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6 Commercial Robot Manipulators

Articulated Arms. The variety of commercial articulated arms, most of
which have six axes, is very large (Fig. 1.2.1). All of these robots’ axes
are re volute. The second and third axes are co-planar and work together
to produce motion in a vertical plane. The first axis in the base is vertical
and revolves the arm to sweep out a large work volume. Many different
types of drive mechanisms have been devised to allow wrist and forearm
drive motors and gearboxes to be mounted close to the first and second
axis of rotation, thus minimizing the extended mass of the arm. The
workspace efficiency of well designed articulated arms, which is the degree
of quick dexterous reach with respect to arm size, is unsurpassed by other
arm configurations when five or more degrees of freedom are needed. A
major limiting factor in articulated arm performance is that the second
axis has to work to lift both the subsequent arm structure and the pay
load. Historically, articulated arms have not been capable of achieving
accuracy as high as other arm configurations, as all axes have joint angle
position errors which are multiplied by link radius and accumulated for
the entire arm.

Type I SCARA. The Type I SCARA (selectively compliant assembly robot
arm) arm, Figure 1.2.2, uses two parallel revolute joints to produce motion
in the horizontal plane. The arm structure is weight-bearing but the first and
second axes do no lifting. The third axis of the Type I SCARA provides work
volume by adding a vertical or z axis. A fourth revolute axis will add rotation
about the z axis to control orientation in the horizontal plane. This type of
robot is rarely found with more than four axes. The Type I SCARA is used
extensively in the assembly of electronic components and devices, and it is
used broadly for the assembly of small- and mediumsized mechanical
assemblies.

Type II SCARA. The Type II SCARA, Figure 1.2.3, also a four axis
configuration, differs from Type I in that the first axis is a long vertical
prismatic z stroke which lifts the two parallel revolute axis and their links.
For quickly moving heavier loads (over approximately 75 pounds) over longer
distance (more than about three feet), the Type II SCARA configuration is
more efficient than the Type I.
Cartesian Coordinate Robots. Cartesian coordinate robots use orthogonal
prismatic axes, usually referred to as x, y, and z, to translate their end-effector
or payload through their rectangular workspace. One, two, or three revolute
wrist axes may be added for orientation. Commercial robot companies supply
several types of Cartesian coordinate robots with workspace sizes ranging
from a few cubic inches to tens of thousands of cubic feet, and payloads
ranging to several hundred pounds. Gantry robots, which have an elevated
bridge structure, are the most common Cartesian style and are well suited to

Copyright © 2004 by Marcel Dekker, Inc.