Open Loop Compliance Model of a 6 DOF Revolute Manipulator to Improve Accuracy Under Load

Abstract

Robotic accuracy has long been limited by the compliance of the manipulator. Whether links under bending loads or backlash in gear trains and stretching of belts, the resulting compliance causes a loss of accuracy at the end-effector. Previous research has investigated accuracy of ideally stiff manipulators from many different points of view; however, an overall compliant modeling technique has not been formulated in the literature. This thesis presents a general technique to develop a compliant model for a general six-degree manipulator with the intent of reducing end-effector error for precision manufacturing.

Experimental and theoretical work was performed on an American Robot Merlin six-degree of freedom robot. The solution technique assumes each link of the manipulator is subject to stiffnesses in three directions, that is, in the direction of motion, laterally and torsionally. Each of the three stiffnesses is assumed constant, but unknown. Three experimental regimes were established, each covering a successively larger region of the workspace, and 243 data samples were taken within each regime. Samples were taken at twenty-seven data points under nine known loads for each of the first two regimes and at nine locations under twenty-seven loads in the third regime. An OPTOTRAK 3020 non-contact distance-measuring system was used to gather data from twelve sensors for each trial. The results were transformed into three displacements and three rotations of the end-effector. A regression algorithm solved for the unknown stiffnesses of the compliant model based on the measured experimental deflection.

Results show that for loads ranging between zero and 445 N, the deflection of the end-effector is predicted within fifteen percent of experimental results for most data points. Furthermore, a load set between zero and 111 N (the stated lift capacity of the manipulator) predicts end point position with an error of less than one-half a millimeter for all tested points.

This research provides a technique to quantify the compliance of a general manipulator and develops a model capable of being implemented with open-loop position control with known compliance.