output to either V+ or GND. If we create three of these
source switching circuits and connect them to the windings,
we can create the high, low, and floating states (Figure 3).
Unfortunately, being able to create the proper
switching sequence isn’t enough. The sequence needs to be
properly timed with the orientation of the motor’s rotor.
Some larger motors have Hall effect sensors near each coil
and can determine the position of the rotor by sensing
which magnets are nearest each coil.
This technique is very reliable and can determine the
position of the rotor at rest.
It does require extra sensors to be built into the motor,
increasing its weight and cost. It also requires more
connections between the motor and ESC. These types of
ESCs have much simpler firmware and are generally more
reliable on startup.
The way that most ESCs work on quadcopters (and the
one we’re looking are here) is with sensorless
orientation determination. Remember the floating
third phase from our power switching network? It can
help us out in the determination of orientation by
measuring the back electromotive force (EMF)
generated on that coil. The back EMF can be modeled
as roughly trapezoidal over the course of one
revolution of the motor (Figure 4). By measuring this,
the ESC can determine the position of the rotor. The
biggest catch is that this method requires the motor to
be rotating. It also requires some signal processing.
The simplest way to determine the orientation of
the rotor involves integrating the signal and
determining when there is a zero-crossing. This can be done
with analog comparators on each of the windings, but is
more commonly done with purpose-built chips or signal
processing on the microcontroller.
For the zero-crossing detection technique to be
effective, the signal needs to be filtered to remove as much
noise as possible; a task that becomes harder at higher
speeds. If you really want to dig into how the orientation
determination works, the gory math is fully discussed in an
app note from NXP ( www.nxp.com/files-
The signal can also be processed using the relatively
modern technique of Kalman filtering. While these filters
could make up an entirely different column (or book for
that matter), the basic idea is using our current noisy
knowledge of the system and some model of how we
expect the system to behave to get a better estimate of the
real state of the system. These are again more complex to
implement, but can provide much better estimates of the
No matter which method is used to calculate the
position of the rotor, it must be turning for some back EMF
to be generated before the ESC can get proper
synchronization established. Generally, the ESC applies
some current to the coils to start rotation, and then gets a
phase determination and locks into the rotor’s position.
Choosing the wrong ESC can result in poor
performance, excessive cost, large releases of magic smoke,
or some combination of these. In selecting your ESC, you
need to consider the voltage rating, current rating, a
battery eliminator circuit (BEC), and firmware.
Selecting an ESC with the incorrect voltage rating is
probably the quickest way to literally watch your hobby
dollars go up in smoke. ESCs will have the battery voltages
they are rated to work with in their description. Often, this
is specified in terms of the battery configuration such as 2-
6S instead of the voltage. For our LiPo batteries, each cell is
3. 7 VDC, so a 2S (two cells in series) is a 7. 4 VDC battery,
and a 6S is 22. 2 VDC. Connecting a 6S battery to a 2S
rated ESC can really ruin your day and potentially be
The second most important rating is the current
capacity of the ESC. Again, this should be specified by the
manufacturer. If your motor is rated to draw up to 10A,
then you need at minimum a 10A ESC. Generally, you will
want to buy an ESC that is a little bigger than you need —
maybe a 15-20A ESC for the 10A motor.
Using an over-rated ESC reduces the risk of an ESC
overheating and/or failing in the middle of a flight and
causing a spectacular crash. Don’t go overboard though, or
SERVO 07.2017 51
Figure 3: A complete ESC power switching network consists of three
MOSFET source inverters. By manipulating the six inputs, each
output can be put into one of the three possible states.
Figure 4: The back EMF measured on each of the sets of coils represents
a trapezoidal shape (hence, BLDCs are sometimes referred to as
trapezoidal motors). (Image courtesy of Freescale Semiconductor.)