Mechanum Wheel Sizes

[Kris Verdeyen]

It's perfectly natural for a team your age to feel things like this. Many of us have experimented with holonomic drive trains at one time or another. It's nothing to be ashamed of, and it's good that you've come to us for help. With hard work, focus and determination, you'll come through this, better than ever.

The thing to remember is this: every single person on every single team, including the best teams in FIRST, have, when first presented with them, looked at mechanum drive trains, and said, "Whoa, cool!", and not one robot built with them has made it to Einstein in at least 2 years (because that's how far back I've checked).

It's not because they suck inherently, it's because it's hard to make them run to their full potential. So, because you'll ignore this warning and build it anyway (because, "Whoa, cool!"), the best advice I can give you is this:

1. Encoders on all 4 wheels
2. Only run it closed loop (typically velocity mode)
3. Make sure each one of the (48) little rollers is free spinning before each run. Bent wheels can cause these to bind.
4. Understand what the code needs to make the math work, especially any code you don't write yourself.
5. It still probably won't work great on anything but a flat floor.

People below me will contradict points 1,2,5, and maybe 4. They will have counterexamples. They also probably have a grandfather who smoked 4 packs a day and lived to be 100. Ignore them, believe me.

[nxtsoccer]

Originally Posted by Kris Verdeyen

1. Encoders on all 4 wheels
2. Only run it closed loop (typically velocity mode)
3. Make sure each one of the (48) little rollers is free spinning before each run. Bent wheels can cause these to bind.
4. Understand what the code needs to make the math work, especially any code you don't write yourself.
5. It still probably won't work great on anything but a flat floor.

Woah! First off, I'd like to thank you all for the amazingly immediate responses! Second, based on recent posts, I'd like to put something out here: we're a one-season team with only two or so of our members having any previous robotic experiences (myself and one or two others). Additionally, this is the first time I (and I believe my teammates) have gone beyond the LEGO Mindstorms NXT (r) level of robotics. In a nutshell: I''m really not sure what to think of some of these posts, much less what they mean! I know what an encoder is (measures/counts rotations), but I don't see how that would be beneficial to a holonomic drive. As for code, if we decide to go with mecanum wheels, they'll almost certainly be driven using four jaguars and the code for holonomic drive found in the WPI Robotics Library in LabView, without any changes.

Thanks for understanding our current lack of experience in pretty much everything at this point!

[Grim Tuesday]

357 doesn't have a big presence on Chief Delphi, but if anyone is going to get to Einstein with mecanums, it will be them. I'd suggest trying to get in contact with them if you're looking for help designing driver practice for this or any other holomic drivetrain.

[KrazyCarl92]

Originally Posted by nxtsoccer

I know what an encoder is (measures/counts rotations), but I don't see how that would be beneficial to a holonomic drive.

Encoders are beneficial to a holonomic drive because these drives largely rely on the kinematics involved with the wheels going at very precise speeds. Since an encoder can measure the revolution (in this case of the drive axle or an axle coming off of the gearbox), it is the proper sensor to close that loop and accurately control the speeds of all of your wheels.

If a mecanum drive is coded for voltage control (no closed loop, just set the motors to a given voltage), you could have a situation where your driver commands the robot to drive straight forward, but only 3 of the 4 wheels are driving close enough to the same speed. That fourth wheel driving at a different speed than the other three will cause a slight rotation/strafe in the driving, resulting in a robot that will not be able to drive in a straight line. This is why speed control of the wheels is very important and encoders can be used to achieve this.

Good luck, and I hope you guys have success with your off season projects! It's this kind of work that will prepare your team for a successful season.

[rocknthehawk]

If it's something your team wants to do, got for it! There will be naysayers that will give every reason not to try it, but what fun is that? Pushing yourselves to try something new, and learning in the process, can be hugely beneficial.

This was our (Team 126's) first year using mecanum wheels. I don't think anyone would have changed this decision. Our camera alignment was fantastic, with the ability to strafe. We spent a lot of time calculating gearing. We ended up with a final drive of about 17.5:1, CIM to wheel speed. Secondly, lots of drivers practice. Usually 4 days a week for 2 hours or more.

We did not run encoders on our robot. We initially made custom mounts for encoders, but ended up finding we did not need them. I don't know the programming side well enough to understand why, or what we did differently. Just that it didn't operate any differently for us not using them.

We used 8" AM wheels. They are heavy. I don't think the washer and brass bushing for the rollers is the best design possible, but it's simple and cost effective.

[nxtsoccer]

Hey, do you guys think we'll need this: http://www.andymark.com/ProductDetai...Code=am%2D0279

It's a hex-bore ball bearing to fit the shaft of the toughbox nano, probably would be mounted on the other side of the shaft. On that note, are the toughbox nano's built-in bearings strong enough to support the weight of the robot, or would they need to be supported by bearings on both sides?

Eh, I think it's a confusing post.... Basically, how many (if any) bearings will we need other than the built-in ones to support the robot's weight without breaking something or unnecessary wear and tear? Any ideas?

[Nemo]

Originally Posted by nxtsoccer

Hey, do you guys think we'll need this: http://www.andymark.com/ProductDetai...Code=am%2D0279

It's a hex-bore ball bearing to fit the shaft of the toughbox nano, probably would be mounted on the other side of the shaft. On that note, are the toughbox nano's built-in bearings strong enough to support the weight of the robot, or would they need to be supported by bearings on both sides?

Eh, I think it's a confusing post.... Basically, how many (if any) bearings will we need other than the built-in ones to support the robot's weight without breaking something or unnecessary wear and tear? Any ideas?

You can go either way. In 2011 we bought the long axle versions and supported the opposite end of the shaft with simple blocks of plastic with 0.5" holes bored in them (we used the keyed versions). We liked that a lot. The disadvantage of doing that is that it's harder to get a wheel off if you need to replace it or repair it. Plus it is extra work to build.

The Toughbox Nano is strong enough to support the robot's weight. Of course, you want to put the wheel as close to the gearbox as is reasonably possible to minimize forces that would try to bend your axles.

[Taylor]

In 2011, we used a drive base very similar to what Scott has described. We then mounted a modified C-Base to the top. We had a nice surface to mount our electronics, and we were careful to leave them easy to reach.

http://www.chiefdelphi.com/media/photos/36120

It was the best, most reliable drive train we've had in 8 seasons, and if the game calls for it again, we'll certainly look at it again in the future. It was built in just a couple hours.

I don't know if this exact setup would work on a non-flat playing field.

[Peck]

first: I (like others who have posted before) would recommend at least a decent amount of experience on your team before dealing with more then 2 axis (x linear and z rotational [or equivalents]) in your drive train. At the very least, make sure someone who has done it before is helping you as it can get very confusing very quickly if u don't.

on topic: what is better depends on the intended use

mechanum is poor at climbing due to the rollers so it has to be significantly taller then whatever u intend to climb. It also stinks at countering gravity. general robustness is well below average for the weight involved. The only real advantage of the system is that it is highly mobile (capable of motion in x linear, y linear, and z rotational [or equivalents] all at once or independent).

finally: if u use it at a comp, bring at least one full set of spares since it takes time to repair and time isn't usually on your side (Murphy's Law is ever so true).

[efoote868]

Speaking from experience of 3 years of robots that used Mecanum wheels (and observing 1 year with swerve).

Mecanum wheels are the most forgiving holonomic drive. If all else fails, slap some skyway wheels on your robot and drive like you were never able to strafe. You can't say the same for kiwi, and you'll be pretty heavily invested in swerve.

Swerve drives have many more points of failure than mecanum wheels. If this is your first season with a non traditional drive train, mecanum will be the easiest to adjust to. Swerve drives have that nasty part where you have to turn your wheels before you go in the direction you want to go, which is another control challenge (having said all that, mad props to the guys on 111 for making it look easy every year).

As far as what you need for a mecanum wheel setup, at the bare minimum get a gyro and learn how to use it. Also learn about trigonometry and the math behind field centric drive.

Design your mecanum drive with either a bit of flex in the frame, or spring loaded wheels. It is very important that each wheel touches the ground, and the carpet at FIRST events typically is not perfectly level.

Last but not least. Buy your mecanum wheels now, along with four sets of gearboxes. Make a drive train with a wood frame and start playing around with it. Don't modify the wheels or the gearboxes, and you'll be able to use them once the next season starts. You'll then have learned valuable things about mecanum wheels that we may have missed here.

Good luck!

[nxtsoccer]

Originally Posted by efoote868

As far as what you need for a mecanum wheel setup, at the bare minimum get a gyro and learn how to use it.

As I said, the team is very new to this stuff... how would a gyroscope help with a mecanum wheel drive?

[ratdude747]

Originally Posted by nxtsoccer

As I said, the team is very new to this stuff... how would a gyroscope help with a mecanum wheel drive?

that's not the Kind of gyro that was being referred to.

A gyro in this case is a sensor that outputs a rate of rotation. There are many types of gyro out there with various benefits and drawbacks, as some are shock-sensitive and others require calibration.

[MichaelBick]

Be very careful when choosing mechanums. Many teams have regreted the decision. On the other hand, there are some that have liked it. Just some things to consider:

a) mechanums while strafing will have much less speed than normal. I believe(anybody correct me if I am wrong) that they have around 60% speed when strafing

b) There is a noticeable decrease in performance in pushing matches compared to a standard traction drive(4, 6, 8, 10+ wheels). While it is true that in some cases you will be able to outmaneuver a traction drive, and avoid the pushing match due to strafing, in 99% of cases you will not be able to, due to the fact that you will not be able to move as fast as the traction drive while strafing(look above).

c) along with the lack of speed and traction, there is a noticeable increase in the amount of time needed for practice.

Therefor, many teams will recommend against mechanums. Though if you are really set on them, then just know what you are getting yourself into.

[ratdude747]

Originally Posted by MICHAELABICK

a) mechanums while strafing will have much less speed than normal. I believe(anybody correct me if I am wrong) that they have around 60% speed when strafing.

Assuming 45 degree rollers, it would be 50% slower. It acts like a gear reduction.

[Ether]

Assuming 45 degree rollers, it would be 50% slower. It acts like a gear reduction.

It's not analogous to a gear reduction.

Neglecting friction, axial free play in the rollers, and compliance of the floor surface, vehicle speed in the strafing direction with 45 degree rollers is exactly the same as the forward direction, for any given wheel speed.

The reason the inexpensive mecanums used in FRC are slower in the strafing direction in actual practice for a given throttle setting is due to friction and axial free play in the roller bearings, friction in the drivetrain, compliance in the flooring (carpet), and open-loop control of speed.