Rod axial position detector including a first scale having equidistant

Abstract

A first magnetic scale consisting of a plurality of non-magnetic parts are arranged axially at equidistant intervals, and a second magnetic scale consisting of a plurality of non-magnetic parts arranged axially at unequal intervals, are formed parallel to each other on a rod consisting of a magnetic material which can be displaced in the axial direction. The non-magnetic parts of the second scale are symmetrically disposed on both side of a predetermined position on the rod. First and second magnetic sensors are disposed in fixed positions facing the scales. A memory which individually memorizes the absolute position of each non-magnetic part on the second magnetic scale in advance, a mechanism which detects the interval between non-magnetic parts passing the second sensor, and a mechanism for distinguishing on which side the second sensor is situated with reference to the aforesaid predetermined position are provided, together with a mechanism for identifying a non-magnetic part which has passed based on the distinguishing result of the distinguishing mechanism and the detected interval, a mechanism for calculating the displacement amount of the rod from the identified magnetic part to its current position based on the output signal from the first sensor, and a mechanism for calculating the current position of the rod from the absolute position of the identified magnetic part and the calculated displacement. In this way, the displacement distance of the rod required to obtain absolute position information is reduced.