This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-304826 filed on Oct. 27, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a detector for detecting the rotational angle of a rotatable object, the detector using a magnetic sensor element.
2. Description of Related Art
An example of rotational detectors of this kind is disclosed in JP-A-5-157506. The essence of the disclosed detector is illustrated in FIGS. 7A and 7B of accompanying drawings. A Hall element 12 is placed in a parallel magnetic field generated by a cylindrical permanent magnet 11, as shown in FIG. 7A. The cylindrical magnet 11 connected to a rotatable object rotates together with the rotatable object, while the Hall element 12 stays in the magnetic field. An angle xcex8 made between an magnetosensitive surface of the Hall element 12 and the magnetic field direction changes according to rotation of the rotatable object, as shown in FIG. 7B. The Hall element 12 generates the following output voltage VH:
xe2x80x83VH=V0. sin xcex8,
where V0 is a maximum value of VH that is generated when the angle xcex8 is 90xc2x0 The output VH is fed to an outside microcomputer that calculates the rotational angle of the rotatable object based on VH.
The output VH, however, is a sinusoidal curve that is not perfectly proportional to the angle xcex8, though it is substantially proportional in a limited narrow range. In other words, the angle xcex8 is not correctly detected in a range beyond the limited narrow range. It may be possible to process the output VH in the computer to convert it into a correct rotational angle, but such conversion process makes the computer more complex.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved rotational angle detector that correctly detects the rotational angle in a wide range without making the detector complex.
The rotational angle detector is composed of a rotor, a magnetic sensor element and circuits for processing the sensor signal. The rotor is connected to a rotatable object such as a throttle valve, the rotational angle of which is to be detected. Magnets are mounted on the rotor to generate a magnetic field therein, and a magnetic sensor element such as a Hall element is disposed in the magnetic field. The magnetic sensor element generates an output in a form of a sine or cosine wave according to the rotational angle of the rotor. The sensor output is not exactly proportional to the rotational angle because it varies in a sinusoidal waveform. A linear converter provided in the circuits converts the sensor output to a signal which is proportional, or linear, to the rotational angle of the rotor. This conversion is preferably performed under arc-sine or arc-cosine transformation. The converted sensor signal represents a correct rotational angle in a wide angle range.
The magnetic sensor element may be rotated relative to the magnetic field while making the magnetic field stationary. The linear converter and the magnetic sensor element may be built in a single-chip-integrated circuit to simplify the detector structure, or they may be separately build in respective integrated circuits if such is convenient for structuring the detector.
Preferably, the analog sensor output is converted into a digital signal, and then the digital signal is converted into the signal linear to the rotational angle under the arc-sine or arc-cosine transformation. The rotational angle detector may include two output terminals, one for an analog signal and the other for a digital signal, so that the output is adaptable to both of analog and digital outside controllers.
The rotational angle detector according to the present invention is able to correctly detect the rotational angle over a wide range without making the detector structure complex.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.