unnecessary for this test, but the test fixture
is not equipped to combat the torque and
lift of the propeller. The setup could wildly
take flight and cause injury, slice wires, and
result in property damage.
Before any motor will turn, we have to
complete two important steps: binding of
the receiver and calibration of the ESC
range. Binding is the same process we used
when building our homemade drone last
year, but the ESC calibration that was
handled by the flight controller is now our
Receiver binding is slightly different
based on which radio you have, but they all
have the same basic procedure. First, set
the transmitter to an unused model profile.
I used the third profile on my remote as the
first two are reserved for our homebuilt
quad and the Parallax ELEV- 8.
Next, plug the “bind” jumper wire onto
the receiver’s bind port. This jumper
connects the signal to ground and puts the
receiver in a binding state. Power-up the
receiver, then turn on the transmitter while
holding down the “bind” button (on the back of the
transmitter on many models.) Finally, power-down the
system and remove the binding jumper.
ESC calibration also varies slightly, but for the ESC used
in this article the setup is powered and the transmitter
turned on with the throttle in the maximum position. Then,
after some beeps from the ESC, the throttle is lowered to
the minimum. The ESC records the width of the pulse width
modulation (PWM) signal at each of these
extents and scales its throttle range to match.
If this step is skipped, it is very likely that only
a small portion of the control stick range will
have an effect on the throttle — a large
apparent deadband in the response.
With the ESC calibration completed, the
motor should respond to the throttle control.
The motor’s speed shouldn’t top out until the
control is all the way at its maximum, and the
motor should cut out and stop at the very
bottom of the range. If not, the calibration
was unsuccessful and needs to be tried
Play with the throttle control some and
observe the motor’s response. Try gradual
changes in throttle as well as rapid changes
— especially from stopped to maximum
throttle. I videoed the response of the system
in its out-of-the-box configuration
I rounded out my test suite by measuring
the minimum stable rotational speed and
maximum rotational speed with a photo
These can be found for less than $20
on Amazon ( http://amzn.to/2ohS9Og)
and are a handy thing to have in your
toolbox (Figure 8). The one I found
included a little cloth carrying case and
some reflective tape.
The tachometer works by shining a light
on the rotating surface and counting the
number of peaks in the reflected light over
a given amount of time. For example, a
motor rotating at 3,600 RPM is rotating at:
So, counting six light pulses per second
would correspond to 3,600 RPM averaged
over that time span. To get a good reading,
a high reflectivity contrast region is needed.
Generally, a white paint pen suffices. In this
case, though, that will not reliably perform.
The outer casing of the motor is a
mirror finish, so we actually need to decrease the reflectivity
of most of the casing, leaving a small strip of reflective
surface. I wrapped painter’s tape around the casing and
trimmed a section about 1 cm wide out with a razor knife. I
tested the motor at its maximum and minimum throttle
settings several times, and reliably obtained numbers of
12,500 RPM and 3,000 RPM, respectively. Arranging
everything on the bench with a helpful adjustable height
SERVO 07.2017 53
Figure 8: For about $15, a decent
handheld tachometer can be
added to your tool bag. These
come in handy more often than
you’d think if you deal with
motor driven equipment.
Figure 9: The ESC and motor ready for testing. Pads of sticky notes make great
adjustable height stands for lining the tachometer up with the motor.
3600 rev 1 minute 6 rev
1 minute 60 seconds second