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FIGURE 4. Servo breakdown
(courtesy of ServoCity).
even use rare earth magnets for a
stronger field, but this technology is
usually limited to larger DC drive
motors.
The newest servo motors use what
is known as ‘coreless’ construction that
dispenses with rotating the core of
heavy magnets, but rotates only the
wires instead. This lightweight ‘basket’
of wired coils accelerates and
decelerates faster. Even newer motors
use a brushless design that has very
little internal drag from brushes, plus
they last longer. These newer designs
provide more torque, quicker response,
and lower power requirements, but as
you might expect, at a higher cost.
An important factor to some is to
determine if it can be used as a drive
motor for a small robot. One very
popular procedure to apply to a
standard servo is to retrofit it to
continuous rotation. There are
numerous sites on the Internet that can
assist you in making a servo
continuously rotate CW or CCW when
a 1.5 ms to 1.0 ms signal (or a 2.0 ms
to 1.5 ms signal) is applied. I won’t go
into the fine details as each servo is
different and requires different
techniques. Figure 4 shows a typical
servo opened up for modification
(courtesy of ServoCity). Talk with
others or search ‘continuous
rotating servo mod’ on the Internet
for help. The second consideration
in servo design and selection are
the mechanical aspects. Stepping
up from bronze bushings to ball
bearings on the output shaft
allows a greater load bearing
capacity. Internal metal gears allow
for a greater torque output with a
longer mean time between failure.
Servos for Robots —
From Very Small to
Very Large
Over the years, I have used
numerous servos in many types of
applications, but mostly in robotics. The
servos that I am reviewing in this article
are all rotary servos. I will review linear
servos/actuators in another article. I
have personally tested all of them for
potential robot applications. Both types
have applications that are best fitted
for their design characteristics and I
saw some features in each one that I
tested that are best met by another
type. Servos come in all sizes,
capabilities, costs, and — most
importantly — applications. A
prospective robot designer should look
at control signal and power
requirements, as well as ways of
mounting, size, torque, output shaft
usability, ruggedness for their
application, and affordability. If a robot
builder remembers that most servos
were/are designed for model aircraft
uses and proceeds with their robot
design with that in mind, the specific
application and appropriate control
methods will become apparent.
Servos come in all sizes and shapes
with unique features that apply to a
narrow type of robot. Hitec has a great
line of robot-specific servos that have
been covered in many robotics blogs,
so I’ll concentrate on servos other than
these. I selected several Hitec and
Futaba standard and digital servos,
several very large Torxis servos, and
several each of the Dynamixel and
Herkulex actuators to test and review
for this article. Just to test torque, I
used a very long armed horn attached
to the servo that had a 1.7 inch arm
length that could represent a 3. 4 inch
diameter reel. Very crude, but effective.
So, for example, if the servo could lift
10 ounces at the 1.7” length, that
represented 17 oz-in of torque. I used
several other jury-rigged arms of 3”, 5”,
and 10” in length with some brass
balance weights from a set that I had
as a reference mass.
Since my weights were in grams
from one to 500, I had to do a bit of
calculating ( 28. 35 G = 1.0 oz/.03527
oz = 1.0 G) to result in oz-in which is
what most US experimenters use. I
used the two different servo testers
shown in Figures 5A and 5B as the
servo drivers and programmers, as well
as a transmitter/receiver setup.
(I used the Hitec HPP- 21 Plus
programmable servo tester and an
inexpensive $3 import.)
Tiny Servos
76 SERVO 11.2012
FIGURE 5A. Hitec
HPP 21-plus digital
servo programmer.
FIGURE 5B. Inexpensive
imported servo tester.
Some very popular servos for tiny
robots weigh only a few grams, such as
the Hitec HS-5035HD digital ultra nano
servo shown in Figure 6 that is perfect
for small robot claws/end-effectors.
