The new design adds a layer of
complexity since electronic speed
controllers (ESCs) must be used to
drive the brushless motors. The
question of using the brushless
motors underwater didn’t seem to
be much of a problem since (without
the brushes) there is not much to
short out underwater. These motors
do have a bearing that could
potentially develop a problem, but
given the motors only cost a few
dollars each, replacement is a simple
solution for any issues that develop.
I gave the inside motor coils and
connections an extra coating of
urethane varnish made for electrical coils for some extra
protection. A little oil on the bearing casing and the iron
laminated stators inside the brushless motors reduces the
problem of rust.
Model boat propellers were obtained that mated with
the brushless motors. Traxxas makes propellers in both right
and left hand propeller screws; numbers 1533 and 1534.
Racerstar BR2205 brushless motors were used that come in
both CW (clockwise) and CCW (counterclockwise) models.
The CW model comes with a normal (right hand)
metric 5 mm thread for propeller attachment, while the
CCW models have a left hand metric 5 mm thread.
Unfortunately, both Traxxas 1533 and 1534 have
normal right hand CW threads for attaching to the shaft of
a motor. Motors like this (sold in groups of four) will have
two each of the CW and CCW threads. If you can’t
purchase motors with right hand threads individually, you
may have to purchase two groups of four to get four with
right hand threads.
The first thing I assembled was a 450 mm quadcopter
frame. Several companies make these kits, and basically
provide you with a nice inexpensive platform to begin
assembly. They can be found on Amazon, eBay, and several
radio control websites.
The four motors are mounted onto the end of the arms
of the frame, except they are mounted upside down from
what is normal. Boat propellers push, while aircraft
propellers pull. Many current versions of the quadcopter kit
have a foot attached just underneath the motor mount; this
will need to be cut off so the brushless motors can be
To reduce vibrations, the circular mounting plates are
not used, and 1/4 inch thick Neoprene rubber is employed
instead. The Adafruit 9-DOF (degrees of freedom) sensor
board is quite sensitive to vibrations, as are most inertial
The arms across from each other
have their motors turning in the
opposite direction, and the motors of
each arm turn in the same direction.
With right and left hand propellers,
all four propellers will be pushing the
Quad_ROV upwards. This
configuration is called Quad-X; refer
to Figure 1.
Attached to the bottom center
plate of the frame kit is a 6 x 6 x 4
inch waterproof plastic junction box:
a Cantex 5133710. These are
normally used for wire connections
outside a house or underground, and
are fitted with a cover containing a
This box houses the printed
circuit board (PCB) which mounts the Teensy 3.1
microcontroller; four ESC units; the Adafruit 9-DOF sensor
board; an Adafruit servo driver board; multiplexer circuit;
five volt regulator; headers for connections between parts;
a pressure sensor; and a miniature NTSC video camera, as
well as other parts and all the needed connections.
This size box works quite well for fitting all the parts
By Theron Wierenga
SERVO 12.2017 27
Figure 1. The Quad-X configuration.
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How a Quadcopter
Moves in Space
A quadcopter's four propellers are arranged so that
two turn in one direction and two in the opposite
From Newton's Third Law, we know that if all four
propellers were turning clockwise, the quadcopter would
spin counterclockwise. With two turning in each direction,
the spin is cancelled out.
If all four motors turn at the same speed, the
quadcopter will rise, hover, or lower, depending on the
If motors 0 and 1 are decreased in speed while
motors 2 and 3 are increased in speed, the quadcopter
will pitch forward and move in the forward direction.
Reversing this will cause the quadcopter to pitch
backward and move backward.
If motors 1 and 2 are decreased in speed while
motors 0 and 4 are increased in speed, the quadcopter
will roll to the right and move in the right direction.
Reversing this will cause the quadcopter to roll left and
If motors 1 and 3 are decreased in speed while
motors 0 and 2 are increased in speed, the quadcopter
will yaw to the left and rotate to the left. Reversing this
will cause the quadcopter to yaw right and rotate right.
This gives us four DOF of motion. As space consists
of three dimensions, we could navigate the quadcopter,
for example, without using yaw. However, it is convenient
to be able to rotate the quadcopter, and yaw is usually
included in the quadcopter's controls.