Development of a spherical stepper wrist motor. - Kok-Meng Lee; George J. Vachtsevanos; Chikong Kwan (J. Intell. Robotic Syst., 1988)

Design concept development of a spherical stepper for robotic applications. - Kok-Meng Lee; Chikong Kwan (IEEE Trans. Robotics Autom., 1991)