Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ncar!gatech!purdue!decwrl!jumbo!dillon
From: dillon@jumbo.dec.com (John Dillon)
Newsgroups: sci.electronics
Subject: Re: A Challenging PWM Design Problem
Summary: Try NE544 servo amp chip
Message-ID: <13464@jumbo.dec.com>
Date: 6 Dec 88 18:02:50 GMT
References: <1663@maccs.McMaster.CA>
Distribution: rec.ham-radio, sci.electronics
Organization: DEC Systems Research Center, Palo Alto
Lines: 42

In article <1663@maccs.McMaster.CA>, darel@maccs.McMaster.CA (Darel
Mesher) writes (I edit the diagrams for brevity):
> 
> I have an interesting Pulse Width Modulation design problem;
>
>     <----------------- 18 mS ------------------->
>  -->.--.<--  1.0 to 1.9 mS                      .--.
> ____|  |________________________________________|  |_____
> 
>              .----------.
>    PWM in    |          |  Analog out
> ------------>|          |------------>
>              |          |
>              `----------'
> 
> where the analog output function looks like;
> 
>    Vcc  |            .
>         |         .  
>         |      . 
>         |   .
>         |.
>        -|-----------------,
>         1.0          1.9     Pulse Width (mSec)
> 
The simplest scheme uses a $30 servo from a radio control (RC) system.
Spare servos, available at many hobby shops, will accept the above
signal waveform (VCC from about 3.5 to 6 volts works OK), and can
accurately position a potentiometer.

If mechanical solutions disgust you, try using the NE544 chip found
in some of these RC servos.  The chip has a full-wave output designed
to drive a low-power (<500 mA) DC servo motor in conjuntion with a
positional feedback pot.  The complementary tri-state outputs pulse
every 18 mS with polarity and width to overcome position errors.

Take one of these outputs and run it through a low-pass filter into
the NE544's position feedback pin.  The feedback voltage level runs
from 1.0 to 2.0 V in response to the input PWM pulse width.

See the NE544 data sheet for specifics.  Unfortunately, the data sheet
does not illustrate this application.