Mecanum. What's Best
 

Our team is trying to build a mecanum drive so we can add it to arsenal of drives. We have never made a successful mecanum drive yet. My freshman year (overdrive) we tried but our gear ratio was miscalculated and we could barely move. So now I want to make one that we can play with and possibly use (rebuilding/coding for the season of course). My questions are:
What's "best?"
-Wide, Long, or Square Frame (wide=wheels parallel to each other are farther apart than the wheels behind/in front of it, long=wheels are closer to the ones in front of/behind it than the ones parallel to it.
-Chain Driven or Direct Driven
-Field Oriented or Robot Oriented

*by best I mean in your experience what have you done and what where the upsides and downsides. what has worked well for you and what has been bad.

I have done a quick search and found how to build mecanums but nothing that shows why one might be better than another. If I missed something Please point me to it.

Re: Mecanum. What's Best
 

Look at this thread: http://www.chiefdelphi.com/forums/sh...hlight=mecanum

Re: Mecanum. What's Best
 

You've heard the saying in real estate: "Location, location, location".

Well, with a mecanum vehicle it's "Wheels, wheels, wheels".

If you don't have well-designed wheels nothing else matters. Well-designed wheels are very expensive. The rollers are crucial: they must have the proper contour, overlap, alignment, and mounting (bearings).

Secondary considerations are wheel mounting (high lateral stiffness) and closed-loop speed control.

Re: Mecanum. What's Best
 

that thread sounds more like the code side of the drivetrain. i think the OP was looking for the physical design of the drivetrain.

i have never built one but i have seen many others. generally, you want to run them with the wheels parallel to the main direction of travel. wide will turn better but if your manipulation design does not allow for it, don't sweat it (true for all drivetrains other than crab).

as for direct vs chain, there are pros and cons:

chain- less foolproof (thown chain= not good), but has greater flexibility of tranny and motor placement. this year chain was a good choice for mecanum (and any other wheel drives). also allows far addition gear reduction via sprocket sizes

direct- very reliable (spur gears are strong and very few teams if any use multi-speed trannies for mecanum). tranny and motor location limited to next to the wheels. depending on wheel size, stock gearing may need to be reduced to give a proper amount of torque (keep in ming mecanums have less traction so torque is less of weapon).

speaking of traction, that is another issue mecanums always seem to have- they do not have much traction compared to traction wheels. this is why teams will still use crab drive even though mecanum is just as mobile and much simpler. this is a good point to keep in mind when 2011 design time come along.

Re: Mecanum. What's Best
 

Not necessarily. For a crab-drive to turn better, it requires 4-wheel independent steering and driving, with complex software and driver interfaces (the second is harder) to align each wheel with it's arc, and drive it at the correct speed.

Most teams that I have seen have independent drive motors (If they have 111-style pods, with integrated motors), and 2 steering motors. If the sides are slaved together (both lefts with 1 steering motor, both rights with 1 steering motor) you can't turn better than a skid-steer. With the front/backs slaved together, you can rotate them in either "car" (front-steer ackerman), "forklift" (rear-steer ackerman), or both, which will help but not be as optimal as 4-wheel independents steering.

Re: Mecanum. What's Best
 

exactly. what i was trying to saw was that wide drive tends to work better on all drives other than crap, since crab can be either depending on what directions the wheels are rotated at any given time. crab tends to throw a good amount of general rules out the window when it come to drivetrains.

crab is one of those things that is awesome if done right but disaster if done wrong... there was a team that shall remain anonymous that tried crab in 2006 and were forced to run it tank-style due to the programmers not having the members and time to program crab. the robot worked, but it could have been better had they opted not to try crab that year.

bottom line: crab takes a lot of effort to do right... if you are not sure if you can do it, it might be best not to.

Re: Mecanum. What's Best
 

Not necessarily. Just add two more wheels and drop the center ones and you can turn like any other skid steer bot.

Re: Mecanum. What's Best
 

that also requires a degree of insanity dillon

Re: Mecanum. What's Best
 

Position the wheels so the contact with the floor makes a square. This means the robot will be a bit longer than it is wide.

We made one with 8" AM wheels, driven by AM gearboxes with a bit of reduction in the chain drive.

Suspension of some sort that lets the wheels all rest on the ground with similar weight load is helpful too, so it will drive where you want it to.

Re: Mecanum. What's Best
 

You can also turn a crab drive well by connecting diagonal pairs of wheels together (like 1717 did last year). This lets you both strafe and orient your wheels in a "diamond" pattern for turning on a dime.

Re: Mecanum. What's Best
 

This configuration lets you crab and turn in place, but turning while translating is difficult.

I actually took some time last year to analyze 25 different possible steering/driving configurations for crab drives. Basically, it combined all of the typical driving and steering configurations in a giant matrix, and figured out what drive modes (crab, tank steer in both long and wide, snake/car steering) would be possible (both with and without skidding the wheels) in each. I should probably wrap it all up into a white paper when I get a chance. I looked at:

4 drive transmissions (one per wheel)
-Steered all together
-Steered in pairs (F and B, L and R, diagonal)
-Steered individually
2 drive transmissions (F and B)
-Steered all together
-Steered in pairs (F and B, L and R, diagonal)
-Steered individually
2 drive transmissions (L and R)
-Steered all together
-Steered in pairs (F and B, L and R, diagonal)
-Steered individually
2 drive transmissions (diagonal)
-Steered all together
-Steered in pairs (F and B, L and R, diagonal)
-Steered individually
1 drive transmission driving all wheels
-Steered all together
-Steered in pairs (F and B, L and R, diagonal)
-Steered individually

The only configuration that gives you perfect (no skidding necessary) crab and turning under all possible circumstances is the 4 drive transmissions/4 steering transmissions case.

Re: Mecanum. What's Best
 

For the mathematically inclined, the above result is equivalent to Equation 3 which is derived in this white paper. It shows the necessary X and Y components of velocity at each wheel to achieve any desired vehicle simultaneous translational and rotational motion.

Using the results from Equation 3, the wheel angle for the nth wheel is -ATAN2(Vyn,Vxn), and the wheel rotational speed for the nth wheel is sqrt(Vxn^2+Vyn^2)/r.

Re: Mecanum. What's Best
 

just remember, you want an X, not an O

Re: Mecanum. What's Best
 

Ok so from what I got it does not necessarily matter your ratio wide-long and the wheels have to be in an X. Thank you so much. Now does the COG have to be in the center of the wheels?

Re: Mecanum. What's Best
 

X from the top, O from the bottom.

CoG doesn't *need* to be in the center, but it helps (when some wheels start slipping before others, control gets difficult - a roughly central CoG will help to evenly distribute your normal force among the wheels).

Re: Mecanum. What's Best
 

Correct.

As viewed from the top. Right way Wrong way

No, but it helps. See Jared341's post.

Re: Mecanum. What's Best
 

above post reported

Re: Mecanum. What's Best
 

My opinion is
- Use a long frame, so that the wheels are square with one another. Having the wheels, in a "long" or "wide" orientation can cause the robots rotation to be skewed from the center due to a different force being applied in rotation of the robot due to an irregular center of gravity.
- Direct driven will be better to use because the signals sent to the motor will be better applied to the wheel, especially in changes in direction due to any slack in the chain causing the wheel to not move for a short interval of time, while the slack shifts.
- During this years game we had the option of using both field-centric and robo-centric drives and switching between both during the same match. Even though we had that option we never switched from robo-centric. field centric can be thrown off and change completely the direction the robot will travel relative to the joystick, this can occur during quick changes in direction due to fast turning, bumping into the field or with other robots. Therefore I would endorse robo-centric, however field centric would be a good thing for your programmers to learn how to do, but I wouldn't implement it until your drivers feel confident with it.

Also as Ether said the wheels can be a major concern. you want to have the sub-wheels all freely spinning, but ideally wheels of this design would be the most prefered, http://www.chiefdelphi.com/media/photos/25159, because they will prevent the wheel from bumping up and down. Similar to the difference in performance between single and double omni wheels.

Re: Mecanum. What's Best
 

This problem can be mitigated by adding a "calibrate" function in your software and binding it to a driver-convenient button or trigger on the joystick.

Whenever the robot's field-orientation sensor loses calibration, the driver can quickly rotate the robot to some pre-determined orientation (most likely aligned parallel to the length of the field) and push the button to re-calibrate.

Re: Mecanum. What's Best
 

Not entirely sure what you mean here by "irregular" CoG. As long as the CoG is equidistant from all 4 wheels it doesn't matter if the wheels are located at the corners of a square or a rectangle (within reason). As Jared341 explained in an earlier post, the purpose of centering the CoG is to distribute the normal force equally on all 4 wheels.

Re: Mecanum. What's Best
 

When irregular CoG I meant non-centered, and that's probably the word I should have used, because I find that on most, if not all, FIRST Robots you will see a non-center CoG. If anyone has proof that they have built a CoG centered robot I would love to here how they did it.

Re: Mecanum. What's Best
 

Yeah, it's pretty tough to control the CoG precisely. But CoG should always be on your mind as the robot is being designed. The design criteria for mec wheels is equal normal force on all 4 wheels, as much as possible. This does not mandate locating the wheels at the corners of a square.

Re: Mecanum. What's Best
 

Well said. The challenge is to try to get as close as possible to the same normal force on each wheel. If your CoG is skewed to the front, move the rear wheels forward some.

From my experience, the normal force does not have to be exact. We had the CoG off by several inches (front biased) and did not have major issues. We had more of a problem with our budget built gearboxes having different amounts of internal friction. If I were to do it again, I would vote for toughbox (standard or nano) driving each wheel.

Re: Mecanum. What's Best
 

You could go one step further and use wheel speed sensors to control wheel speed, thus mitigating not only friction variation but also differences in motor torque gain.

Re: Mecanum. What's Best
 

From our experience with mecanum:
-Make sure all wheels are touching the ground: Aside from wheel orientation (diamond vs. X), its been our biggest problem with mecanum. Make sure your frame doesn't warp or use a "suspension" chassis (we did this with pods and springs).
-Use 8 inch mecanum wheels: The 6 inch ones (back in '08) pinched the rollers. I've heard that the 6" wheels have been improved, but we switched back to 8" since.
-Don't pinch the rollers: Otherwise the wheels WILL NOT WORK (I think most people know that, though).
-As said, orientation (diamond vs X)

All of our mecanums that I can remember have been chain driven so I'm not sure the difference it makes (asides from the general pros/cons of chain vs direct driven).

We've found mecanum to be good for manuverability and its pretty easy to get used to when you drive (we drive with two joy sticks-one for back/forth/left/right, and another for orbiting/point turns). We also think it makes for great demos (people get a kick out of robots driving sideways).

They do, however, get pushed around fairly easy, and messing up wheel orientation is a pain. Also, if your frame warps or rollers get pinched, it doesn't play nice at all.

Re: Mecanum. What's Best
 

My personal preference is the same as forbes' for the Mechanical setup. It's simple and easy to build since it requires very little modification from even the KOP chassis.

My personal preference for Controls is to drive it like a normal bot ("Tank" controls or "Arcade" controls) and use a joystick hat to control 8 possible strafe modes (where "Up" and "Down" on the hat are the same as forward & back). This greatly simplifies the control element and allows a driver to gain practice very early on.

For most teams, I'd say let the rest of the robot integration sway the vote for wheel setup/orientation (wide vs. long). So long as the bottom makes an "O" & c.g. isn't way out of whack, the robot will not have problems turning. "Integration" in this sense is "how wide" the robot needs to be in order to accomodate the forseen mechanisms your team will come up with (intake to conveyor systems are predominantly wide drive, whereas most other years it can flip either way).

Re: Mecanum. What's Best
 

Could you elaborate please? What is it greatly simpler than?

Re: Mecanum. What's Best
 

It greatly simplifies it from control setups can be observed at (I'll conjecture) any FRC Regional, where the drivers themselves do not seem to have a full understanding of the best way to move the bot around. That problem is a result of either complex controls or lack of practice. In either of those cases, it's advantageous to simplify the controls. My case here also assumes that the drive train is geared for a balance of speed/acceleration (10-11 fps) for a typical* 140-150lb robot.

Here are what I consider 'bloated'

• 2 Joysticks that control different degrees of freedom**.
• 1 Joystick that has a "twist" action for a z-axis rotation, in which users (from novices to veterans) unknowingly twist it ever so slightly while trying to strafe
• Control software code where (as an example of negligible amounts) 10% more rotation control input from a joystick translates to 10% more rotational output from the robot even while the robot is strafing
• Overly sensitive controls where fine muscle movement is needed (like game pad thumb sticks) for the difference between (example) strafing left and strafing the forward left diagonal. This is particularly noticeable when combined with the 3rd bullet.

Additional programming or practice may alleviate the problems of the above situations. However, starting with something fundamentally simpler would also alleviate the problem with less impact on the robot's schedule. In other words, the drivers get more practice learning the robot's interaction with game elements rather than learning how to make the durn thing move as expected.

I was able to observe many matches with Mecanum drive trains as Scouting mentor in 2010. I was also able to observe hundreds of little kids at the USASEF last weekend as they drove a couple of Mecanum drive trains on the mini field. So really, this is all just based upon my observations and opinions, for whatever one feels they're worth.

*Typical here is what I've seen on field. A CIM motor that drives a 6" Mecanum wheel that is mounted directly to an AM Toughbox (or Nano) moves the robot at roughly 10.5 ft/s. Under normal conditions, the motor load across 4 motors for such a setup is near peak efficiency of the CIM motor regardless of a Mecanum drive train's wheel base (since all 4 force vectors assist in turning on a Mecanum drive train).

**I've yet to observe a driver who is naturally a master of the Halo-style of driving without having spent many hours playing Halo already.

Re: Mecanum. What's Best
 

Thanks for making that clearer. I completely agree with the general point you are making (about making a driver interface with a quick learning curve).

I think we disagree slightly on the implementation of that goal, specifically the first two bullet points:

2 Joysticks that control different degrees of freedom

Experience has shown that there is a mecanum driver interface with different degrees of freedom on 2 joysticks which has virtually no learning curve for a driver with prior tank-drive experience. It's an implementation of the "Tank-drive" approach that you mentioned. The left and right joystick Y axes control the vehicle just like tank drive, and the right (or left) X axis controls strafe (but only when a button is held down). More detail available here.

1 Joystick that has a "twist" action for a z-axis rotation, in which users (from novices to veterans) unknowingly twist it ever so slightly while trying to strafe

The above objection is valid, but is easily mitigated by adjusting the gain curve of the Z-axis so that small signals have little or no effect. See this post for more detail.

Re: Mecanum. What's Best
 

I think these two points are related in that the second one demonstrates why one would want to control their robots using the first method...

In all seriousness, "splittling" an arcade drive into two joysticks greatly increases drivability. For mecanum drives I happen to be a fan of "halo style" controls with independent joysticks for movement and rotation, and I don't even play Halo, but I also happen to be a very big proponent of not using a mecanum drive. :P I guess it just has always seemed more intuitive to me and whatever random kids drive the demo bot...

Re: Mecanum. What's Best
 

For those of us who don't play video games, could you please state what "halo style controls" means ? i.e. what mecanum function is assigned to each of the joystick axes. Thanks

Re: Mecanum. What's Best
 

Movement is assigned to one joystick (either forward or strafing, the robot moves in the direction the joystick is pressed), and the other joystick controls rotation / orientation. Much like Halo and other console first-person-shooters.

Re: Mecanum. What's Best
 

I agree with your Chris. Since the student sthat are going to drive the robot generally have played halo, or at least a game similar to it, it makes it very easy to pick up the controls. Also, if someone came up with a better way to control omnidirectional drive systems (mecanum, swerve, etc), I would consider using it. But given the number of people who already know how to use "halo controls," I doubt I would use it.

Side note: my team is prototyping a swerve drive this fall and we are using halo controls for the robot.

Re: Mecanum. What's Best
 

I'm a big fan of segregating different degrees of freedom to different joysticks. We currently use arcade drive on a skid steer, with on stick for throttle, and the other stick for turning. A human being is not very good at moving a stick perfectly in one axis.

I feel the same way for using the twist of a stick as the Z, that's just way too much for a driver to do with one hand, after all, he has two.

I believe, as chris described, that translation on one stick, rotation on another, is the way to go. This is the most intuitive, and worked great for us on our prototype crab.

Re: Mecanum. What's Best
 

The suspended chassis is not a bad idea for mecanum wheels, but unless you need a lot of suspension travel I would just use it with a flexible chassis.

Re: Mecanum. What's Best
 

Completely agreed. We do the same, and it has worked very nicely.

Re: Mecanum. What's Best
 

mechanum don't really need any special considerations when mounting, almost any configuration will work, the main difference from tank is in the programming