RADIO CONTROL AEROBATICS - 2003/03

WHEN JR FIRST introduced its digital servos, I was one of the
first in line to take advantage of the new technology. Digital servos
offer increased position-holding ability and precision that is
unmatched by an analog servo. At the time of their introduction I
was flying large aerobatic aircraft, and the need for a stronger, more
precise servo was evident. But when I installed the servos ganged
together to operate a single control surface, it became clear that
there was a downside to precision position-holding with a very low
tolerance.
All servos have manufacturing differences in centering position,
endpoints, and total overall travel. So when a servo that has low
positioning tolerance is coupled to another servo with low tolerance
but differing points of reference, you can see where there would be
a problem. Where analog servos have little problem being forced to
work together when not exactly in synchronization, digital servos
output full power to retain their positioning and only work together
when endpoints and centering are exactly matched.
When I first ran into this problem, I devised a measuring device
that could be used to match individual servos to find ones that could
be used together on a single surface. (See the September 2001 Scale
Aerobatics column.) Although measuring and matching works in
many cases, it is a limited solution at best. One answer would be
servos that were capable of being individually programmed through
your Radio Control transmitter, a computer, or a handheld device.
In fact, programmable servo technology is available and in use by
another manufacturer.
I spoke with JR Research and Development about why they
have chosen not to build programmable servos, and they were very
clear about the answer. JR had sampled and tested the IC chips that
would allow programmability and felt that the compromise in
quality, precision, and reliability would be too great given currently
available parts. R&D felt that they could not create a programmable
servo that had the specifications that JR feels are necessary for its
servo line. Instead JR has chosen to go another route.
JR’s solution to servo adjustability is an external device placed
in line from the receiver to the servo called the MatchBox. Initially
Mike Hurley, 11542 Decatur Ct., Westminster CO 80234; E-mail: [email protected]
RADIO CONTROL SCALE AEROBATICS
To test amperage draw, author assembled MatchBox system with
four high-draw digital servos on bench and measured amperage
with digital meter.
A close-up of the JR MatchBox reveals not only its tiny size but
the intuitive programming dial and buttons.
This MatchBox is installed in the author’s 40% Carden Edge 540
controlling four rudder servos.
130 MODEL AVIATION
I was skeptical. At first glance it seemed to be more equipment to
buy and more electronics thrown into the mix. But for people who
already have a significant investment in digital servos, this is a much
better solution—a cost-effective upgrade that improves the
manageability and flexibility of any JR servo.
The MatchBox works with any JR servo, analog or digital, so
you can use it to solve matching problems with nondigital servos as
well. I also like the fact that you are making a real time adjustment
in the airplane rather than trying to set a point on a test jig then
incorporating that into the aircraft after the adjustments have been
set and stored. It allows you to more easily and accurately adjust
for slight imperfections in your airplane’s construction. I know I
03sig5.QXD 12.20.02 10:50 am Page 130
can use all the help I can get!
The MatchBox is a surprisingly tiny
mechanism weighing just 0.32 ounce. It is
designed to operate from one receiver
channel and can control up to four servos
separately. The programming operation
turns out to be quick and easy once all of the
components are installed.
The MatchBox is operated using a dial to
indicate the servo that is being programmed.
The dial has eight positions so, for example,
1-4 control the four servos and 5-8 control
them while reversing their direction.
Increase and decrease buttons make the
actual adjustments. The radio’s stick
position dictates endpoint or centering
adjustment. To store the setting, simply
rotate the dial past 9 back to 0 before the
unit is powered off. Once the unit is
programmed and the dial is in the 0 position,
no further adjustments can be made;
therefore, vibration cannot affect the
settings.
Since the unit is capable of operating
four servos from one channel, it has a
provision for an external battery that will
isolate the power and supply power only to
those servos attached to the MatchBox.
Many pilots use the MatchBox utilizing only
the battery power from the receiver. To
utilize receiver power, the supplied jumper
must be installed in the auxiliary power
channel of the MatchBox.
I wanted to find out if there were any
issues with four servos being powered
through one receiver channel, so I decided
to conduct a few tests. According to JR, a
fully stalled DS8411 will draw 1.6 amps.
Four of them together will attempt to pull
6.4 amps through one receiver channel. The
device itself uses 12 mA.
Since I don’t have expensive, sensitive
testing equipment, I could not get peak
detected readings. My equipment was not
fast enough to get draw ratings at the
millisecond level, and that is where all the
big numbers come into play. I had to go
with a few very unscientific attempts with
an artificial load to see if I could find some
variations. I was only able to pull
approximately 800 mA out of a single servo
and roughly 3.3 amps from all four stalled
together. At idle the setup would draw
approximately 130 mA.
For the test I hooked up four servos to
the MatchBox with power going through the
receiver with the MatchBox jumpered. I
attempted to put a full stall load on all of
them at once while measuring the amperage
draw. The second test was nearly the same
except I powered the servos directly through
the MatchBox. The only difference was that
the idle draw dropped to 80 mA since I was
no longer powering the receiver.
I also tried the tests with Ni-Cd and
NiMH batteries. The tests yielded identical
results and never exceeded anything that the
receiver bus or the battery packs could
handle. I couldn’t find an instance where the
power supply to the system through a single
channel was even close to faltering, no
matter if the MatchBox itself was powered
or if it was powered through the receiver.
In my conversation with JR we discussed
the fact that the units are set up in aircraft
across the country powering multiple servos
without external power, and they have
performed as expected. It is still not a good
idea to exceed 10 servos on one receiver
through the use of MatchBoxes. When more
than 10 servos are needed, it is a good idea
to use the MatchBox’s external battery port,
an outside power supply such as an isolated
power bus, or split up the load between two
receivers.
I’m currently building a 40% Extra
300SX that will incorporate four
MatchBoxes. I was hoping to have a flight
report before this column was written, but
my building schedule has a way of slipping
away … You know what I mean. When it is
completed, the Extra will have four servos
on the rudder through one MatchBox. There
will be four servos on both elevators with
one MatchBox and two servos on each
aileron with one MatchBox mounted into
the root of each wing so that only one lead is
unplugged for disassembly and there is no
chance of wire mix-up.
There will be two receivers, each with its
own battery and switch incorporating the
rudder and one wing into one and the
elevators and the other wing into the other.
Since the load is divided so completely, I
will take frequent, careful readings of
battery draw to learn how this is working
out.
A setup with a better distribution of the
load is in Mike McConville’s TOC
(Tournament of Champions) airplane. It
uses six MatchBoxes, further dividing the
rudder and elevator servo arrays into two
servos per MatchBox. I’m not privy to
Mike’s exact setup, but with this level of
separation, each set of servos from the
rudder and elevators could be powered
through separate receivers, making the
battery load to each virtually identical and
lowering the load on a single channel.
JR really hit the mark with the MatchBox,
covering all of the bases not only for current
and future JR customers but, because of its
ability to work with any JR servo, for past
customers as well, giving them a means to
enhance and utilize equipment that they’ve
already purchased. The ease of use and
ability to make in-airplane changes coupled
to your transmitter without an external
device makes servo management more
intuitive and user-friendly. If you think it’s
time to move up to the precision of digital
servos but are afraid because of the
problems associated with managing them,
the MatchBox may be your answer. MA
March 2003 131
A partial internal view of Mike McConville’s TOC Extra 330S MatchBox setup. Each of Mike McConville’s airplanes for
the TOC incorporated six MatchBoxes to
control the digital servos.
03sig5.QXD 12.20.02 10:50 am Page 131