South Wales, Australia is a typical
pancake style motor. Figure 5 shows a
side view of the thin armature. Notice
the eight PM magnets and the black
magnetizing wire interlaced around
the magnets. There are also eight
more magnets on the other side of
the case that’s not shown. The two
brushes are placed 45 degrees apart
and rub against the darker center
commutator ring of the armature.
Newer pancake motors use rare earth
magnets. Other motors that I found
to be excellent for large robots were
A-BEC wheelchair motors, especially if
they came with attached wheels and
a hub that could be unlocked so the
robot could be easily pushed along by
hand. They were originally designed
for a British electric wheelchair
manufacturer and the wheels were
made of a non-marring rubber that
was kind to nice floors. I used these
types of motors on the four robots
that I built for the film, ‘Revenge of
the Nerds,’ and also for a promotional
robot that I built for a Beverly Hills
dentist who used the robot to educate
kids in proper dental health care.
coreless, brushless, and pancake style
motors are also popular, as well as
stepper motors. Linear motors are
used in industrial applications but have
found little use in experimenter’s
machines. AC motors also find little use
in today’s hobbyist robots.
The PM DC Motor
Quite often, the first electric motors
that new robot builders are exposed
to are radio control model airplane
servos. The actual motor may be hidden
in a plastic case and is the key
component of the servo. Hi Tec,
makers of popular servos dedicated to
robots speak of their “neodymium
rare earth magnets used in the
coreless motor.” Futaba’s line of digital
servos come with a brushless motor
that uses samarium cobalt magnets
as the field magnets.” This is not just
hype from maker’s advertisements;
this is the way motor technology is
heading today. Yes, these servos are
quite a bit more expensive than the
standard servo of 20 years ago, but
they will last far longer, draw less
current, and react faster.
changed to SM2Co17 that allowed a
higher energy product of 30 MGOe.
In 1983, a new formulation of
neodymium-iron-boron (Nd2Fe14B) was
developed that had an even higher
product of 35 MGOe. Today’s
standard for these magnets seems
to be 40 MGOe, or a grade N40 for
commercially-available super magnets.
As you might expect, there are
numerous patents on these types of
permanent magnets that contain the
above elements, and even lanthanum
and cerium. Most of the rare earth
permanent magnets are of the sintered
or bonded alloy powder types. The more
powerful sintered magnets are compressed powder formed in a die under
heat and are similar to the earlier ceramic
magnets and can be easily broken.
These types are usually plated with a
copper-nickel coating and are shiny. The
bonded magnets use a polymer base
to hold the powder together and are
tougher but weaker. The US patents
expire for most of them in 2014.
DC Motor Types
Electric Motors of
Today
Rare Earth Magnets
The greatest improvement in
today’s PM DC motor is the use of
rare earth field magnets in the place
of the old ferrite and AlNiCo magnets.
There are many varieties of motors
used in robots, and various sub-configurations, but the most popular
is the PM DC motor. This is the one
used in the small model aircraft servos
that are so popular with small robots to
large drive motors for combat robots.
I’ve tossed about the term ‘rare
earth magnets’ a bit when describing
the field magnets of these newer
motors, but the use of these magnets
was the turning point in DC motor
technology. In 1966, the high energy
compound, samarium cobalt (SmCo5)
was discovered that could be
magnetized to an energy product of
18 megagauss oersteds ( 18 MGOe).
In 1972, the ratio of the alloy was
TABLE 1. Three Models of Magmotors.
C40-300
3. 8 Horsepower
4” Diameter
6. 9” Long
3,840 oz-in Torque
84% Efficiency
24 Volts
4,000 RPM
11. 9 Pounds
Built-in Capacitors
Ferrite Magnets
S28-400
4. 5 Horsepower
3” Diameter
6. 7” Long
3,720 oz-in Torque
83% Efficiency
24 Volts
4,900 RPM
6. 9 Pounds
Built-in Capacitors
Neodymium Magnets
S28-150
3 Horsepower
3” Diameter
4” Long
1,970 oz-in Torque
82% Efficiency
24 Volts
6,000 RPM
3. 8 Pounds
Built-in Capacitors
Neodymium Magnets
Listings from Robot Book’s website.
Most of us are fairly familiar with
the popular brushed DC motor used in
tools, toys, and many other lower-cost
applications. This is not to say that
these types of motors may not be best
for a particular robot application as
the following motors are of the
brushed type. The most powerful type
of robot motor is the Magmotor series.
The chart in Table 1 shows the three
models of Magmotors. Notice the
torque at over 3,800 oz-in (that is
without an attached gear train). Can
you imagine the power and torque
output with gearing and over-volting? I
calculate 8. 6 HP at 36 volts for the more
massive C40 (which can take the heat
build-up). Now that’s robot power!
There are many motors that are
great for the larger robot, such as the
large variety sold by National Power Chair
(NPC) Robotics. They remanufacture
many brands and types of electric
wheelchair motors. One great thing
about using wheelchair motors is the
built-in gearbox and wheel attachment.
Considering that these things are used
all day long to move a human-sized
weight around a house and in public,
96 SERVO 05.2008
