Sensors selection

[Akros]

Hi,

I need to measure the displacement of an arm,I need to know the exact position all the time.With a really big precission.

I've been thinking in:

-Rotatory encoder, which I could count revolutions and get the position.
-Magnectic sensor, in this case, im not sure because i need a range between 0 and 30 cm.
-Optical sensor,the same as magnetic sensor.

In this case, which would be the cheapest option? the more reliable?

Thanks!!

[H500]

What mechanism are you using? What resolution is "big precision"? 1cm? 1mm, 1um?

Optical encoders are pretty cheap and reliable. CUI has a 1000 ppr one for less than $50.

[Akros]

The precission is about 0.1 mm.
The mechanism is unknown for the moment, but it would be a DCmotor with a spindle or a Stepper motor.

[Al_The_Man]

.1mm would be easily achieved with a differential encoder, the error usually comes in the mechanical linkage, i.e. if the encoder is on the motor, then the only error is mainly if backlash is present.
Al.