Hi everybody!
Thanks for all the information in this fantastic forum.
I've a problem. I read a lot of post and i don't have solution.
I've to measure teh distance between 2 points.
It's a motorbike suspension like the diagram:
I want to put a sensor to calculate the movement of the suspension but if i use an acelerometer I've the "G" but i don´t have the distance of each movement.
Thanks for your support!
I would transfer the concept of a rotary encoder (quadrature) to this linear case. That way you have an optical reading and it's quite easy to make it weatherproof.
Great solution!
but i can't put anything in contact in the small cilinder.
I think in accelerometer but:
velocity = a * t (meters/sec)
distance = v * t (meters)
t= how can i calculate?
the distance is exactly?
other system:
an IR sensor to measure distance between the 2 big cilinders
??? 
but i can't put anything in contact in the small cilinder.
You can either paint on it or put the whole sensor within the tube.
velocity = a * t (meters/sec)
distance = v * t (meters)t= how can i calculate?
the distance is exactly?
t is the time between two measurements, you have to integrate the curve, so you have to take small time periods, measure the acceleration, subtract the gravitation and sum up the product of that with the square of the time gap. Because there are too many points where you have measurement inaccuracy you won't get an exact distance. With infrared you get an accuracy of a few centimeters, not acceptable for your case in my opinion.