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Modified HS-81 for 180 degree rotation by
twinturbostang
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For several
applications, including FPV flying, it is
advantageous to have a servo that can rotate
through 180 degrees. Typical servos only
rotate about 90 degrees though. One can use
linkage systems to double the "throw" of the
servo. But in cases where this is not
possible (direct drive for example), the
servo either needs to be
modified, or
a "doublers" device needs to be installed.
I've used one of these doublers, and while
it worked good, I couldn't get 180 degrees
(due to a mechanical limit of the servo). I
also wanted to reduce the clutter inside my
plane. So removing the doubler not only
cleared up some space and extra wire, but
also reduced the total weight.
So here is how you modify an
HS81 servo
for 180 degree operation. First to know, is
that the servo IS capable of mechanically
moving through 180 degrees, but there are
two modifications necessary to make this
possible. The first involves removing a
mechanical end stop, or limiting tab located
inside the upper housing. This is there to
protect the electronics inside the servo,
should it try to move more than the normal
amount. However, this usually limits the
travel to somewhere around 150 degrees.
Second involves soldering small resistors to
the potentiometer inside the case.
Start by removing any servo horn attached,
and then the 4 screws located at the bottom
of the case. Once these are removed, you can
pull the servo apart.
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NOTE: This mod can also be done with other
Hitec servos like a standard size HS-322
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Mini-How-To - Modifying an HS-81
servo for 180 degree rotation
photos: twinturbostang
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photo: twinturbostang
Take the upper cover, and turn it over. You should be
able to see a small tab that the output shaft will press against when turned
too far. This is the mechanical stop. Using a sharp X-acto blade, remove the
stop, being careful not to cut into anything else. In particular, if the
output hole is marred, this will interfere with the output shaft and affect
the performance of the servo.
photos: twinturbostang
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Once that's done, the
next modification requires a little bit of
soldering. Inside the case, remove the screw
holding the potentiometer (or pot) in place.
Push the pot out of the case, careful not to
damage any of the wires. You will see three
connections to the pot... red, yellow, and
green wires. We want to install two 2K Ohm
resistors, one on each outer leg of the pot,
in series with the wires. What this does is
trick the servo into thinking it needs to
rotate more, effectively doubling the
travel. Keep in mind, your mileage may vary.
2K Ohm resistors work for me. They may not
in all cases though, so some experimentation
may be required. I used 1/8 watt 1%
resistors. These have low drift, so there
hopefully will not be much change from
varying temperature swings. 1/4 watt can be
used, but 1/8 watt will be easier. There's
not much room in there to work with, so the
smaller the better. What I do is remove the
two outer wires, bend the tab over so it's
flat, and then solder the resistor and wire
on. This keeps everything tucked as far in
as possible. Remember, do one side at a
time, so you don't forget which wire goes
where!
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After you're done,
put the servo back together (don't
forget to lube the gears), and hook it
up on the bench for final adjustments.
The travel limits in the transmitter
need to be set. A computer radio is
highly recommended here. Start with
normal travel limits, and SLOWLY rotate
the servo 90 degrees each way. If it
looks like you're going to go past that,
STOP and reduce the travel. Continue
slowly until you have achieved exactly
90 degrees each way. If you feel ANY
resistance to rotation, STOP and
determine the cause. Remember, when the
pot is centered in it's travel, the
servo horn is actually offset to one
side due to the way the output shaft
spline is molded. This means that you
will have less clearance in one
direction than the other. This is very
important to remember.
Back to the end point adjustment... In
my particular testing with a Futaba 9C,
I ended up with end points of 99/95% and
a sub-trim (for proper centering) of
-25. Again, your mileage may vary, and
it can also depend on which spline
offset you go with for the control horn.
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Re-install the pot and screw into the case being
careful again not to damage anything. Before we put the servo back together,
it is recommended that you mark the output shaft relative to center position
of the pot. The pot has more than 180 degree of travel, but not by a whole
lot. It's VERY IMPORTANT that you not over-extend the range of the pot. In
testing, I did this by accident and wiped out a nylon gear. The output gear
locks into a tab on the output shaft on the pot. And this is very easy to
damage if you go too far. You can also damage the wiper inside the pot. In
the below pictures, I reinstalled the output gear on the pot to show you how
much travel you typically have. It does not give you too much leeway on
either side of 180. Especially since the servo horn does not sit square with
the servo when the pot is centered. So to make sure you know exactly where
the position of the pot is, mark the top of the output shaft.
One thing to look out for. Do NOT use the metal gear
version for 180 degree operation. I know it would be nice to have the
comfort of metal gears, but the reason for not using the metal gear
HS81 is clear in the pictures below. The gear
teeth do not continue all the ways around the gear. When the servo is
rotated 90 degrees to either side, the output gear is dangerously close to
binding or loosing contact with the other gear. The nylon gear however, has
teeth all the ways around, so there is no problem with this one.
For more info check out twinturbostangs
thread at RCGroups:
http://www.rcgroups.com/forums/showthread.php?t=629294
tekst
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