String Potentiometer

[Bmongar]

I have been thinking a little about the possibility of a precision elevator for next year. I know I am not the only one that thinks this may be a likely need for next years game.

One thing I have seen in the past is teams using a string potentiometer for a height sensor instead of motor or shaft encoders. It seems it would not be as susceptible to slack/slop in the system and probably a good idea.

So my question is what string pots have you used and what do you recommend or advise against?

[SLAB-Mr.Thomas]

Some years ago we tried a string. IIRC we used a door key string retractor for your belt. It was too flimsy.

Better is a nice tape measure works well. It might have been this; get a good spring. We printed a case for it to mount it and the potentiometer.

We often use somewhat more rotation than the cheapest pots that have less than 360 deg rotation. If you use, say, 1.5 turns of a 3-turn pot you can change the useful range with a resistor to get a little more precision. That is, change it from linear to non-linear and use the more sensitive part of the range and ignore the less sensitive part of the range.

We also have some Hall effect encoders that are almost indistinguishable from pots except they work smoothly, accurately and cost 10x more ($3, $30).

You can buy a string pot on Amazon; maybe about $100 (okay, now $150 and up). We’ve never been tempted. Few or none are actually pots but are active encoders with a string. Beware the voltages and protocols they use. Few are as easy as plugging into the analog input.

edit - in the spirit of use what you have I think the encoders you use on motors would work fine if you have them in the junk box. Any rotary encoder for shaft end or shaft through hole should work.

[dk5sm5luigi]

We used this String Pot 10 years ago and it worked great for the use case we had: Assembled String Potentiometer - AndyMark, Inc

We have talked about using it again but often find different ways to measure what we want.

[GeeTwo]

3946 used a tape measure in 2015. We disassembled it, glued a radio knob to the spool with the shaft engagement facing out, cut a hole in the side of the tape measure case and reassembled. Finally, mounted a 10 turn pot from (IIRC) Mouser. It worked for a while, but was a bit fragile.