>
>
> On a related matter, I will have to run the controlling lines for the sev=
os
> around 10 to 20 meters from the base controller so I believe I will need
> some sort of line enforcing buffer/booster device to avoid mis-shaping of
> waveforms and/or voltage drops that could lead to bad controlling of the
> servo.


I do not know your experience level - forgive me if I over simplify.

Standard servo has three wires, two for power and one for signal.
Servos were conceived in a world where it was "normal" to power
them from four AA batteries. So about 6 volts. Later various 1.2V
rechargeable cells became popular and 4.8v became the nominal
standard. Basically it comes down to "about 5 volts". Current can
vary widely for different servos and various loads, but with the
S3003 you mentioned, plan for an amp or so while under load.
Notice I did not say "while moving". Servos can draw a lot of power
to hold still if thy have to fight the load - something to address in
the mechanical side if you can.

The signal is a simple square wave - 0 to 5v. The data is encoded
in the ON time of the signal and there is a very loose spec for the
repeat rate. This varies a bit for brand and version, but about 1mS
on time is fully one direction and about 2mS on time is fully the
other direction, and yes 1.5mS is about center. Different manufacturers
made different decisions about which one is clockwise and exactly
where the ends are. This signal needs to be repeated about 20-50
times a second.

So... Lots of ways to drive this, but 20m of wire adds some wrinkles.
The one I would be most concerned about isn't so much "noise" on
the wire, it's voltage drop. With that much wire, 0v on one end is
not 0v on the other end. Not only does that reduce the available
power for the motor, but it also eats into the level available on the
signal line. You could probably find some success using a twisted
pair (ground and signal) and a second twisted pair (ground and power)

I'd seriously consider a point of load switching power supply. Something
along the lines of.
http://www.mpja.com/LM2596-Step-Down-Adjustable-15-37V-DC_DC-Converter/prod=
uctinfo/30148%20PS
Then you can run higher voltage (say 12-24v) over the 20m of wire and the
end
result is considerably less voltage drop.

The other non-obvious thing I would be concerned with is how to drive
the signal line. 20m is a long antenna and I can imagine various
events killing the transmit side of the system. Lots of ways around it
but at these data rates you should be ok with a single transistor to
pull the line low and pull up resistors on _both_ ends. Plan it for 10s of
mA when pulled low.

-Denny
--=20
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