Mechanum Wheel Sizes

[MichaelBick]

Originally Posted by Nemo

Some people always make mecanum sound harder than it is. Let's assume that a team has already figured out how to build, program, and drive a basic 2WD/4WD/6WD FRC robot. For such a team, I would claim the following:

1) Mecanum drives are not difficult to build. You can get preassembled wheels and gearboxes from AndyMark, and hubs to connect them. All you have to do is find a way to fasten the gearbox to your frame. That is about as easy as a drive can get. No chains required. It is not very heavy, either.

2) Mecanum drives are not difficult to program. You don't even have to program it, because holonomic drive code is included in the libraries with our software. All you have to do to turn an arcade drive into a holonomic drive is add "+ x" or "- x" to the motors for the strafing axis joystick input.
Front left motor: y + z + x
Front right motor: y - z - x
Back left motor: y + z - x
Back right motor: y - z + x

Adding gyro feedback prevents the robot from driving crookedly or rotating when you don't want it to. That admittedly adds a bit of complexity, but only a bit. I am not convinced that encoder feedback is necessary if you have a gyro.

3) Mecanum drives are not difficult to drive. I'd like to know how many of the people claiming otherwise have actually driven one. I thought the whole point of a mecanum drive was to make it EASIER to get where you're trying to go, pointing the direction you want to point. A 6WD is less forgiving to the driver than a mecanum drive, because it's faster and easier to correct your slight driving mistakes when you can strafe.

Mecanum can be a reasonable choice for some teams. It is not going to work well for all games, but it was fine for 2011 (flat field, protected offensive zone, and peg goals that were easier to score on with strafing ability).

Mecanum is also not going to be as good as 6WD at the highest level of competition, because a 6WD robot (which has inherently better traction) with a really good driver does not need strafing to quickly get the robot exactly where it needs to go. However, it is okay to focus on getting more competitive at the regional level. The forgiving nature of a Mecanum drive (item 3 above) might, in fact, make some teams more competitive at that level than they would be with 6WD.

1) I agree with mechanum being easy to build(relatively). Our team from last year built one, and last year is a year that is not fun to remember. That being said, you do have reliability problems. If one wheel goes down, you aren't going to be able to strafe.

2) Mechanums are relatively easy to program. But, once you start putting gyros, encoders, etc. on(and you should, you will have problems driving straight otherwise), you then get a lot more complicated, vs the traction drive, where you really don't NEED any of those.

3) While mechanum may be easy to drive(it is basically Halo), you have to make a lot of compromises in strategy and build. You have to give up playing defense, and have to make sure you don't get into a pushing match while playing offense. Therefor, you must be a very good offensive robot, and it is STILL harder to drive, due to the fact that you have to avoid those situations. Furthermore, you must make some compromises in build. If you look at many of the successful swerve bots(just because better teams usually use swerve when they are looking for holonomic robots), all of them have made compromises in build. 148 in 2008 built a robot that was completely round. 118 built a robot in the same year that had a turret. 1717 from 2011 was very effective in the home zone, because they didn't have to turn much, but outside of the home zone, they were much less effective.

[nxtsoccer]

Allright, from what I read (assuming we chose mecanum), we should plan on using a semi-flexible frame that allows for the wheels to raise/lower a little based on terrain, should do most of our prototyping work during the off-season, and use 6" mecanum wheels (AM 6" HD?). Any other suggestions or things I forgot?

So, I suppose all that remains is to figure out how to turn these wheels. Any suggestions for the drive train? We currently only have the frame kit included with the 2012 Rookie KOP, which means 2 CIMs, chain, 4 sprockets, and two CIMple boxes. Would purchasing two more CIMple boxes with accompanying CIMs to use with the existing lengths of chain and sprockets work, or will we need to purchase entirely new drive train parts to make the robot move? (The rookie kit came with four jaguars, so we do have enough motor controllers for the four necessary motors.)

And thanks to everyone for all of these awesome suggestions!

EDIT: And plenty of practice to go with the new axis of travel!

[Tom Ore]

Originally Posted by nxtsoccer

So, I suppose all that remains is to figure out how to turn these wheels. Any suggestions for the drive train?

Look here for our 2011 drive base CAD for one way to set it up.

[Steven Donow]

Originally Posted by nxtsoccer

Allright, from what I read (assuming we chose mecanum), we should plan on using a semi-flexible frame that allows for the wheels to raise/lower a little based on terrain, should do most of our prototyping work during the off-season, and use 6" mecanum wheels (AM 6" HD?). Any other suggestions or things I forgot?

So, I suppose all that remains is to figure out how to turn these wheels. Any suggestions for the drive train? We currently only have the frame kit included with the 2012 Rookie KOP, which means 2 CIMs, chain, 4 sprockets, and two CIMple boxes. Would purchasing two more CIMple boxes with accompanying CIMs to use with the existing lengths of chain and sprockets work, or will we need to purchase entirely new drive train parts to make the robot move? (The rookie kit came with four jaguars, so we do have enough motor controllers for the four necessary motors.)

And thanks to everyone for all of these awesome suggestions!

EDIT: And plenty of practice to go with the new axis of travel!

One thing you're missing is that for mecanum, you need to power each motor seperately, so 2 CIMple boxes won't work. I've seen teams use both these:
http://www.andymark.com/product-p/am-0688.htm
http://www.andymark.com/product-p/am-0482.htm

The 2nd one probably fits your idea of having a flexible frame for going over terrain.

[nxtsoccer]

Originally Posted by stevend1994

One thing you're missing is that for mecanum, you need to power each motor seperately, so 2 CIMple boxes won't work.

I know , that's why I was wondering in the next sentence if we should go with two more CIMple boxes, or scrap the two we have for entirely new drive train parts (hopefully keeping the CIMs).

[Nemo]

Originally Posted by nxtsoccer

I know , that's why I was wondering in the next sentence if we should go with two more CIMple boxes, or scrap the two we have for entirely new drive train parts (hopefully keeping the CIMs).

There is nothing wrong with using the CIMple boxes to save a few bucks. The kit sprocket reduction (26:12) plus the CIMple box gives about a 10:1 gear ratio, which is a nice ratio for 6" mecanum wheels. If we're starting from a 2012 kit of parts, that move saves about $180 as compared to using Toughbox Nano and AndyMark hubs to direct drive. If you want to use the same equipment on a competition robot next year, though, I think the extra $180 is a good move. The Nano boxes have the following advantages over CIMple boxes: takes up less space, no chains to mess with (easier to build + more reliable), Nano is a nice box that runs really smoothly and is probably more efficient than CIMple + chain.

Having said all of this, and everything in my previous post... honestly, I agree with the people who suggest doing a kitbot on steroids (a nice 6WD robot) instead. It sucks when you can't climb an incline very well and other robots can shove you around at will.

[Ether]

Originally Posted by MICHAELABICK

mechanum

Originally Posted by MARS_James

Mechanum

Originally Posted by Peck

mechanum

Originally Posted by Kris Verdeyen

mechanum

Originally Posted by nxtsoccer

Mechanum

Guys,

There is no "h" in mecanum.

[nxtsoccer]

Originally Posted by Ether

There is no "h" in mecanum.

Yeah.... I noticed that after about my second post, sorry

[rocknthehawk]

I still don't understand why everyone is interjecting about why NOT to use mecanum wheels, especially in the offseason. Just answer his questions, and move on. Too many opinions, not enough facts.

We used the kit frame, with cimple boxes to test 8" AM Mecanum wheels this year. It was too fast, the wheels spun a lot before actually gripping. Also, the robot would drift more because of the slipping. As I said, our final ratio was around 17.5:1, which gave us a great mix of speed and maneuverability. I would suggest going with the toughboxes, then a 2:1 sprocket or higher. Something like a 12 tooth on the box and 28 or 32 on the wheel. We used CIM's with Dewalt gearboxes.

My suggestion is to just use the kit frame. Our test frame we just moved the inside rails to fit the wider mecanum wheels, then made new spacers. It's quick, easy, and readily available. It definetely flexes. While yes, it is necessary to have all 4 wheels on the ground, practice will help you the most. We didn't have trouble climbing and balances bridges, or running over the key. And I know there were teams crossing the bump with mecanum wheels.

Someone else mentioned the programming. That's where we started with ours.

[efoote868]

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?

Using a gyro allows you to do something called "field centric drive". Basically, if the robot is in front of you and you push your joystick forward, the robot moves away from you regardless of how it is spun.
In effect, the robot has no front.

This is probably the most useful sensor to use for mecanum wheels. I'd strongly recommend it ahead of encoders and accelerometers.

Also, as far as ramps and inclines go, mecanum wheels perform just fine. In 2006, our robot could climb the diamond plate ramp and get on the platform.
In 2010, our robot could climb over the carpeted bump.
In 2012, our robot could climb on the bridge.


Right now is the time to experiment with drive trains. If you have the time and the money, try building a 6 wheel drive and build a mecanum drive. Learn how to use gyros, accelerometers, and optical encoders. Figure out how to do a good job building each, and experience the strengths and weaknesses of both.

Then, when it comes time to pick a drive train, you won't be second guessing yourself.

[nxtsoccer]

Originally Posted by efoote868

Using a gyro allows you to do something called "field centric drive". Basically, if the robot is in front of you and you push your joystick forward, the robot moves away from you regardless of how it is spun.
In effect, the robot has no front.

Any ideas on how to program this? I have some previous experience with LabView, but I haven't the faintest idea of where to even start with this. Not to mention we don't have a gyro to work with...

Although, we may just end up having a more traditional robot with a front and back. The field centric drive and encoder-based correction sounds more like something to do when we have had more experience with mecanum. I think that four motors with mecanum wheels would be a good starting point, and simply adjust for the lack of a perfect straight line by hand.

[efoote868]

Convert your joystick inputs into an angle and a magnitude. Add or subtract the gyro angle. Convert to wheel speeds.

About as simple as it gets.

[nxtsoccer]

Originally Posted by efoote868

Convert your joystick inputs into an angle and a magnitude. Add or subtract the gyro angle. Convert to wheel speeds.

About as simple as it gets.

Hmm... not sure how to convert like that yet... but it does sound rather simple. Any idea where to get a decent, cheap gyro compatible with the circuits that come with the KOP? As a rookie team, we don't have any custom circuits or boards to work with that would allow different types of connections.

[Ether]

Originally Posted by nxtsoccer

Hmm... not sure how to convert like that yet... but it does sound rather simple.

Even simpler: use the WPILib robotdrive. It has an input for gyro, and will do all the math for you.

Any idea where to get a decent, cheap gyro compatible with the circuits that come with the KOP? As a rookie team, we don't have any custom circuits or boards to work with that would allow different types of connections.

Your 2012 KoP came with a gyro.

[nxtsoccer]

Originally Posted by Ether

Even simpler: use the WPILib robotdrive. It has an input for gyro, and will do all the math for you.



Your 2012 KoP came with a gyro.

..... Nothing else to say... just embarrassed face....

Thanks, though!!