Method for manufacturing a magnetostrictive torque sensor Number:7,386,930 from the United States Patent and Trademark Office (PTO) owispatent For forming an annular magnetostrictive coat on an outer peripheral surface of a rotational shaft associated with a magnetostrictive torque sensor, a magnetostrictive coat forming method comprises a step of fitting a cylindrical masking jig over the outer peripheral surface of the rotational shaft and securing the masking jig to the outer peripheral surface, a step of placing the rotational shaft in a plating tank to thereby form the magnetostrictive coat by plating on the outer peripheral surface, and a step of detaching the masking jig from the rotational shaft.<H magnetostrictive, manufacturing, Method, for, manu, 7386930.html, Patent, pto, 7386930

Title: Method for manufacturing a magnetostrictive torque sensor

Abstract: For forming an annular magnetostrictive coat on an outer peripheral surface of a rotational shaft associated with a magnetostrictive torque sensor, a magnetostrictive coat forming method comprises a step of fitting a cylindrical masking jig over the outer peripheral surface of the rotational shaft and securing the masking jig to the outer peripheral surface, a step of placing the rotational shaft in a plating tank to thereby form the magnetostrictive coat by plating on the outer peripheral surface, and a step of detaching the masking jig from the rotational shaft.

Patent Number: 7,386,930 Issued on 06/17/2008 to Shimizu, et al.

Inventors: Shimizu; Yasuo (Wako, JP), Nakamura; Yoshito (Wako, JP), Sueyoshi; Shunichiro (Wako, JP), Yoshimoto; Nobuhiko (Sayama, JP), Kobayashi; Koji (Sayama, JP), Fukuda; Yuichi (Sayama, JP), Doi; Mizuho (Sayama, JP), Harada; Hitoshi (Sayama, JP), Hoshi; Tomohiro (Sayama, JP)
Assignee: Honda Motor Co., Ltd. (Tokyo, JP)

Appl. No.: 11/983,085

Filed: November 7, 2007

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
10844824	May., 2004	7310870

Foreign Application Priority Data


May 12, 2003 [JP]			2003-133478
May 15, 2003 [JP]			2003-137592
Jun 16, 2003 [JP]			2003-170191

Current U.S. Class: 29/594 ; 205/119; 205/122; 29/592.1; 29/602.1; 427/129; 427/130; 73/862.331; 73/862.335

Current International Class: H04R 31/00 (20060101)

Field of Search: 29/592.1,594,602.1,855 73/862.331-862.335,762 205/119,122 427/129,130

References Cited [Referenced By]

U.S. Patent Documents


3762217	October 1973	Hagen
3823608	July 1974	Pantermuehl et al.

Foreign Patent Documents


62179626	Aug., 1987	JP

Primary Examiner: Kim; Paul D
Attorney, Agent or Firm: Hamre, Schumann, Mueller & Larson, P.C.

Parent Case Text

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of application Ser. No. 10/844,824, filed May 12, 2004, now U.S. Pat. No. 7,310,870 which application is incorporated herein by reference.

Claims

What is claimed is:

1. A method for manufacturing a magnetostrictive torque sensor, said method comprising: a magnetostrictive coat formation step of forming a magnetostrictive coat on a rotational shaft; a heating step of subjecting said magnetostrictive coat to a high-frequency heating process for a predetermined time with predetermined twisting torque kept applied to said rotational shaft; a torque removal step of removing the twisting torque from said rotational shaft to thereby impart a magnetic anisotropy to said magnetostrictive coat; and a coil positioning step of positioning a coiled coil around said magnetostrictive coat for detecting variation in magnetostrictive characteristic of said magnetostrictive coat.

2. The method as claimed in claim 1 wherein said magnetostrictive coat contains an iron-nickel alloy material as a main component thereof, and the predetermined twisting torque is in a range not smaller than 50 Nm but not greater than 100 Nm.

3. A method for manufacturing a magnetostrictive torque sensor, said method comprising: a step of providing a magnetostrictive coat on a rotational shaft; a step of subjecting said magnetostrictive coat to a heating process with predetermined twisting torque kept applied to said rotational shaft; a step of removing the twisting torque from said rotational shaft to thereby impart a magnetic anisotropy to said magnetostrictive coat; a reheating step of subjecting said rotational shaft to a reheating process; and a step of positioning a coiled coil around said magnetostrictive coat for detecting variation in magnetostrictive characteristic of said magnetostrictive coat.

4. The method as claimed in claim 3 wherein a temperature at which said reheating step reheats said rotational shaft is greater than a normal use temperature at which said magnetostrictive torque sensor is actually used for torque detection.

Description

FIELD OF THE INVENTION

The present invention relates generally to a magnetostrictive torque sensor and, more particularly, to a magnetostrictive torque sensor for detecting a steering torque in an electric power steering apparatus for motor vehicles.

BACKGROUND OF THE INVENTION

As well known, the electric power steering apparatus are steering assisting apparatus which are designed to drive an electric motor as a human operator or driver manually operates a steering wheel, during driving of a motor vehicle, to thereby assist the driver's manual steering effort. In such electric power steering apparatus, the steering assisting motor, which provides a steering torque assist (or steering assist torque), is driven by a motor control device in accordance with a PWM (Pulse Width Modulation) scheme, using a steering torque signal generated by a steering torque detector section detecting steering torque that is produced on the steering shaft by driver's operation of the steering wheel and a vehicle velocity signal generated by a vehicle velocity detection section detecting a traveling velocity of the vehicle, so as to reduce manual steering force to be applied by the human driver.

FIG. 38 is a view showing an overall setup of a typical example of the conventional electric power steering apparatus. This electric power steering apparatus 100 includes a steering torque detector section (torque sensor) 102 for detecting steering torque applied by a human driver via a steering wheel 101, an electric steering assisting motor 103 for providing an steering torque assist to the steering by the driver, a power transmission device 104 for boosting the rotational torque of the motor 103, a control device 106 for controlling operation of the motor 103 on the basis of output signals from the torque detector section 102 and vehicle velocity detector section 105, and a rack and pinion mechanism 108 for varying a direction of steerable front road wheels 107.

The electric power steering apparatus 100 is constructed to provide the steering assist torque to an upper steering shaft 109a etc. connected to the steering wheel 101. The upper steering shaft 109a is also connected at its lower end to a lower steering shaft 109b via a universal joint 109c and connected at its upper end to the steering wheel 101. The lower steering shaft 109b is operatively connected at its lower end to a pinion gear 110 meshing with a rack gear 111a provided on a rack shaft 111. The pinion gear 110 and rack gear 111a together constitute a rack and pinion mechanism 108. Tie rods 112 are connected to axial opposite ends of the rack shaft 111, and the front road wheels 107 are connected to respective outer ends of the tie rods 112. The steering assisting motor 103 is operatively connected to the lower steering shaft 109b via the power transmission mechanism 104. The power transmission mechanism 104 comprises a worm gear 104a and worm wheel 104b. The motor 103 generates steering assist torque that is delivered via the power transmission mechanism 104 to the steering shafts 109b and 109a. The steering torque detector section (torque sensor) 102, which is provided on the lower steering shaft 109b, detects steering torque applied to the steering shafts 109a and 109b through driver's operation of the steering wheel 101. The vehicle velocity detector section 105 detects a traveling velocity of the vehicle, and the control device 106 is implemented by a computer. The control device 106 receives a steering torque signal T output from the torque detector section 102, vehicle velocity signal V output from the vehicle velocity detector section 105, etc., on the basis of which it generates a driving control signal SG1 for controlling the rotation of the steering assisting motor 103. The above-mentioned rack and pinion mechanism 108 etc. are accommodated in a gearbox 113 not shown in FIG. 38.

In short, the electric power steering apparatus 100 of FIG. 38 may be constructed by adding, to the construction of the traditional steering apparatus, the torque detector section 102, vehicle velocity detector section 105, control device 106, steering assisting motor 103 and power transmission device 104.

In the electric power steering apparatus 100, the steering assist torque, generated by the steering assisting motor 103 on the basis of the steering torque signal T, vehicle velocity signal V, etc., is boosted via the power transmission device 104 and delivered to a pinion shaft of the rack and pinion mechanism 108 so as to reduce steering torque to be manually applied by the driver. As the driver operates the steering wheel 101 to vary the traveling direction of the motor vehicle, a rotational force based on steering torque applied to the steering shafts 109a and 109b is converted into axial linear movement of the rack shaft 111, via the rack and pinion mechanism 108, to thereby vary the direction of the front road wheels 107 via the tie rods 112. During that time, the torque detector section 102, provided on the lower steering shaft 109b, detects the steering torque applied to the steering shaft 109b to generate an electric steering torque signal T, representative of the detected steering torque, that is supplied to the control device 106, and the vehicle velocity detector section 105 detects a traveling velocity of the vehicle steering to generate an electric vehicle velocity signal V that is supplied to the control device 106. Thus, on the basis of the steering torque signal T and vehicle velocity signal V, the control device 106 generates a motor current for driving the steering assisting motor 103, which in turn provides a steering assist force to the steering shafts 109b and 109a via the transmission mechanism 104. The motor 103 thus driven can reduce the steering force to be manually applied to the steering wheel 101 by the driver.

If the steering torque is represented by "TH" and a coefficient of a steering assist amount AH is assumed to be a constant value "kA", then AH=kA.times.TH.

Thus, if a load or pinion torque is represented by "TP",

.times..times..times..times..times..times..times..times..times..times..tim- es..times..times. ##EQU00001## Therefore, the steering torque TH can be expressed as TH=TP/(1+kA) (2) Namely, the steering torque TH can be reduced to "pinion torque TP/(1+kA)", where kA is equal to or greater than zero.

FIG. 39 shows detailed organization of mechanical and electric components in the electric power steering apparatus 100, where part of left and right end portions of the rack shaft 111 are shown in section. The rack shaft 111 is accommodated in a cylindrical housing 131, disposed in a widthwise direction (left-and-right direction of FIG. 39) of the vehicle, for axial sliding movement therein. Ball joints 132 are secured via screws to opposite ends of the rack shaft 111 that project beyond the housing 131, and left and right tie rods 112 are connected to the ball joints 132. The housing 131 has brackets 133 via which the housing 131 is secured to the body of the vehicle, and stoppers 134 at its opposite ends. In FIG. 39, reference numeral 135 represents an ignition switch, 136 an on-vehicle battery, and 137 an A.C. generator annexed to an engine of the vehicle. The A.C. generator 137 is activated to generate power in response to operation of the vehicle engine. Necessary electric power is supplied to the control device 106 from the battery 136 or A.C. generator 137. Further, reference numeral 138 represents a rack end that abuts against one of the stoppers 134 during axial movement of the rack shaft 138, and 139 a dust-sealing boot for protecting the interior of the gearbox from water, mud, dust, etc.

FIG. 40 is a sectional view taken along the A-A lines of FIG. 39, which clearly shows a structure for supporting the lower steering shaft 109b and detailed organization of the steering torque detector section 102, transmission mechanism 104 and rack and pinion mechanism 108. The lower steering shaft 109b is rotatably supported, via four bearings 141a, 141b, 141c and 141d, within a housing 113a forming the gearbox 113. The transmission mechanism 104 and rack and pinion mechanism 108 are also accommodated within the housing 113a, and the torque detector section 102 is secured to an upper portion of the housing 113a. The steering torque detector section 102 includes magnetostrictive films or coats 102b and 102c that are provided on the outer circumferential surface of the lower steering shaft 109b and surrounded by coils 102d, 102f, 102e and 102f and yoke section 102g; that is, the lower steering shaft 109b are surrounded by the coils 102d, 102f, 102e and 102f and yoke section 102g. The housing 113a has an upper opening closed with a lid 143 bolted thereto. The pinion 110 provided on a lower end portion of the lower steering shaft 109b is positioned between the bearings 141a and 141b. The rack shaft 111 is guided along a rack guide 145 and pressed against the pinion 110 via a compression spring 146. The power transmission mechanism 104 includes the worm gear 104a connected via a transmission shaft 148 to an output shaft of the steering assisting motor 103, and the worm wheel 104b secured to the lower steering shaft 109b. Specifically, the torque detector section 102, which is secured to the lid 143 in the steering gearbox 113, detects steering torque acting on the lower steering shaft 109b and outputs a value of the detected steering torque (steering torque signal) to the control device 106, which in turn supplies a motor signal to cause the motor 103 to generate appropriate steering assist torque.

The steering torque detector section 102 of the electric power steering apparatus 100 comprises a magnetostrictive torque sensor designed to directly detect steering torque applied to the steering shaft 109b, as compared to the traditional torque sensor that detects an twist or torsional angle of a torsion bar, converts the detected torsional angle into axial displacement and detects the converted axial displacement to thereby indirectly detect steering torque.

As illustrated in FIG. 40, the lower steering shaft 109b connected to the steering wheel 101 is rotatably supported, via the bearings 141c and 141d, within the gearbox 113, and two magnetostrictive coats, each in the form of a nickel-iron plating or the like, are provided on two, upper and lower, portions 102b and 102c of the outer surface between the bearings 141c and 141d. The magnetostrictive coats, each having a predetermined thickness, are imparted with opposite magnetic anisotropies and reverse magnetostrictive characteristics, as will be later described in relation to FIG. 40.

FIG. 41 is a diagram showing positional relationship among an exciting coil, detecting coils and magnetostrictive coats in the magnetostrictive torque sensor 102. The magnetostrictive coats 102b and 102c are formed, with a predetermined axial interval therebetween, on the surface of the lower steering shaft 109b, and the exciting coil 102f is disposed near the magnetostrictive coats 102b and 102c with a slight air gap left between the coil 102f and coats 102b and 102c. The exciting coil 102f is connected to an exciting voltage supply source 102h. Further, the detecting coil 102d is disposed near the magnetostrictive coat 102b with a slight air gap therebetween, while the detecting coil 102e is disposed near the magnetostrictive coat 102c with a slight air gap therebetween. When torque acts on the lower steering shaft 109b in the magnetostrictive torque sensor 102, the torque also acts on the magnetostrictive coats 102b and 102c, and reverse magnetostrictive effects are produced in the coats 102b and 102c in accordance with the applied torque. Thus, as a high-frequency A.C. voltage (exciting voltage) is supplied from the exciting voltage supply source 102h to the exciting coil 102f, magnetic field variation due to the reverse magnetostrictive effects of the coats 102b and 102c, based on the input torque, can be detected as variation in impedance or induced voltage. Then, the torque applied to the steering shaft 109b can be detected on the basis of the detected impedance or induced voltage variation.

Example of such reverse magnetostrictive characteristics is shown in FIG. 42, where the horizontal axis represents the steering torque while the vertical axis represents the impedance or induced voltage detected via the detecting coils when an A.C. voltage is applied to the exciting coil. Curve C100 represents variation in the impedance or induced voltage detected via the detecting coil 102d, and a curve C101 represents variation in the impedance or induced voltage detected via the detecting coil 102e. As indicated by the curve C100 corresponding to the detection via the detecting coil 102d, the impedance or induced voltage increases as the steering torque changes from a negative value to a positive value, takes a peak value P1 when the steering torque reaches a positive value T1, and decreases after the steering torque gets greater than the value T1. As indicated by the curve C101 corresponding to the detection via the detecting coil 102e, the impedance or induced voltage increases as the steering torque changes from a positive value to a negative value, takes a peak value P1 when the steering torque reaches a negative value -T1, and decreases after the steering torque gets smaller than the value -T1. As shown, a steering-torque-vs.-impedance (induced voltage) characteristic obtained via the detecting coil 102d and a steering-torque-vs.-impedance (induced voltage) characteristic obtained via the detecting coil 102e present substantial mountain (upwardly-convex) curve shapes that are generally symmetrical with respect to the vertical axis, reflecting the opposite magnetic anisotropies of the upper and lower magnetostrictive coats 102b and 102c. Further, a straight line L100 represents a difference calculated by subtracting the characteristic curve C101, obtained via the detecting coil 102e, from the characteristic curve C100 obtained via the detecting coil 102d. The straight line L100 indicates that, ideally, the difference is zero when the steering torque is zero but varies linearly relative to variation in the steering torque within a steering torque range R. The magnetostrictive torque sensor uses particular regions or ranges of the characteristic curves C100 and C101 which present substantially constant gradients of sensitivity around a neutral torque point, so as to output detection signals corresponding to the direction and intensity of the input torque. Furthermore, using the characteristics of the straight line L100, the magnetostrictive torque sensor can detect the steering torque on the basis of the values detected via the detecting coils 102d and 102e.

From Japanese Patent Publication No. 3268089, there is known a magnetostrictive torque sensor for detecting steering torque input to the steering shaft (rotational shaft), where a magnetostrictive coat is formed by first masking the surface of the rotational shaft and then performing an electroless plating process on the masked surface. However, in the case where an annular magnetostrictive coat is formed on the rotational shaft by first wrapping a masking tape on part of the surface of the rotational shaft to thereby mask the surface and then plating the masked surface as taught in the No. 3268089 patent publication, the magnetostrictive coat would have a greater thickness at its opposite axial end portions than the remaining coat portion, which would unavoidably deteriorate the detection accuracy due to reasons to be described later.

Further, FIGS. 43A-43E show a manner in which the magnetostrictive coats 102b and 102c are imparted with magnetic anisotropies in the conventional magnetostrictive torque sensor, according to which the steering shaft 109b is subjected to a plating process to form the magnetostrictive coats 102b and 102c (see FIG. 43A; however, only the magnetostrictive coat 102b is shown with the other magnetostrictive coat 102c omitted for clarity). After completion of the plating, counterclockwise twisting torque Tq is applied to an upper portion of the steering shaft 109b while clockwise twisting torque Tq is applied to a lower portion of the steering shaft 109b, to thereby impart stress to the circumferential surface of the steering shaft 109b (FIG. 43B). Then, with the twisting torque Tq kept applied, the magnetostrictive coats 102b and 102c are heated in a thermostatic bath (FIG. 43C) and then cooled (FIG. 43D). After the cooling, the twisting torque Tq is removed from the surface of the steering shaft 109b (FIG. 43E), and necessary sensor output setting is performed to manufacture a substantially complete steering shaft 109b. For details of such a magnetostrictive torque sensor manufacturing method, see Japanese Patent Application Laid-Open Publication No. 2002-82000.

In FIGS. 43A-43E, each circle or oval D100 depicted alongside a drawing of the steering shaft 109b represents an enlarged drawing of a minute portion of the magnetostrictive coat 102b, and arrows F1 and F2 represent a tension load and compressing load, respectively. Note that the "minute portion" is a model portion of the magnetostrictive coat assumptively extracted for the purpose of showing typical physical changes occurring in the magnetostrictive coat. In the step of FIG. 43B, the minute portion D100 of the magnetostrictive coat 102b is simultaneously subjected to the tension load F1 and compressing load F2, so that it is deformed into an oval shape with its longitudinal axis extending upward and rightward (i.e., in a lower-left-to-upper-right direction of the figure). In the step of FIG. 43C, undesired creep is produced in the magnetostrictive coat 102b due to the heating, and the minute portion D100 assumes a near-circular oval shape. FIG. 43D shows the minute portion D100 having shrunk after the cooling but still generally keeping the near-circular oval shape of FIG. 43C. Further, FIG. 43E shows a state where torsional torque acting in an opposite direction to the twisting torque Tq has been applied to the magnetostrictive coat 102b due to removal of the twisting torque Tq, and where the minute portion D100 has assumed an oval shape with its longitudinal axis extending upward and leftward, i.e. in a lower-right-to-upper-left direction of the figure.

Specifically, in the conventional magnetostrictive torque sensor, the magnetostrictive coat 102b is formed into a thickness of 40 .mu.m, and the steering shaft 109b is subjected to the twisting torque Tq of 70 Nm, and then heated at a temperature in a range of 150-550.degree. C. for 10-20 minutes with the twisting torque Tq still kept applied thereto.

However, the above-discussed conventional magnetostrictive torque sensor 102 has several drawbacks, such as instable detected values of the steering torque, great hysteresis, a considerably long time for heating the steering shaft (magnetostrictive coats) and poor productivity. FIG. 44 is a graph showing an example of actual reverse magnetostrictive characteristics of the conventional magnetostrictive torque sensor 102, where the horizontal axis represents the input steering torque while the vertical axis represents the impedance or induced voltage detected by the detecting coils when an A.C. voltage has been supplied to the exciting coil. In FIG. 44, a curve C102 represents a difference, i.e. (A-B) value, calculated by subtracting an actual characteristic curve obtained via the detecting coil 102e (corresponding to the curve C101 of FIG. 42) from an actual characteristic curve obtained via the detecting coil 102d (corresponding to the curve C100 of FIG. 42), and the curve C102 corresponds to the straight line L100 of FIG. 42. From FIG. 44, it can be seen that the curve C102 presents occurrence of a hysteresis instead of actually presenting a zero "(A-B)" value when the steering torque is zero. Therefore, in the case where such a magnetostrictive torque sensor is employed in an electric power steering apparatus, the magnetostrictive torque sensor would deteriorate a steering feel and thus can not be suitably put to practical use.

Further, the conventional magnetostrictive torque sensor suffers from another problem that the reverse magnetostrictive characteristics are susceptible to influences of characteristics of the magnetostrictive coat formed by the plating performed prior to the heating process, as shown in FIGS. 45A-45D comparatively illustrating measurements of the reverse magnetostrictive characteristics before and after the heating process. Specifically, FIGS. 45A and 45B illustrate the reverse magnetostrictive characteristics of the magnetostrictive coats 102b and 102c before the heating, where curves C110 and C111 represent variation in the impedance detected when clockwise torque was applied while curves C112 and C113 represent variation in the impedance detected when counterclockwise torque was applied. It can be seen that the magnetostrictive coat 102b presents a greater hysteresis than the other magnetostrictive coat 102c. Further, FIGS. 45C and 45D illustrate the reverse magnetostrictive characteristics of the magnetostrictive coats 102b and 102c detected when the coats 102b and 102c were heated at 300.degree. C. for one hour. In the figures, curves C114 and C115 represent variation in the impedance detected when clockwise torque was applied while curves C116 and C117 represent variation in the impedance detected when counterclockwise torque was applied. It can be seen that the magnetostrictive coat 102b presents a greater hysteresis than the other magnetostrictive coat 102c. Namely, it was found that the magnetostrictive coat (e.g., 102b) having a relatively great hysteresis before the heating would present a great hysteresis even after the heating while the magnetostrictive coat (e.g., 102c) having a relatively small hysteresis before the heating would present a small hysteresis even after the heating; this means that the reverse magnetostrictive characteristics after the heating would be significantly influenced by the reverse magnetostrictive characteristics before the heating. Therefore, there has been a demand for a more sophisticated manufacturing method which can provide a magnetostrictive torque sensor capable of constantly achieving satisfactory torque detection with a small hysteresis without being influenced by characteristics of the magnetostrictive coats present before the heating.

Further, the magnetostrictive torque sensor, made by the above-discussed conventional manufacturing method, has the problem that its zero torque point and sensitivity would vary if it has been exposed for a long time in an engine room heated to a high temperature in the order of 80-100.degree. C. FIG. 46 is a diagram showing high-temperature durability characteristics of the magnetostrictive torque sensor made by the above-discussed conventional manufacturing method. In FIG. 46, a characteristic curve C120 represents variation in the values detected by the detecting coil 102d at the beginning of actual use or operation of the magnetostrictive torque sensor, while a characteristic curve C121 represents variation in the values detected by the detecting coil 102e at the beginning of use of the magnetostrictive torque sensor. Characteristic curve C130 represents variation in the values detected by the detecting coil 102d after exposure, to the high temperature, of the sensor, while a characteristic curve C131 represents variation in the values detected by the detecting coil 102e after exposure, to the high temperature, of the sensor.

The characteristic curves C130 and C131, obtained through detection after the exposure to the high temperature (e.g., after the sensor has been used 1,000 times), each present a greater peak value of the impedance and a peak value of the input torque shifted toward the neutral torque point. By comparison between the characteristic curve C120 at the beginning of the use and the characteristic curve C130 after the exposure to the high temperature, it can be seen that there are a change in the peak impedance value from 26.6.OMEGA. to 26.9.OMEGA., and a change in the peak impedance value from 45.1 Nm to 42.8 Nm. Such changes are due to a creep of the plating (which would remove distortion from the plating), and the characteristics at the beginning of the use can not be restored because the characteristics after occurrence of the creep are retained even after the cooling.

The above-mentioned changes result in a change or shift in the zero point Z200 to a zero point Z210. If such a zero point change occurs, detection values of the detecting coils 102d and 102e would exceed a predetermined range when a failure check is performed to determine presence of any failure in the sensor by ascertaining whether a sum of the detection values of the detecting coils 102d and 102e falls within the predetermined range; as a result, the failure check can not be performed appropriately.

Furthermore, with the conventional technique, where the detected torque value is determined on the basis of the difference between the characteristic curves obtained via the detecting coils 102d and 102e, the sensitivity of the torque sensor would undesirably vary as a gradient of the difference varies in accordance with the above-mentioned changes. Because the sensitivity of the torque sensor is generally set, during manufacture of the torque sensor, so as to achieve an optimal control amount of the torque, variation from the thus-set sensitivity may often lead to an uncomfortable steering feel. Further, if the neutral torque point is erroneously set with a deviation from the optimal point during manufacture of the torque sensor, then the torque sensor would present greater variation in the zero torque point and sensitivity.

SUMMARY OF THE INVENTION

In view of the foregoing prior art problems, it is an object of the present invention to provide a technique for enhancing a torque detecting accuracy by uniformizing a magnetostrictive coat thickness of a magnetostrictive torque sensor.

It is another object of the present invention to provide a method of manufacturing a magnetostrictive torque sensor which presents a small hysteresis and can thereby improve a steering feel of an electric power steering apparatus, and also provide an improved electric power steering apparatus equipped with such a magnetostrictive torque sensor.

It is still another object of the present invention to provide a method of manufacturing a magnetostrictive torque sensor which can constantly achieve suitable detection output characteristics even when it has been exposed for a long time to a high-temperature atmosphere as in an engine room of a motor vehicle and can thereby constantly permit a good steering feel of an electric power steering apparatus, and an improved electric power steering apparatus equipped with the magnetostrictive torque sensor made by the manufacturing method.

In order to accomplish the above-mentioned objects, the present invention provides the following.

According one aspect of the present invention, there is provided a method for forming an annular magnetostrictive coat on an outer peripheral surface of a rotational shaft associated with a magnetostrictive torque sensor so that the torque sensor detects torque of the rotational shaft by detecting variation in magnetostrictive characteristic of the magnetostrictive coat as the rotational shaft is torsionally deformed, which comprises: a step of fitting a cylindrical masking jig over the outer peripheral surface of the rotational shaft and securing the masking jig to the outer peripheral surface; a step of placing the rotational shaft in a plating tank to thereby form the magnetostrictive coat on the outer peripheral surface of the rotational shaft; and a step of detaching the masking jig from the rotational shaft.

In this invention, the rotational shaft associated with the magnetostrictive torque sensor, which has the cylindrical masking jig secured to its outer peripheral surface, is placed or immersed in the plating tank to thereby form the magnetostrictive coat, and then the masking jig is detached from the rotational shaft. Namely, the magnetostrictive coat is formed, as by electro plating, on a portion of the outer peripheral surface of the rotational shaft which is not covered with the masking jig, and the thus-formed magnetostrictive coat can have a thickness that is uniform particularly in an axial direction of the rotational shaft. The magnetostrictive coat of the uniform thickness permits an improved toque detecting accuracy of the magnetostrictive torque sensor.

In one preferred implementation, the step of fitting includes a step of positioning the masking jig on the rotational shaft with respect to the axial direction of the rotational shaft, by fitting an annular ridge, formed on and along an inner peripheral surface of the masking jig, in an annular groove formed in and along the outer peripheral surface of the rotational shaft. Because the annular ridge of the masking jig is fitted in the annular groove of the rotational shaft prior to formation of the magnetostrictive coat, the present invention can accurately position the masking jig relative to the rotational shaft in the axial direction and thereby can control the position of the magnetostrictive coat with a high precision.

In one preferred embodiment, the magnetostrictive torque sensor detects steering torque for controlling an actuator of an electric power steering apparatus. In this case, the actuator of the electric power steering apparatus is controlled using the magnetostrictive torque sensor capable of high-accuracy torque detection owing to the magnetostrictive coat of a uniform thickness, with the result that the power steering apparatus can provide an improved steering feel.

According to another aspect of the present invention, there is provided a method for manufacturing a magnetostrictive torque sensor, which comprises: a magnetostrictive coat formation step of forming a magnetostrictive coat on a rotational shaft; a heating step of subjecting the magnetostrictive coat to a high-frequency heating process for a predetermined time with predetermined twisting torque kept applied to the rotational shaft; a torque removal step of removing the twisting torque from the rotational shaft to thereby impart a magnetic anisotropy to the magnetostrictive coat; and a coil positioning step of positioning a coiled coil around the magnetostrictive coat for detecting variation in magnetostrictive characteristic of the magnetostrictive coat.

With the arrangement that the magnetostrictive coat provided on the rotational shaft is subjected to the high-frequency heating (electromagnetic induction heating) for the predetermined time, the method can simultaneously impart the magnetostrictive coat with both greater residual tensile distortion and twisting distortion and thereby minimize a hysteresis of detected torque values. Further, because magnetostrictive characteristics of the magnetostrictive coat after the heating is not influenced by magnetostrictive characteristics present in the magnetostrictive coat before the heating, there can be achieved stable reverse magnetostrictive characteristics.

In one preferred implementation, the magnetostrictive coat contains an iron-nickel alloy material as its main component, and the predetermined twisting torque is in a range not smaller than 50 Nm but not greater than 100 Nm. Because of such arrangements, subjecting the magnetostrictive coat to the high-frequency heating (electromagnetic induction heating) for the predetermined time, e.g. ten seconds or less, the inventive method can simultaneously impart the magnetostrictive coat of the rotational shaft with both greater residual tensile distortion and twisting distortion within a short time and in a stable manner, with the result that it can significantly enhance the productivity as compared to the conventional technique where the magnetostrictive coat of the rotational shaft is heated in a thermostatic bath for several hours.

According to still another aspect of the present invention, there is provided a method for manufacturing a magnetostrictive torque sensor, which comprises: a step of providing a magnetostrictive coat on a rotational shaft; a step of subjecting the magnetostrictive coat to a heating process with predetermined twisting torque kept applied to the rotational shaft; a step of removing the twisting torque from the rotational shaft to thereby impart a magnetic anisotropy to the magnetostrictive coat; a reheating step of subjecting the rotational shaft to a reheating process; and a step of positioning a coiled coil around the magnetostrictive coat for detecting variation in magnetostrictive characteristic of the magnetostrictive coat.

The arrangement that the rotational shaft with the magnetostrictive coat is reheated after removal of the twisting torque, the inventive method can prevent the magnetostrictive coat from creeping by exposure to a high-temperature atmosphere during actual use or operation of the sensor, thereby avoiding undesired fluctuation in detected steering torque outputs and achieving stable torque detection. Because undesired fluctuation in detected steering torque outputs can be avoided, a reliable failure check can be performed on the magnetostrictive torque sensor by ascertaining whether or not a sum of the detected values falls within a predetermined range.

In one preferred implementation, a temperature at which the reheating step reheats the rotational shaft is greater than a normal use temperature at which the magnetostrictive torque sensor is actually used for torque detection. Because sensor output setting is performed after the magnetostrictive coat is previously caused to creep at a temperature higher than the normal use temperature of the magnetostrictive torque sensor, the inventive method can manufacture a magnetostrictive torque sensor capable of producing detected torque outputs with constant characteristics even when exposed for a long time to a high-temperature atmosphere as in the engine room.

According to still another aspect of the present invention, there is provided an electric power steering apparatus, which comprises: a motor for providing steering assist torque to a steering system; a steering torque sensor for detecting steering torque in the steering system; and a control section for controlling operation of the motor on the basis of at least a steering torque signal output from the steering torque sensor, and in which the steering torque sensor is a magnetostrictive torque sensor including a magnetostrictive coat provided on a rotational shaft, the magnetostrictive coat having a magnetic anisotropy imparted thereto by subjecting the magnetostrictive coat to a high-frequency heating process with predetermined twisting torque kept applied to the rotational shaft and then removing the twisting torque from the rotational shaft. With the magnetostrictive torque sensor including the magnetostrictive coat having a magnetic anisotropy, the electric power steering apparatus can provide an improved steering feel with good turning-back of the steering wheel.

According to yet another aspect of the present invention, there is provided an electric power steering apparatus, which comprises: a motor for providing steering assist torque to a steering system; a steering torque sensor for detecting steering torque in the steering system; and a control section for controlling operation of the motor on the basis of at least a steering torque signal output from the steering torque sensor, and in which the steering torque sensor is a magnetostrictive torque sensor including a magnetostrictive coat provided on a rotational shaft, the magnetostrictive coat having a magnetic anisotropy imparted thereto by subjecting the magnetostrictive coat to a heating process with predetermined twisting torque kept applied to the rotational shaft and then removing the twisting torque from the rotational shaft, the rotational shaft being then subjected to a reheating process at a temperature higher than a normal use temperature at which the magnetostrictive torque sensor is actually used for torque detection. With the magnetostrictive torque sensor capable of producing detected torque outputs with constant characteristics even when exposed for a long time to a high-temperature atmosphere in the engine room, the electric power steering apparatus can constantly provide an improved steering feel with constant sensitivity of the steering torque sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain preferred embodiments of the present invention will hereinafter be described in detail, by way of example only, with reference to the accompanying drawings, in which:

FIGS. 1A and 1B are views illustrating a steering shaft having magnetostrictive coats formed thereon;

FIG. 2 is a view showing construction of a steering torque sensor;

FIGS. 3A and 3B are views showing how uniformity of the magnetostrictive coats influence magnetic permeability;

FIGS. 4A and 4B are views showing how air gaps are influenced by positional relationship between the steering shaft and the steering torque sensor;

FIG. 5 is a perspective view showing a shape of a first masking jig;

FIG. 6 is a perspective view showing a shape of a second masking jig;

FIG. 7 is a view showing the first and second masking jigs attached to the steering shaft;

FIG. 8 is a view explanatory of a step of forming the magnetostrictive coats on the steering shaft by electroplating;

FIG. 9 is a graph showing axial thickness distribution of the magnetostrictive coats formed on the steering shaft;

FIG. 10 is a graph showing relationship between a thickness of the masking jigs and a bulging rate at axial opposite end portions of the magnetostrictive coats;

FIGS. 11A-11C are views showing gaps between the axial ends of the magnetostrictive coats and the first and second masking jigs;

FIG. 12 is a graph showing how a variation rate of the thickness at one of the axial end portions of the magnetostrictive coats is influenced by the gap between the axial end of the coat and one of the masking jigs;

FIG. 13 is a graph showing how electric current density influences a ratio of an iron component in the plating;

FIG. 14 is a view showing a magnetostrictive torque sensor manufactured by a method of the present invention and an electric power steering apparatus equipped with the magnetostrictive torque sensor;

FIG. 15 is a diagram schematically showing positional relationship among an exciting coil, detecting coils and magnetostrictive coats in the magnetostrictive torque sensor of the invention;

FIG. 16 is a graph showing reverse magnetostrictive characteristics of the magnetostrictive torque sensor of the invention;

FIG. 17 is a flow chart explanatory of the method for manufacturing the magnetostrictive torque sensor of the present invention;

FIGS. 18A and 18B are views explanatory of how the magnetostrictive coats are heated in a heating step;

FIG. 19 is a graph showing variation over time in a heating time and torque application time;

FIGS. 20A-20C are graphs showing variation curves of the reverse magnetostrictive characteristics of the torque sensor after completion of the heating process performed on the magnetostrictive coats;

FIGS. 21A and 21B are graphs showing variation curves of the reverse magnetostrictive characteristics of the torque sensor before and after the high-frequency heating process;

FIG. 22 is a view explanatory of portions of the magnetostrictive coats that are heated in the high-frequency heating process;

FIGS. 23A-23C are views illustrating deformation caused in a model portion, originally substantially-circular in shape, of one of the magnetostrictive coats when torque is applied to the rotational shaft;

FIGS. 24A-24C are views illustrating deformation of the model portion caused by a conventional heating process;

FIGS. 25A and 25B are views showing deformation of the model portion caused by the conventional heating process;

FIGS. 26A and 26B are views showing deformation of the model portion caused by the conventional heating process;

FIGS. 27A-27C are views illustrating how the model portion is deformed by a heating process of the present invention;

FIGS. 28A and 28B are views showing deformation of the model portion caused by the heating process of the present invention;

FIGS. 29A and 29B are views showing deformation of the model portion caused by the heating process of the present invention;

FIG. 30 is a sectional view showing part of a magnetostrictive torque sensor manufactured by a method of the present invention and an electric power steering apparatus equipped with the magnetostrictive torque sensor;

FIG. 31 is a flow chart showing an example step sequence of the magnetostrictive torque sensor manufacturing method;

FIGS. 32A-32F are views explanatory of how the steering torque detector section is made;

FIG. 33 is a graph showing variation in twisting torque to be applied to the rotational shaft during a time period from a twisting torque impartment step to a twisting torque removal step, and variation in temperature;

FIG. 34 is a view explanatory of a heating process performed in accordance with the method of the present invention;

FIG. 35 is a graph showing impedance characteristics of the magnetostrictive torque sensor made by the manufacturing method of the present invention and impedance characteristics of the magnetostrictive torque sensor at a stage preceding a reheating step;

FIG. 36 is a graph showing test results pertaining to a zero torque point;

FIG. 37 is a graph showing test results pertaining to a gradient of sensitivity around a neutral torque point;

FIG. 38 is a view showing an overall setup of a conventional electric power steering apparatus;

FIG. 39 is a view showing detailed organization of mechanical and electric components in the conventional electric power steering apparatus of FIG. 38;

FIG. 40 is a sectional view taken along the A-A lines of FIG. 38;

FIG. 41 is a diagram showing positional relationship among an exciting coil, detecting coils and magnetostrictive coats in a magnetostrictive torque detector section of the conventional electric power steering apparatus;

FIG. 42 is a graph showing ideal reverse magnetostrictive characteristics of the magnetostrictive torque detector section in the conventional electric power steering apparatus;

FIGS. 43A-43E are view explanatory of a method for manufacturing a conventional magnetostrictive torque sensor;

FIG. 44 is a graph showing an example of actual reverse magnetostrictive characteristics of the conventional magnetostrictive torque sensor;

FIGS. 45A-45D are graphs illustrating measurements of the reverse magnetostrictive characteristics before and after a heating process performed in accordance with the conventional manufacturing method; and

FIG. 46 is a diagram showing high-temperature durability characteristics of the magnetostrictive torque sensor made by the conventional manufacturing method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a description will be given about a magnetostrictive-coat forming method in accordance with a first embodiment of the present invention.

FIG. 1A is a view illustrating a steering shaft having magnetostrictive coats formed thereon, and FIG. 1B is an enlarged view of a section depicted by reference character 11C in FIG. 1B. As seen in FIGS. 1A and 1B, two annular magnetostrictive coats 12, for example, in the form of Ni--Fe plating, are formed, at a predetermined interval, on the outer circumferential surface of the steering shaft 11 of an electric power steering apparatus. Annular grooves 11a or 11b are formed in and along the outer circumferential surface of the shaft 11 near axial opposite ends of each of the two magnetostrictive coats 12. In the illustrated example, the steering shaft 11 has a diameter D of 20 mm, each of the magnetostrictive coats 12 has a width (axial dimension) of 14 mm, the interval w between the magnetostrictive coats 12 is 8 mm, and a target thickness of each of the magnetostrictive coats 12 is several score .mu.m.

FIG. 2 is a view showing construction of a steering torque sensor, FIGS. 3A and 3B are views showing influences of uniformity of the magnetostrictive coats 12 on magnetic permeability, and FIGS. 4A and 4B are views showing how air gaps are influenced by positional relationship between the steering shaft 11 and the steering torque sensor. As illustrated in FIG. 2, the steering torque sensor 13, provided for detecting steering torque applied or input to the steering shaft 11 of the electric power steering apparatus, includes yokes 14 opposed to the respective magnetostrictive coats 12 (only one of the yokes 14 and one of the coats 12 are shown in FIG. 2 for simplicity) and a coil 15 wound on each of the yokes 14, so as to detect the input steering torque on the basis of variation in magnetic flux along a magnetic path formed by each set of the yoke 14 and coat 12.

Namely, as illustrated in FIG. 3A, as the steering shaft 11 is torsionally deformed along with the magnetostrictive coats 12 due to applied steering torque, the magnetic permeability of the coats 12 varies to cause variation in magnetic flux, so that the applied steering torque can be detected on the basis of the magnetic flux variation. As long as the magnetostrictive coats each have an uniform thickness throughout an axial length thereof, the detecting accuracy of the steering torque is not influenced even where an axial position of either one of the coats 12 relative to the corresponding yoke 14 and coil 15 is displaced by an amount a as illustrated in FIG. 4A.

However, if the thickness of the magnetostrictive coat 12 is not uniform in the axial direction as illustrated in FIG. 3B, then the magnetic permeability of the coat 12 becomes non-uniform in the axial direction, which would result in deterioration in the steering torque detecting accuracy. In this case, if the axial position of the coat 12 relative to the yoke 14 and coil 15 is displaced as illustrated in FIG. 4B, an air gap g between the yoke 14 and the coat 12 changes, which would also deteriorate the steering torque detecting accuracy.

Further, if electrical current distribution at the time of a plating process, to be later described, is not uniform, i.e. if the electrical current density is not uniform on the surface to be plated, a ratio of the iron component in the plating, i.e. magnetostrictive characteristics of the plating, becomes non-uniform as seen in FIG. 13 (that is a graph showing how the current density influences the ratio of the iron component in the plating). In this case too, axial positional displacement (misalignment) of the magnetostrictive coat 12 relative to the corresponding yoke 14 and coil 15 would also adversely influence the steering torque detecting accuracy. Therefore, in order to secure a desired detecting accuracy of the steering torque sensor 13, there is a need to uniformize the current density during the plating so as to achieve a uniform axial thickness of the magnetostrictive coats 12.

Thus, in the instant embodiment, two first masking jigs 16 and one second masking jig 17, each made of electrically-insulating resin, are used in forming the magnetostrictive coats 12 on the steering shaft 11 by means of electro plating, so as to uniformize axial thickness distribution of the coats 12.

FIG. 5 is a perspective view showing one of the two first masking jigs 16 constructed similarly. As shown, the first masking jig 16 is a cylindrical member fitted over and secured to the outer circumferential surface of the steering shaft 11 with no gap therebetween, and it is dividable, along a central dividing plane passing its axis, into first and second halves 18 and 19 for detachment and re-attachment from and to the steering shaft 11. Each of the first and second halves 18 and 19 has a semi-circular ridge 18a, 19a, formed on and along its inner peripheral surface, for fitting in the annular groove 11a of the steering shaft 11. The semi-circular ridges 18a and 19a of the two halves 18 and 19 together constitute a circular or annular ridge. The first half 18 of the first masking jig 16 has two recesses 18b, and two bolt holes 18c formed in its opposite ends to extend between the dividing plane and respective ends of the recesses 18b. The second half 19 has two bolt holes 19b formed in its opposite ends for communication with the bolt holes 18c of the first half 18.

FIG. 6 is a perspective view showing the second masking jig 17. As shown, the second masking jig 17 is a cylindrical member fitted over and secured to the outer circumferential surface of the steering shaft 11 with no gap therebetween, and it is also dividable, along a dividing plane passing its axis, into first and second halves 20 and 21 for detachment and re-attachment from and to the steering shaft 11. Each of the first and second halves 20 and 21 has a semi-circular ridge 20a, 21a, formed on and along its inner peripheral surface, for fitting in the annular groove 11b of the steering shaft 11. The semi-circular ridges 20a and 21a of the two halves 20 and 21 together constitute a circular or annular ridge. The first half 20 of the masking jig 17 has two recesses 20b, and two bolt holes 20c formed in its opposite ends to extend between the dividing plane and respective ends of the recesses 20b. The second half 21 has two bolt holes 21b formed in its opposite ends for communication with the bolt holes 20c of the first half 20.

FIG. 7 is a view showing the first and second masking jigs 16 and 17 attached to the steering shaft 11. As shown, the first and second haves 18 and 19 of each of the first masking jigs 16 are attached to the steering shaft 11 in such a manner that their semi-circular ridges 18a and 19a are fitted in the substantial entire circumferential length of one (lowermost or uppermost one) of the annular grooves 11a of the steering shaft 11, and the first and second haves 18 and 19 are secured to the shaft 11 with two bolts 22 each screwed from one of the recess 18b, through the bolt hole 18c, into the corresponding bolt hole 19b of the second half 19. By the fitting engagement between the semi-circular ridges 18a and 19a and the annular groove 11a, each of the first masking jigs 16 can be positioned accurately on the steering shaft 11 with respect to the axial direction of the shaft 11.

Similarly, the first and second haves 20 and 21 of the second masking jig 17 are attached to the steering shaft 11 in such a manner that their semi-circular ridges 20a and 21a are fitted in the substantial entire circumferential length of the remaining two (i.e., intermediate) annular groove 11a of the steering shaft 11, and the first and second haves 20 and 21 are secured to the shaft 11 with two bolts 23 each screwed from one of the recess 20b, through the bolt hole 20c, into the corresponding bolt hole 21b of the second half 21. By the fitting engagement between the semi-circular ridges 20a and 21a and the annular groove 11b, the second masking jig 17 can also be positioned accurately on the steering shaft 11 with respect to the axial direction of the shaft 11.

Outer circumferential surface portions of the steering shaft 11 located upwardly and downwardly of the two first masking jigs 16 are masked by sticking thereto masking tapes 24.

FIG. 8 is a view explanatory of a step of forming the magnetostrictive coats by electro plating. As shown, the steering shaft 11, having the two first masking jigs 16 and one second masking jig 17 securely attached thereto, is immersed in a plating tank 25, having electrolyte stored therein, and placed between a cathode 26 and an anode 27. Thus, outer circumferential surface portions of the steering shaft 11 which are not covered with the masking jigs 16, 17 and masking tape 24.

FIG. 9 is a graph showing thickness distribution, in the axial direction, of the magnetostrictive coats 12 formed on the steering shaft 11. As indicated by solid lines, the magnetostrictive coats 12 present only slight thickness variation or unevenness in the axial direction in the case where the coats 12 are formed using the masking jigs 16 and 17 as well as the masking tapes. However, in the case where the magnetostrictive coats 12 are formed using the masking tapes alone, the coats 12 present sharp thickness increases at their axial opposite end portions contacting the masking tapes. Namely, the use of the first and second masking jigs 16 and 17 can minimize the thickness increases at the axial opposite end portions of the magnetostrictive coats 12, presumably because the electrical current can be prevented from concentrating at the axial opposite end portions of the coats 12 during the plating.

FIG. 10 is a graph showing relationship between the thickness of the masking jigs and an average bulging rate at the axial opposite end portions of the magnetostrictive coats. If the thickness T (see FIG. 7) of the first and second masking jigs 16 and 17 is increased, the bulging rate at the axial opposite end portions of the magnetostrictive coats 12 decreases sharply, as seen from the graph. It may be seen that, if the thickness T of the first and second masking jigs 16 and 17 is set at 10 mm or more, the bulging rate of the coat thickness decreases to 1.9% or less so that there can be obtained a generally uniform thickness of the coats 12.

It is desirable that the axial ends of the first and second masking jigs 16 and 17 be positioned to conform to the axial ends of the magnetostrictive coats 12 to be formed. Because, if there are gaps between the axial ends of the first and second masking jigs 16 and 17 and the axial ends of the magnetostrictive coats 12, the coats 12 would present an increased bulging rate of the thickness at the axial end portions.

FIGS. 11A-11C are views showing the gaps between the axial ends of the magnetostrictive coats 12 and the first and second masking jigs 16 and 17, and FIG. 12 is a graph showing how a variation rate of the thickness at one of the axial end portions of the magnetostrictive coats 12 is influenced by the gap between the axial end of the coat 12 and one of the masking jigs 16 and 17. Let's assume a case where the first and second masking jigs 16 and 17 each have a smaller height H (see FIG. 7) and gaps .beta. are formed between the axial ends of the magnetostrictive coats 12 to be formed and masking jigs 16 and 17 and where the plating process is performed with the gaps .beta. covered with the making tapes. In FIG. 12, the horizontal axis represents the gap .beta., while the vertical axis represents the variation rate of the thickness at the axial end portion of the magnetostrictive coat 12. Left end point of a curve in the graph represents the thickness variation rate achievable with the instant embodiment of the invention (where the gap .beta. is zero as illustrated in FIG. 11A), which is the minimum thickness variation rate. Further, a black dot separately depicted at a right end point in the graph represents a thickness variation rate achievable with the conventionally-known technique as discussed above (that does not employ the masking jigs 16 and 17 of FIG. 11C), which is the maximum thickness variation rate.

From the graph of FIG. 12, it can be seen that the approach of positioning the axial ends of the first and second masking jigs 16 and 17 to conform to the axial ends of the magnetostrictive coats 12 to be formed is most advantageous for uniformizing the thickness of the magnetostrictive coats 12.

Because a predetermined actuator of the electric power steering apparatus is controlled using the magnetostrictive steering torque sensor 13 that has the magnetostrictive coats 12 of the uniform thickness formed thereon and is thereby capable of high-accuracy steering torque detection, there can be provided an enhanced steering feel. However, it should be appreciated that the magnetostrictive torque sensor of the invention is applicable to any other desired applications than the steering torque detection in electric power steering apparatus.

Namely, according to the present invention set forth above, the cylindrical masking jigs are first fitted over and secured to the ou