Page 134 - Build Your Own Quadcopter_ Power Up Your Designs with the Parallax Elev-8

Chapter 5: Quadcopter Propulsors 113

quadcopter. The basic Elev-8 weighs approximately 1400 grams (including the battery),
which means it will not even lift off unless the propellers are spinning at least 4800 r/min.
That translates to about a 15-min operating cycle, which nicely matches my actual operating
experience. If you add several hundred grams of payload, you should not expect any more
than 10 to 12 min of flight time, assuming you do not operate at maximum power.
Now I want to discuss the timing values of the ESC, as shown in Table 5.3. The T1
column shows the control-pulse width for the various power settings. The pulse ranges from
1.0 ms for zero power to 2.0 ms for 100% power. It is linear and proportional, which means
that a 1.5-ms pulse width would represent 50% power. It is useful to know this relationship,
especially if you decide to create your own flight-control board. The pulse period goes from
21.02 to 22.00 ms because of the way the demo program creates the pulse train. This means
that the frequency ranges from 45 Hz (22 ms) to approximately 47.6 Hz (21.02 ms). Although
it does not matter at all for this demo program, it might affect flight-control responsiveness
if this program code were to be incorporated into a quadcopter flight-control program. The
whole matter of ESC pulse-train frequency is a bit controversial, as there are a number of
quadcopter designers who insist that ESCs should have a 400-Hz operating frequency versus
the standard 50 Hz. Most everyone in the R/C field agrees that the 50 Hz is quite adequate
for the typical, fixed-wing aircraft. However, some insist that it is not nearly responsive
enough to match the desirable quadcopter flight-response characteristics. I am still unsure
about this claim, although I am experimenting with some high-speed ESCs that have been
programmed with the so-called SimonK firmware. The SimonK firmware is named after
Simon Kirby, who created a high-speed software that can be flash-programmed into most
ATmega8L-controlled ESCs. Reprogramming flash memory in an ESC is done by using the
six ISP pins, which are shown at the bottom left of the ESC PCBs in Figure 5.11. I would not
recommend attempting to reprogram your ESCs unless you have successfully done it before.
Otherwise, you will likely brick them, that is, make them inoperative unless they are restored
to their original firmware. You can refer to the following website to learn more about high
speed ESCs: wiki.openpilot.org/display/Doc/RapidESCs.

Battery Eliminator Circuit

The battery eliminator circuit (BEC) refers to the three-wire lead set extending from the ESC.
Typically the wires are color coded as follows:

1. white = signal
2. red = power
3. black = ground

Other colors are also used, including brown, red, and orange, where brown is ground,
red is power, and orange is signal. I have previously referred to the BEC cable as a servo
connection, which is partly true. In a normal servo cable, white is still signal, black is still
ground, but red is a power consumer not a power supplier, as is the case with the BEC cable.
This is why it is called a battery eliminator, as the BEC normally plugs into an R/C receiver
and supplies the power to that receiver. This eliminates the need for a separate receiver
power supply, hence the name BEC. I have extracted a portion of the detailed ESC schematic,
which is posted online at www.mhprofessional.com/quadcopter, as Figure 5.26.
You should be able to identify two 7805 regulator chips whose outputs are connected
in parallel. This combined output is connected to the BEC red wire. The 7805s are linear
regulators that have been manufactured for many years by many companies. Each one can