Magnetic bar code reading system employing phase-shift type sensor

Abstract

A desired coded pattern such as a bar code is formed with two materials which are magnetically different, e.g., in permeability or conductivity. A sensor has primary coils which are excited by reference AC signals which are different in phase from each other, to produce an induced output signal. When at least one of the primary coils is moved relative to the pattern to approach the pattern, the induced signal produced in the primary coil changes in accordance with existence of a material in the pattern and the electrical phase of a composite output signal of the induced signals in the respective primary coils corresponds to the existence of the material in the pattern. By measuring the electrical phase of this induced output signal, therefore, the pattern corresponding to the existence of the material can be read on the basis of the measured data of the electrical phase. The pattern may be formed by partially changing permeability by subjecting a magnetic substance such as iron to hardening by means of laser beam in accordance with a desired pattern. By recording not only a first pattern formed in an arrangement corresponding to desired coded information but also a second pattern formed with opposite characteristics to the first pattern, reading accuracy can be improved. This second pattern can, in a separate embodiment, be formed on both sides of the first pattern.