Six-point Method for Spatial Angle Measurement of a Hydraulic-Driven Underwater Manipulator

Abstract

Underwater manipulator has been widely equipped on remotely operated underwater vehicles (ROV) for underwater operation, which consists of linkages and rotation joints. The maximum rotation range of each joint contributes to the working volume of a manipulator. Therefore, it is necessary to measure the limit range of each rotation joint of a manipulator and determine the deviation between the design and the actual values. A triple-cameras based stereo measurement system is developed in this paper to measure the rotation range of joints for a 7-function underwater manipulator. The obvious way is to attach a marker to the rotating linkage of the joint interested. The
3D coordinate of the marker center is a spatial arc while the joint is rotating. Therefore, measurement of the maximum rotation range of a joint can be regarded as the problem of spatial circle fitting. The rotation angle can be calculated with the starting and end points as well as the center of the circle. However, the above spatial circle fitting method is not applicable for underwater manipulator, which is usually driven by hydraulic power. During the rotation of a joint, other joints of the manipulator may rotate slightly due to pressure fluctuation. The trajectory of the marker center, in fact, is not a spatial arc at all. Therefore, a six-point method is proposed in this paper. Three markers are attached on the reference linkage to detect its slight movement due to pressure fluctuation.