Control theory online courses

I absolutely recommend Russ Tedrake‘s online course on Underactuated Robotics (mostly nonlinear control) and Magnus Egerstedt’s course on linear control. The latter is not a formal prerequisite of the former, but it helps if you know some of the concepts. Russ’ course is just amazing, and extremely rich in mathematical concepts and algorithmic tools. He insists on providing visual intuition for almost every single concept that he introduces, which is great. I’ve recommended it to everyone who cared to listen. Some of my friends and I are currently taking the course.


Kinodynamic motion planning

Or planning that takes into account velocity, acceleration, momentum, force bounds, as well as the dynamics of the vehicle. For example, lateral motion is possible for an omnidirectional robot, but not for a car. This sub-field of planning is very closely related to and borrows ideas from optimal control, however, it always takes into consideration the presence of static or dynamic obstacles in the environment.

  • One issue with existing solutions to this problem, such as kinodynamic RRT and RRT* is that they assume a Steer() function which essentially does optimal control from one state to another nearby one (often without taking into account obstacles). As far as I know, this is hard to do even for Dubins and Reeds-Shepp car dynamics. It would be interesting to see if we could eliminate the need for solving an optimal control problem within a larger-scale optimal control problem. I wonder if there is a way to make RRTs keep their Voronoi bias even in the case where we do not sample a state uniformly, but we appropriately sample from the control space in such a way that the forward integration to the end state results in a uniformly sampled new state. In Lavalle and Kuffner’s Fig. 3 this sampling from the control space was dismissed quickly, because it lead to a random tree that preferred extensions near the already-explored space, but I wonder how they sampled the controls.
  • Another issue is that both RRT and RRT* require a NearestNeighbour() function that returns the closest state to a given state. In the case of omnidirectional robots, one can use a kd-tree to partition the euclidean metric space into regions, such that nearest neighbour queries take O(logn) instead of n in the case of brute-force search. In the case of non-holonomic robots you might have distance functions that are non-euclidean, in which case kd-trees will not work. So, what are some good data structures to use for nearest neighbour queries in non-euclidean spaces? Or, a better question perhaps, can we modify RRT so that it doesn’t need nearest neighbour queries at all?
  • A third issue is that multi-robot kinodynamic planning becomes a bit cumbersome if we think about it in terms of the current formulation of RRT and RRT*. Consider for example the notion of a terminating time: if you are planning the trajectories of multiple robotsĀ in a way that the planning of one robot cannot be separated from the others’, then you will need to consider a different terminating time for each robot if you extend the tree by sampling in state space. On the other hand, if you sampled in control space and decided to extend the state by a time duration dt then this would be avoided.
  • A fourth issue, which sometimes bothers me, is whether long-term planning is really necessary for robots in the field. I agree that it is very useful and in fact mandatory for robotic hands in assembly lines or in labs. But, for robots that operate in natural, unstructured environments it seems like to make an optimal long-term plan and then blindly execute it is foolish, because unexpected circumstances will most likely appear. Plans will have to adapt to incoming streams of sensor feedback. So, why optimize for the optimal plan in a perfectly controlled environment when we should probably be optimizing for the most “adaptable” plan in a “noisy” environment? Finally, if we insist to plan optimal paths for a known environment, how long should the planning horizon be?