Variable Risk Dynamic Mobile Manipulation

The ability to operate effectively in a variety of contexts will be a critical attribute of deployed mobile manipulators. In general, a variety of properties, such as battery charge, workspace constraints, and the presence of dangerous obstacles, will determine the suitability of particular control policies. Some context changes will cause shifts in risk sensitivity, or tendency to seek or avoid policies with high performance variation. We describe a policy search algorithm designed to address the problem of variable risk control. We generalize the simple stochastic gradient descent update to the risk-sensitive case, and show that, under certain conditions, it leads to an unbiased estimate of the gradient of the risk-sensitive objective. We show that the local critic structure used in the update can be exploited to interweave offline and online search to select local greedy policies or quickly change risk sensitivity. We evaluate the algorithm in experiments with a dynamically stable mobile manipulator lifting a heavy liquid-filled bottle while balancing.