Patent ID: 7112956

Claim:
A method of generating an index signal for use in a direct-drive motor having a rotation axis and comprising a stator and a rotor, said stator comprising N stator cores which extend radially outwardly from the rotation axis and which are disposed in one another in first equal angular intervals, where N represents a first positive integer which is multiple of three and which is not less than six, said rotor comprising a ring-shaped permanent magnet faced to an outer periphery surface of said stator with a predetermined gap, said ring-shaped permanent magnet comprising M magnetized elements which are arranged in second equal angular intervals, where M represents a second positive integer which is not less than four, said M magnetized elements alternating equally between a north pole and a south pole, said method comprising the steps of: winding (N−3) driving coils for rotatably driving said rotor around said N stator cores except for particular three cores which are arranged in equal angular intervals of 120 degrees; using a particular at least one of said M magnetized elements as an index detection magnet, said index detection magnet having magnetic force which is different from those in remaining ones of said M magnetized elements; winding an index detection coil around a specific one of said particular three cores, said index detection coil being for detecting variations of the magnetic force generated by said ring-shaped permanent magnet to produce a detected signal; and generating the index signal on the basis of said detected signal.