Torque detecting device having magnetic shield for shielding magnetic noise

A magnetic shield made of magnetic material in the shape of a thin cylinder is externally fitted and fixed on a cylindrical mold member holding magnetic flux collecting rings so that it has extended sections with appropriate length on both sides in the axial direction, and therefore magnetic noise from outside in the radial direction is shielded directly by the outer surface of the magnetic shield, magnetic noise from both sides in the axial direction is shielded by concentrating the magnetic noise on the end faces of the magnetic shield, and the influence on the magnetic flux collecting rings held in the mold member is eliminated.

This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/JP2006/321503 which has an International filing date of Oct. 27, 2006 and designated the United States of America.

BACKGROUND

1. Technical Field

The present invention relates to a torque detecting device for detecting rotational torque applied to a rotary shaft, and more particularly relates to a torque detecting device that can be suitably used to detect steering torque in an electric power steering apparatus.

2. Description of Related Art

In an electric power steering apparatus which assists steering by driving a steering assist motor according to the rotation operation of a steering member, such as a steering wheel, and applying rotational force of the motor to a steering mechanism, it is necessary to detect the steering torque applied to the steering member for use in the drive control of the steering assist motor. For this detection, in the prior art, the electric power steering apparatus uses a torque detecting device which is mounted in the middle of a steering shaft connecting the steering member and the steering mechanism together.

In this torque detecting device, the steering shaft (rotary shaft), which is an object of detection, is divided into a first shaft located on the steering member side and a second shaft located on the steering mechanism side, the first and second shafts are connected coaxially with a small-diameter torsion bar as a torsion spring, and the steering torque (rotational torque) applied to the steering shaft by the rotation operation of the steering member is detected on the basis of a relative angular displacement caused between the first and second shafts with the torsion of the torsion bar.

For the means for detecting the relative angular displacement between the first and second shafts, various kinds of structures have been conventionally proposed. As one example, there is a torque detecting device comprising a cylindrical magnet which rotates together with one of the first and second shafts; a yoke ring which rotates together with the other; and detecting means which uses a change in a magnetic circuit formed between the cylindrical magnet and the yoke ring (see, for example, Japanese Patent Application Laid-Open No. 2004-125717).

In the yoke ring, a plurality of pole claws extending in the axial direction are arranged at equal intervals in the circumferential direction on one side of a ring-shaped yoke body, and a pair of ring-shaped yoke bodies with their pole claws being arranged alternately in the circumferential direction are fixed to the first shaft or the second shaft. The cylindrical magnet is a multi-polar magnet including the same number of pairs of magnetic poles as the pole claws of the yoke rings arranged in the circumferential direction, and positioned and fixed to the second shaft or the first shaft so that, when the cylindrical magnet is in a neutral state in which there is no relative angular displacement between the first and second shafts, the pole claws of the yoke rings are aligned on the boundaries between the N and S poles.

On the outside of the two yoke rings, magnetic flux collecting rings for collecting the magnetic flux generated in these yoke rings are placed closely to face the yoke bodies, respectively. These magnetic flux collecting rings have magnetic flux collecting sections extending in the axial direction from the magnetic flux collecting rings, at positions aligned in the circumferential direction, and a magnetic sensor composed of a magnetic detection element such as a Hall element is placed between these magnetic flux collecting sections opposing each other with a predetermined air gap therebetween. With a mold member formed in a cylindrical shape to cover the outside of the magnetic flux collecting rings, the magnetic flux collecting rings and magnetic sensor described above are integrated while maintaining their positional relationship, and fixed to a housing supporting the steering shaft through the mold member.

In this structure, when steering torque is applied to the steering shaft and a relative angular displacement is caused between the first and second shafts, the positional relationship in the circumferential direction between the pole claws of the two yoke rings and the magnetic poles of the cylindrical magnet changes in mutually opposite direction, and the leakage flux in the air gap between the magnetic flux collecting sections of the magnetic flux collecting rings is increased or decreased by the change of magnetic flux in the respective yoke rings corresponding to the positional change. By extracting a change in the output of the magnetic sensor corresponding to this increase or decrease, it is possible to detect the steering torque.

In the torque detecting device constructed as described above, when magnetic noise applied from outside are superimposed on the magnetic flux collected in the magnetic flux collecting rings, the output of the magnetic sensor suffers from the influence of the noise, and there is a possibility that the accuracy of detecting the steering torque is lowered. Thus, in the prior art, a magnetic shield for shielding magnetic noise is provided in the periphery of the magnetic flux collecting rings.

FIG. 1is a cross sectional view showing the structure of a conventional magnetic shield disclosed in Japanese Patent Application Laid-Open No. 2004-125717.FIG. 1shows a magnetic flux collecting ring assembly comprising a pair of magnetic flux collecting rings6and6and a magnetic sensor7which are integrated with a mold member61formed by molding a resin into a cylindrical shape. The magnetic flux collecting rings6and6are integrated so that they are separated from each other by a predetermined distance in the axial direction and exposed to the inner circumferential surface of the mold member61. The magnetic flux collecting rings6and6have, at corresponding positions in the circumferential direction, magnetic flux collecting sections60and60extending toward each other in the axial direction, and the magnetic sensor7is positioned between the opposing surfaces of the ends of the magnetic flux collecting sections60and60which are bent outward in the radial direction.

As shown inFIG. 1, the magnetic shield9is a thin plate of magnetic material composed of an outer circumferential plate9afor covering the outer circumferential surface of the mold member61and linked end-face plates9band9b, which are formed by bending both sides of the outer circumferential plate9ainward at substantially right angles for covering the end faces, is mounted to cover three surfaces (the outer circumferential surface and both end faces) except the inner circumferential surface of the mold member61in which the magnetic flux collecting rings6and6made of magnetic material are exposed, and performs the function of shielding magnetic noise coming from various directions as shown by the arrows inFIG. 1and eliminating the influence on the magnetic flux collecting rings6and6.

As disclosed in Japanese Patent Application Laid-Open No. 2004-125717, such a magnetic shield9is mounted by a procedure in which a belt-like plate having a U-shaped cross section consisting of the outer circumferential plate9aand end-face plates9band9bshown inFIG. 1is wound around the outside of the mold member61, cut at an appropriate position, and joined to the beginning end of the belt-like plate. During this mounting, however, a lot of power and work are required to bent and wind the belt-like plate having the U-shaped cross section and further work is required for joining after winding, and thus there is a problem that the product cost is increased by an increase in the number of assembly steps.

It is also possible to construct the outer circumferential plate9aand the end-face plates9band9bas separate rings and mount them by a procedure in which the outer circumferential plate9aand the end-face plates9band9bare positioned separately on the outside of the mold member61and then joined together. In this case, however, it is necessary to join the outer circumferential plate9aand the end-face plates9band9bover the entire circumference, and thus there is no effect of reducing the number of assembly steps.

SUMMARY

An object of the present invention is to provide a torque detecting device comprising a magnetic shield capable of being easily mounted while maintaining the magnetic noise shielding effect, and capable of reducing the number of assembly steps.

A torque detecting device according to a first aspect of the invention is a torque detecting device comprising: a cylindrical magnet rotating together with one of a first shaft and a second shaft which are coaxially connected to each other; a pair of yoke rings rotating together with the other one of the first shaft and second shaft within a magnetic field formed by the cylindrical magnet; a pair of magnetic flux collecting rings surrounding the outside of the yoke rings separately; and a magnetic sensor placed between opposing surfaces of magnetic flux collecting sections provided on the respective magnetic flux collecting rings, wherein the torque detecting device detects torque applied to the first shaft and second shaft, based on leakage magnetic flux between the magnetic flux collecting sections which is detected by the magnetic sensor, and is characterized by comprising a magnetic shield for covering the outside of a mold member holding the magnetic flux collecting rings integrally, the magnetic shield being made of magnetic material and having extended sections in an axial direction on both sides of the mold member.

A torque detecting device according to a second aspect of the invention is characterized in that the mold member and the magnetic shield of the first aspect are cylindrical members, and the magnetic shield is externally fitted and fixed on the mold member.

In the torque detecting device according to the first aspect of the invention, the outside of the mold member holding the magnetic flux collecting rings is covered with a simple-shaped magnetic shield having extended sections in the axial direction on both sides of the mold member. The magnetic shield is able to be mounted easily, shields directly magnetic noise from outside in the radial direction, and shields magnetic noise from both sides in the axial direction by concentrating the magnetic noise on the end faces of the extended sections on both sides. It is thus possible to eliminate the influence of magnetic noise on the magnetic flux collecting rings and achieve highly accurate detection of torque.

In the torque detecting device according to the second aspect of the invention, the magnetic shield in a cylindrical shape is externally fitted and fixed on the cylindrical mold member. Hence, it is possible to effectively shield magnetic noise by simply mounting the magnetic shield, and it is possible to achieve highly accurate detection of torque.

DETAILED DESCRIPTION

The following description will explain the present invention in detail with reference to the drawings illustrating a preferred embodiment thereof.

FIG. 2is an exploded perspective view of a torque detecting device of the present invention, andFIG. 3is a vertical cross sectional view showing an assembled state of the torque detecting device of the present invention.

The torque detecting device of the present invention aims to detect torque applied to two shafts (the first shaft1and second shaft2) connected coaxially to each other through a torsion bar3, and comprises a cylindrical magnet4which rotates together with the first shaft1and a pair of yoke rings5and5which rotate together with the second shaft2. The torque detecting device also comprises magnetic flux collecting rings6and6, which are placed closely to surround the outside of the yoke rings5and5separately, for collecting magnetic flux generated in the respective yoke rings5and5, and two magnetic sensors7and7placed between the magnetic flux collecting rings6and6in the manner described later.

The torsion bar3comprises large-diameter short connection sections30and30at both ends of a small-diameter round bar functioning as a torsion spring to connect the first and second shafts1and2. The connection of the first shaft1and the second shaft2with the torsion bar3is realized by inserting the connection sections30and30at both ends of the torsion bar3into connection holes10and20formed in the axial center of the respective shafts, performing positioning in the circumferential direction as to be described later, and then inserting separate connection pins11and21to integrate them. When rotational torque is applied to the first shaft1and second shaft2thus connected, the torsion bar3is torsionally deformed by the function of the rotational torque, and a relative angular displacement with a magnitude corresponding to the rotational torque is caused between the first shaft1and the second shaft2.

As shown inFIG. 2, the cylindrical magnet4which rotates together with the first shaft1is constructed as a multi-polar magnet in which a plurality of magnetic poles (a plurality of N poles40,40. . . and S poles41,41. . . ) are aligned in the circumferential direction and the end faces and inner surface are covered with a mold member42made of a resin having an appropriate thickness, and, as shown inFIG. 3, the cylindrical magnet4is coaxially fitted and fixed on the first shaft1through the mold member42.

As shown inFIG. 2, each of the yoke rings5and5which rotates together with the second shaft2is a ring made of magnetic material and comprising a plurality of pole claws51,51, . . . which extend in the axial direction and are arranged at equal intervals in the circumferential direction in the inner surface of a ring-shaped yoke body50. Each of the pole claws51,51. . . has a triangular shape narrowed toward the extending edge. The two yoke rings5and5are positioned by arranging protruding ends of their pole claws51,51. . . to oppose each other and arranging their pole claws51,51. . . alternately in the circumferential direction, and then the outside of these yoke rings5and5is covered with a mold member52made of a resin molded into a cylindrical shape to integrate these components.

The yoke rings5and5thus constructed are mounted by coaxially fitting and fixing them to the shaft end of the second shaft2through a boss section53formed by extending the mold member52to one side, so that their inner surfaces face the outer circumferential surface of the cylindrical magnet4fitted and fixed to the first shaft1, with a slight air gap therebetween as shown inFIG. 3.

FIGS. 4A to 4Care explanatory views showing the positional relationship between the pole claws of the yoke rings and the magnetic poles of the cylindrical magnet in the circumferential direction.FIG. 4Bshows the positional relationship at the time of mounting in which the yoke rings5and5and the cylindrical magnet4are positioned in the circumferential direction so that the pole claws51,51, . . . are aligned with the boundaries between the N poles40and S poles41arranged along the circumference of the cylindrical magnet4as shown inFIG. 4B. This positioning is realized by adjusting the circumferential positions of the cylindrical magnet4and yoke rings5and5as well as the two shafts1and2when connecting the first shaft1and second shaft2with the torsion bar3.

In such a mounted state, the pole claws51,51, . . . of the two yoke rings5and5are positioned under the same conditions in a magnetic field formed between adjacent N pole40and S pole41on the circumference of the cylindrical magnet4, and equal magnetic flux is produced in the yoke bodies50and50connecting the base sections of the pole claws51,51, . . . .

The positional relationships between such pole claws51,51. . . and the N poles40and the S poles41are displaced in opposite directions as shown inFIG. 4AorFIG. 4C, according to the relative angular displacement caused with the torsion of the torsion bar3between the first shaft1to which the cylindrical magnet4is fixed and the second shaft2to which the yoke rings5and5are fixed. When this displacement occurs, lines of magnetic forces having mutually opposite polarities increase in the pole claws51,51. . . of one yoke ring5and the pole claws51,51. . . of the other yoke ring5, and positive magnetic flux and negative magnetic flux are generated in the respective yoke bodies50and50. The positive and negative of the magnetic flux generated at this time depends on the direction of the relative angular displacement caused between the cylindrical magnet4and the yoke rings5and5, that is, between the first shaft1and the second shaft2, and the density of positive or negative magnetic flux depends on the magnitude of the relative angular displacement.

The magnetic flux collecting rings6and6for collecting magnetic flux generated in the yoke rings5and5as described above are rings made of magnetic material having an inner diameter slightly larger than the outer diameter of each yoke body50and placed closely to face the outside of the yoke bodies50and50of the respective yoke rings5and5. As shown inFIG. 2, the magnetic flux collecting rings6and6have magnetic flux collecting sections60and60, which extend from the magnetic flux collecting rings in the axial direction and have ends bent outward at substantially right angles, at two corresponding positions in the circumferential direction.

These magnetic flux collecting rings6and6are positioned coaxially so that the extended sides of the magnetic flux collecting sections60and60face each other and the ends of the respective magnetic flux collecting sections60and60oppose each other with a predetermined air gap therebetween in the axial direction, and then integrated by covering the outside thereof with a mold member61made of a resin molded into a cylindrical shape. The magnetic sensors7composed of magnetic detection elements, such as the Hall elements, are placed in the air gap between the ends of the magnetic flux collecting sections60and60of the magnetic flux collecting rings6and6thus positioned.

Note that the magnetic flux collecting rings6and6have the magnetic flux collecting sections60and60at two positions in the circumferential direction, and, as shown inFIG. 2, two magnetic sensors7and7are placed in the air gap between the ends of the magnetic flux collecting sections60and60at two positions. The reason why two magnetic sensors7and7are provided is to use one magnetic sensor7for torque detection and the other for failure determination. The failure determination is performed by a publicly known procedure in which, for example, the outputs of the two magnetic sensors7and7are compared in time course and, when there is a clear difference between the outputs, the magnetic sensor7which shows an unsteady output change before or after this time is determined to be in a failed state.

Further, a magnetic shield8which is a characteristic feature of the present invention is wound on the outside of the mold member61.FIG. 5is a cross sectional view showing the structure of the magnetic shield in the torque detecting device of the present invention, and illustrates a magnetic flux collecting ring assembly comprising a pair of magnetic flux collecting rings6and6integrated with the mold member61as described above, and the magnetic sensors7and7placed between the magnetic flux collecting sections60and60.

The magnetic shield8is a cylindrical member made of a thin plate of magnetic material and mounted to cover entirely the outside of the mold member61. The magnetic shield8has a length appropriately longer than the mold member61and is mounted so that, as shown inFIG. 5, there are extended sections with a predetermined length on both sides of the mold member61in the axial direction.

FIG. 6is an explanatory view showing the procedure of mounting the magnetic shield8. As shown inFIG. 6, it is possible to mount the magnetic shield8by shaping it into a cylinder in advance, positioning it coaxially on one side of the mold member61and fitting it over the mold member61as shown by the arrows inFIG. 6. In the circumferential surface of the magnetic shield8, a window hole80is formed at a substantially center in the axial direction. As shown by the alternately long- and short-dashed line inFIG. 6, the window hole80is positioned between the opposing surfaces of the magnetic flux collecting sections60and60by fitting the magnetic shield8over the mold member61, and used as an insertion hole for the magnetic sensor7which is to be placed between the opposing surfaces.

Mounting of such a magnetic shield8is easily realized by simple fitting. Moreover, the magnetic shield8as a simple cylindrical member can be easily formed by shaping a belt-like plate, or by using a tube material with an appropriate diameter. Thus, it is possible to easily mount the magnetic shield8without requiring a large number of steps.

Further, the magnetic shield8may also be mounted by procedures other than the above-mentioned procedure, for example, by a procedure in which the magnetic shield8is positioned together with the magnetic flux collecting rings6and6in a mold for molding the mold member61and integrated while maintaining a predetermined positional relationship with the magnetic flux collecting rings6and6. In this case, needless to say, it is also possible to easily mount the magnetic shield8without requiring a large number of steps.

The magnetic shield8mounted by the above-mentioned procedure performs the function of shielding magnetic noise from various directions and eliminating the influence on the magnetic flux collecting rings6and6in a later-described use condition. Here, magnetic noise from outside in the radius direction is directly shielded by the magnetic shield8covering the outer surface of the mold member61entirely like a conventional magnetic shield9shown inFIG. 1, but, unlike the conventional magnetic shield9, magnetic noise from both sides in the axial direction is shielded by concentrating the magnetic noise on the end faces of the magnetic shield8made of magnetic material as shown by the arrows inFIG. 5.

Such concentration is effectively realized because the end faces of the magnetic shield8are extended from both sides of the mold member61. In order to enhance the shield effect, the length X of the extended sections (seeFIG. 5) is preferably made as long as possible within the limit of the length of the magnetic flux collecting ring assembly in the axial direction.

The magnetic flux collecting ring assembly with the magnetic shield8thus mounted is fitted into a housing H shown in part inFIG. 3and fixed at a position so that the inner circumferential surfaces of the magnetic flux collecting rings6and6exposed to the inner surface of the mold member61closely face the outer circumferential surfaces of the yoke bodies50and50of the respective yoke rings5and5. Hence, the magnetic flux generated in the yoke bodies50and50placed close to the inner sides of the respective magnetic flux collecting rings6and6is guided to the magnetic flux collecting rings6and6, converged onto the respective magnetic flux collecting sections60and60, and leaks into the air gap secured between the magnetic flux collecting sections60and60, and then the magnetic sensors7and7placed in the air gap give outputs corresponding to the density of the leakage magnetic flux.

The magnetic flux density thus detected as the outputs of the magnetic sensors7and7changes depending on the magnetic flux generated in the yoke bodies50and50facing the magnetic flux collecting rings6and6. Since the generated magnetic flux corresponds to the relative angular displacement with respect to the cylindrical magnet4as described above, that is, the relative angular displacement between the first shaft1and second shaft2, the outputs of the magnetic sensors7and7correspond to the direction and magnitude of rotational torque which is applied to the first shaft1and second shaft2and causes the relative angular displacement, and thus it is possible to detect the rotational torque applied to the first shaft1and second shaft2, based on a change in the outputs of the magnetic sensors7and7.

The outputs of the magnetic sensors7and7may possibly contain errors due to the influence of magnetic noise generated in the periphery of the mount position. However, in the torque detecting device of the present invention, since the magnetic shield8surrounding the outside of the magnetic flux collecting rings6and6performs the function of shielding magnetic noise as described above, it is possible to reduce the influence of magnetic noise on the outputs of the magnetic sensors7and7, and it is possible to realize highly accurate detection of rotational torque based on the outputs.

In the torque detecting device of the present invention, the magnetic shield8for shielding magnetic noise in such a manner has a simple cylindrical shape and is easily mounted by the above-mentioned procedure, and thus it is possible to highly accurately detect the torque without increasing the product cost due to an increase in the number of assembly steps.

Therefore, the torque detecting device of the present invention is suitably used as a torque detecting device capable of achieving high detection accuracy in an environment including a lot of magnetic noise and capable of being constructed at low costs, such as a torque detecting device for detecting steering torque applied to a steering member for use in the drive control of a steering assist motor in an electric power steering apparatus.