Control a servo by a small voltage signal

[madhippiescientist]

I have a magnet inducing up to 50mv in a coil, and
want to convert the signal to drive a servo. I've
looked up V/F conversion, VCO and the like, but
just don't know where to begin. I know I may need
to amplify the signal to begin with because most V/F
chips need more voltage than my coil supllies,
but most of those chips also specify high frequencies
(so I thought maybe if I could simply control a 555
output I would do better, re the servo's 1-2ms
pulse width for the control signal). Can anybody
point me in the correct direction for the circuitry I
need to build, or a means of simply controlling
the 555's output <I would set it at 1.5ms and have
my -50mv to +50mv signal alter the 555 between 1
and 2 ms> ???

 

[Nigel Goodwin]

For a start, use an opamp to amplify the sensor output to something more sensible.

 

[jpanhalt]

Then, if you want to use a 555, go to the NE555 datasheet (I checked ST's) and look at using pin 5 (control voltage) to control pulse width as a function of voltage. Pin 5 controls the level at which triggering occurs. That datasheet implies you need a continuous pulse on trigger, so you may need two 555, one to generate the 50 Hz pulse and one to control the pulse width. It may be doable with just one, but I have never tried to do that. You will also need to level shift your amplified voltage as pin 5 needs to be between 45% and 90% of the supply voltage (see: AN170 from Philips).

John

 

[madhippiescientist]

Some of my ideas so far...

I figured I would likely need to amplify
the signal, and I also "planned" on the 556
"Astable/Monostable" thing in order to get
a good square wave output.
Pin 5 (of the 555), the "Control V" pin,
only sets off the timer to cycle (if the V is
too low, the timer doesn't cycle). I need the
timer to continuously generate a pulse, that
varies with the detected V generated by the
coil/magnet (-50mV to +50mV). Then even
when the magnet isn't moving, the timer still
sends a (center) signal to the servo. I need
the servo to move as the magnet does. I'm
supposing also that since the induced signal
navigates - and +, that I would need to have
a circuit (which I found but haven't tried) to
create a "center + voltage" that goes up/down
depending on the coil/magnet generation (V
goes up/down exponentially as the coil's V
goes up/down in either the + or - range). I
just need, I'm guessing, to convert the 4-6V
output from that circuit, to a frequency in
the range I need (1.0ms to 2.0ms), and allow
me to adjust the frequency so I can center
the servo.

 

[madhippiescientist]

Also, I meant to mention...

I forgot, also...

Most of the V/F circuits I see need a
negative supply voltage as well as the
positive, which I do not have available.

I was also contemplating, with the
"center voltage circuit" I found, to maybe
somehow tie it into the RC part of the
Astable 555 and then the mV fluctuation
might affect the outputs enough to cause
it to navigate between 1.0 and 2.0ms.
Even if I can get it to fluctuate the signal
at a higher frequency, I could always
divide the output before the Monostable.

 

[madhippiescientist]

Well, I think I solved it, almost all of it.
I utilized the circuit and my noted alterations
as mentioned in user "Danielsmusic"'s "servo" post
from 7:00 "tomorrow", and simply wired my 49ohm
coil in series with my specified R4 replacement. It
seems to work, quite sensitively, but I think I
need to create a coil with higher resistance so I
get more fluctuation in the monostable's output,
creating more throw in the servo. I never thought
that a coil's resistance changes depending upon the
presence of magnetism, or that I could finitely
adjust such a circuit with it in order to actually do
something with it (the coil's resistance only changes
up to 10 ohms depending upon how quick the magnet
moves, not it's position...it's for a "Lehman" style
seismograph and use for the helicorder's stylus).

In case anyone's interested...