Guest author explains RC electronic system - 201209

Hi, everyone. It is good to see that you have returned to the “RC Helicopters”
column again this month. Some of you have requested more technical
information within the heli pages here in MA. With that in mind, I want to
introduce my guest author, David Buxton.
David is an electrical engineer who has worked in that field since 1973. He is an
avid helicopter pilot with a clear understanding of the electrical components used
in our machines.
Many pilots have experienced various issues setting up their onboard electronics
in electric-powered helicopters. David agreed to share some of his experience to
help us better understand an RC electronic system.
Is My BEC Adequate?
My T-Rex 600 E dropped out of the sky like a shot goose. A couple
of weeks and several flights later, it happened again. So what could
be the problem? An important clue was that I had recently replaced
a failed tail servo. Also, low voltages were reported in the Mikado
VBar’s flybarless controller data log.
I previously performed numerous bench tests and in-flight data
logging which convinced me that the 3-amp battery eliminator
circuit (BEC) built into the electronic speed controller (ESC) had
plenty of excess capacity.
For example, I found that manually stressing the servos brought on
loads that were twice what was logged in flight. Did I need a higher
amp BEC? Was my VBar failing or was it something else? Was there
anything about the new tail servo that could explain
the problem?
Figure 1 shows the various components discussed in this column.
BECs
Castle Creations high-input voltage BECs are good for 10 and 20
Figure 1: A Battery Eliminator Circuit (BEC) is a component that collects power from the main
system battery and then adjusts the voltage to power the onboard receiver and servo group.
amps respectively. The small one in Figure
2 has a 220 micro-Farad (uF) capacitor at
its output.
First determine if the BEC can supply the
necessary average current consumed by the
BEC load (radio, servos, etc.). You can use
the Hangar 9 Digital Servo and Receiver
Current Meter.
This will give you a ballpark idea of
whether your BEC is adequate. These will
be average measurements—offering no
insight into the peak transient currents that
can crash your radio.
Voltage Measurements
These are important and easier to
perform than current measurements. Some
RC transmitters now feature telemetry,
including voltage readings and alarms.
Several in-flight logging devices are also
available and can download to a PC for
graphing after a flight.
The problem is that peak loads are higher
than what the instrumentation can reveal.
BEC voltage can briefly drop below 3 volts
and these instruments won’t see it happen.
Servo Dynamics
Figure 3 shows what happens when a
servo is moved to a new position. At first,
the servo motor acts similar to a short
circuit until it gets moving. There is also
plenty of inertia to get spinning and moving
(the motor, tall stack of gears, swash,
and rotor blades). Because of overshoot
recovery, the servo consumes more energy
coming to a stop, starting up the other
direction, and then stopping.
These pulses of energy explain why
vigorously stirring the sticks will create
stronger surges of load current than
manually forcing the servo arms. Start and
stop currents are higher than what it takes
for servos to hold position under load.
Analog Servos
The typical receiver will send its control
signals to each servo one at a time. Analog
servos will draw their pulse of current
accordingly, one at a time. If the servos
are drawing 1 amp pulses, then the peak
current drawn from the BEC will be 1
amp. If the receiver is programmed for
servo sync, then the associated servo
current will add up accordingly (see
Figure 5).
Digital Servos
The pulses of current drawn by digital
servos are shorter, at a higher rate, and are
not synchronized. Occasionally all four
servos (three swash servos and one tail
servo) will fire in unison (see Figure 4),
resulting in the occasional 4-amp pulse
(assuming one amp per servo).
Caution: not all digital servos behave
the same with regard to modulation,
such as pulse width, pulse rate, and
pulse amplitude (see Figure 4). Beware
that some high-speed digital servos draw
more than 1-amp pulses (such as 4- and
10-amp start/stop transients).
Capacitor Analogy
Move out to the country and drill a
well. To your great disappointment, it
only produces two gallons per minute.
Do you drill a deeper well or install a
storage tank?
With a tank installed, you can quickly
fill the bathtub despite the slow flow of
water from the well. A larger tank will
allow you to quickly fill the bathtub
after watering the lawn and garden. A
large capacitor for your helicopter can
accomplish much the same.
Plug-In Capacitors
The system solution for transient
high-current pulses is to plug a capacitor
into an unused receiver channel. Using
one with a value of 4700 uF at 10 volts
and an application of 1000 uF per servo
might be a good rule of thumb. Double
or triple that for aggressively high-speed
servos and servos for Giant Scale aircraft.
The ideal value is a function of a long list
of things.
For most heli pilots, a 4700 uF
capacitor stands a good chance of making
a 3-amp BEC safe to use. There are tests
to make sure, just in case.
How can we be sure that our radios’
battery supply systems are adequate
when we don’t have oscilloscopes and
other test equipment to monitor while
our helis are dashing in the sky? Some
products have been developed for that purpose.
Testing for Low-Voltage Glitches
I bought a VoltMagic (shown in Figure
6) and it was ready to do what I wanted.
It is designed to capture and report lowtransient
voltages and those below 3.8
volts by default.
I experimented with it on my T-Rex
600. The VoltMagic reported glitch
voltages on the bench when I vigorously
stirred the sticks. It also indicated a clean
bill of health when I plugged a 2000 uF
capacitor into the VBar and receiver.
After flying the helicopter, the VBar data
log was the cleanest I’ve seen.
Following in the footsteps of my
larger heli, I upgraded
the small T-Rex 450
to a VBar flybarless
system, which required
upgrading to digital
servos. There was no
glitching with the
analog servos, but there
was low-voltage glitching with the new
servos. The problem was solved with a
1000 uF capacitor.
VoltMagic is the best low-voltage glitch
detector for our application, but some
pilots find it difficult to decipher.
Other good devices include the
Fromeco DC-UP Mark II Voltage
Monitor and Capacitor System, which
uses a LED display to report the detected
voltage levels. It also has a huge supply of
capacitance onboard, adding up to 1.25
Farad (266 times larger than the 4700
uF Spektrum Voltage Protector). That is
huge!
The Fromeco device is also available
without the 1.25 Farad capacitor bank.
I don’t know how good it is at detecting
glitches.
Conclusions
Helicopter pilots with paddles and
analog servos should do fine with 3-amp
BECs. Upgrade to digital servos, and you
need to plug in a 4700 uF capacitor. If
you like to hammer the envelope using
the hottest digital servos, then get a
VoltMagic and learn how to use it. It
can help you decide if you need more
capacitance and/or a larger BEC.
Servo behavior is more of an issue
than heli size when it comes to sizing
the BEC and the plug-in capacitor.
Most measuring and logging devices for
voltage and current cannot adequately
ensure that your receiver and flybarless
systems are safe from low-voltage supply
transients.
A big capacitor will do more than a
higher-amp BEC to provide adequate
punch for your sizzling-fast servos. BEC
manufacturers have no clue whether the
customer flies an easy-going airplane or
a rocket-propelled helicopter, so don’t
expect to see a big output capacitor on
your BEC.
Retest your BEC system for potential
glitches, especially when changes have
been made. Learn how to perform the
necessary tests.