I have attached a diagram for your reference. The flexible arm will be floating in water. So I need some sort of position feedback sensor or algorithm to detect the position with reference to the fixed axis of the flexible arm. Any suggestions?

 

Maybe a rotary optical encoder? Absolute measurement type with grey code might be a good way to do it.

Steve

 

thank you. any other method or is there anyway that the position can be calculated manually?

 

You can use the arm to pull a cable, which is moves some indicator up and down a scale. You can also use a potentiometer, either linear or rotary for this. There are a lot of ways, there is no best method unless you clearly state your performance requirements.

Steve