Re: Swerve drive 4, 2+2?
 

I would definitely go with a 2+2 steering style as opposed to all wheels steered dependently. Our team used a swerve style drivetrain this year, and we used a effective 2+2 system. One disadvantage to the dependent steering is that it makes changing the orientation of the robot very difficult. If you want to have a tank steer mode, you must make sure that the treads on your wheels have a low coefficient of friction. Even with the 2+2 steer this is a problem we encountered this year. In fact, we had to change our treads between the Pittsburgh and Boston regionals in order to allow our tank steer mode to work (even though we never actually used it in competition). The 2+2 steer was effective for Overdrive because we had a variety of useful steering modes.
1. Front steering (for normal "car" style drive)
2. All wheel steering (for very tight turns. This was useful when coming around the center divider)
3. Swerve steering (for translating sideways without changing the orientation of the robot relative to the field)
4. Tank steering (the standard form of steering)

With a system where all wheels are steered together, you only get steering mode number 3. Unquestionably there are great advantages to choosing 2+2. We did encounter problems, however. The robot could not drive straight because the potentiometers did not provide a high enough resolution to make the robot align the wheels perfectly opposite to one another. We are going to experiment with a gyroscope in order to rectify this.
One option I would not leave out of the equation is steering each wheel independently. This would eliminate the need for tank steer and provides almost limitless potential for the robot's performance. We plan to do something like this next year.
Another thing to keep in mind is the location of your powertrain. If you have a centrally located powertrain, you can save weight and add a multi-speed transmission (another thing we plan to do next year). Otherwise you could mount the motor vertically and use beveled gears. We chose to do a direct drive style this year (you can see our hinged bumpers, they pop up every time we go into 4-wheel or swerve mode) this does tend to leave the motors exposed, however.
Obvioulsy there are other things that must be considered when choosing a drivetrain, but these are some of the basics. It is something into which ALARM puts much thought and energy.:cool:

Re: Swerve drive 4, 2+2?
 

This year our team went with mecanum wheels and I was the driver this year and I found them to be very helpful, alot. Easy to move around obstacles and other robots. I would go with mecanum wheels over omni and we went with the AndyMark 8" wheels. Talking with other teams that went with the 6" ones weren't as satisfied. To check out some videos of our bot moving in all directions there are videos of it on our website at http://www.whitebearlakerobotics.com

Re: Swerve drive 4, 2+2?
 

If all of your wheels are moving indepentently then you have all the capability of a 2+2 and more, using a 2+2 you have all the capability of a 4 and more. I would consider that none of these are any more complex than the others and go with each module being moved indepentently. An easy way to make this work is to vertically mount an FP motor and attach it to the wheel module and do this to all of the modules. Make sure the wheels are straight when you start a match because if they are a little off it can mess up hybrid mode. My team ran into this issue, it wasn't that bad in tele op but we had a good hybrid mode, but if the wheels weren't perfectly aligned the robot would cross one line and before it could get a second teh unstraightenedness would make it go into a wall.(it didn't help that our sonars died in eliminations)

Re: Swerve drive 4, 2+2?
 

A 2+2 system with dependent steering and drive is a lot simpler than a fully independent system. On a fully independent system, you would need to do feedback on 4 modules instead of 2. That means another 4 PIDs (2x forward 2x reverse). It also makes the precise control logic a lot more complicated.

A big challenge for us in previous years was to pack both the motor and any transmission inside the module itself, so we do not suffer efficiency loss through use of bevel gears. That, IMO, was one of the biggest challenges for us in these years. Additionally, remember it is vital to keep the wheel base as large as possible, and therefore, the modules as small as possible.