Getting the bot to go straight?

[JVN]

The past two seasons, team 148 has utilized an articulating drivetrain. The robot shifts from one set of wheels to another via pneumatics. One of these sets of wheels consists of (4) high-traction wheels, spread to the outer 4 corners of the robot in "narrow/long body" configuration. This configuration of wheels makes it VERY difficult for the robot to turn due to the scrub forces of these wheels -- in addition... it makes it hard for the robot to BE turned. This "traction mode" keeps the robot driving straight as an arrow.

If we are ever in a situation where we need to push forward in a straight line (like autonomous this year), we run in this traction mode. For normal moving around the field, we use our other drivetrain mode.

For more information on drivetrain turning, and scrub forces... check out the math in Chris Hibner's whitepaper:
http://www.chiefdelphi.com/media/papers/1443

Note... articulating drivetrains aren't for everyone. There is some added complexity over a "standard" style drive. However, we on 148 like the benefits that come with such a drive.

Whatever solution you decide to go with for driving straight, be it a software or mechanical solution, I strongly urge you to work to develop / test it during the off-season. What else were you going to do this summer anyways? Sleep?

-John

[Eric222]

P.S. - It always helps to check and make sure that the jumpers are all in the same position on the victors (either coast or brake).

[JamesTerm]

"
The old Drill Motors were biased to run faster forwards than in reverse
"

We found a similar symptom to happen with our robot and I believe we are using CIM. Basically when appling equal voltage say 0.08 on both motors only one side would run while the other did not. I've applied a software solution that removes the "dead-zone" like what you can do for joysticks, and then linear interpolate the rest of the values. With the wheels on drive blocks I can see this appears to work, but I have not yet tested on the ground. I feel confident that a software solution could work even if it requires an exhaustive attempt to plot a graph of several points to obtain the derivative for each side. I think voltage applied to the motors only are linear, but due to the chains and other static friction it makes it appear more logrithmic. Unfortunately one of our encoders is still defected (returns NAN), and we are all too mechanically challenged to fix it. So I am hoping for a miracle with tweaking the voltage distribution (or hope to get team to install a gyro).

I've added more details (and videos) on my blog here:
http://www.termstech.com/articles/