22
These chains are shown in Figs. 2.7 and 2.8, where the order
of (R-L) can be reversed as (L-R).
2.4 Technical Discussion
Actuator. One of the major concerns in the design of
serial open-loop manipulators is to maximize the actuator
power/weight ratio, since some of the actuators must be
mounted on the moving links, as shown in Fig. 2.9, which
adds to the inertia of the actuators to the links' inertia
and decreases payload capacity. Therefore, the actuator
size increases from the distal joint to the proximal joint.
The manipulator becomes a massive linkage requiring bigger
actuator sizes and resulting in smaller payloads. If one-
degree-of-freedom actuators are to be used, this will
result in a five-loop linkage for a six-degree-of-freedom
robot manipulator. Further, if only one-degree-of-freedom
joints are used, the number of links in the linkage becomes
quite large. The number of links can, however, be reduced
by using joints with more than one degree of freedom, such
as cylinder and spherical pairs. The number of loops can
also be reduced, thereby reducing the number of links
further by devising and using two- or more-degree-of-
freedom self-actuated joints. A six-degree-of-freedom
parallel manipulator, where all actuators are ground-
mounted, is shown in Fig. 2.10.
Computation. The computation of inverse kinematics and
dynamics requires considerable time for serial-link