Criteria for Maintaining Desired Contacts for Quasi-Static Systems

In robotic manipulation, finding a feasible motion plan doesn't guarantee a successful execution. The real world could bring all kinds of unexpected changes to the planned motion, the most deadly ones are usually marked by or caused by unexpected changes of contacts (object slipping away between fingers; getting stuck somewhere, etc). We notice that some actions are more likely to maintain desired contacts than others. To help finding these actions, in this work we propose a set of criteria to quantify the robustness of contacts against modeling uncertainties and disturbance forces. Under the quasi-static assumption, we analyze the causes of contact mode (sticking, sliding, disengaged) transitions and discuss how to endure larger uncertainties and disturbances. We summarize our results into several physically meaningful and easy-to-compute scores, which can be used to evaluate the quality of each individual contacts in a manipulation system. We illustrate the meaning of the scores with a simple example.

I. Introduction

A contact could have one of three modes: sticking, sliding or disengaged. Robot’s failures in manipulation are usually associated with unexpected modes of some contacts. To successfully execute a task, it is important to maintain desired contact modes under real world uncertainties and disturbances. Many factors can influence the robustness of maintaining a contact mode against modeling uncertainties and disturbance forces, such as the choice of contact locations, robot forces and/or robot velocities.