Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

16 Chapter 1 Motor and Motion Control Systems
mand signals for the motor driver or drivers that control the motor or
motors driving the load.
Motor Selection
The most popular motors for motion control systems are stepping or stepper
motors and permanent-magnet (PM) DC brush-type and brushless DC
servomotors. Stepper motors are selected for systems because they can run
open-loop without feedback sensors. These motors are indexed or partially
rotated by digital pulses that turn their rotors a fixed fraction or a revolution
where they will be clamped securely by their inherent holding torque.
Stepper motors are cost-effective and reliable choices for many applications
that do not require the rapid acceleration, high speed, and position
accuracy of a servomotor.
However, a feedback loop can improve the positioning accuracy of a
stepper motor without incurring the higher costs of a complete servosystem.
Some stepper motor motion controllers can accommodate a closed
loop.
Brush and brushless PM DC servomotors are usually selected for
applications that require more precise positioning. Both of these motors
can reach higher speeds and offer smoother low-speed operation with
finer position resolution than stepper motors, but both require one or more
feedback sensors in closed loops, adding to system cost and complexity.
Brush-type permanent-magnet (PM) DC servomotors have wound
armatures or rotors that rotate within the magnetic field produced by a
PM stator. As the rotor turns, current is applied sequentially to the appropriate
armature windings by a mechanical commutator consisting of two
or more brushes sliding on a ring of insulated copper segments. These
motors are quite mature, and modern versions can provide very high performance
for very low cost.
There are variations of the brush-type DC servomotor with its ironcore
rotor that permit more rapid acceleration and deceleration because of
their low-inertia, lightweight cup- or disk-type armatures. The disk-type
armature of the pancake-frame motor, for example, has its mass concentrated
close to the motor’s faceplate permitting a short, flat cylindrical
housing. This configuration makes the motor suitable for faceplate
mounting in restricted space, a feature particularly useful in industrial
robots or other applications where space does not permit the installation
of brackets for mounting a motor with a longer length dimension.
The brush-type DC motor with a cup-type armature also offers lower
weight and inertia than conventional DC servomotors. However, the tradeoff
in the use of these motors is the restriction on their duty cycles because