Robotic jamming-free and precise assembly for peg-in-hole operation by multi-DOF force-controlled parallel end-effector

In smart manufacturing, the precision peg-in-hole assembly under contact-unknown conditions poses a major challenge, which has spurred growing interest in enhancing active compliance in robotic assembly systems. Multi-Degree-of-Freedom (Multi-DOF) compliance provides high adaptability for addressing this assembly challenge. However, problems such as excessive size, insufficient dynamics, and complex jamming mechanisms have emerged. To solve these problems, we propose a jamming-free strategy. This strategy uses a lightweight, compact multi-DOF end-effector with a 3-Prismatic-Revolute-Spherical (3-PRS) parallel configuration, along with an active force control algorithm. The force control result of an absolute mean error of 1 N is observed for a setpoint value of 100 N along the z-axis. Meanwhile, the moments about the x- and y-axis are controlled within an absolute mean error of 0.04 N.mm. With the accurate compliance provided by the proposed force-controlled end-effector, the peg-in-hole operation is considered as a reciprocal transition between a transient one-point contact and a steady two-point jamming. Based on admittance control, a control strategy is designed to transform the two-point jamming into one-point contact, thereby preventing jamming during the precision peg-in-hole operation with significant uncertainties, eliminating the need for repeated positional adjustments during insertion. Experimental results show that the peg can be inserted into different holes with various pose errors and unknown dimensional tolerances. These results validate the rationality of the proposed jamming-free strategy for precision peg-in-hole assembly. The jamming-free peg-in-hole assembly employing a force-controlled parallel end-effector exhibits superior robustness and operational stability when it is applied into a satellite assembling task.