Need an advice on a distance sensor

[miceuz]

I was offered to make a solution for a theatre - they have three separate platforms which can rise up and down. All of them are driven by a single induction motor thru a reduction box, each platform has a magnetic clutch to couple it to a motor.

The vision is to put a separate induction motor for each platform driven by a variable frequency drive and provide a user interface to specify specific height for each platform and move them up and down when keeping relative height the same.

I need to measure absolute height of a platform within 5mm. I'm afraid I can't just count revolutions of a motor shaft because of freedom in a reduction box. What else? Ultrasonic pingers? Laser distance sensors? The range is 0.5 - 2m

[digsys]

For a 5mm resolution, what max height distance are you looking for ?? A system I've worked on with huge trucks and injection moulding
machines is linear transducers. Very easy to use and all lengths. You don't need one the full length, I've seen many clever methods of
doing "mechanical" ratio reduction. Those things can get you >1mm resolution and better, so you're good for 5:1 reduction +

[fcb]

Google "draw wire encoder". These are very simple to install and use, and plenty accurate for 5mm.

[miceuz]

Google "draw wire encoder". These are very simple to install and use, and plenty accurate for 5mm.

can somebody advice me on the good source of those?