Mecanum wheels

[TheJoe]

Our team has used a mecanum drive train for the last three years and we've had a lot of fun with them.

As the robot driver for the past two years, I can assure you they're wonderful to control, and driving becomes a lot easier with mecanum.

Controlling the a mecanum-powered robot is a lot like moving a character in a first person shooter video game, which I'd assume many students are familiar with already.

Couple a mecanum drive train with gyroscope technology and now your robot can always move relevant to the driver (No matter the orientation), which in my personal opinion makes controlling the robot even easier.

While a downfall may be that you lose a lot of pushing power, the versatility gained to out-maneuver the defense matches said downfall quite well.

[Donut]

Our VexPro mecanums have held up well. They survived one regional and some off season demos with no problems, and in our shop we typically run them on concrete. We are planning to use them again this year.

[Keegbot]

Originally Posted by BEN35678

Last year we used Andymark mecanum and this year we were looking at the vex pro mecanums, and a couple of our main mentors were worried about them cracking because they are plastic. Did you have anything that looked like it was about to crack or anything like that?

Yes, our VEXpro wheels did crack, but that only happened this past December at a fundraiser where we were showing off our robot. We went to two competitions last year and had several other fundraisers and expos before they broke. I believe they are durable enough. You shouldn't have to worry about cracks during competition. I believe they also sell replaceable wheels for the mecanum if any of them seem to have significant wear or break.

[lcoreyl]

Mecanum wheels are very nice for alignment of your robot even when the driver has not had a lot of practice time. I would imagine this year aligning your robot in order to gain possession of a ball would be important to some teams. Practicing gaining possession in a real-game situation is difficult for a lot of teams, so mecanum can be handy here.

Also, I always throw in my vote for don't believe that mecanum can easily be pushed around. Wait...am i part of the majority on this vote??

[bEdhEd]

At the moment, my team is not a fan of using mecanum, unless it is undeniably the absolute and only way to win matches. With mecanum wheels you can lose up to 50% of your power when doing moves like strafing, whereas with swerve drive, all the power is used more efficiently. Mecanum wheels are indeed expensive, and I don't think that they're worth the price unless you are absolute that they are the best choice. From my experience here in the Californian bay area/central valley, the best teams stick with drop center six wheel drive treaded wheels. Why? They are inexpensive, offer the best traction, best pushing power, least complex in programming and build, easy to fix, and provide all the maneuverability necessary. Name one team that has made it to the Einstein field using mechanum wheels. None, because this has never happened in FRC history, but hey, there's always a first, and it could be anyone!

If your philosophy isn't all about winning, then have at it with the mecanum wheels! Mecanum robots are really fun to drive, too. Still, they are a pretty penny, but 100 bucks per wheel isn't a bad deal. I'm not saying that you can't win with mecanum wheels, many regionals have been won by these types of drive bases, but what I'm saying is that treaded wheels in general are a better choice in my opinion. Treaded wheels are also LIGHTER! And that could be the difference between passing and not passing inspection if you cut it really close with your other robot components.

[thefro526]

There's a bit of mis-information in this thread, and while I don't want to join in the debate, I'd like to set the record straight for any of the newer teams that might be using this thread for information.

Mecanum Wheels can be purchased from two different FRC Suppliers.

- AndyMark http://www.andymark.com/Mecanum-s/53.htm

AndyMark's Mecanums were first released for the 2007 Season and have been evolving since then. They're offered in FRC relevant sizes of 6" and 8" with a Standard Duty and Heavy Duty version of each. The Pricing Varies from ~$250 for a set of 6" Standard Duty to in excess of $400 for a set of 8" Standard Duty.

- VexPro http://www.vexrobotics.com/vexpro/wh...um-wheels.html

VexPro's Mecanum Wheels are relatively new compared to AM's Mecanum Wheels, but have proven themselves to hold up reasonably well in the FRC Environment. They're a bit Cheaper than AM's Mecanum Wheels and rely on a primarily plastic construction in place of Metal.

As far as Mecanum's implementation in FRC, they're a decent way to make your drivetrain more maneuverable, as well as adding omni directional functionality within a traditional frame. The increased agility awarded by Mecanum wheels should allow you to move around traditional defense to the point where 'pushing' isn't necessarily required.

All of that being said, when comparing two similar drives, with the only difference being that one Uses Mecanum wheels in place of a traditional wheel, the Mecanum Drive will almost always have less traction than the traditional drive train. This is due in part to a couple of factors, the wheels always being in a constant state of slip (in that they're applying force in a non-fore/aft direction) being one of them, and the other being that each wheel's motor is independent. This means that if one of your wheels leaves the floor, or starts spinning, you're not only putting down the power of (3) motors.

[TedG]

We have used Mecanum wheels the last 3 years with great success.
Great for manouvering in all directions without haveing to turn.
Last year we used the 6" dia heavy ones from AM and could push our way around as well as resist being pushed around. It helps if you have a heavy robot though.
And it's a good idea to buy a spare wheel or two, but they are expensive.

[BEN35678]

Originally Posted by Keegbot

Yes, our VEXpro wheels did crack, but that only happened this past December at a fundraiser where we were showing off our robot. We went to two competitions last year and had several other fundraisers and expos before they broke. I believe they are durable enough. You shouldn't have to worry about cracks during competition. I believe they also sell replaceable wheels for the mecanum if any of them seem to have significant wear or break.

What are the advantages of using the vex pro mecanums then why not just go with the AndyMark then?

[efoote868]

Mecanum wheels have a short history in FRC. They tend to have bad stereotypes as being easy to push around and difficult to control, particularly among individuals that have never used them.

The way people write about mecanum wheels, you'd think they were describing casters on an ice-rink.

The easiest drive-train I've ever used in FRC was a set of field-centric mecanum wheels. I've watched mecanum wheels be used for defense effectively, and I've seen times where the maneuverability and ease of use of mecanum wheels won matches.


To answer the OP's question, I think $400 is a bargain for a set of mecanum wheels. Whether your team's strategy calls for them is a different question.

[purduephotog]

Could anyone point me to resources about the maximum speed you can run Mecanum wheels at ?

I'm wondering (and trying) to find out if all the gear boxing is really necessary. Once they're slipping they're slipping, and the friction factor for all four wheels would be ~equal.

[Ether]

Originally Posted by purduephotog

Once they're slipping they're slipping, and the friction factor for all four wheels would be ~equal.

What do you mean by "slipping" in this context?

[Ether]

Originally Posted by bEdhEd

...my team is not a fan of using mechanum...

With mechanum wheels you can lose up to 50% of your power...

...Name one team that has made it to the Einstein field using mechanum wheels...

...If your philosophy isn't all about winning, then have at it with the mechanum wheels...

...Mechanum robots are really fun to drive...

...I'm not saying that you can't win with mechanum wheels...

There is no "h" in mecanum.

[JesseK]

Can anyone speak to the wear of VEXPro 4" Mecanum rollers compared to the 6" wheel rollers over the course of multiple competitions?

In an average year we compete in 2-3 official and 2 off-season competitions, perform in several demos & run a lot of driver practice for the next year's team. So the wheels really need to last. Yet it'd be nice to know what spare parts to get ahead of time (i.e. 1 set of spare rollers per 1 event or 2 events?) when comparing wheels. If we have to replace the 4" wheel's rollers every 1 event, we may go the 6" route so we don't have rollers that are wearing thin at the end of an event.

My team's poking at the idea of Mecanum for a few reasons, and it's easy to implement on our current drive frame.

About Mecanum in general:
The only thing I can say without reading a metaphorical earful from Ether (ha, hi Ether) is that the default field-centric (or robot-centric with no deadband) control algorithm robs the drive train of power just by the nature of the code. When translating on the 45-degree relational diagonal (i.e. 0 degrees = full forward, 90 degrees = full right, so at 45 degrees the bot goes forward & right), 2 motors are completely turned off by the algorithm.

If we implemented Mecanum, we'd most likely do a dual-mode driver control. In the first mode, it'd be field-centric holonomic drive. In the 2nd mode, the driver would be limited to robot-centric tank drive with 90-degree strafing. The driver would be in the 2nd mode most of the time.

[Ether]

Hi Jesse :-)

Originally Posted by JesseK

When translating on the 45-degree relational diagonal (i.e. 0 degrees = full forward, 90 degrees = full right, so at 45 degrees the bot goes forward & right), 2 motors are completely turned off by the algorithm.

Yup. And so the available driving torque at the wheels to keep the bot moving against the rolling resistance is half. And so the top speed in that direction will be substantially less than that predicted on the basis of kinematics assuming no load.

In the 2nd mode, the driver would be limited to robot-centric tank drive with 90-degree strafing. The driver would be in the 2nd mode most of the time.

What driver interface do you have in mind for this? Perhaps divide this 2nd mode into 2 modes?

Mode 2a: Standard 2-joystick Y-axis-only tank drive (X-axes disabled)

Mode 2b: Right (or left) X-axis commands strafe (disable Y-axes)

[TheMadCADer]

Originally Posted by azcalg

In this game the only big advantage I see for them is with catching the ball from the truss, so unless that's a big part of your strategy, I wouldn't get them. Another downside to them is that they're quite heavy.

I'd actually disagree. I wouldn't want a catcher to have mecanum wheels, since they are so easily pushed.

Looking at the scoring for this year, a catch is the same as a high goal score (disregarding assist points, since you can score low and get the same assist score), so if teams know you can catch, they will try to defend it.

Hitting a moving target is added complexity you don't need. I'd pick a catcher that has no wheels at all before I'd pick one with mecanums.