" way of mounting Mecanum wheels. The wheels on my robot will allow the "coasting" on just the rollers in a diagonal line passing through opposite corners of the robot.
As I try to imagine the way the wheels would roll if the robot were upside down, I can see what you mean by an appearent circular rotation path. (Though the path isn't really circular, just perpendicular to the "usual" way.)
As the wheel rotates the contact point moves with respect to the axle. This movement with respect to the axle also changes the apparent pivot point of the set of wheels.
I think the average aspect ratio created by this apparent pivot point would be a square but this shape would change depending on where the contact point is with regards to the axle. I thing the aspect ratio would likely be both a stretched and squashed square depending on the wheel positions.
I can't think of an advantage one aspect ratio would have over another.
I'm inclined to think the "usual" orientation of the wheels (with rollers in line with opposite corners of the robot) has an advantage over the orientation you initially tried but I can't really think of a reason why one orientation would be favored over the other.
I'm very interested in seeing how others are mounting the Vex Mecanum wheels to motors. I'm satisfied with how my wheels worked out but I think Cliff improved on my method when he made his robot.
BTW, Have you seen Vex's new 6" and 8" Mecanum Wheels? The new ones look pretty tough (and cool) but the prices are much higher than the 4" version we know and love.
I think this "pivot" point isn't stationary. As the wheels rotate (about the axle) the place of contact of the roller changes. It possible to have two rollers on one wheel in contact with floor at the same time. These two contact points (of two different rollers on one wheel) do not share a common "pivot point". I think the pivot point moves (with respect to the axle) as the wheel rotates. When the center of a roller is in contact with the ground and both halfs of the roller make contact with the ground, I think the pivot point would be in the center of a square wheel layout. This wheel position also causes the contact point to be directly under the axle.
I used four of my favorite 24V gearmotors http://www.mpja.com/24VDC-45-RPM-Gearhead-Motor-with-Wheel/productinfo/17971+MD on this bot, running 8x 18650 cells and two L298N modules. My buddy rapid prototyped custom hubs for the vex mecanum wheels and motors that fit beautifully. Got the bot running last night. My ghetto build was quick and it's fabulous. Plywood and... super glue. Seriously, the four motors and my central pivot are (neatly) superglued on. Nary a screw. No one would suspect watching it work from the topside. It's not nearly as slick as Rich's, but quite presentable and fun to watch. Working up an interesting combo video now. Should have it up in a day or two.
Edit, oh it looks like most of the plastic wheels come in four packs, although I still think a 3 wheel bot would be cools
"First test of a simple homemade robot chassis with VEX mecanum wheels driving a preprogrammed pattern while a robot arm on top moves some blocks around. Custom 4WD chassis: no wheel encoders, two L298N motor controllers, BASIC Stamp 2. 5-axis arm is controlled by a PicAxe 20M2, modified from a kit available on Ebay."
http://letsmakerobots.com/node/35872
Very Cool!! That's a really nice bot!! You plan to have it carry things across the house? Maybe clean up pet toys? I always thought it would be a neat idea to have a bot scoop all of my dogs toys in to there toy basket, I bet this would drive them nuts.
Way to go erco.
I'm still hoping you have time to document the build sometime (particularly the weel mounts).
I can hear the figure 8 thread calling to your robot; you hear it too don't you?
BTW, no encoders were harmed (nor used) in the construction of that robot (so far, anyway). As I found on Retrobot many moons ago, those 24V motors are pretty well matched in RPM: http://www.youtube.com/watch?v=cfMFO-ee6ak
Assuming Rich is on schedule with his software, Kepler should be tracking a perfect figure 8 while rotating about any given axis...
Edit: "Document the build?" Ha... tons of SUPERGLUE! The coolest things on the bot are the RP wheel mounts.
It depends on how you define "around". I thought around and/or over was the idea? (And yes, the robot has gotten around cleanly but not on video.)
There is another...
My ghetto Figure 8 was crazy simple: turn on 2 motors for 11 seconds, then turn on the other 2 motors for 11 seconds. http://www.youtube.com/watch?v=BrsVI6nX6xQ
If you ever get a chance to take some picture of those wheel mounts, I'd love to see them.
From what I can see in the video, it looks like your friend did a great job making them.
Will do, they are very nice.
You, Rich & Duane have made a “school” for mecanum bots construction and development!
Can I gues a mecanum bot competition in the next EXPO?
Your research will be my “base” if I decide to convert my 4wd rover to a mecanum bot!
If you design any new part for your robot and you want access to 3d printer I can help you. My 3d printer will be into your service!
I don’t know if you have made any 3d model using SketchUP or other software, but I think it would be very useful!
I do, but the 4MB file is too large to upload here.
as I saw the chassis of your mecanum rover is made by a Laser cutter or CNC machine. Isnt it? Have you seen Thingiverse site? You can post the 3d model of your robots parts there if you want, and then you can give the link here. This site has a lot of hardware parts made by CNC, laser cutters and 3d printers. I think is a very good idea for hardware robotics parts. Other users can extend or improve your model or make useful changes.
I think you have a 3d printer dont you?
Yes, certainly. I have several things there.
I have not for many months, and will not upload any more things to thingiverse until I see a change it how it is managed. It is too much of a free-for-all. When obvious copyright violations are brought to the attention of thingiverse their response is to say it is not their job to follow up - even though they are hosting the material. They advised to contact the uploader and take it up with them. Some users will take someone else's work, scale it, and call it a derivative. Or worse yet, not give credit to the original designer. There is no recourse other than to convince the uploader to change it - which I have had to do on several occasions. Everything that is uploaded has a license attached to it. Not everyone honors those licenses and you cannot depend upon the site owners to step in. There is also a whole lot more spam than there used to be. I'm finding that there are less things being posted that are interesting to me than there used to be - even though there is a great deal more traffic on the site.
Months ago when the MakerBot guys (they run thingiverse) were in Seattle I spoke to them about this. They acknowledged the problem and gave examples of some changes that were coming. There have been changes in the site layout but not the kind of changes that make it a better place to share your designs.
Not to mention drug paraphernalia, sex toys, and other inappropriate items that a few people like to upload.
Then there is MakerBot themselves. They used to be all about open source. Not anymore. You can't purchase the parts that make up their new printers. Weeks ago I asked about buying a replacement part - they haven't yet replied.
It sure looks like he's doing it the hard way.
Pretty funky looking wheels.
You're right, I did miss that link. Sorry.
I found the printed wheels while trying to figure out what kind of plastic is used to make the Rover 5 chassis. I never found out but I have found super glue works reasonably well on it. (I had stripped some threads that hold the encoder PCBs in place.)
Isn't that 3D printer plastic kind of expensive? Would printing those wheels really cost less than buying the $60 Vex wheels?
Spot on. Even the cheapest 3D printing isn't cheap. IMO it's novel that the guy took the time to "reinvent the wheel", but those wheels are relatively weak, bumpy and a poor substitute for real mecanums.
In comparison, the $60 VEX wheels are very strong (and heavy), roll very smoothly and they have excellent rubber and bushings. Friction only where you need it. At $15 a wheel, they are a conspicuous bargain in every way.
Having played around with mine for a while now, I can say mecanums are magical to watch: definitely mind blowers and crowd pleasers. BUT... they operate exactly like 4 omniwheels in an X configuration. I'll have to build one of those too at some point.