32 SERVO 02.2017
has all the power it needs. If you are unsure of how much
power your mechanism will require, I suggest that more is
better! I have not ever had this philosophy fail me.
Servo speed is defined as how long it takes to move
the servo horn 60 degrees, again at a certain voltage.
While not usually as critical of a design factor as the
torque, it still needs to considered. I have found that faster
is not always better.
When assembling Jarvis (see the September 2016
issue of SERVO Magazine), I found that I was able to get
smoother body movements when using a slower servo.
We will discuss how you can program your digital
servos speed in a bit. Sometimes it takes some
experimenting to achieve the exact motion you are looking
for. There is a trade-off between speed and torque, so you
need to consider where you are willing to compromise.
Servos can usually be powered by a range of voltages;
often between 4. 5 all the way up to 7. 4 volts. By using a
higher voltage, your servo is able to deliver more torque.
Improvements in battery performance have allowed us to
use higher voltages and remain within our build
The internal components used in the construction of a
servo vary as well. Motors can be coreless or brushless,
with the brushless models getting
the nod for increased performance.
The material used for the gears
can be plastic, Karbonite, metal, or
titanium. The performance
differences can include wear
resistance, strength, and how
quietly they operate. Titanium gears
offer the best performance but you
will, of course, have to pay a
premium for them.
The number and type of
bearings will contribute to the
smoothness of operation as well.
You can choose from plastic, metal,
or ball bearings. It is hard to beat
the smoothness of dual ball
As with most things, if you
want more features, you are going
to pay a bit more. Digital servos are more expensive than
their analog cousins, but in my opinion are worth the
Selecting a servo that is within your budget is an
important consideration. Take into account such things as
the difficulty in replacing a faulty servo. Is this a temporary
project or something that is expected to last long term? Is
the continual operation of a particular servo of high
Sometimes it is advantageous to pay a higher upfront
cost than to make repairs later.
Have You Considered These?
I would like to take this opportunity to point out
several specialized servos that you may not have
considered before. These fulfill some unique requirements
and you should at least be aware that they are available if
you need them.
Flat — When your only available space is a narrow
slot, the HS-7115H servo may fit the bill (Figure 5). It is
super slim and still provides 50 oz-in at 7.4V. It includes a
titanium gear train and dual ball bearings, giving it plenty
Continuous Rotation — Have you ever wanted to be
able to create a winch for a project? Maybe a continuous
rotation servo such as the HSR-2645CR will solve your
problem. While it has no potentiometer to provide
feedback, it can still be a useful device to add to your
Waterproof — Some applications may require you to
have a waterproof servo. The D845WP model delivers
Figure 6. Who’s afraid of a little rain?
Figure 5. Servos can be installed in