he motion controller is part of a complete closed-
loop system which also includes the power amplifi-
er, motor, and an encoder. Such systems, also
known as servo systems, sense the motor position and
feed back the position signal to the controller. A typical
servo system is shown in the figure below.
The operation of the various system elements may be
described by comparing the servo system with the human
body. The combination of the motor and power amplifier
is analogous to the muscle that moves the human arm.
The motor or actuator is the device that generates the
motion; the power amplifier generates the current
required to drive the motor. For example, the amplifier
takes a low-current signal and amplifies it to a higher level
of current of 10 A.
The controller is the intelligent element that com-
mands the motion; as such, it operates as the brain of the
system. It generates a signal, referred to as the motion
command, which is applied to the power amplifier. The
function of the position sensor is analogous to that of the
human eyes. It senses the position of the motor and
reports the result to the controller, and so closes the loop.
A closed-loop system receives its command from an
outside source, often a host computer. Continuing our
human society analogy, the command source may be seen
as the Boss, generating commands, and often requesting
M O T I O N C O N T R O L S Y S T E M S
Motion Control Systems
97
DC motors convert electrical energy to mechanical energy,
or, more specifically, convert current into rotational torque.
The key parameters of a DC motor are the torque constant,
K
t
, the armature resistance, r, the moment of inertia, J
m
, and
the maximum torque levels. The torque constant is expressed
in units of Nm/A or oz-in/A, and indicates the amount of
torque that the motor generates for a unit of current. For
example, a DC motor with a torque constant of 0.1 Nm/A
converts a current of 2 A to a torque of 0.2 Nm.
The armature resistance is the total resistance of the arma-
ture winding and the brushes; it is expressed in ohms. The
moment of inertia, Jm, is the sum of the moments of inertia
of the rotating parts of the motor and is expressed in units of
kg-m
2
or oz-in-s
2
.
A motor is also characterized by the level of torque it can
produce. The motor torque capability is expressed by two
parameters: the continuous value and the peak value. The
continuous torque is the torque that the motor can produce
continuously, often at any speed, without overheating. The
peak torque, on the other hand, is the maximum torque that
can be generated for short periods of time without causing
mechanical damage or demagnetization. The peak torque is
several times the continuous torque. The motor can generate
any level of torque below the peak torque so long as the
root-mean-square (RMS) value of the torque is within the
continuous torque level.
The current that drives the motor is generated by a power
amplifier. The operation of such amplifiers is described
below.
DC Motors
OVERVIEW
T
Elements of a Servo System
MOTION
CONTROLLER
POWER
AMPLIFIER
MOTOR
POSITION
SENSOR
POSITION
POSITION
FEEDBACK
POSITION
COMMAND
MOTION
COMMAND
CURRENT
HOST
COMPUTER
status reports. The commands can also be generated by
other sources, such as a programmable controller, a termi-
nal, or just a set of switches.
The following discussion briefly describes system ele-
ments. The focus is on digital position systems controlling
DC motors and utilizing incremental encoders, the most
common type of control systems. However, most of the
discussion applies to systems utilizing AC or hydraulic
motors and resolvers or absolute encoders, etc.
