Cheap and simple way to measure distance / absolute position with ~5mm accuracy?

[alm]

If you put an accurate clock on it, you could measure the difference in height through time dilation .

Depending on the mechanical construction, hacking cheap electronic calipers could be an easy way to get an accurate linear position sensor for cheap. Some, if not most of them, have a serial output you could monitor.

[Nusa]

I wonder if cheap electronic Vernier callipers could hacked to provide an output?

Easy, since most of them already have output pins under the cover. But they aren't so inexpensive if you need more than 6 or 12 inches!

[Kalvin]

Or use a rubber band / spring connected to a force sensing resistor or similar, giving analog output read by the Arduino

http://www.robotshop.com/en/force-sensors.html

Edit: The rubber bands may work as springs, but the results are not that reliable and repeatable as with springs: http://www.wired.com/wiredscience/2012/08/do-rubber-bands-act-like-springs/

[Ian.M]

I do believe if we continue giving suggestions sooner or later there will be a suggestion about an AI powered robot that would autonomously and without direct command push the button and move the table to the desired positions after analyzing OPs daily routines.

https://www.theregister.co.uk/2017/07/28/bofh_2017_episode_10/ 

More seriously, if there's a flat surface that moves past another part of it parallel to the surface,  a small rubber tired idler wheel carrying a small magnet, in sprung contact with the surface + a hall sensor could work.   Its far more dust resistant than an opto.   Add at least one fixed magnet mid travel with another hall sensor so you can reset the position as it goes past to eliminate any errors due to culminative slip.

[alexanderbrevig]

Maybe use time? Chances are you get within 5mm just by'holding' it for x ms for uip, and y for down

Edit: was on the phone, did not see page two and all my typos. I'll keep them for future embarrassment

[Kalvin]

Maybe use time? Chances are you get within 5mm just by'holding' it for x ms for uip, and y for down

Edit: was on the phone, did not see page two and all my typos. I'll keep them for future embarrassment

One mechanical limit switch for getting the home position and then use time would be an easy solution.

[alm]

Time would suffer from drift without a position sensor/top on at least one end. Unless you would periodically do a calibration that goes all the way to the min or max height and then back again. I.e. throw it over the wall and let the software people solve it .

[NivagSwerdna]

I have a height adjustable desk which can be moved up and down to switch between standing and sitting. I like to do that at least few times a day so that I'm not sitting the entire time...

This is definitely a first world problem....
I would use a hall-effect sensor on the moving part and two magnets which I would move up and down for the limits... the logic is simply move until you meet a magnet.  The only twist is the extreme limits... microswitch, another magnet or timeout/load limit for those?
The Cs133 clock suggested earlier is a good idea though as long as you don't bring heavy objects to work.

[andyturk]

Are there any good, cheap linear distance sensors from ~60cm to ~120cm with at least 5mm accuracy?

How about this one?

http://www.st.com/content/st_com/en/products/imaging-and-photonics-solutions/proximity-sensors/vl53l1.html

[_Wim_]

A diy string encoder (https://www.pc-control.co.uk/string_encoders.htm) seems ideal for this job. The sensor could be a 10-ruen pot. You could also look for a "draw wire encoder" (industrial device, same principle) on Ebay...

[David Hess]

I agree with _Wim_; wrap a thin cable around a pulley attached to a spring loaded ten turn potentiometer.

https://en.wikipedia.org/wiki/String_potentiometer

[ThomasDK]

Not clear if this is for work or home. If for work, depending on local regulations you will need a way to detect and stop it if something gets stuck as it moves down.