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Mecanum Revelations for Rich & Duane — Parallax Forums

Mecanum Revelations for Rich & Duane

ercoerco Posts: 19,860
edited 2013-04-02 21:48 in Robotics
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.


  • W9GFOW9GFO Posts: 4,006
    edited 2013-01-30 20:58
    It sounds like you may have the wheels on backwards. Viewing from the top, the rollers should make an "x" pattern. Viewed from the bottom, a diamond.

    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
  • ercoerco Posts: 19,860
    edited 2013-01-30 22:22
    Good call Rich, I may have them swapped (left 'em at work, will see tomorrow). Curiously, I just found this image online which shows the goofy aspect ratio I mentioned. Third image down. Hmmm...

    And this working robot is even more ridiculous:
  • ercoerco Posts: 19,860
    edited 2013-01-30 22:28
  • W9GFOW9GFO Posts: 4,006
    edited 2013-01-30 22:51
    Yeah, that is a pretty squashed aspect ratio. I'm sure it doesn't matter. As long as the wheels aren't fighting each other there should be no scrubbing or breaking of traction. It's when you use encoders to tightly regulate the movement that you would need to know the distances between wheels and compensate in software.

    For a sanity check regarding wheel orientation and aspect ratio just look at the Airtrax forklifts.
  • Roy ElthamRoy Eltham Posts: 2,992
    edited 2013-01-31 01:07
    Don't forget this one:
  • Duane DegnDuane Degn Posts: 10,443
    edited 2013-02-04 22:33

    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.
  • ercoerco Posts: 19,860
    edited 2013-02-05 08:41
    Yessir, I did have the wheels reversed, per Rich's comment. Now the center of rotation for the front axle is well in front of the robot. I did copy Rich's general shape & proportions. (Good designers borrow, great designers steal!)

    I used four of my favorite 24V gearmotors 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.
  • rwgast_logicdesignrwgast_logicdesign Posts: 1,464
    edited 2013-02-05 22:49
    Have any of you guys thought about using two mecanum wheels with an omni directional skid wheel in back? This may be a fairly inexpressive way to experiment with these things, since the mecanum's are so expensive.

    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
  • ercoerco Posts: 19,860
    edited 2013-02-05 23:14
    For Rich, Duane, Martin_H, and robot lovers everywhere. I can't believe this all worked together on the very first time! The arm & chassis were both slam-built for a tech show tomorrow. I thought tonight would be an all nighter for sure. Got lucky, robotics sure is fun when it all works. Next I'll have to do Towers of Hanoi on the mecanum platform!

    "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."
  • rwgast_logicdesignrwgast_logicdesign Posts: 1,464
    edited 2013-02-05 23:37
    Wow I dont know how many of you guys have access to 3d printers, but this is definitely cool! Homemade mecanums
  • rwgast_logicdesignrwgast_logicdesign Posts: 1,464
    edited 2013-02-05 23:44

    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.
  • Duane DegnDuane Degn Posts: 10,443
    edited 2013-02-06 11:03
    That robot is cooler than cool!

    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?
  • ercoerco Posts: 19,860
    edited 2013-02-06 11:14
    Yes, fig 8 is calling my name. I hope to be first. Unless you have had your mecanum around the plates yet?

    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:

    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.
  • Duane DegnDuane Degn Posts: 10,443
    edited 2013-02-06 12:54
    erco wrote: »
    Unless you have had your mecanum around the plates yet?

    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.)
  • ercoerco Posts: 19,860
    edited 2013-02-06 13:25
    OK, that's the obvious (and more interesting) way to do the "8".

    There is another...
  • ercoerco Posts: 19,860
    edited 2013-02-06 22:12
    For Duane: Here are the gory details of my hastily-built mecanum bot. As promised, the underside is quite underwhelming. :)

    My ghetto Figure 8 was crazy simple: turn on 2 motors for 11 seconds, then turn on the other 2 motors for 11 seconds.
  • Martin_HMartin_H Posts: 4,050
    edited 2013-02-07 16:24
    I can't watch this video as I will be tempted to buy Mecanum wheels. My 4x4 could obviously be converted into one of these nifty vectorbots with a rather expensive new set of wheels. What would I do with my old $20 set of off road wheels? I must resist!
  • ercoerco Posts: 19,860
    edited 2013-02-07 19:18
    In "The Ring", people were held at wrenchpoint and forced to buy mecanum wheels exactly 10 days after watching a video.
  • Duane DegnDuane Degn Posts: 10,443
    edited 2013-02-08 10:17
    Thanks for the video erco.

    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.
  • ercoerco Posts: 19,860
    edited 2013-02-08 11:39
    Duane: but then everyone would want them...! :)

    Will do, they are very nice.
  • NikosGNikosG Posts: 702
    edited 2013-02-10 03:13
    Amazing robot erco! Congratulations!
    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!
  • ercoerco Posts: 19,860
    edited 2013-02-10 07:55
    NikosG, you always have kind and swift words. Considering much of the bad news that we hear coming from Greece, your good thoughts, creativity and enthusiasm are always welcome here. Efharisto poli!
  • W9GFOW9GFO Posts: 4,006
    edited 2013-02-10 08:54
    NikosG wrote: »
    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.

    851 x 686 - 105K
  • NikosGNikosG Posts: 702
    edited 2013-02-10 12:15
    Rich ,
    as I saw the chassis of your mecanum rover is made by a Laser cutter or CNC machine. Isn’t it? Have you seen Thingiverse site? You can post the 3d model of your robot’s 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 don’t you?
  • W9GFOW9GFO Posts: 4,006
    edited 2013-02-11 17:04
    NikosG wrote: »
    Have you seen Thingiverse site?

    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.
  • Duane DegnDuane Degn Posts: 10,443
    edited 2013-02-11 20:12
    Speaking of Thinkiverse and Mecanum wheels, have you guys seen this robot?


    It sure looks like he's doing it the hard way.

    Pretty funky looking wheels.
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  • ercoerco Posts: 19,860
    edited 2013-02-11 21:19
    Cool looking, but hard way is right! Eleven wheels is an odd (ak, prime!) number. Awkward to machine (thus, he used rapid prototyping). And it would be a clunky ride with flat disk rollers, like rolling on 11-sided polygonal wheels.
  • rwgast_logicdesignrwgast_logicdesign Posts: 1,464
    edited 2013-02-11 23:59
    Guess you guys didnt see it, i posted a link to the LMR including video of that rover with printed mecanums. It acually does a really decent job, the guy made them becuase he was on a budget, and was able to print them at a local hacker space, oh how i wish there was a place with a laser cutter and printer around here.
  • Duane DegnDuane Degn Posts: 10,443
    edited 2013-02-12 09:13
    Guess you guys didnt see it, i posted a link to the LMR including video of that rover with printed mecanums. It acually does a really decent job, the guy made them becuase he was on a budget, and was able to print them at a local hacker space, oh how i wish there was a place with a laser cutter and printer around here.

    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?
  • ercoerco Posts: 19,860
    edited 2013-02-12 11:07
    Duane Degn wrote: »
    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.
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