Precision Assembly of Underconstrained Heavy Shafts Suspended By Multiple Cables From A Robotic Crane

Abstract

This work presents a new approach to precision mating of heavy objects suspended from overhead cranes. Overhead cranes have been used extensively in heavy industry for transporting heavy objects across a factory floor. The current work aims to make such cranes more dexterous and capable of performing precision assembly tasks using the WinchBot system, which suspends an object with tension-controlled cables. Analysis and new control strategies are presented for coordinating multiple winches axed to a crane, so that, despite a small clearance, a) a suspended shaft (peg) can be smoothly inserted into a hole without jamming due to ill-proportioned insertion forces, and b) even in the case a shaft gets wedged inside a hole, the shaft can be recovered and re-inserted.

First, the physics regarding the chamfer crossing and one point contact are explored. Using static force balance and kinematic analyses, it was shown that there are two primary ranges of cable mounting angles in which a fixed-length cable-suspended peg can successfully cross the chamfered surface of a hole, enter the hole and proceed up to two point contact. The first region, where the cables are nearly vertical and the second where the cable mounting angles are moderately small, greater than 85 degree sand less than 65 degrees for our specific experimental conditions.

Further, the introduction of linear actuators to the WinchBot system to results in the expansion of the the semi-dexterous workspace such that successful peg insertion can occur if the hole is not directly located beneath the center of the WinchBot System. It was shown that for a given hole location, there may be a certain winch configuration that allows the WinchBot to position the peg directly above the hole albeit while avoiding unnecessary peg tilt.

Finally, analysis and new control strategies are presented for coordinating multiple winches axed 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 multicable crane prototype that is able to demonstrate the successful insertion of a 15 kg shaft into a hole with 120 µm of clearance. The incorporation of the WinchBot system into existing factories will allow for the automation of precision assembly tasks. In turn, this will reduce the reliance on skilled workers, reduce damage to parts in increase factory throughput.