Motor rotation angle detection device

A motor rotation angle detection device includes a resolver which detects a rotation angle of a motor generating an assist torque for steering road wheels. The resolver includes an annular rotor disposed inside an annular stator with an air gap interposed in between an outer circumferential surface of the rotor and an inner circumferential surface of the stator, so that the stator and the rotor are rotatable relative to each other. A rotor supporting member is connected to a rotary shaft of the motor, and the rotor extends out from an axial end portion of the rotor supporting member so as to surround an outer periphery of the rotary shaft. With the described motor rotation angle detection device, it is possible to stabilize the magnetic permeability of the rotor, and accordingly to increase an accuracy with which the rotation angle of the motor is detected.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority under 35 USC §119 based on Japanese patent application No. 2007-277179 filed 25 Oct. 2007. The subject matter of this priority document is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a motor rotation angle detection device, for detecting, using a resolver, a rotation angle of a motor generating an assist torque for steering road wheels, wherein the resolver is configured by arranging an outer circumferential surface of an annular rotor inside an inner circumferential surface of an annular stator with an air gap interposed in between, so that the stator and the rotor are rotatable relative to each other.

2. Description of the Related Art

Japanese Patent Application Laid-open No. 2005-247079 has made publicly-known an electric power steering system for assisting the steering operation of a driver by use of a torque generated by an assist motor which is operated depending on a steering torque which is inputted into a steering wheel by the driver and then is detected by a steering torque sensor.

In a case where a DC brushless motor is employed as the assist motor for the electric power steering system, it is necessary to detect a rotation angle (phase) of a rotary shaft of the assist motor for the purpose of controlling the application of currents to the U-phase, V-phase and W-phase coils of the stator of the motor. A variable reluctance resolver, generally used to detect the rotation angle, includes a stator and a rotor which are opposed to each other. The stator is fixed to a housing of the assist motor, and the rotor is provided on a rotary shaft of the assist motor. The variable reluctance resolver is designed to detect a rotation angle of the rotary shaft based on change in the air gap between the outer circumferential surface of the rotor and the inner circumferential surface of the stator. In this respect, concaves and convexes are formed on the outer circumferential surface of the rotor, and the stator is formed of multiple coils.

However, problems arise in using a resolver such as that disclosed in JP 2005-247079. For example, when the rotor is press-fitted and fixed to the rotary shaft of the assist motor, magnetic permeability becomes lower in some parts of the rotor due to residual stress caused by the press-fit. This lower magnetic permeability leads to reduction in the accuracy of the resolver in detecting the rotation angle. Moreover, when a distortion torque is applied to the rotary shaft of the assist motor, a stress is generated by the distortion torque, thus decreasing the magnetic permeability. This decrease in the magnetic permeability reduces the accuracy of the resolver in detecting the rotation angle.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances, and it is an object of the present invention to prevent the rotor of the resolver from being distorted, and thus to detect the rotation angle of the motor with high accuracy.

In order to achieve the object, according to a first feature and aspect of the present invention, there is provided a motor rotation angle detection device, for detecting, by a resolver, a rotation angle of a motor generating an assist torque for steering road wheels, the resolver being configured by arranging an outer circumferential surface of an annular rotor inside an inner circumferential surface of an annular stator with an air gap being interposed in between, so that the stator and the rotor are rotatable relative to each other, wherein a rotor supporting member is connected to a rotary shaft of the motor, and the rotor is extended out from an axial end portion of the rotor supporting member so as to surround an outer periphery of the rotary shaft.

With the configuration according to the first feature and aspect of the present invention, when the rotation angle of the motor for generating the assist torque for steering the wheels is detected by use of the resolver configured by arranging the outer circumferential surface of the annular rotor inside the inner circumferential surface of the annular stator with the air gap interposed in between so that the rotor and the stator are rotatable relative to each other, it is possible to stabilize the magnetic permeability of the rotor, and accordingly to increase the accuracy with which the resolver detects the rotation angle of the motor. This happens because the rotor supporting member is connected to the rotary shaft of the motor, and because the rotor of the resolver is extended out from the axial end portion of the rotor supporting member so as to surround the outer periphery of the rotary shaft of the motor. Thereby, neither the load which occurs due to press-fitting when the rotor supporting member is connected to the rotary shaft nor the load which occurs due to torsional load acting on the rotary shaft as a result of a torque of the motor is transmitted to the rotor.

According to a second feature and aspect of the present invention, in addition to the first feature and aspect, the rotor supporting member is a coupling for connecting the rotary shaft of the motor to an input shaft of a decelerator or reduction gear.

With the configuration according to the second feature and aspect of the present invention, the rotor supporting member is configured of the coupling for connecting the rotary shaft of the motor to the input shaft of the decelerator. For this reason, the coupling can be also used as the rotor supporting member, which leads to a reduced number of parts.

According to a third feature and aspect of the present invention, in addition to the second feature and aspect, the coupling is an Oldham coupling.

With the configuration according to the third feature and aspect of the present invention, the rotary shaft of the motor is connected to the input shaft of the decelerator by use of the Oldham coupling. For this reason, an imbalanced load is prevented from being applied to the rotary shaft by causing the Oldham coupling to absorb any misalignment between the axis of the rotary shaft of the motor and the axis of the input shaft of the decelerator. Thereby, distortion of the rotor of the resolver can be further reduced to further increase the detection accuracy.

According to a fourth feature and aspect of the present invention, in addition to the second feature and aspect, the rotary shaft of the motor is connected to the coupling by press-fit.

With the configuration according to the fourth feature and aspect of the present invention, the rotary shaft of the motor is connected to the coupling by press-fit. This makes it easy to connect the rotary shaft and the coupling together. Although, on the other hand, the coupling is elastically deformed in a way that the diameter of the coupling is enlarged, the distortion of the resolver rotor can be reduced because the rotor extends from an axial end portion of the coupling so as to surround the outer periphery of the rotary shaft. Thereby, the detection accuracy can be increased.

According to a fifth feature and aspect of the present invention, in addition to the fourth feature and aspect, a diameter of an inner circumferential surface of the rotor is larger in its radial direction than a diameter of an inner circumferential surface of the coupling.

With the configuration according to the fifth feature and aspect of the present invention, although press-fit load of the rotary shaft of the motor affects the inner circumferential surface of the coupling most strongly, the configuration makes the rotor less susceptible to the influence of the press-fit load, and accordingly makes it possible for the rotation angle of the rotary shaft to be detected with accuracy. This happens because a diameter of the inner circumferential surface of the rotor is larger in its radial direction than a diameter of the inner circumferential surface of the coupling.

The above description, other objects, characteristics and advantages of the present invention will be clear from detailed descriptions which will be provided for the preferred embodiments referring to the attached drawings.

DESCRIPTION OF THE PRESENT EMBODIMENTS

A first exemplary embodiment of the present invention will be described based onFIGS. 1 to 6.

As shown inFIG. 1, an upper steering shaft12, designed to rotate together with a steering wheel11, is connected to a pinion shaft17protruding from a decelerator or reduction gear16via an upper universal joint13, a lower steering shaft14and a lower universal joint15. Tie rods19,19protruding from left and right ends of a steering gear box18connected to the bottom tip of the decelerator or reduction gear16are connected to unillustrated knuckles of the left and right road wheels WL, WR. An assist motor M, configured of a DC brushless motor, is supported by the decelerator or reduction gear16. The operation of this assist motor M is controlled by an electronic control unit U into which a signal is inputted from a steering torque detecting device St housed in the decelerator or reduction gear16.

As shown inFIG. 2, the decelerator16includes: a lower housing21integrated with the steering gear box18; an intermediate housing23connected to the top surface of the lower housing21with bolts22; and an upper housing25connected to the top surface of the intermediate housing23with bolts24. The pinion shaft17is rotatably supported by the lower housing21with a ball bearing26being interposed in between, and also is rotatably supported by the upper housing25with a ball bearing27being interposed in between. A pinion28, provided at the bottom tip of the pinion shaft17, meshes with a rack30provided on a rack bar29which travels leftward and rightward inside the steering gear box18. A substantially cylindrical rack guide31is slidably contained in a guide hole21a, which has a circular cross-section. The guide hole21ais formed in the lower housing21, and constitutes a part of the structure of the steering gear box18. A pressing surface of the rack guide31is biased toward a rear of the rack bar29by use of a spring33arranged between a concave part formed in the rack guide31and a concave part formed in a nut member32for closing the guide hole21a. This makes it possible to restrain the bend of the rack bar29, and thus to cause the rack30to mesh with the pinion28tightly.

A worm35provided to an input shaft34, which extends into the inside of the decelerator16, meshes with a worm wheel36fixed to the pinion shaft17. For this reason, once the assist motor M is driven, the pinion shaft17of the decelerator16rotates via the worm wheel36which meshes with the worm35provided to the input shaft34of the assist motor M, and thus the rack30, which meshes with the pinion28, is driven. Thereby, the steering torque, which a driver applies to the steering wheel11, is assisted by the assist motor M.

As shown inFIGS. 3 to 6, the outer covering of the assist motor M is configured to include: a bottomed cylindrical motor housing41; and an attachment flange43fixed to the motor housing41with multiple bolts42in such a way as to close the opening end of the motor housing41. The attachment flange43is fastened to the lower housing21with multiple bolts44. A rotary shaft47of the motor M is rotatably supported by paired ball bearings45,46which are respectively provided to the motor housing41and the attachment flange43. A column-shaped rotor49is fixed to the outer circumferential surface of the rotary shaft47. Inside the rotor49, multiple permanent magnets48are arranged in the circumferential direction of the rotor49. A stator51is fixed to the inner circumferential surface of the motor housing41. The stator51is configured to include multiple coils50which are arranged in the circumferential direction of the stator51.

The rotary shaft47of the assist motor M and the input shaft34of the decelerator16are connected together by use of an Oldham coupling52. The Oldham coupling52is configured including: a metallic first hub53to which the front end of the rotary shaft47is connected by spline connection; a metallic second hub54to which the front end of the input shaft34is connected by spline connection; and a synthetic-resin-made insert55held between the first and second hubs53,54.

Paired parallel first guide plates55a,55aare projected on a side surface of the insert55. Paired parallel second guide plates55b,55bare projected on the other side surface of the insert55in such a way as to be orthogonal to the first guide plates55a,55a. First projections53a,53aare formed in the first hub53, and slidably guide the paired first guide plates55a,55aof the insert55. Second projections54a,54aare formed in the second hub54, and slidably guide the paired second guide plates55b,55bof the insert55.

For this reason, even in a case where the axis of the rotary shaft47of the assist motor M and the axis of the input shaft34of the decelerator16have unparallel misalignment, a periodic bending moment can be prevented from acting on the rotary shaft47and the input shaft34. This happens because the torque can be transmitted from the rotary shaft47to the input shaft34while absorbing the unparallel misalignment by causing the insert55to slide over the first and second hubs53,54in the respective two directions which are orthogonal to each other.

A resolver56is supported by a retainer57inside the attachment flange43of the assist motor M. The resolver56detects a rotation angle of the rotor49of the assist motor M, and thereby controls the phases of the U-phase, V-phase and W-phase currents supplied to each coil50of the stator51depending on the rotation angle detected.

The resolver56is a variable reluctance type, and is configured to include a stator59and a rotor60. The stator59comprises multiple coils58(10 coils in the present exemplary embodiment) fixed to the inner circumferential surface of the attachment flange43. The rotor60is configured in such a way as to be integrated with the first hub53of the Oldham coupling52, and is configured of a cylindrical magnetic material which is opposed to the inner circumferential surface of the stator59by an air gap α. The rotor60is a short cylindrical member in which multiple convex parts (7 convex parts in the present exemplary embodiment)60aand multiple concave parts (7 concave parts in the present exemplary embodiment)60bare alternately formed. The rotor60protrudes from the assist-motor-M-side end portion of the first hub53of the Oldham coupling52in the axial direction with a gap being interposed between the rotor60and the outer periphery of the rotary shaft47. Furthermore, the air gap α formed between the outer circumferential surface of the rotor60and the inner circumferential surface of the stator59is narrow in the convex parts60aand is wide in the concave parts60b.

Note that first hub53and the coupling61of the exemplary embodiments disclosed herein correspond to the rotor supporting member of the present invention, and assist motor M of the exemplary embodiments corresponds to the motor of the present invention.

Next, descriptions will be provided for how this exemplary embodiment of the present invention including the foregoing configuration operates.

Once the driver operates the steering wheel11, the steering torque is transmitted to the rack30via the upper steering shaft12, the upper universal joint13, the lower steering shaft14, the lower universal joint15, the pinion shaft17and the pinion28. Thus, the steering torque transmitted drives the rack bar29leftward or rightward inside the steering gear box18. At this time, if the assist motor M operates, the assist torque is transmitted to the pinion shaft17via the worm35and the worm wheel36, and thus assists the steering operation of the driver.

When the rotary shaft47of the assist motor M rotates, the rotor60of the resolver56integrated with the first hub53of the Oldham coupling52fixed to the rotary shaft47rotates, and the air gap α, located between the stator59and the alternate series of convex parts60aand concave parts60bwhich are arranged in the outer periphery of the rotor60, changes its width periodically. When change in the magnetic property is outputted as a serrate voltage waveform, the rotational position of the rotary shaft47can be detected.

Once the resolver56detects the rotation angle of the rotary shaft47of the assist motor M, the phases of the respective U-phase, V-phase and W-phase currents of the DC brushless motor are controlled depending on the rotation angle detected. At this time, if the rotor60of the resolver56was to be hypothetically, directly fitted to the outer periphery of the rotary shaft47of the assist motor M by press-fit, it is likely that stress may act on the rotor60, and that the magnetic permeability may accordingly decrease in some parts of the rotor60. In addition, the rotary shaft47of the assist motor M would be distorted and deformed due to the torque. The distortion angle constantly changes depending on the torque. Moreover, when the rotary shaft47of the assist motor M and the input shaft34of the decelerator16are not fully coaxially arranged, a periodic bending moment acts on the rotary shaft47. These factors may cause a triangular waveform, which is outputted from the resolver56, to be out of shape, and that the cycle may change. Such factors make it likely that the rotation angle of the rotary shaft47of the assist motor M may not be detected with accuracy.

By contrast, in the present exemplary embodiment, the rotary shaft47of the assist motor M is connected to the first hub53of the Oldham coupling52by spline-connection, and the rotor60of the resolver56is arranged to extend out from the end surface of the first hub53so as to surround the outer periphery of the rotary shaft47with the gap being interposed in between. For this reason, the rotor60is no longer directly affected by the stress coming from the press-fitting of the rotary shaft47and the stress coming from the torsion of the rotary shaft47. In addition, the influence of the stress coming from the press-fitting of the rotor60and the influence of the stress coming from the torsion of the rotary shaft47are eliminated. Thereby, the rotation angle of the rotary shaft47can be detected with accuracy. Moreover, because the rotor60is supported by use of the first hub53of the Oldham coupling52, specialized members for supporting the rotor60are no longer necessary, leading to a reduced number of parts.

Furthermore, because the Oldham coupling52is arranged between the rotary shaft47and the input shaft34, the rotary shaft47can smoothly transmit the driving force of the assist motor M to the decelerator16without receiving the periodic bending load even when the axis of the rotary shaft47and the axis of the input shaft34have the unparallel misalignment. As a result, the bending moment is no longer directly transmitted to the rotor60of the resolver56and the rotor60is prevented from making a twisting rotational motion. By this, the rotation angle of the rotary shaft47can be detected with higher accuracy.

Next, descriptions will be provided for a second exemplary embodiment of the present invention with reference toFIGS. 7 and 8.

In the first exemplary embodiment, the rotary shaft47of the assist motor M and the input shaft34of the decelerator16are connected together by use of the Oldham coupling52. By contrast, the second exemplary embodiment adopts a cylindrical coupling61in lieu of the Oldham coupling52. A collar62is fitted to an end side of the coupling61by press-fit, and the front end of the rotary shaft47of the assist motor M is fitted into the collar62by press-fit. In addition, the input shaft34of the decelerator16is connected to the other end side of the coupling61by spline-connection.

The rotor60of the resolver56extends from an end portion of the coupling61in the axial direction in such a way as to surround the outer periphery of the rotary shaft47. In this respect, a diameter of the inner circumferential surface60cof the rotor60is larger in its radial direction with a distance δ than a diameter of the inner circumferential surface61aof the coupling61into which the collar62is fitted by press-fit.

According to the second exemplary embodiment, the rotation angle of the rotary shaft47can be detected with accuracy even if the coupling61is deformed due to the press-fitting of the rotary shaft47of the assist motor M. This happens because neither stress coming from the press-fit nor stress coming from the torque of the assist motor M is transmitted to the rotor60, which is formed so as to extend out from the end surface of the coupling61. Furthermore, although the press-fit load of the rotary shaft47affects the inner circumferential surface61aof the coupling61most strongly, the rotor60is less susceptible to the influence of the press-fit load because the inner circumferential surface60cof the rotor60is separated away from the inner circumferential surface61aof the coupling61outward in the radial directions with the distance δ in between. Thereby, it is possible to detect the rotation angle of the rotary shaft47with accuracy.

The foregoing descriptions have been provided for the exemplary embodiments of the present invention. However, the present invention is not limited to the above exemplary embodiments. Various design modifications can be carried out without departing from the present invention set forth in the scope of claims.

For example, the present invention may be applied to a cable steering apparatus and a steer-by-wire steering apparatus, although the shaft steering apparatus has been exemplified for the exemplary embodiments.

Further, as to the resolver56, the number of poles of the stator59and the number of poles of the rotor60can be changed depending on the necessity.