Before revision: In robotics the common normal of two non-intersecting joint axes is a line perpendicular to both axes.Introduction to Robotics by Saeed Niku ISBN 0470604468 page 75 The common normal can be used to characterize robot arm links, by using the "common normal distance" and the angle between the link axes in a plane perpendicular to the common normal.Robot manipulators: mathematics, programming, and control by Richard P. Paul 1981 ISBN 026216082X page 51 The common normal is widely used in the representation of the frames of reference for robot joints and links, and the selection of minimal representations with the Denavit-Hartenberg parameters.
After revision: When two consecutive joint axes are parallel, the common normal is not unique and an arbitrary common normal may be used, usually one that passes through the center of a coordinate system.<ref>Foundations of Robotics: Analysis and Control by Tsuneo Yoshikawa 1990 ISBN 0262240289 page 33In robotics the common normal of two non-intersecting joint axes is a line perpendicular to both axes.Introduction to Robotics by Saeed Niku ISBN 0470604468 page 75 The common normal can be used to characterize robot arm links, by using the "common normal distance" and the angle between the link axes in a plane perpendicular to the common normal.Robot manipulators: mathematics, programming, and control by Richard P. Paul 1981 ISBN 026216082X page 51 The common normal is widely used in the representation of the frames of reference for robot joints and links, and the selection of minimal representations with the Denavit-Hartenberg parameters.