Control Strategy For Jam And Wedge-Free 3d Precision Insertion Of Heavy Objects Suspended With A Multi-Cable Crane

Overhead cranes have been used extensively in heavy industry for transporting heavy objects across a factory floor. This work aims to make such cranes more dexterous and capable of performing precision assembly tasks by suspending an object with tension-controlled cables. New control strategies are presented for coordinating multiple winches affixed to a crane, so that a) a suspended shaft (peg) can be smoothly inserted into a hole without jamming despite a small clearance, and b) even in the case a shaft gets wedged inside a hole, the shaft can be recovered from wedging. It is shown through analysis, and validated with experiments, that simply equalizing the three cable tensions allows the shaft to be inserted smoothly into a hole. If the difference between the cable tensions is large, the shaft may experience large contact frictional forces, which may cause the shaft to jam. If wedging occurs, two particular proportions of cable tensions are obtained to break wedging. A process monitor is designed to detect wedging, estimate the location and orientation of the shaft wedged within the hole, and confirm whether the shaft has recovered from wedging. The tilt of the shaft can then be adjusted and re-insertion can occur. The effectiveness of the proposed control strategies are validated on a 3D multi-cable crane prototype that is able to demonstrate the successful insertion of a 15 kg shaft into a hole with 120 mu m of clearance.