crab drive vs. mecanum drive system

[daltore]

Originally Posted by superbotman

3. Use mecanum drive with one motor per side.

Do you mean omnidrive? This is not with mechanum wheels, it uses omniwheels, and generally uses either three (kiwi drive) or four. Probably the simplest holonomic drive train is the 4-wheel version with two on the sides, and then one each for front and back, so you just feed the front and back ones values from the X-axis of a joystick, and the ones on the side values from the Y-axis.

However, whether you want to use any of these systems depends on what your team is aiming for. If you just want to learn how to build and drive a holonomic platform, do it. One deciding factor might be the game challenge. Last year, we played around with several strategies before we settled on needing a very maneuverable robot, and because of field losses, we decided to use swerve drive to keep the wheels pointed in the same direction as the driving vectors most of the time. Definitely not the only way to do it, but we did learn a lot.

Here's the run-down in my opinion:

1) Most complicated. If you use full swerve drive (wheels rotate independently), it will require a lot of motors, a lot of power, and careful design to get things to line up. Not to mention, the code is fairly hard to do if you want position feedback. If you use crab instead of full swerve, you may want some kind of turret, because rotating can be VERY important.

2) Mechanum has fewer losses than omnidrive doing forward, but has huge losses strafing. Going diagonally can cause interesting vectors to result accidentally. Definitely the loudest of the drive trains when strafing. Simple to build, more expensive than most of the others (depending on how you make swerve drive, and if you make your own mechanum wheels), hard to conceptualize. Possibly hard to program, I don't know, I haven't tried.

3) My personal favorite. Omnidrive results in what looks like a hockey puck on wheels, it moves almost eerily, and I've played around with it on Vex many times before. If you have the wheels mounted at 90-degree angles from the center (on the front and back and the two sides), your efficiency is 100% of a two-motor drive train, in both the X and Y directions. If you mount at 45-degree angles (on the corners), you get a theoretical maximum of 70% efficiency going straight forward or sideways, but you get more power behind it. However, you will not win any pushing matches, I guarantee. It can also be a problem to mount wheels at odd angles if you go that route. Fairly simple conceptually, the code isn't that complicated (I have a library for Vex if you want to see it).

4) Generally, skid steer/tank drive/differential steering/whatever you want to call it is the simplest, easiest, and most of the time, most effective. You have to remember that anytime you embark on a more complicated project, there will be a learning curve, and it will consume time. If you care more about learning the mechanical/electrical/control systems side of things, go with a holonomic drive train. If you care more about autonomous routines, camera interpretation, and perfecting manipulators and driver skill, go with something a little simpler.