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'Servo control signals to analogue voltage conversi'
1999\12\21@141356 by

Hi

If anyone has any documentation on servo control signals, would you be so
kind
as to mail it to me please.
(Assume I know nothing about servo control signals).

I want to read a servo control signal and convert it into an analogue
voltage (say
0 to 5V) - using perhaps pwm with a pic.

If anyone has done anything similar please e-mail me direct.

Regards,
Darren Logan BSc

Darren,
By servos I assume you mean hobby servos like those used in radio control
cars and airplanes.  To control these with a PIC (or read the signal) is
very simple.  The R/C receiver sends an on pulse to the servo that lasts
between 1 and 2 milliseconds depending on the position of the
corresponding stick on the transmitter.  It repeats these pulses every
20ms (the total period is 20ms, so if the on signal is 1ms the off signal
is 19ms and if the on pulse is 2ms the off period is 18ms).  So the
pulses are being transmitted at a frequency of 50Hz.  A 1ms on pulse will
rotate the servo all the way in one direction, a 2ms pulse all the way in
the other direction, and a 1.5ms pulse will center the servo. So a
centered servos control pulse will look like:

-----_______________-----________________

On for 1.5ms, off for 18.5ms on for 1.5ms, off for 18.5, etc.

Some servos can be driven beyond the 1 and 2ms limits described here, but
1 and 2ms are the normal limits.  I have also seen pulse frequencies as
high as 80Hz and as low as 5Hz.  With some servos the higher the pulse
frequency the more torque and holding power the servo provides.

To read the incoming pulses I would hook the line coming from the R/C
receiver to an input port on the PIC that can trigger an interrupt.
Set/reset a counter when the interrupt occurs, then watch for the next
change on that port.  I have use the PORTB change interrupts on a
16C(F)84 quite successfully for this measurement.

Someone else will have to help with converting to a voltage.

Hope this helps,

On Tue, 21 Dec 1999 14:12:15 EST Darren Logan <DAZLOGANAOL.COM> writes:
{Quote hidden}

Parker, CO, USA
Robotics, RC Airplanes, anything using a PIC

___________________________________________________________________
Why pay more to get Web access?
Try Juno for FREE -- then it's just \$9.95/month if you act NOW!
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Well, I put together some informations about servos (controlled by lpt
port on a pc running win 95, but still interesting if you know nothin

Phil

Darren Logan a Žcrit :
{Quote hidden}

--
---------------------------------------------------------------------
Belgium, Europe
Simple yet usefull  Robot stuff goto
For low-cost web design goto http://users.skynet.be/clairetnet

"Paul B. Webster VK2BZC" wrote:
>
>
> > Well, I put together some informations about servos (controlled by lpt
> > port on a pc running win 95, but still interesting if you know nothin
>
>
>   OK, firstly your webpage omits the ground connection to the printer
> port connector.  I think you should have shown that clearly as some
> people will otherwise complain of very peculiar behaviour.

You also talk on your web page of pulse lengths of 1.5 to 2 millisecs.
AFAIK 1.5 millisecs is servo centre with 1 millisecs minimum and 2
millisecs maximum position of the servo. Also don't use 1 millisec and 2
millisecs as absolutes, as it can damage some servo's. Rather keep it
between 1.2 and 1.8 millisecs to be safe.

Quentin

Servo signals vary widely among manufacturers.  I took an article on faith
that claimed 0.5 to 2.5 milliseconds was the right signal range.  Worked
great on one servo.  Bought another servo, and it was acting a little funny
around the end of travel.  the second time it hit the end stop it went
"POP!" and never worked again.  OOPS  it was a 1 to 2 ms servo.  You can
NEVER tell this info from the packages, you have to just call the mfr and

SILVER LINING:  So I took apart the servo, and removed the busted pot and
the stops.  Replaced it with an external 5K linear pot.  Now my servo drives
a little nonlinear mechanism (cams and stuff)  that is linearized because
the feedback is an external linear pot!  Much improved.  I have another
servo that I replaced the pot with fixed resistors and removed the stops.
Instead of position, the input signal determined speed and direction.  If
you hack a servo this way (they are EMINENTLY hackable!) you can go outside
the mfrs. signal specs quite a bit.

I fell in love with servos about a year ago, having ignored them all this
time.  Servos are great little packages of cheap mass produced technology!
They were born to marry the PIC.

{Original Message removed}
> You also talk on your web page of pulse lengths of 1.5 to 2 millisecs.
> AFAIK 1.5 millisecs is servo centre with 1 millisecs minimum and 2
> millisecs maximum position of the servo. Also don't use 1 millisec and
> 2 millisecs as absolutes, as it can damage some servo's. Rather keep
> it between 1.2 and 1.8 millisecs to be safe.
>
> Quentin
>
Have you measured the pulse width? 1.0-2.0 has been standard for
some time. A lot of newer servos will go a bit beyond that as some
new r/c TX's let you set travel over 100%. As long as you don't run
the output gear up against the stops you will be OK.

Mike DeMetz sect. Michiana R/C Choppers
mikedtechnologist.com
http://www.elkhart.net/~miked/

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