CAREER: Planning and Control for Overconstrained Mechanisms

Northwestern University, Evanston IL

Investigators

Abstract

The goal of this Faculty Early Career Development (CAREER) Program research is to produce hybrid estimators, motion planning algorithms, and control strategies that will work in concert to guarantee performance and stability in the face of multiple contact interfaces in an unstructured environment. An overconstrained multiple point manipulator prototype will be developed and planning and control methods will be applied to this system to demonstrate manipulation that is not sensitive to the particulars of the frictional interfaces. Multiple point contact is common in many manipulation tasks. Examples include arrays for distributed or micro manipulation, vehicles, and traditional robotic grasping. Although all of these applications have received a great deal of attention, no previous works have provided an analytical approach capable of dealing with the inherent uncertainties in the contact state--the state that represents whether a given contact is sticking, slipping, or is out of contact. These nonsmooth transitions between contact states have a dramatic impact on the dynamics; hence, it is important to systematically mitigate the negative performance these nonsmooth effects induce. Moreover, these systems are often overactuated, so that each contact interface is independently articulated. This leads to mechanisms that are nominally kinematically overconstrained--that is, the kinematic relationships between all the point contacts cannot be simultaneously satisfied. Which constraint is broken is sensitive to details of friction and normal force modeling, so it is necessary to estimate the current contact state and incorporate the contact state into the motion planning and control. Manufacturing often involves the need to reposition and reorient objects for purposes of assembly. To accomplish this, multiple actuators are often used, and these actuators experience stick and slip contact with the object due to frictional interactions. The physics of the actuators and, in particular, their interaction with the object are notoriously difficult to model accurately. Hence, there is a strong need for manipulation strategies that are not sensitive to these low-level details. Moreover, many vehicles, such as the original Mars rover, have a mechanical design that guarantees that some of the wheels must slip during operation. However, which wheels slip is dependent on unknown environmental conditions. Hence, in this situation as well there is a need to develop motion planning strategies that are guaranteed to work even in the presence of substantial uncertainty arising from the environment. This project will contribute to these needs by developing strategies that have guaranteed performance in the face of inherent uncertainty. In the short term this project will contribute to macro-scale manufacturing and vehicle control, and in the long-term will likely impact micro-scale manufacturing.