Programming Crab Drive

[MikeE]

I agree with Dave - it's a worthwhile exercise to think through the whole of this problem with the particulars of your drive system. This is the first year our team has attempted a swerve/crab system so I'll say a little about how we approached it. But first I'd like to put in a plug for not using standard kit joysticks.

We have had some success in using the Logitech Dual Action Game Controller (~$20 from Target) to control the crab/swerve drive system we developed this year. The Logitech device gives you a pair of thumbsticks along with numerous buttons, and our drivers seem to prefer it to traditional full-size joysticks. Being hand-held you can stand slightly away from the team station and also stand at full height which helps a little in seeing the action on the field.

The original idea was to use the control sense of video games such as Halo, where the left stick controls forward/reverse left/right movement (translation) and the right stick controls rotation http://halo.wikia.com/wiki/Halo_Controls
We changed that a bit during development, and ended up with a control model where the left stick controls speed and the right stick controls the various forms of steering.

[big1boom]

This year was also our first year with a crab/swerve type drive.

We had the left stick controlling CIM power, and the right stick controlling various types of rotation.

If the driver pointed the right stick +45 degrees, all the wheels pointed to +45 degrees, etc (Translation)

However, if the driver pulled the trigger, and twisted the stick full right, then the front wheels pointed 45 right, the back pointed 45 left
If the driver twisted full left and pulled the trigger, the front pointed 45 degrees left, the back 45 degrees right. (Rotation)

With this, we were able to "Warthog" our drive system for turning



Make sure that you have quality sensors for your wheel rotation. If your program doesn't know where your wheels are, crab or swerve is going to be VERY hard on your driver.

[Tom Line]

We're strongly considering doing an off-season swerve/crab drive this year.

We have some questions about the programming as well. Namely:

Does your joystick relate to the field, or to the robot? For instance, if you push to the left with your "crab" joystick, does your robot go to it's left, or does it go to the left on the field?

If it goes to the left on the field, I assume you're using a gyro to keep track of absolute angular orientation versus the field: do you have any problems with collisions throwing your gyro readings off?

Thanks!

[Dave Scheck]

Originally Posted by Tom Line

Does your joystick relate to the field, or to the robot?

All of our desired crab angles have always been relative to the robot. It's the simplest, most intuitive solution in my opinion.

We've thought about making it relative to the field, but a good driver will beat out a noisy gyro reading any day. Unless you do some processing to keep it in check, gyros can be more and more unreliable the longer that it is being used because of the noise being included in the double integration.

Other than noisy data, a major downside to being relative to the field is starting position. If you don't line up exactly right, you're going to be off for the entire match.