Patent ID: 7447580

Claim:
A method for measuring an absolute steering angle of a steering shaft for a vehicle using a first rotatable body and a second rotatable body that rotate together with the steering shaft of the vehicle at a predetermined rotation ratio, respectively, the method comprising: obtaining a measured angle Ψ M ′ by measuring a relative rotational angle Ψ′ of the first rotatable body with a first angle sensor whose measurement range is Ω, and obtaining a measured angle θ M ′ by measuring a relative rotational angle θ′ of the second rotatable body with a second angle sensor whose measurement range is Ω; calculating a plurality of relative rotational angles θ′s of the second rotational body that correspond to the Ψ M ′, by using a functional relation between the relative rotational angle Ψ′ of the first rotatable body and the relative rotational angle θ′ of the second rotatable body, and obtaining calculation of angles θc′s, the θc′s being calculated by using a functional relation defined by the relative rotational angles of the first and second rotatable bodies; obtaining a frequency i of the first rotatably body by comparing the plurality of θc′s to the θ M ′; obtaining an absolute rotational angle Ψ of the first rotatable body by using the i, and obtaining an absolute rotational angle Φ 1 of a steering shaft out of a relation between the Ψ and an absolute rotational angle θ of the second rotatable body; and obtaining an absolute steering angle Φa of a steering shaft that corresponds to the Ψ M ′ and the θ M ′, by using a functional relation between Ψ and θ, a difference between the absolute rotational angle of the first rotatable body Ψ and the absolute rotational angle of the second rotatably body θ, and the absolute steering angle of the steering shaft Φa, and if a difference between the Φ 1 and the Φa is greater than a predetermined value, adding/subtracting to/from the Φ 1 a value that is obtained by dividing the Ω by a rotation ratio of the first rotatably body.