1. Field of the Invention
The invention relates to a torque detection device that detects, for example, a steering torque of a steering wheel and an electric power steering system that includes the torque detection device.
2. Discussion of Background
A torque sensor described in US 2006/0137474 A is formed of a torsion bar, a ring-shaped magnet, a pair of annular magnetic yokes, a pair of magnetic flux concentration rings, a magnetic sensor, and the like. The torsion bar couples an input shaft to an output shaft such that the input shaft and the output shaft are coaxial with each other. The magnet is connected to an end portion of the input shaft. The magnetic yokes are connected to an end portion of the output shaft. The magnetic flux concentration rings are arranged in proximity to the outer peripheries of the magnetic yokes. Each of the magnetic yokes has lugs all around. The number of the lugs is equal to the number of N-poles and S-poles of the magnet. The magnetic flux concentration rings concentrate magnetic fluxes induced from the magnet by the magnetic yokes. The magnetic sensor is placed between the magnetic flux concentration rings, and detects the magnetic flux density generated due to the magnetic fluxes concentrated by the magnetic flux concentration rings.
When torque (steering torque) is input into a portion between the input shaft and the output shaft through a steering operation of a steering wheel, the torsion bar is twisted, and the relative position between the magnet and the magnetic yokes in the circumferential direction changes. The torque sensor detects the steering torque input into the portion between the input shaft and the output shaft on the basis of the magnetic flux density that changes in the magnetic yokes in accordance with the change in the relative position.
In the torque sensor described in US 2006/0137474 A, a portion of each magnetic yoke, at which each lug is provided, has an L-shape such that, when viewed from the circumferential direction of the magnetic yoke, the portion extends radially inward of the magnetic yoke, bends at a substantially right angle and then extends in the axial direction of the magnetic yoke (see FIG. 5 of US 2006/0137474 A). The distal end portions of the lugs of the respective magnetic yokes are arranged alternately in the circumferential direction. In the torque sensor, the lugs through which magnetic fluxes flow are desired to be elongated by extending the lugs in the axial direction in order to improve torque detection capability and torque detection accuracy. However, if the size of each lug in the axial direction (hereinafter, in the specification, referred to as “effective length” where appropriate) is elongated excessively, the distal end of the lug excessively approaches the opposed magnetic yoke. Thus, the magnetic yokes may interfere with each other portions close to each other. In addition, the magnetic flux is likely to unexpectedly leak from the magnetic yoke at that portion of each lug. Therefore, it is difficult for the magnetic sensor to highly accurately detect the magnetic flux density in the magnetic yokes (the density of magnetic fluxes concentrated by the magnetic flux concentration rings). That is, it is difficult for the torque sensor to highly accurately detect a torque.
In addition, it is always desired to reduce the size of the torque sensor.