Robotic Assembly Strategy With Wrist Force Sense for Narrow Clearance Peg-in-Hole

Narrow clearance peg-in-hole assembly is a typical industrial assembly scene. Due to the motion accuracy limitation of industrial robot system and the uncertainty of the assembly circumstance, robotic autonomous assembly of narrow clearance peg in hole assisted by machine vision and force feedback has always been a research focus in this field. However, few studies focused on accomplishing it in a short time while maintaining minimal forces. On the other hand, in some extreme scenarios, such as nuclear, camera lenses may age quickly when exposed to high-dose radiation, requiring a non-visual peg-in-hole method. In this article, aiming at peg-in-hole assembly tasks without vision, and inspired by wrist force sense of human, a robotic narrow clearance peg-in-hole location and insertion methods with wrist force sensation are proposed. The collaborative robot obtains the accurate location of the hole by touching the hole edge several times, named "force-sense-based location". And then, the peg is automatically inserted into the hole by the collaborative robot with four basic motions named "approach"", slippage"", rolling" and "drilling". A geometric theoretical model was deduced to guarantee the feasibility of the developed method. To avoid excessive contact force, all four motions were designed with admittance hybrid/position control in different directions. The experimental results demonstrate the effectiveness of the proposed method, while the key factors affecting their performance were analyzed. Finally, we compared the methods with three peg-in-hole methods and comprehensively analyzed their performances in terms of assembly time, assembly force, and sensitivity to initial bias. This article provides a new effective method for automatic assembly of narrow clearance peg-in-hole tasks.