E l3 B or CMC7 cheque code MR and conventional reading heads comparison

Magnetoresistive heads are very attractive for cheque reading. They are speed unsensitive and output signal is large compared to conventional coil heads. Many papers describe MR heads and some theoretical calculations predict output signal. In order to use MR head to read printed code on cheque with magnetic ink we need to know fringing field produced by printed characters with various geometries. We calculate the magnetic equilibrium when the character has been previously saturated. We use an iterative method based on field formulas and hysteresys loop characteristics. We found virtual magnetic charges localised on each side of the small magnetic bars. Complete field maps are given. In a second part we will discuss the magnetic behaviour concerning MR strip compared to conventional reading head. Variations of output signal according to different parameters (head to cheque distance, side definition of small bar, magnetic stain) are compared for MR and conventional heads. In conclusion MR strip exhibits many advantages compared to conventional gap head.