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4 Commercial Robot Manipulators

individual joints. Some robots will have unusable spaces such as dead zones,
singular poses, and wrist-wrap poses inside of the boundaries of their reach.
Payload weight is specified by the manufacturers of all industrial robots.
Some manufacturers also specify inertial loading for rotational wrist axes. It
is common for the payload to be given for extreme velocity and reach
conditions. Weight and inertia of all tooling, workpieces, cables and hoses
must be included as part of the payload.
Quickness is critical in determining throughput but difficult to determine
from published robot specifications. Most manufacturers will specify a
maximum speed of either individual joints or for a specific kinematic tool
point. However, average speed in a working cycle is the quickness
characteristic of interest.
Precision is usually characterized by measuring repeatability. Virtually
all robot manufacturers specify static position repeatability. Accuracy is
rarely specified, but it is likely to be at least four times larger than
repeatability. Dynamic precision, or the repeatability and accuracy in
tracking position, velocity, and acceleration over a continuous path, is not
usually specified.

Common Kinematic Configurations
All common commercial industrial robots are serial-link manipulators,
usually with no more than six kinematically coupled axes of motion. By
convention, the axes of motion are numbered in sequence as they are
encountered from the base on out to the wrist. The first three axes account
for the spatial positioning motion of the robot; their configuration
determines the shape of the space through which the robot can be positioned.
Any subsequent axes in the kinematic chain generally provide rotational
motions to orient the end of the robot arm and are referred to as wrist
axes. In a robotic wrist, three axes usually intersect to generate true
independent positioning in terms of 3-D orientation. See Appendix A for a
kinematic analysis of the spherical robot wrist mechanism. Note that in
our 3-dimensional space, one requires three degrees of freedom for fully
independent spatial positioning and three degrees of freedom for fully
independent orientational positioning.
There are two primary types of motion that a robot axis can produce
in its driven link- either revolute or prismatic. Revolute joints are
anthropomorphic (e.g. like human joints) while prismatic joints are able
to extend and retract like a car radio antenna. It is often useful to classify
robots according to the orientation and type of their first three axes.
There are four very common commercial robot configurations:
Articulated, Type I SCARA, Type II SCARA, and Cartesian. Two other
configurations, Cylindrical and Spherical, are now much less common.

Copyright © 2004 by Marcel Dekker, Inc.