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By replacing the third joint, or elbow joint, in the revolute manipulator by a prismatic joint, one obtains the spherical manipulator shown in Figure 1.13. The term spherical manipulator derives from the fact that the joint coordinates coincide with the spherical coordinates of the end effector relative to a coordinate frame located at the shoulder joint. Figure 1.13 shows the Stanford Arm, one of the most well-known spherical robots.

Figure 1.13 Schematic representation of an RRP manipulator, referred to as a spherical robot (left), and the Stanford Arm (right), an early example of a spherical arm. (Photo courtesy of the Coordinated Science Laboratory, University of Illinois at Urbana-Champaign.)