Active Control of a Compliant Wrist in Manufacturing Tasks

Abstract

A robotic wrist unit has been constructed and is functioning on a large industrial robot used to load, assemble, and finish parts in an automated metal working cell. The wrist uses force feedback and remote-center-compliance techniques similar to those that have been developed for assembly robots. The wrist employs four spherical hydraulic actuators that behave as adjustable springs with a stiffness that varies between 5.8 × 103 and 7.9 × 103 N/m. This allows the center of compliance to be projected over a range from 25 to 157 mm out from the upper platform of the wrist. The wrist has 5 1/2 degrees-of-freedom, being compliant in each direction except axial extension. Deflections of ± 4.6 mm in the radial plane and 5.1 mm in compression are possible. The accuracy of the sensors over this working range is within 0.025 mm for translations and within 0.001 rad for rotations.