Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 10 Manipulator Geometries 247
arm segments are supported, gantry and cantilevered.
Mounted on the front of a robot, the first two
DOF of a cantilevered Cartesian manipulator can
move left/right and up/down; the Y-axis is not
necessarily needed on a mobile robot because the
robot can move back/forward. Figure 10-5 shows
a cantilevered layout with three DOF. Though not
the best solution to the problem of working off
the front of a robot, it will work. It has the benefit
of requiring a very simple control algorithm.
CYLINDRICAL
The second type of manipulator work envelope is
cylindrical. Cylindrical types usually incorporate
a rotating base with the first segment able to telescope
or slide up and down, carrying a horizontally
telescoping segment. While they are very
simple to picture and the work envelope is fairly
intuitive, they are hard to implement effectively
because they require two linear motion segments,
both of which have moment loads in them caused
by the load at the end of the upper arm.
In the basic layout, the control code is fairly
simple, i.e., the angle of the base, height of the
first segment, and extension of the second segment.
On a robot, the angle of the base can simply
be the angle of the chassis of the robot itself,
leaving the height and extension of the second
segment. Figure 10-6 shows the basic layout of a
cylindrical three-DOF manipulator arm.
A second geometry that still has a cylindrical
work envelope is the SCARA design. SCARA
means Selective Compliant Assembly Robot
Arm. This design has good stiffness in the vertical
direction, but some compliance in the horizontal.
This makes it easier to get close to the
right location and let the small compliance take
up any misalignment. A SCARA manipulator
replaces the second telescoping joint with two
vertical axis-pivoting joints. Figure 10-7 shows a
SCARA manipulator.
Figure 10-6
Figure 10-7
Three-DOF cylindrical manipulator
A SCARA manipulator