Interaction Force Experiments for Force-Guided Assembly

Insertion tasks, such as cable and electrical connector insertion, are performed primarily by humans on assembly lines. The flexibility of the cables and wires, and the tight-tolerance insertions, with likely visual occlusions, make the automation of these tasks difficult. Force-guided insertion is a robust alternative for performing insertion tasks under occlusion conditions. However, the capabilities of force-guided approaches are compromised when there is no proper alignment of the object with the hole/connector. In this note, we aim to infer orientation and/or position errors in the object alignment using only force and moment measurements, which can be useful for corrections during force-guided insertion. We analyze the interaction forces and moments acting during the insertion of a peg/cable into a tight hole with a diameter approximately equal to the peg. The forces are recorded using a UR5e robot with a Robotiq Hand-E parallel gripper (with a force/torque sensor) grasping a cable/peg rigidly, and a hermetic pass-through with a hole of the same diameter as the cable/peg. Several insertion tests were performed considering rotation and position alignment errors. The experimental results present analogous behaviour of the force in the direction of the insertion, and the other two orthogonal forces, no matter if there is orientation or position error. Then, the moments were considered, and a hint about the rotation error was found. The torque measurements and their principal component analysis (PCA) show a completely different direction when positive and negative rotation errors occur.