Fixture loading with sensor-based motion plans

Abstract

By necessity, the clearance between fixtures and the parts they hold is small. When loading is performed by a robot, uncertainty in the initial position and orientation of the part and uncertainty in robot velocity as the part is moved make it a challenge to reliably and repeatedly load parts into fixtures. The "3-2-1" approach to fixture loading is well-known in industry: load the part into 3-point contact with a reference surface, slide it into 2-point contact with the fixture, then pivot to make 1-point contact, and finally apply a clamp. This method implicitly assumes force feedback (since different part surfaces are emphasized as contacts are detected) and has not been geometrically formalized. The authors propose a related method that uses simple binary sensors to detect contact. The authors describe an algorithm for generating such loading plans for functionless planar fixtures using the theory of compliant motion planning. The algorithm has 2 phases: phase 1 plans purely translational compliant motions and phase 2 plans motions assuming compliance in both rotation and translation. The authors show that a simple selective compliance mechanism (SCM) permits separate treatment of these phases. Also, the authors give an exact representation for directions in C-space that partition the continuous range of part orientations into equivalence classes. Finally, the authors describe how plans generated with their algorithm perform in 200 laboratory trials with a 5-axis robot.