Conventionally, a rotary actuator is employed as a driver device for a mechanism such as a shift-by-wire system. For example, Patent Document 1 discloses a rotary actuator including a motor accommodated in a housing formed of a resin material. More specifically, the motor includes a rotor spindle rotatably supported by a first bearing, which is equipped to a rear housing, and a second bearing, which is equipped to an output shaft. The output shaft is rotatably supported by a bush, which is equipped to a front housing. The first bearing is fitted to the rear housing such that the first bearing is movable in the axial direction. The second bearing is fitted to the output shaft such that the second bearing is movable in the axial direction. Therefore, the rotor spindle is movable in the axial direction by a gap between the first bearings and the rear housing in the axial direction and a gap between the second bearings and the output shaft in the axial direction.
A rotation angle detection unit is equipped between a rotor core and the rear housing for detecting the rotation angle of the rotor core relative to the rear housing. The rotation angle detection unit includes a magnet and a magnetic sensor. The magnet is in an annular shape and is fixed to the rotor core. The magnetic sensor is fixed to the rear housing to form a gap with the magnet in the axial direction. The magnetic sensor receives magnetic field caused by the magnet and sends a pulse signal in synchronization with rotation of the rotor core.
The gap formed between the magnet and the magnetic sensor varies among products, due to an accumulated error in dimensions (axial dimensions) of multiple components of the rotary actuator in the axial direction. The variation in the gap coincides with accumulation of dimensional tolerances in (1) the axial dimension of the magnetic sensor, (2) the axial dimension between a magnetic sensor mount surface of the rear housing and an opening end surface of the rear housing, (3) the axial dimension between an opening end surface of the front housing and a bush contact surface of the front housing, (4) the axial dimension of a collar portion of the bush, (5) the axial dimension between a bush contact surface of the output shaft and a second bearing contact surface of the output shaft, (6) the axial dimension of the second bearing, (7) the axial dimension of a second bearing contact surface of the rotor spindle and a rotor core contact surface of the rotor spindle, and (8) the axial dimension of the magnet.
The rear housing, the front housing, the bush, the output shaft, the second bearing, and the rotor spindle are designed, such that variation in the gap is reduced as much as possible, and such that the magnet does not make contact with the magnetic sensor when the rotor spindle moves most closely to the first bearing. The dimensions of these components are determined in consideration that the rotor spindle is movable by a predetermined length in the axial direction and that the gap between the magnet and the magnetic sensor varies among products.
[Patent Document 1] Publication of Japanese unexamined patent application 2009-177982
In the rotary actuator disclosed in Patent Document 1, the gap needs to be secured enough in order to prevent the magnet from making contact with the magnetic sensor when the rotor spindle moves most closely to the first bearing, in a case where the variation in the gap between the magnet and the magnetic sensor is the smallest. Therefore, in the configuration of Patent Document 1, magnetism applied from the magnet to the magnetic sensor may be insufficient.
Furthermore, in the configuration of Patent Document 1, the gap largely varies, since the rotor spindle and the magnet are movable by a large length in the axial direction. Therefore, the magnetism applied from the magnet to the magnetic sensor may be unstable.
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