1. Technical Field
The present invention relates to a rotation sensor for detecting a relative rotation angle between two relatively rotating members or a rotation angle.
2. Background Art
As a rotation sensor for detecting a relative rotation angle between two relatively rotating members, a rotation sensor is known which, as shown in FIG. 12, comprises a stationary magnetic member 1 having an exciting coil, a magnetic rotor 2 having an indented outer circumference, and a metal rotor 3 with a plurality of metal teeth 3a placed between the stationary magnetic member 1 and the magnetic rotor 2 with predetermined spaces, and which is used, for example, for detecting torque acting on a steering shaft of an automobile.
Here, the exciting coil is electrically connected to an oscillation circuit and a signal detection circuit. When alternating current is supplied to the coil at a fixed frequency, an ac magnetic field is produced in a magnetic circuit formed around the coil. The sensor has the metal teeth 3a provided at equal spaces in the circumferential direction. As the rotors 2, 3 rotate relatively and the metal teeth 3a traverse the ac magnetic field, eddy currents are induced in the metal teeth 3a. The magnitude of the eddy currents varies depending on the relative rotation angle between the rotors 2 and 3. As the magnitude of the eddy currents varies, the impedance of the coil varies.
Thus, the sensor is designed to detect the variation in the impedance of the coil by the signal detection circuit, to thereby detect the relative rotation angle between the rotors 2, 3, namely, the two relatively rotating members.
The problem with the above-described conventional rotation sensor is that since the impedance of the coil varies due to disturbances such as variation in ambient temperature, electromagnetic noise, variation in the oscillation frequency of the oscillation circuit, power supply voltage, assembly error, etc., it is difficult to detect the relative rotation angle or the rotation angle accurately.
The present invention has been made in view of this problem. The object of the present invention is to provide a rotation sensor that shows little variation in detection accuracy even in the presence of disturbances and can detect the relative rotation angle or the rotation angle accurately.
In order to attain the above object, a first rotation sensor for detecting a relative rotation angle between first and second relatively rotating shafts according to the present invention comprises a first rotor having an insulating ferromagnetic layer and a first conductive layer, the first rotor being fixed to one of the first and second shafts at a predetermined position in an axial direction of the one of the first and second shafts; a stationary core having two exciting coils arranged in an axial direction of a rotation axis of the rotor with a predetermined space between, and a core body for holding the exciting coils, the stationary core being fixed to a stationary member; a second rotor having a second conductive layer, the second rotor being fixed to the other of the first and second shafts and arranged between the first rotor and the stationary core; and oscillation means for producing an oscillating signal of a particular frequency, the oscillation means being electrically connected to each of the exciting coils; wherein the rotation sensor further comprises variation detecting means for detecting a variation in impedance of each of the two exciting coils due to eddy currents induced in the first and second rotors; difference detecting means for detecting a difference in the detected amount of variation in impedance between the two exciting coils; and determining means for determining a relative rotation angle based on the detected difference; wherein the first conductive layer of the first rotor is provided on the insulating ferromagnetic layer, on at least one of the opposite sides of the first rotor as viewed in the axial direction of the rotation axis, and consists of a plurality of portions arranged in a circumferential direction of the first rotor with predetermined spaces between, the stationary core is so arranged that the core body holding the two exciting coils is symmetrical with respect to a plane normal to the rotation axis, and the second conductive layer of the second rotor is provided on an outer circumferential surface of the second rotor and consists of a plurality of portions arranged in a circumferential direction of the second rotor with spaces between, the spaces between the portions of the second conductive layer corresponding to the spaces between the portions of the first conductive layer.
In order to attain the above object, a second rotation sensor for detecting a rotation angle of a rotating shaft according to the present invention comprises a rotor having an insulating ferromagnetic layer and a first conductive layer provided to cover the insulating ferromagnetic layer in a range corresponding to a center angle of 180xc2x0, the rotor being fixed to the rotating shaft; a stationary core having two exciting coils arranged in an axial direction of a rotation axis of the rotor with a predetermined space between, a core body for holding the exciting coils, and a second conductive layer provided on at least one of the opposite sides of the stationary core as viewed in the axial direction of the rotation axis to cover at least one of the exciting coils and a corresponding portion of the core body in a range corresponding to a center angle of 180xc2x0, the stationary core being fixed to a stationary member in a manner that the core body holding the exciting cores is symmetrical with respect to a plane normal to the rotation axis; oscillation means for producing an oscillating signal of a particular frequency, the oscillation means being electrically connected to each of the exciting coils; variation detecting means for detecting a variation in impedance of each of the two exciting coils due to eddy currents induced in the rotor; difference detecting means for detecting a difference in the detected amount of variation in impedance between the two exciting coils; and determining means for determining a rotation angle based on the detected difference.
In order to attain the above object, a third rotation sensor according to the present invention has a structure such that the first rotation sensor and the second rotation sensor are combined together.