Static Friction Effects During Hard-on-Hard Contact Tasks and Their Implications for Manipulator Design

In this article, we present the results and implications of our experimental study into the effects of static, nonlinear, fric tion during hard-on-hard contact tasks where making a stable contact is problematic. Experiments have been conducted with a PUMA 560 manipulator to address the hard-on-hard con tact stability problem and to stress the importance of damping (active-velocity feedback, and passive-nonlinear friction) for hard-on-hard applications typified by robotic drilling. Experi mental results reveal that during hard-art-hard contact, active damping is inoperative because of quantization, leaving only passive damping to stabilize the system. However, passive damping characteristics vary between joints. Most importantly, the last three joints of the manipulator, which are more af fected by contact force disturbances than others. lack sufficient passive damping, which we argue should match their respec tive joint stiffnesses to achieve contact stability. Therefore, we conclude that manipulator joints with low passive damping should have high-resolution encoders for velocity feedback. These are required to stabilize a force control system making a hard-on-hard constrained contact. The lower the joint friction, the higher the resolution required. This constitutes the main contribution of this article to manipulator design.