Position measure of cylinder piston rod

?There are a lot of methods of measuring position of cylinder piston's rod,but they all change the structure of cylinder,the technology of cylinder is complicated ,therefor the processing of cylinder is more difficult,it isn't easy to put it into effect.The position of cylinder piston's rod is measured by rotary encoder and magnetic steel wheel.By this method,the structure of original cylinder isn't changed.By experiment,the error of this measure method is lower than 1?mm.The old products can be transformed by this method,and the process of new product isn't complicated,therefore manpower and material resources are cut down.