Alignment of threaded parts using a robot hand: theory and experiments

Abstract

Techniques for determining and correcting threaded part alignment using minimal force and angular position data are developed to augment currently limited techniques for aligning threaded parts. These new techniques are based on backspinning a nut with respect to a bolt and measuring the force change that occurs when the bolt "drops" into the nut. Kinematic models that describe the relationship between threaded parts during backspinning are presented and are used to show how angular alignment may be determined. The models indicate how to distinguish between the aligned and misaligned cases of a bolt and a nut connection by using axial force data only. In addition, by tracking the in-plane relative attitude of the bolt during spinning, data can be obtained on the direction of the angular misalignment which, in turn, is used to correct the misalignment.\nExperiments using a three fingered Stanford/JPL robotic hand validate portions of the derived kinematic models while a higher resolution fixture provides additional data. Evaluation of the correlation between the test data and the kinematic models lead to strategies for identifying and correcting misalignment. One technique that tracks the inplane attitude when a drop occurs at moderate to large misalignments is evaluated as an improvement to the existing techniques for automated alignment of threaded fasteners.