Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

24 Chapter 1 Motor and Motion Control Systems
Figure 1-19 Exploded view of a
permanent-magnet DC servomotor
with a disk-type armature.
bell, which completes the magnetic circuit. The spring-loaded brushes
ride directly on stamped commutator bars.
These motors are also called pancake motors because they are housed
in cases with thin, flat form factors whose diameters exceed their
lengths, suggesting pancakes. Earlier generations of these motors were
called printed-circuit motors because the armature disks were made by a
printed-circuit fabrication process that has been superseded. The flat
motor case concentrates the motor’s center of mass close to the mounting
plate, permitting it to be easily surface mounted. This eliminates the
awkward motor overhang and the need for supporting braces if a conventional
motor frame is to be surface mounted. Their disk-type motor form
factor has made these motors popular as axis drivers for industrial robots
where space is limited.
The principal disadvantage of the disk-type motor is the relatively
fragile construction of its armature and its inability to dissipate heat as
rapidly as iron-core wound rotors. Consequently, these motors are usually
limited to applications where the motor can be run under controlled
conditions and a shorter duty cycle allows enough time for armature heat
buildup to be dissipated.
Cup- or Shell-Type PM DC Motors
Cup- or shell-type PM DC motors offer low inertia and low inductance
as well as high acceleration characteristics, making them useful in many