Passive Alignment Principle for Robotic Assembly between a ring and a shaft with Extremely Narrow Clearance

This paper deals with precise insertion processing (peg-in-hole task) for speeding up and increasing flexibility of assembly process in factory automation. When a gap between a peg and a hole is narrower than positioning accuracy of robotic manipulator, it is impossible to succeed the peg-in-hole task by conventional position control methods. Moreover, in the case that a robotic hand holds a deformable thin ring and put it around a shaft, the insertion process becomes increasingly difficult because the gripping force deforms the ring and the narrow gap is crushed. We propose a novel mating technique based on passive alignment principle (PAP) in order to solve the twin problems of accuracy and deformation. The PAP mechanism can correct the ring position in nanoorder and remove the ring deformation. We derived the limitations of the PAP, which are remaining position errors after the PAP applied, based on Hertz stress. We also analyzed accuracy requirements of industrial robotic manipulator for applying the PAP and calculated allowance diagrams. We conducted 280 mating and insertion experiments with four kinds of materials of rings and shafts with three different insertion directions. The success rate 97.8 % validates our proposed PAP-based mating algorithm.