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.