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8 Commercial Robot Manipulators

functional single-axis actuator; the modules may be custom assembled for
special-purpose applications.

Spherical and Cylindrical Coordinate Robots. The first two axes of the
spherical coordinate robot, Figure 1.2.5, are revolute and orthogonal to
one another, and the third axis provides prismatic radial extension. The
result is a natural spherical coordinate system with a spherical work volume.
The first axis of cylindrical coordinate robots, Figure 1.2.6, is a revolute
base rotation. The second and third are prismatic, resulting in a natural
cylindrical motion. Commercial models of Spherical and Cylindrical robots
were originally very common and popular in machine tending and material
handling applications. Hundreds are still in use but now there are only a
few commercially available models. The decline in use of these two
configurations is attributed to problems arising from use of the prismatic
link for radial extension/retraction motion; a solid boom requires clearance
to fully retract.

Figure 1.2.5: Hydraulic powered spherical robot (courtesy Kohol Systems, Inc.).

Parallel-Link Manipulators. For some special purpose applications, parallel-
link robots are more suitable than serial link robots. These robots generally
have three or six links in parallel, each link attached to a fixed base and to a
moving working platform. See Figure 1.2.7. With proper design, a six-link
parallel-link manipulator can have six degrees of freedom motion of the
working platform. The military Link trainer is a large parallellink robot
moving a pilot’s seat. These robots have greater stiffness and precision than
serial-link robots, where the positioning errors of each link are compounded
as one moves outwards from the base. Thus, lightweight parallel-link robots

Copyright © 2004 by Marcel Dekker, Inc.