On every power up, you cause the door to move until the "it has reached the end" sensor is tripped, and then count from "there".... until the whole thing has to be repeated after the next power failure. (You also use the "reache end" indication to take care of "drifting" between power failures.)
hmm, the strip is a good idea, but cant be used, the port is moving longer than its own length, so it would move off the scale. And it folds too, so a big area isn't possible to use. Should then use two sensors, but that might be a mess to work with too.
I should try to see if I could count the rotations of the motor, to see if it would be possible to use a multi turn pot.
Since that is something that has to be done safely you'd want an optical incremental rotary encoder with a home position.http://www.arduino.cc/playground/Main/RotaryEncoders This gives a basic understanding of them. Industrial ones are available on ebay for around $25. I picked up one that does 1024 pulses per revolution and works at 5-24 volts which allows it to work with the Arduino or with 24volt industrial controllers. You want the line driver version.
Yes it is a door... Language confusion
The joint panels flex as they are moving which could be monitored. I think a martini is in order to get the creative juices flowing.
I would put a layer of insulating tape down the side of the aluminum channel (the one with the lead-screw inside). Then I would put a length of carbon conductive tape to form a 3-meter long "slide pot". That would give you an absolute reading on position and you can connect it directly to an Arduino analog input.
A distance sensor mounted at the bottom edge of the door pointing downwards would allow you to read the door position relative to ground at all times.