Patent Application
20070089538
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
Preferably, the demagnetization step comprises initializing remanent magnetization created in the rotating shaft by steps prior to the demagnetization step.
In a preferred form, the demagnetization step is carried out after the magnetostrictive film formation step.
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.
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.
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.
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.
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:
A magnetostrictive torque sensor will be described with reference to
A magnetostrictive torque sensor 10 is configured from a rotating shaft 11, and one excitation coil 12 and two sensor coils 13A, 13B disposed around the periphery of the rotating shaft 11, as shown in
Referring to the example of utilization shown in
For the sake of convenience in the description below, the terms “magnetostrictive films 14A, 14B” and “magnetostrictive plating parts (14A, 14B)” 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 14A, 14B,” and prior to this step these parts are referred to as “magnetostrictive plating parts.”
The excitation coil 12 and the sensor coils 13A, 13B are provided for both of the two magnetostrictive films 14A, 14B formed on the surface of the rotating shaft 11, as shown in
In
The magnetostrictive films 14A, 14B 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 14A, 14B 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 14A, 14B are detected using the sensor coils 13A, 13B disposed around the periphery of the magnetostrictive films 14A, 14B.
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
In
The steering shaft 21 is rotatably supported by two shaft bearings 32, 33 in a housing 31a that forms a gear box 31. The rack-and-pinion mechanism 34 and the drive force transmission mechanism 35 are accommodated inside the housing 31a.
The steering torque sensor 20 is provided for the steering shaft 21. The previously described magnetostrictive films 14A, 14B are formed on the steering shaft 21, and the excitation coil 12 and sensor coils 13A, 13B corresponding to the magnetostrictive films 14A, 14B are supported by the supporting frames 15A, 15B and yokes 36A, 36B.
The top opening of the housing 31a 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 37a of the lid 37.
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 14A, 14B that correspond to the torsion in the steering shaft 21. The changes are sensed as changes in the induced voltages VA, VB from the output terminals of the sensor coils 13A, 13B.
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 14A, 14B. 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 14A, 14B is detected by the sensor coils 13A, 13B as a change in the induced voltages VA, VB. According to the steering torque sensor 20, the difference between the two induced voltages VA, VB is outputted as a detected voltage value on the basis of the change in the induced voltages VA, VB. Therefore, the direction and extent of the steering torque (T) applied to the steering shaft 21 can be sensed based on the outputted voltage (VA−VB) of the steering torque sensor 20.
The magnetostrictive characteristic curves 51A, 51B for the magnetostrictive films 14A, 14B simultaneously show the detection output characteristics of the sensor coils 13A, 13B. Specifically, an excitation AC current is supplied by the shared excitation coil 12 to the magnetostrictive films 14A, 14B that have the magnetostrictive characteristic curves 51A, 51B, and the sensor coils 13A, 13B respond to this excitation AC current by outputting induced voltages. Therefore, the changing characteristics of the induced voltages of the sensor coils 13A, 13B correspond to the magnetostrictive characteristic curves 51A, 51B of the magnetostrictive films 14A, 14B. In other words, the magnetostrictive characteristic curve 51A shows the changing characteristics of the induced voltage VA outputted from the sensor coil 13A, while the magnetostrictive characteristic curve 51B shows the changing characteristics of the induced voltage VB outputted from the sensor coil 13B.
According to the magnetostrictive characteristic curve 51A, the value of the induced voltage VA outputted from the sensor coil 13A 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 T1, then peaks when the steering torque reaches the positive value T1, and gradually decreases as the steering torque increases past T1. According to the magnetostrictive characteristic curve 51B, the value of the induced voltage VB outputted from the sensor coil 13B gradually increases as the value of the steering torque approaches the negative value −T1, then peaks when the steering torque reaches the negative value −T1, and decreases in substantially linear fashion as the steering torque further increases past −T1 and changes from negative to positive.
As shown in
The line 52 shown in
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.
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 13A, 13B by using the characteristics of the vertical line 52.
The following is a description, made with reference to
In
First, the magnetostrictive film formation step P1 is performed. In the magnetostrictive film formation step P1, 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.
In the magnetostrictive film formation step P1, washing or another such preparatory process is first performed on the rotating shaft 11 (step S11). Electroplating is then performed (step S12). 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 14A, 14B by a post-process to be described later. Drying is then performed (step S13).
In the magnetostrictive film formation step P1, an electroplating method was used to form the previously described magnetostrictive films 14A, 14B on the surface of the rotating shaft 11. However, the base portions that form the magnetostrictive films 14A, 14B 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.
Next, the magnetic anisotropy formation step P2 is performed. The magnetic anisotropy formation step P2 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 14A, 14B. The magnetic anisotropy formation step P2 has a step S21 of high-frequency heating performed on the top magnetostrictive plating part, and a step S22 of high-frequency heating performed on the bottom magnetostrictive plating part.
As shown in
In the heating step S202, 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.
Magnetic anisotropy is created in the top magnetostrictive plating part of the rotating shaft 11 in steps S201 through S204, whereby a magnetically anisotropic magnetostrictive film 14A is formed.
The steps S201 through S204 are similarly performed in the high-frequency heating step S22 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 14B 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 14B.
The following is a description, made with reference to
In
In
In
In
When the magnetostrictive film 14B 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 14A.
In
The characteristic stabilization step P3 is performed after the magnetic anisotropy formation step P2. In the characteristic stabilization step P3, first the annealing step P31 is performed. In the annealing step P31, 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 P31 is not absolutely necessary and can be omitted.
The demagnetization step P32 is performed after the annealing step P31. The demagnetization step P32 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 P32, all of the magnetized portions created in the entire surface of the rotating shaft 11 (including the surfaces of the magnetostrictive films 14A, 14B) are demagnetized, and remanent magnetization is initialized.
In the previously described magnetostrictive film formation step P1, various electromagnetic generation devices are provided to electrolytic degreasing or another preparatory step S11, the electroplating step S12, step P2 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
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 14A, 14B 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.
In view of this, the rotating shaft 11 is demagnetized in the demagnetization step P32 in the state described above. The state of the rotating shaft 11 after demagnetization is as shown in
The demagnetization device 81 shown in
When the rotating shaft 11 is demagnetized using a demagnetization device 81 as described above, the shaft is moved parallel to the container surface 81a 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
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
In the example described above, the demagnetization step P32 is provided after the magnetic anisotropy formation step P2, but the demagnetization step P32 can also be provided after the magnetostrictive film formation step P1, or as part of the preparatory process (not shown) for the rotating shaft 11. Furthermore, the demagnetization step P32 may be performed any number of times in any steps after reheating to alleviate stress. Performing the demagnetization step P32 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.
The demagnetization step P32 is followed by the testing step P4, which is performed as sampling testing.
A sensor mounting step P5 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 14A, 14B of the rotating shaft 11. The magnetostrictive torque sensor 10 is completed by the steps described above.
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.
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 P32, and also ten rotating shafts (11) manufactured by the manufacturing method of the present invention that does have the demagnetization step P32. A comparison of the results of two tests will now be described.
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 P32 is not used, and the word “with” in the “demagnetization step” column indicates that the demagnetization step P32 is used. The symbols 14A and 14B in the “plating” column correspond to the magnetostrictive films 14A, 14B shown in
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 P32, as well as a magnetostrictive torque sensor that uses a rotating shaft manufactured by a manufacturing method provided with the demagnetization step P32.
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)
It is generally acknowledged that process capability is satisfactory if the process capability index Cp satisfies the relationship 1.33<Cp<1.67, and not necessarily satisfactory when Cp<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 P32 as previously described, and for ten rotating shaft samples manufactured by the manufacturing method of the present invention provided with the demagnetization step P32; 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).
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 P32 is less than 1.33 in terms of sensor sensitivity. Therefore, it is apparent that a manufacturing method devoid of the demagnetization step P32 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 P32 is greater than 1.33 in terms of sensor sensitivity, and it is clear that this sensor has satisfactory process capability.
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
As shown in Table 2 and in
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.
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.
Obviously, various minor changes and modifications of the present invention are possible in light of the above teaching. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-288653 | Sep 2005 | JP | national |
