LVDT's are VERY RUGGED... we use them in the paper industry to measure the position of a lip on the headbox where the paper stock comes out onto a moving screen. Ours are run with a single ended 24v supply and measure down to 0.0005 inches. An excellent choice, but not as cheap as a pot. Wes - kd4rdb@qsl.net http://www.qsl.net/kd4rdb Where am I? http://www.aprs.net:8000/kd4rdb-9 http://www.aprs.net:8000/kd4rdb-10 Stupidity should be painful -----Original Message----- From: Michael Rigby-Jones To: PICLIST@MITVMA.MIT.EDU Date: Friday, September 24, 1999 5:30 AM Subject: Re: Cheap Linear potentiometer >You could use a rotary pot with some kind of rack & pinion system. If you >can eliminate non-linearities (via e.g. table lookup), then you could use a >rortary pot with a lever on the shaft, the longer the lever, the less the >linearity, but also the smaller the change in resistance. > >The main thing to bear in mind is that you will need a high quality pot >rated for a lot of operations. Typical high quality pots for proffesional >audio mixers may be up to the job, but they are anything but cheap! > >Hmmm...thinking too much in terms of resistive sensors here. Ok, how about >a hall efect sensor that measures the field strength of a small permanent >magnet that is free to move? Or some kind of graduated translucent material >(maybe from a pair of graduated sunglasses?) and a LED/phototransistor combo >to measure posistion. > >Something from my college days is ringing a bell. The LVDT ot linear >variable differential transformer. Basically it uses 3 fixed coils and a >moveable magnetic core. One coil is energised by an AC signal. Moving the >core in and out of the coils produces a differential signal in the other two >coils. Obviously some signal conditioning will be needed, but these devices >are available commercially and pretty rugged. We built ours at college on a >simple plastic former and used a piece of ferrite rod as the core. > >Hope this gives you a few ideas. > >Mike Rigby-Jones > > > Jon Petty wrote: > >> Hi >> >> I need a cheap linear potentiometer for position feedback for a linear >> stepper motor. >> It needs to be small, stroke is 0 -.6 inches and output prefferably 0-5 v >> for >> a/d converter. Also needs to be suitable for automotive underhood >> temperatures. It also needs to slide easily to not use up motor torque. >> >> It doesn't have to be real accurate though. >> >> It could almost be a DIY system since I can calibrate the output. Maybe a >> resistive strip and a wiper, but I don't know where I can find any. >> >> Do you think one of those flex or stretch sensors could be modified for >> this? >> >> Any ideas or suggestions? >> >> You know a linear r/c servo would solve all these problems! Anyone make >> one? >> >> Thanks >> >> Jon > ________________________________________________________ NetZero - We believe in a FREE Internet. Shouldn't you? Get your FREE Internet Access and Email at http://www.netzero.net/download/index.html