BACKGROUND
The history of industrial automation is characterized by periods of rapid change in
popular methods. Either as a cause or, perhaps, an effect, such periods of change in
automation techniques seem closely tied to world economics. Use of the industrial
robot, which became identifiable as a unique device in the 1960s, along with
computer-aided design (CAD) systems and computer-aided manufacturing (CAM)
systems, characterizes the latest trends in the automation of the manufacturing
process. These technologies are leading industrial automation through another
transition, the scope of which is stifi unknown.
In North America, there was much adoption of robotic equipment in the early
1980s, followed by a brief pull-back in the late 1980s. Since that time, the market has
been growing, although it is subject to economic swings, as are all markets. The number of robots being installed per year in the major
industrial regions of the world. Note that Japan reports numbers somewhat differently
from the way that other regions do: they count some machines as robots
that in other parts of the world are not considered robots (rather, they would be
simply considered "factory machines"). Hence, the numbers reported for Japanare
somewhat inflated.
A major reason for the growth in the use of industrial robots is their declining
cost. Through the decade of the 1990s, robot prices dropped
while human labor costs increased. Also, robots are not just getting cheaper, they
are becoming more effective—faster, more accurate, more flexible. If we factor
these quality adjustments into the numbers, the cost of using robots is dropping even
faster than their price tag is. As robots become more cost effective at their jobs,
and as human labor continues to become more expensive, more and more industrial
jobs become candidates for robotic automation. This is the single most important
trend propelling growth of the industrial robot market. A secondary trend is that,
economics aside, as robots become more capable they become able to do more and
more tasks that might be dangerous or impossible for human workers to perform.
The applications that industrial robots perform are gradually getting more
sophisticated, but it is stifi the case that, in the year 2000, approximately 78%
of the robots installed in the US were welding or material-handling robots.
A more challenging domain, assembly by industrial robot, accounted for 10% of
installations.
This book focuses on the mechanics and control of the most important form
of the industrial robot, the mechanical manipulator. Exactly what constitutes an
industrial robot is sometimes debated. Devices such as that shown in are
always included, while numerically controlled (NC) milling machines are usually
not. The distinction lies somewhere in the sophistication of the programmability of
the device—if a mechanical device can be programmed to perform a wide variety
of applications, it is probably an industrial robot. Machines which are for the most
part limited to one class of task are considered fixed automation. For the purposes
of this text, the distinctions need not be debated; most material is of a basic nature
that applies to a wide variety of programmable machines.
By and large, the study of the mechanics and control of manipulators is
not a new science, but merely a collection of topics taken from "classical" fields.
Mechanical engineering contributes methodologies for the study of machines in
static and dynamic situations. Mathematics supplies tools for describing spatial
motions and other attributes of manipulators. Control theory provides tools for
designing and evaluating algorithms to realize desired motions or force applications.
Electrical-engineering techniques are brought to bear in the design of sensors
and interfaces for industrial robots, and computer science contributes a basis for
programming these devices to perform a desired task.
THE MECHANICS AND CONTROL OF MECHANICAL MANIPULATORS
The following sections introduce some terminology and briefly preview each of the
topics that will be covered in the text.
Description of position and orientation
In the study of robotics, we are constantly concerned with the location of objects in
three-dimensional space. These objects are the links of the manipulator, the parts
and tools with which it deals, and other objects in the manipulator's environment.
At a crude but important level, these objects are described by just two attributes:
position and orientation. Naturally, one topic of immediate interest is the manner
in which we represent these quantities and manipulate them mathematically.
In order to describe the position and orientation of a body in space, we wifi
always attach a coordinate system, or frame, rigidly to the object. We then proceed
to describe the position and orientation of this frame with respect to some reference
coordinate system. Any frame can serve as a reference system within which to express the
position and orientation of a body, so we often think of transforming or changing
the description of these attributes of a body from one frame to another. Chapter 2
discusses conventions and methodologies for dealing with the description of position
and orientation and the mathematics of manipulating these quantities with respect
to various coordinate systems.
Developing good skifis concerning the description of position and rotation of
rigid bodies is highly useful even in fields outside of robotics.
Forward kinematics of manipulators
Kinematics is the science of motion that treats motion without regard to the forces
which cause it. Within the science of kinematics, one studies position, velocity, acceleration, and all higher order derivatives of the position variables (with respect
to time or any other variable(s)). Hence, the study of the kinematics of manipulators
refers to all the geometrical and time-based properties of the motion.
Manipulators consist of nearly rigid links, which are connected by joints that
allow relative motion of neighboring links. These joints are usually instrumented
with position sensors, which allow the relative position of neighboring links to be
measured. In the case of rotary or revolute joints, these displacements are called
joint angles. Some manipulators contain sliding (or prismatic) joints, in which the
relative displacement between links is a translation, sometimes called the joint
offset.
The number of degrees of freedom that a manipulator possesses is the number
of independent position variables that would have to be specified in order to locate
all parts of the mechanism. This is a general term used for any mechanism. For
example, a four-bar linkage has only one degree of freedom (even though there
are three moving members). In the case of typical industrial robots, because a
manipulator is usually an open kinematic chain, and because each joint position is
usually defined with a single variable, the number of joints equals the number of
degrees of freedom.
At the free end of the chain of links that make up the manipulator is the endeffector.
Depending on the intended application of the robot, the end-effector could
be a gripper, a welding torch, an electromagnet, or another device. We generally
describe the position of the manipulator by giving a description of the tool frame,
which is attached to the end-effector, relative to the base frame, which is attached
to the nonmoving base of the manipulator. A very basic problem in the study of mechanical manipulation is called forward
kinematics. This is the static geometrical problem of computing the position and
orientation of the end-effector of the manipulator. Specifically, given a set of joint angles, the forward kinematic problem is to compute the position and orientation of
the tool frame relative to the base frame. Sometimes, we think of this as changing
the representation of manipulator position from a joint space description into a
Cartesian space description.
