This all happens hundreds of times a
second!
If you have any aerospace
engineer friends, you’ll hear them
describe the current state of a vehicle
by its state vector. This is just a vector
of numbers describing the
orientation of a vehicle in
space and the state of its
systems. If you don’t have
any aerospace engineer
friends, I highly recommend
getting some.
Using double-stick tape,
I placed the receiver in the
airframe cavity. I connected
power from the BEC to the
middle (positive) and lower
(negative) pins of the “bat”
terminal, right below the
bind terminal.
The wires from the RC
lead of the flight controller
connect to the signal (top)
line of channels 1-6 on the
receiver. These are single leads since
we just need signal as there is no
servo to power.
These can really be
connected in any order
(we’ll fix it in software
later), but I connected
them to the channels in
the same order they
were on the connector
to make any
troubleshooting easier.
I decided to mount
the flight controller on
top of the drone. A
model with side facing
connections for the servo
would easily fit in the
airframe, but I’d like to
be able to get to the
USB port and other
connectors quickly when
experimenting.
The flight controller
also has a couple of
status LEDs that tell us if
the system is armed; I
like visual confirmation of
40 SERVO 07.2016
Figure 11. There is plenty of
room inside the drone for all
of the equipment! Check that
everything is secure and
insulated before you attempt
to power-up the vehicle.
Figure 10. I drilled 1/2" holes to allow me to pass the ESC
wires and the battery lead to the top deck of the drone. This
also makes room for any future equipment to get power and
signal wires.
You may wonder why I wanted to use
a battery eliminator (i.e., power supply)
for test powering the equipment instead
of just connecting up the real battery. This
is because large RC batteries contain an
alarming amount of energy.
If you connect them to a system with
a short or with faulty equipment, they can
provide many amps, resulting in melted
wires, destroyed equipment, and possibly
even a dangerous fire. A good bench
power supply with a current limit setting
is always handy to have, but not totally
necessary if you check for shorts with a
meter and are very cautious during initial
testing.
To demonstrate how much power is
in a battery, let's take the total energy
stored in a 3,000 mAh lithium polymer
battery and make it relatable. If you get
lost in the math, don’t worry. Hang in
there.
First, we need to get battery capacity
into energy. The first step is determining
the average voltage during discharge. As
batteries discharge, their voltage does not
go down in a linear fashion (part of why
battery life predictions on products can
be off). That’s a whole other story, but
we’ll assume that our three-series cell
battery has an average voltage of 11.1V.
Multiplying 11.1V by 3 Ah, we get a
capacity of 33. 3 watt-hours.
Multiply this by 3,600 seconds/hour
to get to 119,880 watt-seconds. Reaching
way back to high school physics, we can
remember that a watt-second is a Joule
of energy. If you’re like me, 119,880
Joules doesn’t mean much.
Let’s convert that potential energy to
something we can visualize. We know
that potential energy of a mass is the
mass times the acceleration of gravity
times the height at which it sits.
Assuming an average person mass of 80
kg (~175 lbs), let's see how high 11,880
Joules could lift us in an ideal system:
80 kg 9.81 m/s^ 2 / 119880 J = 153 m
Wow! That is about 500 feet.
So remember, batteries hold a lot of
energy and should be treated with
immense amounts of respect.