Rod axial position detector including plural scales wherein

Abstract

A magnetic scale is formed by a plurality of non-magnetic parts arranged at equidistant intervals in all axial direction on a magnetic rod which displaces in the axial direction. The non-magnetic parts consist of first non-magnetic parts having a constant depth, and second non-magnetic parts having a different depth. A magnetic sensor which outputs a signal according to the depth and positional change of the non-magnetic parts is installed at a fixed position opposite the magnetic scale. Further provided are a mechanism for identifying a second non-magnetic part which has passed the magnetic sensor based on the output of the sensor, a memory for individually storing the absolute positions of the second non-magnetic parts, and a mechanism for calculating the displacement of the rod from the identified second non-magnetic part to its current position based oil the sensor output. The absolute position of the rod is detected, in terms of a small displacement from its stationary position, by computing it from the absolute position of the identified second non-magnetic part stored in the memory and the displacement of the rod from this second non-magnetic part. This assembly comprises only a single magnetic scale having several types of non-magnetic parts, and is therefore more economical to manufacture than the conventional assembly having a plurality of magnetic scales.