5448Re: Remote tuning
- Aug 1, 2008I received a good follow up question from one of the members.
"Do servos need a continuous pulse stream to maintain a position?"
No, but if you just turn off the transmitter, the momentary drop in
voltage distorts the pulses just enough to make the servos jump a
bit, often as much as 15*-20* off their original position. (I'm
using * for degree)
With a wheel type car/boat transmitter, the centering spring would
have to be removed from the steering wheel. With a "two stick"
transmitter, such as used with gliders, the centering spring would be
removed from one stick, and set so it will stick in position, such as
used for a throttle. This is easy to do.
If you have 12 v going out to the antenna, such as to operate an
active antenna, you could come off the 12 v with a 7805 (5v) or 7806
(6v) regulator chip (TO-220 type) with a capacitor accross in and out
legs to ground. I've done this with R/C gear run with higher voltage
battery packs used for electric boats and with an electric powered
glider. A receiver only draws about 50-80 ma, and servos draw about
50 ma at idle, and 100-200 ma moving, so this won't tax a 7805 at
all, which will put out 1 amp easily. And a tuning cap has almost no
resistance, so, moving the servo won't draw but about 80-100 ma.
I may be wrong on the resistors added... if the pot is 5k, use 2.2 k
resistors on each side. If 10 k, use 4.7 k on each side. The
feedback pot is simply a voltage divider.
The way a servo works, input pulse is stretched, and turned into a
voltage. Longer the pulse, the longer the voltage. The servo
circuit compares the pulse/voltage with the voltage on the feedback
pot via an op amp. If one voltage is higher than the other, the op
amp produces a + or - voltage, which drives the motor one way or the
other until once again the two voltages are equal. So, doubling the
resistance of that feedback pot will double the throw of the servo
from 90* to 180*, which is the rotation of the tuning cap. Putting a
resistor on each end of the feedback pot will keep it approximately
Stick movement to servo movement is very precise and repeatable,
otherwise it wouldn't make a good aircraft or whatever control.
Now, here's another possibility...
Instead of the whole receiver/servo in the antenna... just a servo
and supply it with 5v to operate. No radio, but control it with a
servo tester, which is just a little box with a knob and a battery.
You would probably have to use an optocoupler or op amp buffer due to
the long line.
So, no tx, no rx, just 5v for the servo tester, 5v for the servo.
See the leads? Black is common or -, red is 5v, white is signal (or
pulse). It would be easy to put an op amp buffer inline.
In a second email he commented:
"I asked the question as I was concerned in case the servo needed a
continuous pulse stream. As you would be sending LF square waves out
to control the servo I was thinking of any possible broadband
interference that might result."
That is valid a concern.
If the pulses are cut off cleanly, that is, just turn off the white
signal wire, the servo stays in the last position. If the
transmitter is turned off, as the voltage falls, the servos jump.
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