Patent ID: 8089269

Claim:
An annular magnetic encoder comprising an annular magnetic body having magnetic N poles and S poles alternately arranged around a circumference of the annular magnetic body, the annular magnetic body comprising: at least one equal pitch annular section wherein N poles and S poles are alternately arranged circumferentially with equal pitch; at least one unequal pitch annular section of a size equal to the size of a plurality of said N or S poles and including one pole portion in the form of a single pole and other pole portions having a polarity opposite the polarity of said one pole portion, said other pole portions being disposed at the periphery of said one pole portion; and wherein the magnetization pattern defined by said one pole portion and said other pole portions are symmetrical relative to a virtual central traverse line central to said unequal pitch annular section, and said one pole portion has an area which is 40 to 60% of the area of said unequal pitch annular section; and wherein said one pole portion is shaped in the form of the letter H which is symmetrical relative to the virtual central traverse line.