Patent Application
20130152703
The present invention relates to a magnetostrictive torque sensor and a method of manufacturing such magnetostrictive torque sensor.
Magnetostrictive torque sensors are employed in, for example, an electric power steering apparatus for motor vehicles. A typical example of such magnetostrictive torque sensors is disclosed in Japanese Patent Application Laid-open Publication (JP-A) No. 2007-292727, corresponding to U.S. Pat. No. 8,225,483.
The magnetostrictive torque sensor disclosed in JP 2007-292727A includes a magnetostrictive element provided on a rotary shaft, first and second coil units mounted to surround the magnetostrictive element, and a housing accommodating therewithin the magnetostrictive material and the coil units. The housing has an end flange for connection to another case member. The housing including the end flange is molded of synthetic resin. The coil units are supported by the housing as they are molded integrally with the housing.
In the manufacture of the disclosed magnetostrictive torque sensor, first and second coil units are fitted onto a cylindrical centering rod in tandem relation to each other. Then, a molding die assembly is set to enclose the coil units such that a mold cavity which is complementary in contour to a housing to be produced is defined within the molding die assembly. A molten resin is injected into the mold cavity, so that a plastic housing is produced with an end flange formed integrally with a housing body and with the coil units molded integrally with the housing body. Since the molding process is carried out while the coil units are fitted onto the centering rod, the coil units and the housing are concentric with each other.
In the molding process, the molten resin injected into the mold cavity undergoes thermal contraction as it cools down and becomes solidified. The amount of thermal contraction increases with an increase in the amount of synthetic resin material used. The end flange of the housing, which is adapted to be connected to another case member, requires a high mechanical strength and hence is made thicker than other parts of the housing. This means that due to a relatively large amount of synthetic resin material used, the end flange may undergo relatively large thermal contraction during injection molding process. When such thermal contraction occurs, a relatively large compressive force is applied to a part of one coil unit which is located adjacent to the end flange. This will deteriorate the sensing accuracy of the coil units because the coil units operate as a member for detecting variations in magnetic property of the magnetostrictive element.
It is, therefore, an object of the present invention to provide a magnetostrictive torque sensor having high detection accuracy and a method of manufacturing such magnetostrictive torque sensor.
According to a first aspect of the present invention, there is provided a magnetostrictive sensor comprising: a rotary shaft; a magnetostrictive element provided on the rotary shaft; a single coil bobbin arranged to face the magnetostrictive element; a plurality of detection coils wound on the coil bobbin and spaced apart in an axial direction of the coil bobbin for detecting a change in magnetic characteristic of the magnetostrictive element; and a housing configured to house the magnetostrictive element, the coil bobbin and the detection coils, wherein the housing includes a plastic housing body having a generally cylindrical shape and molded integrally with the coil bobbin with the detection coils wound on the coil bobbin, a metal flange portion disposed on an axial end portion of the generally cylindrical plastic housing body, and a plastic fastener member firmly connecting the metal flange portion to the axial end portion of the plastic housing body.
With this arrangement, since the housing body and the flange portion are formed of different materials and structurally independent from each other, and since the plastic housing body is molded integrally with the coil bobbin with the detection coils wound on the coil bobbin, the housing body can be formed into a generally cylindrical shape having a uniform thickness throughout an axial length thereof. This means that the amount of thermal contraction occurring when the plastic housing body is formed by molding is substantially uniform with respect to the detection coils, which will insure highly accurate detection accuracy of the magnetostrictive torque sensor. Additionally, integral formation of the housing body and the coil bobbin having the detection coils wound thereon can obviate the need for screws and a collar that are conventionally used for mounting the coil bobbin and the detection coils in the housing body.
In one preferred form of the invention, the axial end portion of the plastic housing body has a cylindrical surface and a circumferential groove formed in the cylindrical surface, the metal flange portion has a central boss fitted with the cylindrical surface of the axial end portion of the plastic housing body and a plurality of through-holes extending radially through the central boss, the through-holes having one end connected with the circumferential groove of the axial end portion of the plastic housing body, and the plastic fastener member has a first ring-shaped portion disposed on a radial outer side of the central boss of the metal flange portion, a second ring-shaped portion disposed in the circumferential groove of the axial end portion of the plastic housing body and a plurality of radial arms extending between the first and second ring-shaped portions and disposed in respective ones of the through-holes of the metal flange portion. By virtue of the radial arms disposed in the through-holes of the metal housing portion, the plastic fastener member is locked in position against rotation relative to the metal flange portion and the plastic housing member.
Preferably, the plastic fastener member is molded integrally with the plastic housing body and the metal flange portion. The molded plastic fastener member undergoes thermal contraction as it cools down and becomes solidified during a molding process. However, the thermal contraction of the fastener element gives no direct effect on the detection coils and, hence, the magnetostrictive torque sensor can retain a good temperature characteristic.
The magnetostrictive torque sensor may further comprise an elastic member disposed between the axial end portion of the plastic housing body and the metal flange portion in an elastically distorted condition such that an elastic restoring force of the elastic member acts in a direction to urge the plastic housing body and the metal flange portion to move away from each other along an axis of the rotary shaft. This arrangement ensures that when the magnetostrictive torque sensor is subjected to heat, the plastic housing body, which has a larger linear expansion coefficient than the metal flange portion, is allowed to move relative to the metal flange portion in an axial direction of the rotary shaft so as to follow thermal expansion of the rotary shaft on which the magnetostrictive element is provided. Thus, the torque sensor can retain good temperature characteristic.
Preferably, the axial end portion of the plastic housing body has an end surface disposed in abutting contact with a surface of the metal flange portion and an annular groove formed in the end surface, and the elastic member is an O-ring disposed in the annular groove of the plastic housing body. The annular groove can readily be formed when the plastic housing body is formed by molding.
According to a second aspect of the present invention, there is provided a method of manufacturing a magnetostrictive torque sensor including a housing having a plastic housing body and a metal flange portion firmly connected to an end portion of the plastic housing body by a plastic fastener member with a coil bobbin firmly held within the plastic housing body with a plurality of detection coils wound on the coil bobbin, the method comprising the steps of providing a single coil bobbin having a plurality of detection coils wound thereon, and a metal flange portion; setting the coil bobbin in a first molding die assembly such that a mold cavity which is complementary in contour to the plastic housing body of the housing to be produced is formed within the first molding die assembly; filling the mold cavity with a molten synthetic resin material to thereby form a plastic housing body having the coil bobbin firmly held therein with the detection coils wound on the coil bobbin; setting the plastic housing body and the metal flange portion in a second molding die assembly such that a mold cavity which is complementary in contour to the plastic fastener member of the housing to be produced is formed within the second molding die assembly; and filling the mold cavity of the second molding die assembly with a molten synthetic resin material to thereby obtain a housing having the metal flange portion firmly connected to an axial end portion of the plastic housing body by a molded plastic fastener member.
Since the plastic housing body is formed integrally with the coil bobbin having the detection coils wound thereon in a first or primary molding process before it is joined with the metal flange portion, it is possible to form the plastic housing body into a generally cylindrical shape having a substantially uniform thickness throughout an axial length thereof. The cylindrical plastic housing body with uniform thickness, as it cools down and becomes solidified during the molding process, may undergo thermal contraction occurring uniformly throughout the axial length of the housing body. This will ensure that an air-gap between the coil bobbin and the magnetostrictive element on the rotary shaft remains constant with respect to each of the detection coil elements. The detection coils can retain high detection accuracy. Furthermore, since the metal flange portion is firmly connected to the axial end portion of the plastic housing body by the plastic fastener member molded in a secondary molding process, thermal contraction of the molded fastener member gives no direct effect on the performance of the detection coils. Thus, highly accurate detection sensitivity of the detection coils can be maintained.
It is preferable that the setting the plastic housing body and the metal flange portion in the second molding die assembly is carried out while an elastic member is held between the axial end portion of the plastic housing body and the metal flange portion in an elastically distorted condition such that an elastic restoring force of the elastic member acts in a direction to urge the plastic housing body and the metal flange portion to move away from each other along an axis of the plastic housing body.
Certain preferred structural embodiments of the present invention will be described in detail herein below, by way of example only, with reference to the accompanying sheets of drawings, in which:
Referring now to the drawings and
The electric power steering apparatus 10 shown in
In the steering system 20, a pinion shaft (input shaft) 24 is coupled to the steering wheel 21 via a steering shaft 22 and universal joints 23, and a rack shaft 26 is coupled to the pinion shaft 24 via a rack-and-pinion mechanism 25. Further, the right and left steerable road wheels 29 are coupled to opposite ends of the rack shaft 26 via right and left tie rods 27 and knuckle arms 28. The rack-and-pinion mechanism 25 includes a pinion 31 formed on the pinion shaft 24 and a rack 32 formed on the rack shaft 26.
With the steering system 20 thus arranged, when a human operator or driver of the vehicle operates the steering wheel 21, steering torque is delivered from the steering wheel 21 to the right and left steerable road wheels 29 via the rack-and-pinion mechanism 25, right and left tie rods 27 etc. and thereby steers the road wheels 29.
The steering torque assist mechanism 40 includes a magnetostrictive torque sensor 60 for detecting steering torque applied by the driver to the steering system 20 through operation of the steering wheel 21, a control unit 42 for generating a control signal on the basis of a torque detection signal from the magnetostrictive torque sensor 60, an electric motor 43 for generating steering assist torque corresponding to the driver-applied steering torque on the basis of the control signal, and a worm gear mechanism 44 for transmitting the motor-generated steering assist torque from the motor 43 to the pinion shaft 24. The steering assist torque transmitted to the pinion shaft 24 is further transmitted to the rack-and-pinion mechanism 25.
With the steering torque assist mechanism 40 thus arranged, the right and left steerable road wheels 29 are steered by a combination of (i.e., composite torque composed of) the driver-applied steering torque and motor-generated steering assist torque via the rack shaft 26.
As shown in
The magnetostrictive elements 61, 62 are magnetostrictive films formed on the outer circumferential surface of the pinion shaft 24 and having a residual strain in opposite directions along a longitudinal axis of the pinion shaft 24. The magnetostrictive films 61, 62 are formed of a material which can create a magnetic flux density variable greatly with a change in distortion. The magnetostrictive elements 61, 62 may have a magnetic anisotropy in opposite directions to each other and, hence, they can be formed by a single magnetostrictive film having two magnetostrictive parts or regions separated in the axial direction of the pinion shaft 24.
The housing body 71 has a generally cylindrical shape and is connected at the lower axial end portion 71a (
As better shown in
As shown in
The plastic fastener member 72 is configured to extend between a radial outward side of the central boss 82 of the flange portion 73 and an internal space of the circumferential groove 80a of the boss 85 of the housing body 71 through the radial through-holes 83 of the flange portion 73. More specifically, as shown in
The pinion shaft 24 is rotatably supported by the lower housing 51 via a pair of bearings 54 and 55 at a longitudinally intermediate portion 24c and an output end portion 24a (lower end portion in
The plastic housing body 71 has a larger linear expansion coefficient than the metal flange portion 73. The plastic housing body 71 and the plastic fastener member 72 may be formed of a same synthetic resin material.
The lower housing 51 has an annular groove 79 formed in an upper end face thereof, and an elastic seal member 78 such as an O-ring is disposed in the annular groove 79. The flange portion 73 of the upper housing 70 and the upper end of the lower housing 51 are joined together by the bolts 84 (one being shown in
The terminal 74 is formed as an integral part of the housing body 71 and projects in a radial outward direction of the housing body 71. The terminal 74 includes a central current-carrying portion 74a and a cylindrical wall 74b disposed circumferentially around the current-carrying portion 74a.
With the magnetostrictive torque sensor 60 thus constructed, because the plastic housing body 71 and the metal flange portion 73 are structurally independent from each other, the plastic housing body 71 can be formed into a cylindrical shape having a substantially uniform thickness throughout an axial length thereof. This means that the amount of synthetic resin material used to form the plastic housing body 71 is substantially uniform throughout the axial length of the housing body 71, and the amount of thermal contraction occurring during the production of the plastic housing body 71 is substantially uniform throughout the axial length of the plastic housing body 71. The plastic housing body 71 is molded integrally with the coil bobbin 63 with the detection coils 65, 66 wound on the coil bobbin 63. By virtue of the substantially uniform thermal contraction throughout the axial length of the plastic housing body 71, a compressive force applied to the upper yoke 67 and a compressive force applied to the lower yoke 68 are substantially equal in magnitude and the upper and lower yokes 67, 78 have substantially the same magnetic permeability. This will ensure that the detection coils 65, 66 have substantially the same level of sensitivity. Additionally, because a first portion of the plastic housing body 71 corresponding in position to the upper detection coil 65 and a second portion of the plastic housing body 71 corresponding in position to the lower detection coil 66 contract in a radial inward direction of the housing body 71 by substantially the same amount, an air-gap between the magnetostrictive elements 61, 62 on the pinion shaft 24 and the coil bobbin 63 remains constant with respect to each of the detection coils 65, 66. This will insure highly accurate detection sensitivity of the magnetostrictive torque sensor 60.
The metal flange portion 73 is firmly connected to the axial end portion 71 of the plastic housing body 71 by the plastic fastener member 72. The plastic housing body 71 is molded integrally with the coil bobbin 63 having the detection coils 65, 66 wound thereon in a first or primary molding process, and the plastic fastener member 72 is molded integrally with the plastic housing body 71 and the metal flange portion 73 in a secondary molding process. With this arrangement, thermal contraction of the plastic fastener member occurring when the molded plastic fastener member 72 cools down and becomes solidified gives no direct effect on the performance of the detection coils 65, 66. The detection coils 85, 66 can thus retain their high detection sensitivity.
In the magnetostrictive torque sensor 60, the pinion shaft 24 provided with the magnetostrictive elements 61, 62 is rotatably supported by the lower housing 51 via the bearings 54, 55 at the longitudinally intermediate portion 24c and the output end portion 24a (lower end portion in
A method of manufacturing the magnetostrictive torque sensor 60 will be described below with reference to
As shown in
Then, as shown in
Subsequently, as shown in
Then, after solidification, a molded product 98 is removed from the first molding die assembly 78, as shown in
Subsequently, as shown in
Then, as shown in
Subsequently, as shown in
Since the coil assembly composed of the coil bobbin 63, the detection coils 65, 66, and the yokes 67, 68 is integrally molded with the plastic housing body 71 in a first or primary molding process which is achieved before the metal flange portion 73 is connected to the axial end portion of the plastic housing body 71, the housing body 71 can be formed into a generally cylindrical shape having a substantially uniform thickness throughout an axial length thereof. When such cylindrical plastic housing body 71 having a uniform thickness undergoes thermal expansion, the amount of thermal expansion is substantially uniform throughout the axial length of the housing body 71. This will insure high detection sensitivity of the detection coils 65, 66. Additionally, because the metal flange portion 73 is firmly connected to the axial end portion of the plastic fastener member 72 formed by molding during a secondary molding process, thermal contraction of the molded plastic fastener member 72 gives no direct effect on the performance of the detection coils 65, 66. The magnetostrictive torque sensor 60 can achieve toque detection with high accuracy.
Although in the illustrated embodiments, the magnetostrictive torque sensors 60, 110 are used in an electric power steering apparatus 10 of a vehicle, the present invention can be applied to any other apparatus in which detection of a torque is a major requirement.
Obviously, various minor changes and modifications of the present invention are possible in light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-273181 | Dec 2011 | JP | national |
