- Another planner that uses optimization to minimize cost
- Performs local search by starting with an initial candidate trajectory and then introducing noise to sample nearby it:
STOMP is an algorithm that performs local optimization, i.e. it finds a locally optimum[al] trajectory rather than a gobal one. Hence, performance will vary depending on the initial trajectory used for optimization. STOMP cannot be expected to solve typical motion planning benchmark problems like the alpha puzzle in a reasonable amount of time.
- Compares itself to the algorithm CHOMP, which is similar but CHOMP is a gradient-based method. Because STOMP is not a gradient method, and has some randomized exploration, it has a better ability to escape local optima than CHOMP
- Because it is not a gradient method, STOMP can be used with wider classes of cost functions (such as those hard constraints on angles or torques which many method have trouble with), and more general constraints
- Hard limits can also cause local optima, so while the method doesn’t just break in these settings, it is possible convergence will not be to optimal resutls
- Basic motion planning algorithms generally require smoothing and other further transformations after a solution is found in order to maintain energy efficiency, minimize jerk (and motor wear), among humans abrupt changes caused by classical motion planning methods can be disconcerting.
- A result from a classical motion planner could be used as the initial proposal for this algorithm
- In contrast to the classical motion planning approach (which they call sampling-based), are optimization based planners which minimize cost
- Seems like this method also uses inverse kinematics, runs on a domain that does is *not* underactuated or exhibit drift: “Specifically, we consider trajectories of a fixed duration T, discretized into N waypoints, equally spaced in time.”
- Also does an EM-style update, and weighs samples according to their goodness: “The trajectory updates can be considered safer than a standard gradient descent update rule. The new trajectory is essentially a convex combination of the noisy trajectories which already been evaluated, i.e. there are no unexpected jumps to unexplored parts of the state space due to a noisy gradient evaluation.”
- The only open parameter to the algorithm is the exploration noise
- In settings where optimization is done in a multidimensional space, each dimension is dealt with independently
- They do a grasping task on a PR2 in simulation and on the real robot
- CHOMP fails to produce an effective plan about 30% of the time, while STOMP is always successful, probably due to the bit of randomized exploration STOMP does.
- They mention a paper that introduces noise into CHOMP to help get around local minima, but that it was difficult to use and implement, with many parameters that needed to be tuned
STOMP: Stochastic Trajectory Optimization for Motion Planning. Kalakrishnan, Chitta, Theodorou, Pastor, Schaal