PID Control and the Navigation Stack
Hello, I'm trying to learn how to properly set up a standard ROS navigation and control stack to drive a field robot (one that operates outdoors, navigating primarily by GPS and possibly a compass). My own previous experience with other systems keeps getting in the way and so I have some questions about how this is meant to work.
1) This system is not a wheeled system, (it's a boat) and so there's not a direct measure of "odometry", which move_base seems to require. Even with speed measured by the GPS and heading measured by a separate compass, the combination don't typically reflect the actual movement of the vessel, since the vessel's direction of travel rarely matches it's heading. The speed from the GPS is course made good, not speed in the vessel's reference frame. My questions is, how should one best set this up?
2) I understand the canonical output from move_base (from the local_planner specifically) is the Twist message, containing linear and angular velocity information. Having been told the robot's position, given a local cost map and a goal, the local planner calculates the optimal velocity message for the vehicle. What is confusing to me is that the output of all this. What the robot needs (it seems to me) is a direction to travel and a speed at which to go. But a Twist message outputs not a desired direction of travel, but the angular velocity or the change in direction of travel. It seems to me move_base is acting like a controller (think PID controller) on some level by not specifying the desired direction of travel directly. Do I understand this correct?
3) Finally, ros_control can implement a PID (or other) controller to adjust the speed of motors (or maybe maneuver a rudder on a boat) to match the desired setpoint. But in this case, it is matching a heading rate, not a heading directly. Is this not backwards? Shouldn't the output of the local_planner be a desired heading and since the motors, driving wheels or a rudder, actually change the heading rate directly, a PID controller in ros_control would then take adjust the heading rate to achieve the desired heading published by the local planner?
I hope this all makes some sense. I can't help but feel like the roles of the local_planner and ros_control are reversed in some sense (at least with respect to the desired direction of travel).
Thanks for your thoughts, in advance. Val
