Im looking for a sensor to measure linear position between 0 and 1000mm. The sensor is for a college project so has to be as cheap as possible. I wanted to but a draw wire potentiometer but they seem to costing around €300 and my budget is under 100. I was loooking at making a string pot using a 10 turn pot, im not sure of the electronics? would it be a difficult build?

or is there other options?

 

I'm sure someone will be by to tell you a pic could do exactly what you need.

And they are 100% correct.

Buut..if it where me, i would use your variable resistor as a voltage divider.

Then build one of the many voltage meter circuits out there to display distance in terms relative to voltage.

-luvv-

 

sully,

Since you posted this on another forum, and got many questions there, you may want to post all the asked for info here, as well as the photo. The more info you give, the easier it is to help you.

Ken

 

I would use a drum of about 10 cm circumfrence, note that the circum. lies part way into wire; use a measuring tapes spring return spring to rewind wire on drum. Attach a slotted disc, 10 slots, on one end of drum Use 2 optical slot detectors, 90 deg out of phase. I believe there is an explination in AACs Volume 4.
Or, can use drum & 10, or greator.turn pot, with a stronger return spring. Use an OP Amp, to adj. output V to cover 0 to 5, or as desired.
Vol IV- Ch 11- synchronous counters- last page

 

I would use a drum of about 10 cm circumfrence, note that the circum. lies part way into wire; use a measuring tapes spring return spring to rewind wire on drum. Attach a slotted disc, 10 slots, on one end of drum Use 2 optical slot detectors, 90 deg out of phase. I believe there is an explination in AACs Volume 4.
Or, can use drum & 10, or greator.turn pot, with a stronger return spring. Use an OP Amp, to adj. output V to cover 0 to 5, or as desired.
Vol IV- Ch 11- synchronous counters- last page

Actually, the OP only needs one opto-detector, since they are only measuring in one direction (that, from an earlier post). The "tape measure" spring is called a "constant force" spring in the industry.

Ken

 

It could be as simple as this.

 

How accurately does it need to measure the position?

 

" as simple as this " ; if it is a 10 turn pot, but believe a little under & over shoot would be prudent.

 

It's a ten turn pot. That's why it is labeled, "10 turns". The unlabeled pot is for overshoot compensation. If it needs to be compensated for undershoot (below zero volts), that's a bit complicated for a concept drawing with no accuracy requirements specified by the OP.

 

Don't you think you would need a level wind mechanism to keep the cable/wire/string on the surface of the drum (think bait casting reel)? Also, the fact that the cable is wound in a helix has to be considered when designing the drum, in addition to the diameter of the cable.
These things are expensive for a reason.

 

Ron, point well taken & considered. Wire, or cable. that I am looking at is the type used on drafting board sliding arm & is .o3 in, ss. Ten turns only takes .3 in, so with constant tension should lay down in one layer. If required, a simple level wind could be added; two gears. threaded rod [ screw], & traveling nut.

 

Our OP hasn't come back. I'm wondering if 1cm resolution would be sufficient.
I have always found that "simple" mechanisms like this are not so simple, unless you have access to a machine shop. Where do you get a ≈10cm circumference drum that will couple to the shaft on the pot? Where do you get the slotted disk? I suppose it could be printed on clear mylar. Attaching the spring might even be a challenge.

 

....Where do you get the slotted disk? I suppose it could be printed on clear Mylar. Attaching the spring might even be a challenge.

There are free programs out there to create and print any encoder pattern on Mylar with a laser printer.
You don't actually have to attach the constant force spring to the cylinder. If you maintain >1.5 wraps minimum it will hold tight.

(S)He's also posted over on the Electro-Tech Forum. Someone there suggested an accelerometer. Sounds like a good, inexpensive, high-tech solution.

Ken

 

Re Ron H: Where do you get a drum; If the max equipment is a drill press, then a 1.5 in hole saw makes a nice 1.25 in drum, which can be filed down or built up with thin metal, ie beer can. Looks like " Gym equipment project" & this one are kaput.