Patent ID: 8183857

Claim:
A rotation angle detection device for detecting a rotation angle of a rotating body to be measured that rotates with respect to a stator, comprising: magnetic detection elements which are fixed to the stator and detect magnetic flux density of a magnet which is rotated with said rotating body to be measured, said magnet having a ring shape, the North Pole and South Pole of said magnet being formed in the direction parallel to the rotation axis of said magnet, and the North Pole and South Pole being formed so that the flux density is periodically varied also along the rotating direction of said magnet; and magnetic bodies having two edge portions in positions where said magnet is inserted and arranged along an area where a magnetic line of force of said magnet is formed, wherein there are gaps between one edge portion of said magnetic bodies and one side of said magnet in the direction of the rotation center axis of said magnet, and between another edge portion of said magnetic bodies and another side of said magnet in the direction of the rotation center axis of said magnet, said magnetic detection elements are intermediated in the magnetic line of force which passes through said magnetic bodies, and the magnetic flux density detected by the magnetic detection elements is sinusoidal with respect to the rotation angle of the magnet, and the forming areas of the North Pole, one side of the magnet, and South Pole, opposite edge portion to one edge portion in a diameter direction, are gradually exchanged between the one edge portion and the another edge portion of the magnet in the rotation center axis in a thickness direction of the magnet, said magnetic detection elements comprising a pair of hall elements arranged with the arranging positions where output properties detected by the elements are shifted in phase by an angle of 90 degrees, the output property by the respective hall elements is a sinusoidal wave having a rotation angle in 360 degree cycle, the rotation angle is detected based on tan −1 (X/Y), where X and Y are respective outputs of the hall elements, and a part where the magnet is inserted between said magnetic bodies is only a fraction part of the magnet.