1. Field of the Invention
The present invention relates to a rudder angle sensor detecting a rotation angle of a steering shaft mounted on a vehicle such as an automobile.
2. Description of the Related Art
As a rudder angle sensor which is known up to now (hereinafter, referred to as “conventional rudder angle sensor”), for example, there is the one disclosed in Patent Document 1. The conventional rudder angle sensor is constituted by including a first gear interlocked with a steering shaft to rotate and a second gear interlocked with the first gear and rotating in higher speed than the first gear. Generally, it is required for a rudder angle sensor including the conventional rudder angle sensor to detect a rotation angle of the steering shaft with high accuracy. Therefore, usually a third gear engaging with the first gear is provided in the conventional rudder angle sensor as means to accomplish the requirement.
[Patent document 1] Japanese Patent Application Laid-open No. 2004-198287 (refer to Paragraph Number 0016, FIG. 3)
However, it is well known that a backlash (play) is necessary between gears to rotate paired gears smoothly and easily. Here, a case is considered when a steering shaft is, for example, rotated in a counterclockwise direction under a state that it is rotated for a predetermined angle in a clockwise direction, namely the case when it is reversed. As a more concrete example, the above-stated angle when it is reversed is assumed to be a position of three o'clock of a clock. That is, a state is assumed, in which the steering shaft is rotated in the clockwise direction, then the rotation is stopped when it reaches the position of three o'clock passing through the positions of one o'clock and two o'clock, and it is rotated in one o'clock direction which is a reverse direction from the position of three o'clock where it is stopped. Teeth of the first gear (interlocked with a steering shaft) and teeth of the third gear are in contact state while it is rotated from passing over the position of one o'clock to reaching the position of three o'clock, and therefore, the backlash between both gears does not become a problem. This is because the third gear conforms to the rotation of the first gear to be interlocked to rotate. Meantime, the third gear when it is reversed to head in one o'clock direction does not start to conform to the rotation of the first gear immediately. The third gear conforms to the first gear behind for the extent of the above-stated backlash. Namely, the steering shaft is reversed from the position of three o'clock to one o'clock direction, and then, the first gear is reversed with conforming to the steering shaft. However, the third gear engaging with the first gear does not start to rotate until the steering shaft comes to, for example, the position of two o'clock if figuratively speaking. This is a rotational delay caused by the backlash, namely, a rotational displacement or discrepancy. Such naturally arising rotational discrepancy prevents a high accurate detection of the rotation angle of the steering shaft. For example, there are a doubling structure in which two gears are overlapped and one gear is displaced in a rotational direction to adjust, a doubling structure in which a gear with a twist such as a helical gear is divided into two pieces, and one piece is displaced in the rotational direction to adjust, and so on, as the known methods to reduce the backlash. However, a requirement for the rudder angle sensor to be lighter and more compact is extremely strong as a steering structure disposed inside of a limited space of a vehicle complicates. Consequently, it is generally impossible to make the gear for detecting the rudder angle doubling structure and so on to prevent the backlash, because it leads to a large sizing of the rudder angle sensor. A problem to be solved by the present invention is to provide a rudder angle sensor maintaining a small size as much as possible and having high accuracy. Namely, it is to provide the rudder angle sensor with high accuracy while avoiding leading to the large size, by eliminating a detection error caused by the backlash existing between the gears in the rudder angle sensor including the paired gears.