Team 254 Presents: FRC 2016 Code

[Jared Russell]

Originally Posted by ranlevinstein

Why did you choose to follow a path instead of a trajectory during auto this year?

Great question! I assume you are referring to (my) definition of path vs. trajectory from the motion profiling talk (these definitions are hardly universal).

Path: An ordered list of states (where we want to go, and in what order). Paths are speed-independent.

Trajectory: A time-indexed list of states (at each time, where we want to be). Because each state needs to be reached at a certain time, we also know get a desired speed implicitly (or explicitly depending on your representation).

In 2014 and 2015, our controllers followed trajectories. In 2016, our drive followed paths (the controller was free to determine its own speed). Why?

Time-indexed trajectories are planned assuming you have a pretty good model of how your robot will behave while executing the plan. This is useful because (if your model is good), your trajectory contains information about velocity, acceleration, etc., that you can feed to your controllers to help follow it closely. This is also nice because your trajectory always takes the same amount of time to execute. But if you end up really far off of the trajectory, you can end up with weird stuff happening...

With a path only, your controller has more freedom to take a bit of extra time to cross a defense, straighten out the robot after it gets cocked sideways, etc. This helps if you don't have a good model of how your robot is going to move - and a pneumatic wheeled robot climbing over various obstacles is certainly hard to model.

Originally Posted by ranlevinstein

Why did you choose the adaptive pure pursuit controller instead of other controllers?

Simplicity. A pure pursuit controller is basically a P-only controller on cross track error, but somewhat easier to tune. Adaptive pure pursuit is sort of akin to a PD controller (the only difference is a fudge factor in how far you look ahead). If you only have a day to get auto mode working, and the robot is being repaired up on a table while you are coding, then pure pursuit requires very little time to get tuned once you are back on the floor