Re: Mechanum Wheel Sizes
 

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).

Re: Mechanum Wheel Sizes
 

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!

Re: Mechanum Wheel Sizes
 

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

Re: Mechanum Wheel Sizes
 

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.

Re: Mechanum Wheel Sizes
 

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.

Re: Mechanum Wheel Sizes
 

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

Re: Mechanum Wheel Sizes
 

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.

Re: Mechanum Wheel Sizes
 

While I still think your team's curiosity in mecanum would be a GREAT offseason project to learn and grow as a team, I firmly believe that THIS drive train will bring you a much more successful robot during the FRC competition season. It is simple, low-cost, and easy to build and maintain. This will let you spend far more time on your game piece manipulator and allow you to have a much better competition robot.

Re: Mechanum Wheel Sizes
 

When it comes to Mechanum Drive some of the things you would think and have been stated are issues can be overcome easily as we learned the last two years since we have used the drive for both Logomotion and Rebound Rumble.

Size argument for the wheels: 10 inchers are way to big for FRC we bought a set of 8 and 6 inch mechanums for testing purposes this year and discovered that the cost of the 8 inch were not worth the increase of skill they gave us

Below are some of the standard issues that have been brought up

Speed: We never experienced any issues with speed in the standard directions and in the strafing this year we had to use controls to slow down the speed because of fear of tipping (the wheels have less surface area in that direction of movement so they aren't as stable)

Climbing: We thought me might have issues this year with climbing the ramps but we were pleasantly surprised, we didn't have enough traction/force to push someone up the ramp but we were able to balance on our own several times with literally seconds left on the clock without issue. The wheels also make it very very easy to be pushed up the ramp.

Pushing: Last year we figured that our maneuverability would be more then enough that it wouldn't matter but this year since we had to score at the fender and were worried so we ran some tests with our tank drive bots and discovered that in the standard direction (as in both robots wheels in the same direction) we could hold our own for about 10 seconds before eventually succumbing and losing the contest. However once we came at the tank drive in the strafing direction (the mechanum robots wheels perpendicular to the tank) we were evenly matched

Driving: The best thing for this type of drive is to give the driver a video game controller and program it to run like a standard FPS game does, but nothing will replace practice for any person driving a robot.

"Einstein" Argument: The amount of teams utilizing Mechanum or any omnidirectional drive is less then the number of teams utilizing a more traditional drive, so they have less representatives competing for a place on Einstein, statistics and probability show that the smaller group has a smaller chance of being selected (Also in judging the amount of time passed since someone with omnidirectional drive got on Einstein try to ignor 2009 since lunacy didn't allow for mechanum or oni wheels).

The most important thing to consider though is what the game is asking you to accomplish and if the wheels will help you to do it. These last two years we felt it was worth it so we did just my two cents

Re: Mechanum Wheel Sizes
 

You seem to be saying that you were able to achieve speed and quite a bit of pushing power. I don't doubt that you may have been able to achieve this in quals. But in elims and if you get to championship, you will see a lot more robots that have 2 speeds, which you will never be able to push and outrun. A 2 speed traction bot done right with the same amount of practice against the best mechanum you will see, will be driven better(more driving practice), will take less time to build and program, be able to outrun the mechanum while strafing and sometimes while driving regularly(due to the fact that you gan gear for speed in the first gear), and outpush the mechanum anytime(due to the fact they have 50% power while strafing, and you have the second speed)

Re: Mechanum Wheel Sizes
 

Well, bomb squad did pretty well at Einstein this year with an omni directional drive train, however, it wasn't meccanum.

On a side note: In my engineering drawing class we were told to make bikes. (using inventor), so I decided it would be an interesting idea to make a mecanum bike. Originally I planned to use 26" diameter wheels, but accidentally plugged 26" in as the radius, but I've decided to roll with 52" diameter wheels. While I wouldn't be confident enough to actually use mecanum on a competition bot, especially considering that my team now has experience with swerve, I do think that mecanum is an interesting challenge and learning experience.

Re: Mechanum Wheel Sizes
 

Steps to having a great season as a newer team.

Step 1: Build this
Step 2: Use all the extra build season time you now have to build something awesome on top of it.
Step 3: Have a great competition season.

Regards, Bryan

Re: Mechanum Wheel Sizes
 

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.

Re: Mechanum Wheel Sizes
 

If you're dead set on using mecanum wheels, but still want to be in the "small wheel" category of offensive teams, these may be if use. If not, then you can probably use a 3D printer to print out some 4" ones. I've heard 3D printer wheels can actually be quite strong if you use the right material.

All in all, a good 6wd, drop center wheel will be the best way to get your team headed in the right direction. Why? 1 word: Traction. You don't know how useful it is until you're without it.

Re: Mechanum Wheel Sizes
 

Emphasis mine. We use plenty of ABS plastic on our bot, and I can't imagine that printing our own rollers would hold up for the season.

One thing we considered last season was making our own mecanum wheels; a plan we had to integrate our 3D Printer into it was making a mold to make "polysomething"(I don't remember what...) rollers.