Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

32 Chapter 1 Motor and Motion Control Systems
Figure 1-27 Operating principles
of a linear servomotor.
Before the invention of linear motors, the only way to produce linear
motion was to use pneumatic or hydraulic cylinders, or to translate rotary
motion to linear motion with ballscrews or belts and pulleys.
A linear motor consists of two mechanical assemblies: coil and magnet,
as shown in Figure 1-27. Current flowing in a winding in a magnetic
flux field produces a force. The copper windings conduct current (I ), and
the assembly generates magnetic flux density (B). When the current and
flux density interact, a force (F ) is generated in the direction shown in
Figure 1-27, where F = I × B.
Even a small motor will run efficiently, and large forces can be created
if a large number of turns are wound in the coil and the magnets are powerful
rare-earth magnets. The windings are phased 120 electrical degrees
apart, and they must be continually switched or commutated to sustain
motion.
Only brushless linear motors for closed-loop servomotor applications
are discussed here. Two types of these motors are available commercially—steel-core
(also called iron-core) and epoxy-core (also called
ironless). Each of these linear servomotors has characteristics and features
that are optimal in different applications
The coils of steel-core motors are wound on silicon steel to maximize
the generated force available with a single-sided magnet assembly or
way. Figure 1-28 shows a steel-core brushless linear motor. The steel in
these motors focuses the magnetic flux to produce very high force density.
The magnet assembly consists of rare-earth bar magnets mounted
on the upper surface of a steel base plate arranged to have alternating
polarities (i.e., N, S, N, S)