Abstract:
A method for manufacturing a magnetostrictive torque sensor compriseis the steps of forming magnetostrictive films on a rotating shaft of a magneto-strictive torque sensor, creating magnetic anisotropy in the magnetostrictive films formed in the magnetostrictive film formation step, and demagnetizing the rotating shaft. The demagnetization step is provided in any of the stages after the magnetostrictive film formation step, and comprises initializing the remanent magnetism created in the rotating shaft.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for manufacturing a magnetostrictive torque sensor, and particularly relates to a method for manufacturing a magnetostrictive torque sensor that is suitable for reducing nonuniformities induced in the sensitivity characteristics of different sensors by magnetizing effects in the various steps, and for increasing the efficiency of assembling the sensor in an electrically powered steering apparatus or the like.  
       BACKGROUND OF THE INVENTION  
       [0002]     In an electrically powered steering apparatus that is provided as a steering system in an automobile, for example, a steering torque sensor commonly senses a steering torque applied to a steering shaft from a steering wheel by the steering operation of the driver. In the prior art, the steering torque sensor is normally configured from a torsion bar torque sensor, and magnetostrictive torque sensors have recently been proposed. The steering shaft functions as a rotating shaft that rotates due to rotational force from the steering operation. The steering shaft constitutes a rotating shaft in the steering torque sensor. The electrically powered steering apparatus controls the driving of a steering force auxiliary motor according to a torque signal detected from the steering torque sensor, and reduces the steering force for the driver to provide a pleasant steering feel.  
         [0003]     As described above, magnetostrictive torque sensors are well known as steering torque sensors used in electrically powered steering apparatuses. In such a magnetostrictive torque sensor, magnetostrictive films that are magnetically anisotropic with respect to each other are formed at two specific locations on the surface of the steering shaft. The magnetostrictive torque sensor has a configuration in which a non-contact system is used to detect changes in the magnetostrictive characteristics of the magnetostrictive films that correspond to the torsion of the steering shaft when torque is applied to the steering shaft from the steering wheel.  
         [0004]     In the process for manufacturing a magnetostrictive torque sensor, a magnetostrictive film is formed over the circumferential surface in a specific surface in part of the steering shaft; i.e., over a specific axial width in the rotating shaft; and then a process must be performed to provide this magnetostrictive film with magnetic anisotropy. Conventional methods for providing the magnetostrictive film with magnetic anisotropy in the manufacture of a magnetostrictive torque sensor involve applying a twisting torque to a rotating shaft on which a magnetostrictive plating (magnetostrictive film) is formed by an electroplating process, for example, thus creating stress in the circumferential surface of the rotating shaft. This is followed by heat-treating the rotating shaft in a thermostat while the shaft is kept under stress (see JP 2002-82000 A, for example).  
         [0005]     In a conventional method for manufacturing a magnetostrictive torque sensor, electromagnetism acts on the rotating shaft because a device is provided for generating electromagnetic action in an electroplating step for forming magnetostrictive films on the rotating shaft, or in a heating step for creating magnetic anisotropy in the magnetostrictive films formed on the surface of the rotating shaft. As a result, nonuniform irregular magnetization occurs at numerous locations on the surface of the rotating shaft or the surfaces of the magnetostrictive films in these steps. Therefore, in a magnetostrictive torque sensor manufactured by a conventional magnetostrictive torque sensor manufacturing method, the magnetic anisotropy characteristics in the magnetostrictive films have been subject to the effects of irregular magnetization created in the rotating shaft surface or the magnetostrictive film surfaces, resulting in non-uniform sensor sensitivity when torque is sensed. Particularly, since the magnetization in the rotating shaft surface or the magnetostrictive film surfaces is not uniform, problems have arisen with nonuniform torque sensing sensitivity among magnetostrictive torque sensors. When nonuniform torque sensing sensitivity occurs among magnetostrictive torque sensors in this manner, the operator must adjust the sensor sensitivity when a magnetostrictive torque sensor is assembled in an electrically powered steering apparatus. Therefore, the sensitivity of magnetostrictive torque sensors produced by such methods for manufacturing a magnetostrictive torque sensor must be exhaustively tested, and the sensors must be individually adjusted according to the sensitivity characteristics determined by testing when the sensors are assembled in apparatuses.  
         [0006]     It can therefore be expected that if the steps of conventional methods for manufacturing magnetostrictive torque sensors are improved and nonuniformities in the sensitivity of completed magnetostrictive torque sensors are resolved, then the step for exhaustively testing sensor sensitivity can be omitted, and the operation of adjusting the sensitivity of the sensors when the sensors are assembled apparatuses can be simplified.  
         [0007]     Because of the matters described above, a need exists for a method whereby nonuniformities induced in the sensitivity characteristics of different sensors by the steps involved in the methods for manufacturing magnetostrictive torque sensors can be reduced by improving the steps of these manufacturing methods.  
         [0008]     A need therefore exists for establishing a method for manufacturing a magnetostrictive torque sensor wherein nonuniformities induced in the sensitivity characteristics of different sensors by the process for manufacturing the magnetostrictive torque sensor are reduced by improving the manufacturing process, the step for exhaustively testing the torque sensing sensitivity if each sensor can be omitted, the load of adjusting the sensor during apparatus assembly can be reduced, and the efficiency of the operation of assembling the sensor in an electrically powered steering apparatus or the like can be increased.  
       SUMMARY OF THE INVENTION  
       [0009]     According to the present invention, there is provided a method for manufacturing a magnetostrictive torque sensor comprising the steps of forming magnetostrictive films on a rotating shaft, creating magnetic anisotropy in the magnetostrictive films formed on the rotating shaft, and demagnetizing the rotating shaft.  
         [0010]     The demagnetization step for demagnetizing the rotating shaft allows nonuniform and irregular remanent magnetization to be removed. This type of magnetization is created in the surface of a rotating shaft in the magnetostrictive film formation step or the magnetic anisotropy formation step. The magnetostrictive characteristics of the magnetostrictive films formed on the rotating shaft are thereby not susceptible to the effects of the remanent magnetization in the rotating shaft, and nonuniformities in the sensitivity and other such sensor characteristics can be reduced when the torque is sensed.  
         [0011]     Preferably, the demagnetization step comprises initializing remanent magnetization created in the rotating shaft by steps prior to the demagnetization step.  
         [0012]     In a preferred form, the demagnetization step is carried out after the magnetostrictive film formation step.  
         [0013]     Desirably, the method further comprises, after the demagnetization step, providing sensor means around the peripheries of the magnetostrictive films for sensing changes in the magnetostrictive characteristics of the magnetostrictive films as a torque is applied to the rotating shaft. The magnetostrictive torque sensor is completed by adding excitation coils or other such sensor means to the completed rotating shaft.  
         [0014]     In the inventive method, the demagnetization step is provided either after or immediately before the step of creating magnetic anisotropy in the magnetostrictive films, and repeated irregular magnetization created in the rotating shaft surface or the magnetostrictive film surfaces by the electroplating step or the magnetic anisotropy formation step are removed to the fullest extent possible. Therefore, effects of irregular magnetization, such as those seen in conventional practice, on the magnetostrictive characteristics of the magnetostrictive films are reduced, and nonuniformities in the sensor sensitivity characteristics during torque sensing are reduced.  
         [0015]     Since the torque sensing characteristics of different magnetostrictive torque sensors are made uniform, there is no need for exhaustive testing on the manufactured magnetostrictive torque sensors. As a result, a sampling test is sufficient to test the quality of the magnetostrictive torque sensors, whereby the number of steps in testing the quality of the magnetostrictive torque sensors can be reduced, and the testing process can be accomplished in less time.  
         [0016]     Furthermore, since the characteristics of the magnetostrictive torque sensors are made uniform, the quality of the magnetostrictive torque sensors is improved, the sensor sensitivity is more easily adjusted when a magnetostrictive torque sensor is assembled in an electrically powered steering apparatus or the like, the operating load and operating time can be reduced, and operating efficiency can be improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     Certain preferred embodiments of the present invention will be described in detail below, by way of example only, with reference to the accompanying drawings, in which:  
         [0018]      FIG. 1  is a partial cross-sectional side view showing the basic structure of a magnetostrictive torque sensor manufactured by the method for manufacturing a magnetostrictive torque sensor according to the present invention;  
         [0019]      FIG. 2  is a side view schematically showing the basic configuration of the magnetostrictive torque sensor;  
         [0020]      FIG. 3  is a fragmentary longitudinal cross-sectional view of a specific structure in which the magnetostrictive torque sensor is incorporated as a steering torque sensor into the steering shaft of an electrically powered steering apparatus;  
         [0021]      FIG. 4  is a graph showing the magnetostrictive characteristic curves and sensor characteristics of sensor coils in a magnetostrictive torque sensor;  
         [0022]      FIG. 5  is a view showing the process for manufacturing a rotating shaft as part of the method for manufacturing a magnetostrictive torque sensor according to the present invention;  
         [0023]      FIG. 6  is a flowchart of the magnetic anisotropy formation step;  
         [0024]      FIGS. 7A through 7D  are views showing the temperature distribution and torsion distribution in the radial direction in a rotating shaft in the steps of the magnetic anisotropy formation step;  
         [0025]      FIG. 8  is a view showing the impedance characteristics of a magnetostrictive torque sensor immediately after magnetostrictive plating parts have been formed, as well as the impedance characteristics of a magnetostrictive torque sensor that uses a magnetically anisotropic magnetostrictive films in the method for manufacturing a magnetostrictive torque sensor according to the present invention;  
         [0026]      FIGS. 9A and 9B  are schematic views showing the magnetization and other such states of the rotating shaft after the demagnetization step;  
         [0027]      FIG. 10  is a perspective view of a demagnetization device for demagnetizing the rotating shaft;  
         [0028]      FIG. 11  is a perspective view of a process in which the rotating shaft is demagnetized using the demagnetization device;  
         [0029]      FIG. 12  is a graph showing the change over time in a normal magnetic field in the container surface of the demagnetization device;  
         [0030]      FIG. 13  is a graph showing the change over time in the magnetic field when the rotating shaft is demagnetized using the demagnetization device;  
         [0031]      FIG. 14  is a changing characteristics view showing the change in magnetization in the rotating shaft in relation to the change in the magnetic field; and  
         [0032]      FIG. 15  is a graph showing a comparison between the process capability index of a magnetostrictive torque sensor manufactured by a manufacturing method devoid of a demagnetization step, and the process capability index Cp of a magnetostrictive torque sensor manufactured by a manufacturing method provided with a demagnetization step. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]     A magnetostrictive torque sensor will be described with reference to  FIGS. 1 through 3 .  FIGS. 1 through 3  show a structural example of a magnetostrictive torque sensor manufactured by the method for manufacturing a magnetostrictive torque sensor according to the present invention.  
         [0034]     A magnetostrictive torque sensor  10  is configured from a rotating shaft  11 , and one excitation coil  12  and two sensor coils  13 A,  13 B disposed around the periphery of the rotating shaft  11 , as shown in  FIGS. 1 and 2 . For the sake of convenience in the description, the rotating shaft  11  is shown without the top and bottom parts in  FIGS. 1 and 2 .  
         [0035]     Referring to the example of utilization shown in  FIG. 3 , the rotating shaft  11  is configured as part of a steering shaft  21 , for example. The rotating shaft  11  is subjected to the rotational force (torque) of right-hand rotation (clockwise) or left-hand rotation (counterclockwise) around the axis  11   a , as shown by the arrow A. The rotating shaft  11  is formed from a metal rod made of chromium-molybdenum steel (SCM) or the like, for example. Magnetostrictive films  14 A,  14 B are provided to the rotating shaft  11  at two locations aligned vertically in the axial direction. The magnetostrictive films  14 A,  14 B both have specific widths in the axial direction of the rotating shaft  11 , and also are formed over the entire circumferential periphery of the rotating shaft  11 . The width dimension of the magnetostrictive films  14 A,  14 B and the dimension of the space between the two magnetostrictive films  14 A,  14 B are arbitrarily set in accordance with certain conditions. In practice, the magnetostrictive films  14 A,  14 B are formed on the surface of the rotating shaft  11  as magnetostrictive plating parts by an electroplating process. The magnetostrictive films  14 A,  14 B, which are magnetically anisotropic, are formed by processing the magnetostrictive plating units so that magnetic anisotropy is obtained.  
         [0036]     For the sake of convenience in the description below, the terms “magnetostrictive films  14 A,  14 B” and “magnetostrictive plating parts ( 14 A,  14 B)” denote the same items, but are used for different purposes depending on the steps and conditions of manufacturing. In principle, the completed products after magnetic anisotropy has been created are referred to as the “magnetostrictive films  14 A,  14 B,” and prior to this step these parts are referred to as “magnetostrictive plating parts.” 
         [0037]     The excitation coil  12  and the sensor coils  13 A,  13 B are provided for both of the two magnetostrictive films  14 A,  14 B formed on the surface of the rotating shaft  11 , as shown in  FIG. 1 . Specifically, the sensor coil  13 A is disposed with an interposed gap from the periphery of the magnetostrictive film  14 A, as shown in  FIG. 1 . The ring-shaped sensor coil  13 A encircles the entire periphery of the magnetostrictive film  14 A, and the axial width dimension of the sensor coil  13 A is substantially equal to the axial width dimension of the magnetostrictive film  14 A. Also, the sensor coil  13 B is disposed with an interposed gap from the periphery of the magnetostrictive film  14 B. The ring-shaped sensor coil  13 B similarly encircles the entire periphery of the magnetostrictive film  14 B, and the axial width dimension of the sensor coil  13 B is substantially equal to the axial width dimension of the magnetostrictive film  14 B. Furthermore, a ring-shaped excitation coil  12  is disposed around the peripheries of two sensor coils  13 A,  13 B. In  FIG. 1 , ring-shaped excitation coils  12  are illustrated as being provided separately to the magnetostrictive films  14 A,  14 B, but this is a depiction of two portions of what is actually one excitation coil  12 . The sensor coils  13 A,  13 B and the excitation coil  12  are wound in the peripheral space around the magnetostrictive films  14 A,  14 B using ring-shaped supporting frames  15 A,  15 B that are provided to the periphery of the rotating shaft  11  so as to encircle the rotating shaft  11 .  
         [0038]     In  FIG. 2 , the excitation coil  12  and the sensor coils  13 A,  13 B are schematically shown in terms of their electrical relationship to the magnetostrictive films  14 A,  14 B of the rotating shaft  11 . An alternating-current (AC) power source  16  that constantly supplies an AC excitation current is connected to the excitation coil  12  shared by the magnetostrictive films  14 A,  14 B. Also, induced voltages V A , V B  corresponding to the torque to be detected are outputted from the output terminals of the sensor coils  13 A,  13 B provided to the magnetostrictive films  14 A,  14 B, respectively.  
         [0039]     The magnetostrictive films  14 A,  14 B formed on the surface of the rotating shaft  11  are magnetically anisotropic magnetostrictive films formed by an electroplating process using Ni—Fe plating, for example. The two magnetostrictive films  14 A,  14 B are formed so as to be inversely magnetically anisotropic to each other. When torque is applied to the rotating shaft  11  by a rotational force, the reverse magnetostrictive characteristics produced in the magnetostrictive films  14 A,  14 B are detected using the sensor coils  13 A,  13 B disposed around the periphery of the magnetostrictive films  14 A,  14 B.  
         [0040]     The magnetostrictive torque sensor  10  is incorporated as a steering torque sensor into the steering shaft of an electrically powered steering apparatus, for example, as shown in  FIG. 3 . Elements in  FIG. 3  that are substantially identical to those described in  FIGS. 1 and 2  are denoted by the same numerical symbols.  FIG. 3  shows the specific configuration of a steering torque sensor  20 , a supporting structure for a steering shaft  21  (corresponding to the rotating shaft  11 ), a rack-and-pinion mechanism  34 , a drive force transmission mechanism  35 , and a steering force auxiliary motor  42 .  
         [0041]     In  FIG. 3 , the top of the steering shaft  21  is joined to the steering wheel (not shown) of the vehicle. The bottom of the steering shaft  21  is configured so as to transmit steering force to a vehicle shaft comprising a rack shaft, via the rack-and-pinion mechanism  34 . The steering torque sensor  20  provided at the top of the steering shaft  21  is configured using the magnetostrictive torque sensor  10 . The steering torque sensor  20  corresponds to the magnetostrictive torque sensor  10 , and the portion of the steering shaft  21  on which the magnetostrictive films  14 A,  14 B are formed corresponds to the rotating shaft  11 .  
         [0042]     The steering shaft  21  is rotatably supported by two shaft bearings  32 ,  33  in a housing  31   a  that forms a gear box  31 . The rack-and-pinion mechanism  34  and the drive force transmission mechanism  35  are accommodated inside the housing  31   a.    
         [0043]     The steering torque sensor  20  is provided for the steering shaft  21 . The previously described magnetostrictive films  14 A,  14 B are formed on the steering shaft  21 , and the excitation coil  12  and sensor coils  13 A,  13 B corresponding to the magnetostrictive films  14 A,  14 B are supported by the supporting frames  15 A,  15 B and yokes  36 A,  36 B.  
         [0044]     The top opening of the housing  31   a  is closed by a lid  37 . A pinion  38  provided at the bottom end of the steering shaft  21  is positioned between the shaft bearings  32 ,  33 . A rack shaft  39  is guided by a rack guide  40  and is urged by a compressed spring  41  to press against the side of the pinion  38 . The drive force transmission mechanism  35  is formed by a worm gear  44  fixed on a transmission shaft  43  that is joined to the output shaft of the steering force auxiliary motor  42 , and also a worm wheel  45  fixed on the steering shaft  21 . The steering torque sensor  20  is attached to the interior of a cylindrical part  37   a  of the lid  37 .  
         [0045]     The steering torque sensor  20  senses the steering torque applied to the steering shaft  21 . The sensed value is inputted to a control apparatus (not shown) and is used as a reference signal for generating a suitable auxiliary steering torque in an electric motor  42 . When the steering torque from the steering wheel is applied to the steering shaft  21 , the steering torque sensor  20  electrically senses changes in the magnetic characteristics of the magnetostrictive films  14 A,  14 B that correspond to the torsion in the steering shaft  21 . The changes are sensed as changes in the induced voltages V A , V B  from the output terminals of the sensor coils  13 A,  13 B.  
         [0046]     Torsion occurs in the steering shaft  21  when steering torque acts on the steering shaft  21 . As a result, a magnetostrictive effect is created in the magnetostrictive films  14 A,  14 B. Since an excitation electric current is constantly supplied to the excitation coil  12  from the AC power source  16  in the steering torque sensor  20 , the change in the magnetic field resulting from the magnetostrictive effect in the magnetostrictive films  14 A,  14 B is detected by the sensor coils  13 A,  13 B as a change in the induced voltages V A , V B . According to the steering torque sensor  20 , the difference between the two induced voltages V A , V B  is outputted as a detected voltage value on the basis of the change in the induced voltages V A , V B . Therefore, the direction and extent of the steering torque (T) applied to the steering shaft  21  can be sensed based on the outputted voltage (V A −V B ) of the steering torque sensor  20 .  
         [0047]      FIG. 4  will now be described in further detail. In  FIG. 4 , the horizontal axis represents the steering torque applied to the steering shaft  21 , wherein the positive side (+) corresponds to right-hand rotation, while the negative side (−) corresponds to left-hand rotation. The vertical axis in  FIG. 4  represents a voltage axis.  
         [0048]     The magnetostrictive characteristic curves  51 A,  51 B for the magnetostrictive films  14 A,  14 B simultaneously show the detection output characteristics of the sensor coils  13 A,  13 B. Specifically, an excitation AC current is supplied by the shared excitation coil  12  to the magnetostrictive films  14 A,  14 B that have the magnetostrictive characteristic curves  51 A,  51 B, and the sensor coils  13 A,  13 B respond to this excitation AC current by outputting induced voltages. Therefore, the changing characteristics of the induced voltages of the sensor coils  13 A,  13 B correspond to the magnetostrictive characteristic curves  51 A,  51 B of the magnetostrictive films  14 A,  14 B. In other words, the magnetostrictive characteristic curve  51 A shows the changing characteristics of the induced voltage V A  outputted from the sensor coil  13 A, while the magnetostrictive characteristic curve  51 B shows the changing characteristics of the induced voltage V B  outputted from the sensor coil  13 B.  
         [0049]     According to the magnetostrictive characteristic curve  51 A, the value of the induced voltage V A  outputted from the sensor coil  13 A increases in a substantially linear fashion as the value of the steering torque changes from negative to positive and approaches the positive steering torque value T 1 , then peaks when the steering torque reaches the positive value T 1 , and gradually decreases as the steering torque increases past T 1 . According to the magnetostrictive characteristic curve  51 B, the value of the induced voltage V B  outputted from the sensor coil  13 B gradually increases as the value of the steering torque approaches the negative value −T 1 , then peaks when the steering torque reaches the negative value −T 1 , and decreases in substantially linear fashion as the steering torque further increases past −T 1  and changes from negative to positive.  
         [0050]     As shown in  FIG. 4 , the magnetostrictive characteristic curve  51 A pertaining to the sensor coil  13 A and the magnetostrictive characteristic curve  51 B pertaining to the sensor coil  13 B reflect that the magnetostrictive films  14 A,  14 B are inversely magnetically anisotropic to each other, and have a relationship of substantially linear symmetry about the vertical axis that includes the point where the two magnetostrictive characteristic curves intersect.  
         [0051]     The line  52  shown in  FIG. 4  indicates a graph that is created based on values obtained in a region that is common to the magnetostrictive characteristic curves  51 A,  51 B and that has substantially linear characteristics. The values of this line are obtained by subtracting the corresponding values of the magnetostrictive characteristic curve  51 B obtained as output voltages of the sensor coil  13 B from the values of the magnetostrictive characteristic curve  51 A obtained as output voltages of the sensor coil  13 A. When the steering torque is zero, the induced voltages outputted from the sensor coils  13 A,  13 B are equal, and their difference is therefore zero. In the steering torque sensor  20 , the line  52  is formed as being a substantially straight line by using the region in the magnetostrictive characteristic curves  51 A,  51 B that is considered to have a substantially constant slope near the mean point (zero) of the steering torque. The vertical axis in  FIG. 4  represents an axis that indicates a voltage difference value for the characteristic graph of the line  52 . The line  52 , which is a characteristic graph, is a straight line that passes through the origin ( 0 ,  0 ) and lies on the positive and negative sides of both the vertical and horizontal axes. Since the detection output values of the steering torque sensor  20  are obtained as the difference (V A −V B ) between induced voltages outputted from the sensor coils  13 A,  13 B as previously described, the direction and extent of the steering torque applied to the steering shaft  21  can be detected based on the use of the straight line  52 .  
         [0052]     As described above, it is possible to obtain a sensor signal that corresponds to the rotational direction and extent of the steering torque inputted to the steering shaft  21  (rotating shaft  11 ). The signal is obtained based on the output values of the steering torque sensor  20 . Specifically, the rotational direction and extent of the steering torque applied to the steering shaft  21  can be known from the sensor values outputted from the steering torque sensor  20 .  
         [0053]     In other words, the sensor values of the steering torque sensor  20  are outputted as any of the points on the vertical line  52  in accordance with the steering torque. The steering torque is determined to be rotating to the right when the sensor value is on the positive side of the horizontal axis, and the steering torque is determined to be rotating to the left when the sensor value is on the negative side of the horizontal axis. The absolute value of the sensor value on the vertical axis is the extent of the steering torque. Thus, it is possible, with the steering torque sensor  20 , to sense the steering torque on the basis of the output voltage values of the sensor coils  13 A,  13 B by using the characteristics of the vertical line  52 .  
         [0054]     The following is a description, made with reference to  FIGS. 5 through 15 , of the method for manufacturing the magnetostrictive torque sensor  10  previously described. The main part of the method for manufacturing the magnetostrictive torque sensor  10  in  FIG. 5  shows the steps for manufacturing the rotating shaft  11 ; i.e., the steering shaft  21  of the magnetostrictive torque sensor  10 .  FIG. 5  primarily shows all the steps for manufacturing the rotating shaft  11 .  
         [0055]     In  FIG. 5 , broadly classified, the process for manufacturing the rotating shaft  11  comprises a magnetostrictive film formation step P 1 , a magnetic anisotropy formation step P 2 , a characteristic stabilization step P 3 , and a testing step P 4 . The characteristic stabilization step P 3  comprises an annealing step P 31  and a demagnetization step P 32 . The testing step P 4  is a step for inspecting the quality of the manufactured rotating shaft. To complete the magnetostrictive torque sensor  10 , a detection device mounting step is provided after the testing step P 4 , wherein the excitation coil  12 , the sensor coils  13 A,  13 B, and the other detection devices are mounted to the rotating shaft  11 .  
         [0056]     First, the magnetostrictive film formation step P 1  is performed. In the magnetostrictive film formation step P 1 , magnetostrictive plating parts are formed by electroplating as base portions for the magnetostrictive films at specific locations on the surface of the rotating shaft  11 .  
         [0057]     In the magnetostrictive film formation step P 1 , washing or another such preparatory process is first performed on the rotating shaft  11  (step S 11 ). Electroplating is then performed (step S 12 ). This electroplating is performed so that the magnetostrictive material reaches a specific thickness at the top and bottom locations on the rotating shaft  11 . The upper and lower magnetostrictive plating parts are formed into magnetically anisotropic magnetostrictive films  14 A,  14 B by a post-process to be described later. Drying is then performed (step S 13 ).  
         [0058]     In the magnetostrictive film formation step P 1 , an electroplating method was used to form the previously described magnetostrictive films  14 A,  14 B on the surface of the rotating shaft  11 . However, the base portions that form the magnetostrictive films  14 A,  14 B on the rotating shaft  11  can also be formed by methods other than electroplating, such as sputtering, ion plating, or another such PVD method; plasma spraying; or the like.  
         [0059]     Next, the magnetic anisotropy formation step P 2  is performed. The magnetic anisotropy formation step P 2  is a step for creating magnetic anisotropy in the magnetostrictive plating parts formed at the two top and bottom locations on the rotating shaft  11 , thus forming the previously described magnetostrictive films  14 A,  14 B. The magnetic anisotropy formation step P 2  has a step S 21  of high-frequency heating performed on the top magnetostrictive plating part, and a step S 22  of high-frequency heating performed on the bottom magnetostrictive plating part.  
         [0060]      FIG. 6  shows a flowchart of the processing steps performed in steps S 21  and S 22  in the magnetic anisotropy formation step P 2 .  FIGS. 7A through 7D  are views showing the temperature distribution in the axial and radial direction, and the torsion distribution in the axial and radial direction, in the magnetostrictive plating parts on the rotating shaft  11  in steps S 21  and S 22  in the magnetic anisotropy formation step P 2 .  
         [0061]     As shown in  FIG. 6 , step S 21  of the high-frequency heating of the top magnetostrictive plating part in the magnetic anisotropy formation step P 2  comprises step S 201 , which is performed first to apply a specific twisting torque to the rotating shaft  11  via a torque application device; a heating step S 202  performed next to heat the top magnetostrictive plating part of the rotating shaft  11  by magnetic induction, wherein high frequency waves are supplied for a specific amount of time while the specific twisting torque is being applied; a subsequently performed step S 203  of naturally cooling the heated rotating shaft  11 ; and the finally performed torque releasing step S 204  of creating magnetic anisotropy in the top magnetostrictive plating part by releasing the twisting torque, thus forming the magnetostrictive film  14 A.  
         [0062]     In the heating step S 202 , an induction heating coil is placed on the top magnetostrictive plating part of the rotating shaft  11 , and specific high-frequency waves are supplied to this induction heating coil from a high-frequency power source to perform high-frequency heating on only the top magnetostrictive plating part.  
         [0063]     Magnetic anisotropy is created in the top magnetostrictive plating part of the rotating shaft  11  in steps S 201  through S 204 , whereby a magnetically anisotropic magnetostrictive film  14 A is formed.  
         [0064]     The steps S 201  through S 204  are similarly performed in the high-frequency heating step S 22  for the bottom magnetostrictive plating part of the rotating shaft  11 , creating magnetic anisotropy in the bottom magnetostrictive plating part, whereby a magnetically anisotropic magnetostrictive film  14 B is formed. In this case, magnetic anisotropy is created in the bottom magnetostrictive plating part, whereupon the direction in which torque is applied to the rotating shaft  11  is reversed so as to achieve inverse magnetic anisotropy in the magnetostrictive film  14 B.  
         [0065]     The following is a description, made with reference to  FIGS. 7A through 7D  and to  FIG. 8 , of the mechanism whereby magnetic anisotropy is created in the magnetostrictive plating parts, and the magnetostrictive film  14 A is formed in the magnetic anisotropy formation step P 2 .  
         [0066]      FIGS. 7A through 7D  show the temperature distribution in the radial direction of the rotating shaft  11  at the top of the view, and the torsion distribution in the radial direction of the rotating shaft  11  at the bottom of the view.  FIGS. 7A through 7D  also show a state of torque application ( FIG. 7A ), a state of induction heating ( FIG. 7B ), a state of releasing plating torsion ( FIG. 7C ), and a state of releasing torque ( FIG. 7D ), respectively. The state of torque application ( FIG. 7A ) corresponds to step S 201  shown in  FIG. 6 , the state of induction heating ( FIG. 7B ) corresponds to step S 202  in the same view, the state of releasing the plating torsion ( FIG. 7C ) corresponds to step S 203  in the same view, and the state of releasing the torque ( FIG. 7D ) corresponds to step S 204  in the same view. The axis  61  in  FIG. 7A  indicates temperature, and the axis  62  indicates torsion. The axis  61  that expresses temperature and the axis  62  that expresses torsion are used in the same manner in  FIGS. 7B through 7D .  
         [0067]     In  FIG. 7A , a twisting torque Tq is applied to the rotating shaft  11 , and stress is created in the circumferential surface of the rotating shaft  11 . The twisting torque Tq is thereby applied. In this case, the torsion distribution in the radial direction of the rotating shaft  11  is a distribution ST 1  that increases outward towards the periphery away from the axis  11   a  in the middle of the rotating shaft  11 . In the distribution ST 1 , the direction of the torsion distribution is opposite on the right and left sides of the axis  11   a , and therefore the torsion distribution on the right side is shown as positive (+), and the torsion distribution on the left side is shown as negative (−). Furthermore, the temperature distribution in the radial direction of the rotating shaft  11  in  FIG. 7A  is shown by the broken line, and is a constant distribution T 1  at room temperature from the axis  11   a  of the rotating shaft  11  outward to the periphery. This room temperature is a reference for the temperature of the rotating shaft  11 .  
         [0068]     In  FIG. 7B , while twisting torque Tq is being applied to the rotating shaft  11 , the periphery of the magnetostrictive plating part is placed inside an induction heating coil, a high-frequency electric current is supplied to the induction heating coil, and the magnetostrictive plating part is heated. In  FIG. 7B , the torsion distribution in the radial direction of the rotating shaft  11  is the same as in  FIG. 7A . Also, the temperature distribution in the radial direction of the rotating shaft  11  is a distribution T 2  wherein the temperature abruptly increases towards the outer peripheral edge of the rotating shaft  11  from a point near the outer peripheral edge.  
         [0069]     In  FIG. 7C , cooling is performed, causing cleaving to occur in the magnetostrictive plating part, and the torsion in the magnetostrictive plating part to reach zero. The torsion distribution in the radial direction of the rotating shaft  11  at this time is shown by the numerical symbol ST 2 . The step showing the state in  FIG. 7C  is step S 203  of naturally cooling after the heating process. There is no substantial change in the shape of the temperature distribution T 2  in the radial direction of the rotating shaft  11 , and the temperature decreases as a whole as cooling proceeds.  
         [0070]     In  FIG. 7D , torque is released, wherein the twisting torque Tq applied to the rotating shaft  11  is released after cooling. The torsion distribution in the radial direction of the rotating shaft  11  thereby reaches zero, as shown by the torsion distribution ST 3 . Conversely, a torsion distribution is seen only in the magnetostrictive plating part as shown by the torsion distribution ST 3 . As a result, magnetic anisotropy can be created in the magnetostrictive plating part by means of this torsion distribution ST 3 , and a magnetostrictive film  14 A having magnetic anisotropy can thereby be formed. The temperature distribution in  FIG. 7D  is reduced so as to generally be smoothly distributed, as shown by T 3 .  
         [0071]     When the magnetostrictive film  14 B is created, the process previously described is performed by applying a clockwise twisting torque in the opposite direction of the twisting torque Tq to create magnetic anisotropy in the opposite direction of the magnetostrictive film  14 A.  
         [0072]      FIG. 8  shows the impedance characteristics Z 0  of the magnetostrictive plating parts provided at the two top and bottom locations on the rotating shaft  11 , and the impedance characteristics Z A , Z B  of the magnetostrictive films  14 A,  14 B formed by creating magnetic anisotropy in the magnetostrictive plating parts. In  FIG. 8 , the horizontal axis represents torque (relative units), and the vertical axis represents impedance (relative units). The impedance characteristics Z 0  of the magnetostrictive plating parts prior to the creation of magnetic anisotropy change to the impedance characteristics Z A  in the case of the magnetostrictive film  14 A, and to the impedance characteristics Z B  in the case of the magnetostrictive film  14 B. The change is brought about by the creation of magnetic anisotropy. Since the magnetostrictive film  14 A has the impedance characteristics Z A , the sensor coil  13 A corresponding to the magnetostrictive film  14 A has the previously described magnetostrictive characteristic curve  51 A. Also, since the magnetostrictive film  14 B has the impedance characteristics Z B , the sensor coil  13 B corresponding to the magnetostrictive film  14 B has the previously described magnetostrictive characteristic curve  51 B.  
         [0073]     In  FIG. 8 , the range  73  is a range wherein the impedance characteristics Z A  and Z B  overlap, and substantially linear changes are obtained. This range  73  is used as the usable range of the magnetostrictive torque sensor  10 .  
         [0074]     The characteristic stabilization step P 3  is performed after the magnetic anisotropy formation step P 2 . In the characteristic stabilization step P 3 , first the annealing step P 31  is performed. In the annealing step P 31 , a heating process is performed for a specific amount of time at a temperature equal to or greater than the service temperature under conditions in which the steering torque sensor  20  is used, for example. This annealing step P 31  is not absolutely necessary and can be omitted.  
         [0075]     The demagnetization step P 32  is performed after the annealing step P 31 . The demagnetization step P 32  is a step for applying an AC magnetic field to the rotating shaft  11  to remove the magnetization created in the surface of the rotating shaft  11 . As a result of the demagnetization step P 32 , all of the magnetized portions created in the entire surface of the rotating shaft  11  (including the surfaces of the magnetostrictive films  14 A,  14 B) are demagnetized, and remanent magnetization is initialized.  
         [0076]     In the previously described magnetostrictive film formation step P 1 , various electromagnetic generation devices are provided to electrolytic degreasing or another preparatory step S 11 , the electroplating step S 12 , step P 2  for endowing the magnetostrictive plating parts with magnetic anisotropy, or another production process. Therefore, numerous unplanned magnetized parts MS are formed in the surface of the rotating shaft  11  (including the surface of the magnetostrictive films) as shown, for example, in  FIG. 9A . In  FIG. 9A , in the stage prior to the demagnetization step P 32 , numerous magnetized parts MS and strains MK are created in the surface of the rotating shaft  11 , as are magnetically anisotropic parts MM scattered across the surface of the magnetostrictive films  14 A,  14 B of the rotating shaft  11 . The magnetically anisotropic parts MM, the magnetized parts MS, and the strains MK are formed in an irregular manner.  
         [0077]     The presence of magnetized parts MS and the like in the rotating shaft  11  in an irregular manner causes the magnetostrictive characteristics of the magnetostrictive films  14 A,  14 B to become affected in an unstable manner when changes occur in the magnetostrictive characteristics in accordance with the applied torque. If this rotating shaft  11  is used in a magnetostrictive torque sensor  10 , the sensitivity of the magnetostrictive torque sensor  10  becomes unstable. Since the rotating shaft  11  is irregularly magnetized during the manufacturing process for the various reasons described above, the state in which the magnetized parts MS and the like are created differs for each rotating shaft  11 , nonuniformities occur among rotating shafts  11 , and nonuniformities also occur in the output sensitivity of the magnetostrictive torque sensors  10 .  
         [0078]     In view of this, the rotating shaft  11  is demagnetized in the demagnetization step P 32  in the state described above. The state of the rotating shaft  11  after demagnetization is as shown in  FIG. 9B . In the demagnetized rotating shaft  11 , the magnetized parts MS and the strains MK present in the surface are initialized, and inversely magnetically anisotropic parts MM having stable sensor output sensitivity are formed in each of the two magnetostrictive films  14 A,  14 B. As a result, the previously described problems of unstable sensor sensitivity and nonuniformities in the sensor output sensitivity are resolved.  
         [0079]     The demagnetization device  81  shown in  FIG. 10  is used to demagnetize the rotating shaft  11 . An AC magnetic field generator is disposed inside the demagnetization device  81 . An AC magnetic field  82  is generated from the container surface  81   a  of the demagnetization device  81  by this AC magnetic field generator. A known example of the demagnetization device  81  is a capacitor-type demagnetization power source device that has a resonance circuit configured from a capacitor and a coil. Typical variations over time in the strength of the AC magnetic field  82  are shown in  FIG. 12 . In  FIG. 12 , the horizontal axis represents time, and the vertical axis represents magnetic field strength. Variations in magnetic field strength in a normal AC magnetic field  82  are AC variations having a constant amplitude.  
         [0080]     When the rotating shaft  11  is demagnetized using a demagnetization device  81  as described above, the shaft is moved parallel to the container surface  81   a  of the demagnetization device  81  in the direction of the arrow D relative to the region in which the AC magnetic field  82  is created, as shown in  FIG. 11 . The AC magnetic field  82  initially has the state shown in  FIG. 11  when the rotating shaft  11  is demagnetized, but is finally varied so that the amplitude of the AC magnetic field  82  gradually decreases as shown in  FIG. 13 .  
         [0081]     When the AC magnetic field  82  gradually decreases and the surface of the rotating shaft  11  is demagnetized, the magnetized state of the irregular magnetized parts MS in the surface of the rotating shaft  11  varies and decreases as shown in  FIG. 14 . As the amplitude of the AC magnetic field  82  decreases, the magnetized strength of the magnetized parts MS of the rotating shaft  11  gradually approaches zero over time in accordance with the hysteretic characteristics. As a result of the demagnetization step P 32 , the magnetized state of the magnetized parts MS created in the rotating shaft  11  reaches zero as shown in  FIG. 9B , and the irregular magnetized parts and strains created in the rotating shaft  11  in the magnetostrictive film formation step P 1  are demagnetized. Stable characteristics can thereby be maintained in the magnetically anisotropic parts MM of the magnetostrictive films  14 A,  14 B formed on the rotating shaft  11 , and nonuniformities in the sensor sensitivity during torque sensing be reduced.  
         [0082]     In the example described above, the demagnetization step P 32  is provided after the magnetic anisotropy formation step P 2 , but the demagnetization step P 32  can also be provided after the magnetostrictive film formation step P 1 , or as part of the preparatory process (not shown) for the rotating shaft  11 . Furthermore, the demagnetization step P 32  may be performed any number of times in any steps after reheating to alleviate stress. Performing the demagnetization step P 32  after reheating to alleviate stress is preferred because there is no danger of magnetization in the subsequent steps, and the magnetization created in the previous steps can be completely eliminated; i.e., initialized.  
         [0083]     The demagnetization step P 32  is followed by the testing step P 4 , which is performed as sampling testing.  
         [0084]     A sensor mounting step P 5  for mounting excitation coils or other such sensors is provided thereafter, wherein sensor devices for sensing changes in the magnetostrictive characteristics are placed around the peripheries of the magnetostrictive films  14 A,  14 B of the rotating shaft  11 . The magnetostrictive torque sensor  10  is completed by the steps described above.  
         [0085]     The following is a description of the test results pertaining to nonuniformities in the sensor characteristics of a magnetostrictive torque sensor  10  manufactured by the previously described method for manufacturing a magnetostrictive torque sensor.  
         [0086]     The test of nonuniformities in the sensor characteristics of the magnetostrictive torque sensor  10  involves using, as samples, ten rotating shafts manufactured by a conventional manufacturing method devoid of the demagnetization step P 32 , and also ten rotating shafts ( 11 ) manufactured by the manufacturing method of the present invention that does have the demagnetization step P 32 . A comparison of the results of two tests will now be described.  
         [0087]     Table 1 below shows the standard deviation (σ) of “sensor sensitivity.” In Table 1, the word “without” in the “demagnetization step” column indicates that the demagnetization step P 32  is not used, and the word “with” in the “demagnetization step” column indicates that the demagnetization step P 32  is used. The symbols  14 A and  14 B in the “plating” column correspond to the magnetostrictive films  14 A,  14 B shown in  FIG. 1 . The words “sensor sensitivity” refer to the value obtained by dividing the amount of change in impedance per 1 N m of input torque in the input torque impedance characteristics by “0-point Z,” which is the impedance value when the input torque is 0. Also, in Table 1, the standard deviation (σ) of sensor sensitivity is 1, referring to the value without the demagnetization step, and the values for when the demagnetization step is used are expressed as ratios. As is made clear in Table 1, the standard deviation (σ) of “sensor sensitivity” of a sample manufactured by a manufacturing method without the demagnetization step P 32  is 1, while the standard deviation (σ) of “sensor sensitivity” of a sample manufactured by a manufacturing method with the demagnetization step is smaller, at 0.658 and 0.591. Therefore, it is clear that nonuniformities are smaller for the sensor sensitivity of a magnetostrictive torque sensor that uses a rotating shaft manufactured by the manufacturing method having the demagnetization step P 32  than for the sensor sensitivity of a magnetostrictive torque sensor that uses a rotating shaft manufactured by a manufacturing method devoid of the demagnetization step P 32 .  
                               TABLE 1                                   Demagnetization step   Plating   Sensor sensitivity σ                           without   14A   1.000               14B   1.000           with   14A   0.658               14B   0.591                      
 
         [0088]     Table 2 below shows a process capability index Cp related to the sensor sensitivity of a magnetostrictive torque sensor that uses a rotating shaft manufactured by a manufacturing method devoid of the demagnetization step P 32 , as well as a magnetostrictive torque sensor that uses a rotating shaft manufactured by a manufacturing method provided with the demagnetization step P 32 .  
                               TABLE 2                                   Demagnetization step   Plating   Sensor sensitivity Cp                           without   14A   1.09               14B   1.01           with   14A   1.66               14B   1.72                      
 
         [0089]     In Table 2, the “demagnetization step” column and the “plating” column have the same contents as those in Table 1 above. Also, the term “process capability index Cp” refers to an index by which evaluations are made as to the degree in which the quality of the products made from the manufacturing steps conforms to standards. This index expresses the extent to which nonuniformities occur in the above-described steps in relation to a standard step. Furthermore, the “process capability index Cp” is given by the following formula (1).
 
 Cp =(standard upper limit−standard lower limit)/6σ  (1)
 
         [0090]     It is generally acknowledged that process capability is satisfactory if the process capability index Cp satisfies the relationship 1.33&lt;Cp&lt;1.67, and not necessarily satisfactory when Cp&lt;1.33. The values in Table 2 above are obtained by calculating the sensor sensitivity (Table 1) for ten rotating shaft samples manufactured by a conventional manufacturing method devoid of the demagnetization step P 32  as previously described, and for ten rotating shaft samples manufactured by the manufacturing method of the present invention provided with the demagnetization step P 32 ; setting the standard upper limit and standard lower limit on the basis of these sensitivity values; and calculating the values according to the above formula (1).  
         [0091]     As is made clear in Table 2, the process capability index Cp of a magnetostrictive torque sensor that uses a rotating shaft manufactured by a manufacturing method devoid of the demagnetization step P 32  is less than 1.33 in terms of sensor sensitivity. Therefore, it is apparent that a manufacturing method devoid of the demagnetization step P 32  does not necessarily have satisfactory process capability. The process capability index Cp of a magnetostrictive torque sensor that uses a rotating shaft manufactured by the manufacturing method of the present invention provided with the demagnetization step P 32  is greater than 1.33 in terms of sensor sensitivity, and it is clear that this sensor has satisfactory process capability.  
         [0092]      FIG. 15  shows a bar graph of the process capability indexes Cp of sensor sensitivity in Table 2. The horizontal axis represents the type of plating, and the vertical axis represents the process capability index Cp. The graphs B 10  and B 11  pertain to a magnetostrictive torque sensor that uses a rotating shaft manufactured by a manufacturing method devoid of the demagnetization step. The graphs B 20  and B 21  pertain to a magnetostrictive torque sensor that uses a rotating shaft manufactured by a manufacturing method provided with the demagnetization step.  
         [0093]     The process capability index Cp for sensor sensitivity in a magnetostrictive torque sensor manufactured by a manufacturing method devoid of the demagnetization step is less than 1.33, as shown in  FIG. 15 . Therefore, this manufacturing method does not necessarily have satisfactory process capability. The process capability index Cp for sensor sensitivity in a magnetostrictive torque sensor manufactured by the manufacturing method of the present invention is greater than 1.33, and it is clear that this method has satisfactory process capability.  
         [0094]     As shown in Table 2 and in  FIG. 15 , the process capability index Cp for sensor sensitivity in a magnetostrictive torque sensor manufactured by a manufacturing method provided with the demagnetization step P 32  according to the present invention is greater than 1.33, which is a satisfactory process capability, making exhaustive testing unnecessary. Quality can be assured by a sampling test. Therefore, the number of steps can be reduced.  
         [0095]     Thus, according to the present invention, nonuniformities in the characteristics of the magnetostrictive torque sensor can be reduced, the number of steps can be reduced, and quality can be improved because magnetization and the like induced in the rotating shaft by various conditions in the manufacturing process can be initialized.  
         [0096]     The present invention can be used as a method for manufacturing a magnetostrictive torque sensor for sensing the steering torque in an electrically powered steering apparatus or the like.  
         [0097]     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.