Post by thread:Re: Cheap Linear potentiometer

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

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: {Quote hidden}> 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

At 03:16 AM 9/24/99 -0400, Jon wrote: >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. Make a dark mark on paper with a graphite pencil! Works great, but it's not too durable. Using pencil leads may also work for you, depending on your resistance needed. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: spam_OUTralTakeThisOuTlcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causae scire" ================================================================

In a message dated 9/24/99 2:30:40 AM US Mountain Standard Time, .....mrjonesKILLspam@spam@NORTELNETWORKS.COM writes: << 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. >> These ideas sound like a good option. The hall effect sensor sounds like the most rugged. How to implement becomes the question? and What is the output of the sensor? I have never worked with a hall effect sensor. I am also going to do some research on LVDT Thanks for the suggestions Jon

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 - kd4rdbKILLspamqsl.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 removed}

Systron Donner makes some wonderful, accurate, low force needed linear pots, with a (IIRC) 1/4" or 1/8" or so shaft - 4" travel or something. It's been years (These were used to measure ground movement under a snow covering, with teflon shafts through the snow) - I know they coped with LOW temperatures well, anyways - Give 'em a look. Omega should have something, I can stare at my set of books here if that'd help (I'll be needing something like this soon...) OTOH, I would not call those pots "cheap"! - Could use the "Fader" pots they use for audio boards, even tho audio taper, for this & just calibrate 'em? Optical encoders, grey coded, also <G> Mark