Servo motors are used in closed
loop control systems in which work is the control variable. The
digital servo motor controller directs operation of the servo
motor by sending velocity command signals to the amplifier,
which drives the servo motor. An integral feedback device
(resolver) or devices (encoder and tachometer) are either
incorporated within the servo motor or are remotely mounted,
often on the load itself. These provide the servo motor's
position and velocity feedback that the controller compares to
its programmed motion profile and uses to alter its velocity
signal. Servo motors feature a motion profile, which is a set of
instructions programmed into the controller that defines the
servo motor operation in terms of time, position, and velocity.
The ability of the servo motor to adjust to differences between
the motion profile and feedback signals depends greatly upon the
type of controls and servo motors used.
Three basic types
of servo motors are used in modern servo systems: ac servo
motors, based on induction motor designs; dc servo motors, based
on dc motor designs; and ac brushless servo motors, based on
synchronous motor designs.
motors are used in ac servo mechanisms and
computers which require rapid and accurate response
characteristics. To obtain these characteristics, servo motors
have small-diameter high-resistance rotors. The small diameter
provides low inertia for fast starts, stops, and reversals,
while the high resistance provides a nearly linear speed-torque
relationship for accurate control.
In an ideal servo motor, torque at any
speed is directly proportional to control-winding voltage. In
practice, however, this relationship exists only at zero speed
because of the inherent inability of an induction motor to
respond to voltage input changes under conditions of light load.
Servomotors are wound with two phases
physically at right angles or in space quadrature. A fixed or
reference winding is excited from a fixed voltage source, while
the control winding is excited by an adjustable or variable
control voltage, usually from a servo amplifier. The windings
are usually designed with the same voltage-turns ratio, so that
power inputs at maximum fixed-phase excitation and at maximum
control-phase signal are in balance.
The inherent damping of servo motors
decreases as ratings increase, and the motors have a reasonable
efficiency at the sacrifice of speed-torque linearity. Most
larger motors have integral auxiliary blowers to maintain
temperatures within safe operating ranges. Servomotors are
available in power ratings from less than 1 to 750 W, in sizes
ranging from 0.5 to 7-in. OD. Most designs are available with
modular or built-in gear heads.
motors are normally used as prime movers in
computers, numerically controlled machinery, or other
applications where starts and stops are made quickly and
accurately. Servo motors have lightweight, low-inertia armatures
that respond quickly to excitation-voltage changes. In addition,
very low armature inductance in these servo motors results in a
low electrical time constant (typically 0.05 to 1.5 msec) that
further sharpens servo motor response to command signals. Servo
motors include permanent-magnetic, printed-circuit, and
moving-coil (or shell) dc servo motors. The rotor of a shell dc
servo motor consists of a cylindrical shell of copper or
aluminum wire coils which rotate in a magnetic field in the
annular space between magnetic pole pieces and a stationary iron
core. The servo motor features a field, which is provided by
cast AlNiCo magnets whose magnetic axis is radial. Servo motors
usually have two, four, or six poles.
Dc servo motor characteristics include
inertia, physical shape, costs, shaft resonance, shaft
configuration, speed, and weight. Although these dc servo motors
have similar torque ratings, their physical and electrical
Motor Selection: The first selection
approach is to choose a servo motor large enough for a machine
that has already been designed; the second is to select the best
available servo motor with a specific feature and then build the
system around it; and the third is to study servo motor
performance and system requirements and mate the two.
The final servo motor system design is
usually the least sophisticated that meets the performance
specifications reliably. Servo motor requirements may include
control of acceleration, velocity, and position to very close
tolerances. This says that the servo designer must define the
system carefully, establish the servo motor's performance
specifications, determine critical areas, and set up tolerances.
Only then will the designer be able to propose an adequate servo
system and choose a servo motor type.