We are considering having mecanum this year and were wondering what the subtleties of it are. Anything and everything would be helpful such as what gear boxes to avoid, what ratios would be best or various problems with programing.
This is our (461) second year in a row that we are using mecanum; i don’t know a whole lot about it, but i do know that:
Much better maneuverability
Less pushing force
Harder to program/longer
Problems can occur with a direct drive setup and thoughbox nanos, because it pushes the bearings with a force and direction they’re not designed to take.
But all in all, if you’re programmers and drivers are up for it, it is a great choice
You ask a broad question with many details that is difficult to answer succintly. Specific questions are better in the forum format. However, there have been quite a few threads that address ALL of your sub-questions in some form or another. The technology hasn’t changed since those discussions were posted. Also, there are a couple of white papers that discuss the physics behind mecanum drives.
We were considering using the nanos. Have you tried using them? What went wrong if you did?
Team 2950 used a 4 wheel drive combination of two Mecanums in the front and two Plactions in the back for a mix of great maneuverability AND traction in Breakaway last year. It worked out very well for us and could be good for your team as well.
I’ve heard good things about direct drive + Toughbox Nanos in mecanum drives, as well as regular Toughboxes.
Well, we haven’t tested them out yet, but the mentors noticed that the bearings might not be able to take the 45 degree angle load from the direct mecanum drive, at least for the nanos. we aren’t 100% sure yet, but it’s definitely something to look into
If you want an agile bot, go ahead. The team I’m with wanted to do mecanum wheels, but decided not to because of programing issues tied up in it. The ideas are great, but are not very good for newer teams. The frame might have to modified in order to do this.
Last year 1279 used 8" mecanums, direct drive to long output shaft nanos, 12.5:1. We used a bearing on the other side of the long shaft.
My suggestion is get it done ASAP, so the driver(s) have time to train, and programers have time to program it. We actually built a test bot using a second set of Mecanums, using a 3/4" plywood base fot driver training and programming.
We are planning to do the same thing except with a legit second drivetrain.
Our plan is to use the 6 inch mecanum. Is the 8.45:1 ratio to slow? I know that mecanum is inefficient and will lose some of that speed but how much?
Nope. For reference, 12.75:1 with 6" wheels is ~9 feet per second.
If you have an algorithm that translates in every direction the same, you will get about 71% ( sqrt(2)) of full power from your motors when going forward with no other modifications. 1675 usually includes a “beast mode” for going stright forward/back with 100% power.
Going on the topic of bearings, and this might sound like a stupid question, are spherical bearings ever used (as opposed to the traditional ball bearings)? and if so, what are the pros and cons respectively?
thanks
-duke
Can you further explain this? Why would you only get 71% when going forward?
If you are direct driving a mecanum drive or any drive for that manner, make sure you support the shaft in two places. If you don’t you’ll bend your shaft and destroy your drive train. The tough box series from andy mark does have two support points internally and you can get away with no external supports on the shaft for short distances. However if you use an extended shaft you’ll just kill the gear box and the shaft if there is no support on the other end of the shaft. My team used mechanum last year and used the P80 gear boxes with an extended shaft. It was extremely efficient, had good mount points for the encoder, and we had no problems. Hope this helps
We (1002) ran 8" mecanums last year off standard Toughboxes. They’re surprisingly quick and we were very pleased with how well they worked. I’d recommend avoiding chain if you can, the added strains from the different types of motion didn’t agree with it. Also, once you’ve got them assembled, check that all the rollers spin freely.
Good luck!
If I may ask, where did you mount your encoders on the p80?
love that
I don’t see why the longer shaft would destroy the gearbox unless using that long shaft meant mounting your wheel far away from the gearbox which creates problems. Also wouldnt a third mounting point create more problems because a third mounting point gives the shaft a place to bend.
Not sure how much you know overall about Mecanum so I’ll lay it all out.
Mecanum wheels produce a force at a 45% angle when driven. The x and y components respectively equate to roughly 71~% of the total vector. (Actually thinking about it my sqrt(2) comment makes no sense. it’s actually 1/sqrt(2) for each side. 1/1.414 ~= .71)
In my head, without looking at our algorithm it isn’t making a ton of sense (since the forward vectors would be 71% too), but I know that when we’re running “normally”, which is translating at the same speed in every direction, our motors run at only that 71% when going forwards.
EDIT2: It is because the motor outputs in our algorithm are limited by a trigonometric function and motor offsets. It just so happens that our motor outputs when going forwards or backwards are +/- sqrt(2)/2, which is the same ~71%.
We give the drivers the option to drive our motors at 100% forward/backward.
If you use the provided library functions (such as LabVIEW Holonomic vi), you don’t have to worry about this and there is no need for a “beast mode”, because the normalization is performed on the wheel speeds after the inverse kinematic calculations are performed, not on the joystick commands. You get the full range of motor command to the wheels in all directions.
Now, don’t confuse this with the pushing force of a mecanum, which is a separate issue. In a traction-limited situation, mecanum has less pushing force than a standard-wheel vehicle using the same tread material, because the floor reaction force (for a given forward force) is higher on a mecanum wheel so it loses traction more easily.
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