Patent Description:
<CIT> and its family member <CIT> disclose a magnetostrictive torque sensor. The magnetostrictive torque sensor uses a shaft having magnetostrictive characteristic whose magnetic permeability changes when stress is applied, and detects a torque applied to the shaft by detecting a change in magnetic permeability of the shaft when the torque is applied and the shaft is twisted as a change in inductance of a detecting coil. The shaft is manufactured by subjecting a steel shaft material to carburizing, quenching and tempering, and performing shot peening on the shaft material after the heat treatment step at least on a position where a sensor portion of the magnetostrictive torque sensor is to be mounted. Shot media formed of steel with a Rockwell hardness of <NUM> HRC is used for shot peening.

<CIT> discloses a method for manufacturing a magnetostrictive torque sensor shaft as specified in the preamble of claim <NUM>.

<CIT> discloses a method of treating a substance made of steel by shot peening, using iron-based amorphous spherical particles having a Vickers hardness in the range of <NUM> to <NUM> HV.

It has been known that a steel material used for the shaft comprises a non-magnetic structure called retained austenite. The retained austenite is gamma-iron (y-Fe) with face-centered cubic lattice structure in which the other elements are dissolved.

When the shaft includes a large amount of the non-magnetic retained austenite, the change in magnetic permeability when the torque is applied reduces, and thus it leads to decrease in sensitivity of the magnetostrictive torque sensor.

However, if the amount of the retained austenite in the entire shaft reduces by adjusting the conditions of heat treatment or the like, the toughness of the shaft may decrease, so that the shaft may be cracked.

In addition, the magnetostrictive torque sensor is used for, e.g., a transmission stroke control or an engine output control. Therefore, it is desirable to have a small error as much as possible to perform a reliable shift transmission.

Therefore, it is an object of the present invention to provide a method for manufacturing a magnetostrictive torque sensor shaft capable of improving sensor sensitivity and reducing an error while securing toughness.

For achieving the above-mentioned object, the present invention provides a method for manufacturing a magnetostrictive torque sensor shaft as specified in claim <NUM>. Advantageous Effects of Invention.

According to the present invention, it is possible to provide a method for manufacturing a magnetostrictive torque sensor shaft capable of improving sensor sensitivity and reducing errors while securing toughness.

Embodiments of the present invention will be explained in conjunction with appended drawings.

Firstly, a magnetostrictive torque sensor (hereinafter simply referred to as a torque sensor) will be explained. <FIG> is a side view showing an example of a sensor portion of the torque sensor that is attached to a shaft. <FIG> is a cross-sectional view taken along line A-A in <FIG>. <FIG> is a plan view schematically showing a state in which a bobbin is deployed, which illustrates a first detecting coil and a fourth detecting coil. <FIG> is a plan view schematically showing a state in which the bobbin is deployed, which illustrates a second detecting coil and a third detecting coil. <FIG> is a circuit diagram showing an example of a measuring part to measure torque applied to the shaft by detection signal of the torque sensor.

As shown in <FIG>, the sensor portion <NUM> of the torque sensor <NUM> is attached around a magnetostrictive torque sensor shaft (hereinafter, simply referred to as a shaft) <NUM> having magnetostrictive characteristics. The torque sensor <NUM> measures torque (rotational torque) applied to the shaft <NUM>.

The shaft <NUM> comprises a material having magnetostrictive characteristics (such as SCr<NUM>, SCM<NUM>, SNCM<NUM>, SUS<NUM>C, and SUS<NUM>), and is formed in a cylindrical shape (rod shape). For example, the shaft <NUM> is used for torque transmission in a powertrain system of a vehicle or a torque transmission in an engine of a vehicle.

The sensor portion <NUM> comprises a coil <NUM> and a magnetic ring <NUM>. The magnetic ring <NUM> comprises a magnetic material (a ferromagnetic material) and is formed in a hollow cylindrical shape. The coil <NUM> is inserted into a hollow portion of the magnetic ring <NUM>. The magnetic ring <NUM> serves to suppress leakage of a magnetic flux generated by the detecting coil <NUM> of the coil <NUM> to the outside, thereby decreasing the sensitivity.

The coil <NUM> comprises a bobbin <NUM> comprising a non-magnetic resin, and a plurality of detecting coils <NUM> formed by winding insulated wires around an outer periphery of the bobbin <NUM>. The bobbin <NUM> is provided coaxially with the shaft <NUM> in such a manner as to be separated from the shaft <NUM>, and is formed in a hollow cylindrical shape. A plurality of first inclined grooves <NUM> inclined at a predetermined angle (herein +<NUM>°) with respect to an axial direction of the shaft <NUM> and a plurality of second inclined grooves <NUM> inclined at a predetermined angle (herein -<NUM>°) with respect to a direction opposite to the first inclined grooves <NUM> along the axial direction are formed on an outer peripheral surface of the bobbin <NUM>. The first inclined grooves <NUM> and the second inclined grooves <NUM> are formed of grooves concaved in a radial direction of the bobbin <NUM>.

As shown in <FIG>, the coil <NUM> comprises first to fourth detecting coils <NUM> to <NUM> as the detecting coils <NUM>. The first detecting coil <NUM> and the fourth detecting coil <NUM> are formed by winding insulated wires around the bobbin <NUM> along the first inclined grooves <NUM>. The second detecting coil <NUM> and the third detecting coil <NUM> are formed by winding insulated wires around the bobbin <NUM> along the second inclined grooves <NUM>.

In <FIG>, reference signs 31a and 31b respectively indicate input and output ends of one layer of the first detecting coil <NUM>, and reference signs 34a and 34b respectively indicate input and output ends of one layer of the fourth detecting coil <NUM>. In <FIG>, reference signs 32a and 32b respectively indicate input and output ends of one layer of the second detecting coil <NUM>, and reference signs 33a and 33b respectively indicate input and output ends of one layer of the third detecting coil <NUM>. In addition, <FIG> shows winding of the insulated wires for one turn. Each of the detecting coils <NUM> to <NUM> is formed by repeatedly winding the insulated wires to a target number of turns. In addition, the method for winding the insulated wires shown in <FIG> is merely an example. The detecting coils <NUM> to <NUM> may be formed by using the other methods of winding.

The first detecting coil <NUM> and the fourth detecting coil <NUM> are provided for detecting a change in magnetic permeability of the shaft <NUM> in a first direction inclined at a predetermined angle (herein +<NUM>°) with respect to the axial direction of the shaft <NUM>. The second detecting coil <NUM> and the third detecting coil <NUM> are provided for detecting a change in magnetic permeability of the shaft <NUM> in a second direction inclined at the predetermined angle (herein -<NUM>°) opposite to the first direction along the axial direction of the shaft <NUM>.

As shown in <FIG>, a measuring part <NUM> measures the torque applied to the shaft <NUM> by detecting changes in inductance of the first to fourth detecting coils <NUM> to <NUM>.

The measuring part <NUM> comprises a bridge circuit <NUM> configured by annularly arranging the first detecting coil <NUM>, the second detecting coil <NUM>, the fourth detecting coil <NUM>, and the third detecting coil <NUM> in this order, a transmitter <NUM> for applying alternating current (AC) voltage between a contact point a arranged between the first detecting coil <NUM> and the second detecting coil <NUM> and a contact point b arranged between the third detecting coil <NUM> and the fourth detecting coil <NUM>, a voltage measuring circuit <NUM> for detecting voltage between a contact point c arranged between the first detecting coil <NUM> and the third detecting coil <NUM> and a contact point d arranged between the second detecting coil <NUM> and the fourth detecting coil <NUM>, and a torque calculating part <NUM> for calculating a torque applied to the shaft <NUM> based on the voltage measured by the voltage measuring circuit <NUM>. The bridge circuit <NUM> is configured by arranging the first detecting coil <NUM> and the fourth detecting coil <NUM> at one of two sides facing each other, and arranging the second detecting coil <NUM> and the third detecting coil <NUM> at the other of the two sides facing each other.

In the measuring part <NUM>, inductances L1 to L4 of the first to fourth detecting coil <NUM> to <NUM> are equal when the torque is not applied to the shaft <NUM>, and the voltage detected by the voltage measuring circuit <NUM> becomes approximately zero.

When the torque is applied to the shaft <NUM>, the magnetic permeability in the direction of +<NUM>° with respect to the axial direction decreases (or increases) while the magnetic permeability in the direction of -<NUM>°with respect to the axial direction increases (or decreases). Thus, the inductances in the first detecting coil <NUM> and the fourth detecting coil <NUM> decrease (or increase) while the inductances in the second detecting coil <NUM> and the third detecting coil <NUM> increase (or decrease), when the torque is applied to the shaft <NUM> while the AC voltage is applied from the transmitter <NUM>. As a result, the voltage detected by the voltage measuring circuit <NUM> changes, and thus the torque calculating part <NUM> calculates the torque applied to the shaft <NUM> based on the change in voltage.

Since the first detecting coil <NUM> and the fourth detecting coil <NUM> have the same structure as the second detecting coil <NUM> and the third detecting coil <NUM> except the winding direction being different, the effects of the temperature or the like on the inductances of the first to fourth detecting coils <NUM> to <NUM> can be canceled by using the bridge circuit <NUM> shown in <FIG>. And thus, the torque applied to the shaft <NUM> can be accurately detected. In addition, for the torque sensor <NUM>, when the inductances in the first detecting coil <NUM> and the fourth detecting coil <NUM> increase (or decrease), while the inductances in the second detecting coil <NUM> and the third detecting coil <NUM> decrease (or increase). Thus, the detection sensitivity can be further improved by using the bridge circuit <NUM> shown in <FIG>.

In the present embodiment, an iron-based shaft member subjected to a carburizing, quenching and tempering process, thereafter shot peening, and then surface polishing is used as the shaft <NUM>.

That is, the method for manufacturing the shaft <NUM> according to the present embodiment comprises a heat treatment step of subjecting an iron-based shaft member to a carburizing, quenching, and tempering process, a shot peening step of performing shot peening at least in a position on the shaft member, after the heat treatment step, to which the sensor portion is to be attached, and a surface polishing step of subjecting the shaft member after the shot peening to surface polishing.

For the shaft member used for the shaft <NUM>, iron-based materials such as SCr<NUM> (chromium steel), SCM<NUM> (chromium-molybdenum steel), SNCM<NUM> (nickel-chromium molybdenum steel), SUS440C (martensitic stainless), SUS<NUM> (precipitation hardening stainless steel), and the like can be used. In the present embodiment, SCr<NUM> is used for the shaft member.

The mechanical strength including the toughness of the shaft <NUM> can be enhanced by performing the carburizing, quenching, and tempering process on the entire shaft member. In the present embodiment, surface hardness of the shaft member after the heat treatment step is determined as not less than <NUM> HV (Vickers hardness). This is because if the surface hardness of the shaft member after the heat treatment step is low, the sensor sensitivity may decrease, or the hysteresis error may increase. The surface hardness of the shaft member after the heat treatment step is preferably at least equal to <NUM> HV in order to suppress deterioration of the sensor sensitivity and the hysteresis error. That is, in the heat treatment step, it is preferable to perform heat treatment in such a manner that the surface hardness of the shaft member after the heat treatment step is not less than <NUM> HV. In the present embodiment, the heat treatment step is performed by gas carburization (at <NUM>°C for <NUM> hours, then at <NUM>°C for <NUM> min, and then oil quenching and tempering at <NUM>°C for <NUM> hours).

The shaft member is crooked when being subjected to the heat treatment step. Thus, it is preferable to perform a step of press-unbending the shaft member and a grinding step of grinding the surface of the shaft member to provide the shaft member with a straight shape, in addition to the heat treatment step. In the grinding step, it is preferable to perform grinding in such a manner that a grinding amount (grinding depth) at the part (referred to as the measuring part), to which the sensor portion <NUM> is to be attached, is even in the circumferential direction. This is because that a composition varies along the depth direction (radial direction) of the shaft member, and when the grinding amount (grinding depth) is uneven in the measuring part, portions with different compositions are generated on the surface of the measuring part and may cause the angle errors to be described later. In order to obtain the desired outer diameter after the grinding step, the shaft member before the heat treatment is preferably slightly larger than the finished outer diameter (for example, the shaft member before the heat treatment having a diameter of about <NUM> mm).

In the shot peening step, the shaft member after the heat treatment is subjected to the shot peening. Martensitic transformation (diffusionless transformation) occurs at the surface (a region distant with a predetermined depth from the surface) of the shaft <NUM> by shot peening the shaft member after heat treatment, so that non-magnetic retained austenite decreases, and ferromagnetic martensite can increase. As a result, the change in magnetic permeability when the torque is applied increases by reducing the non-magnetic area on the surface of the shaft <NUM> and increasing the magnetic area on the surface of the shaft <NUM>. And thus, the sensitivity of the torque sensor <NUM> can be improved. The retained austenite is iron (y-Fe) with face-centered cubic lattice structure in which the other elements are dissolved. The martensite is a solid solution in which carbon enters into a crystal of iron with body-centered tetragonal lattice structure.

In addition, the surface of the shaft member is work hardened by shot peening the shaft member after the heat treatment and thus the hysteresis characteristic is improved. More specifically, by shot peening the shaft member after the heat treatment, the magnetic domains (areas in which the direction of the magnetic moment is aligned) on the surface of the shaft <NUM> are subdivided, and the retained austenite serving as a pinning site for preventing movement of the magnetic domain wall, which is the boundary of the magnetic domains, is reduced. As a result, when the applied torque is released, the magnetic domains and the magnetic walls will be easily returned to the original state, and thus the hysteresis error decreases.

As shown in <FIG>, the sensitivity of the torque sensor <NUM> is defined by dividing the sensor output V (mV) by the torque T (Nm) applied to the shaft <NUM>, and expressed as V/T (mV/Nm). The sensor sensitivity is expressed as (V<NUM>-V<NUM>)/(T<NUM>×T<NUM>) = Vs/Ts, where the torque T applied to the shaft <NUM> is not less than -T<NUM> and not more than +T<NUM>, the sensor output corresponding to the torque -T<NUM> is V<NUM>, and the sensor output corresponding to the torque +T<NUM> is V<NUM>. The hysteresis error, which is also referred to as a linearity error, is a ratio of the maximum value Vh of a sensor output difference at the same torque between the torque increase and the torque decrease with respect to the sensor output Vs (= V<NUM>-V<NUM>), and is expressed as Vh/Vs (%FS, FS means a full-scale).

In the present embodiment, in addition to the hysteresis error, the variation of the sensor output in the circumferential direction of the shaft <NUM> (hereinafter referred to as the angle error) is evaluated. As shown in <FIG>, the angle error means an error generated when the shaft is rotated. The angle error is expressed as (Vmax-Vmin)/Vs (%), where the maximum value of the sensor output is Vmax and the minimum value is Vmin. Hereinafter, the sum of the hysteresis error and the angle error is referred to as a total error. In the present embodiment, a target value of the total error is not more than <NUM>%.

In the present embodiment, chromium steel (SCr<NUM>) was used as the shaft member of the shaft <NUM>, and optimum shot peening conditions was studied. In the present embodiment, a steel shot media having a particle diameter of <NUM> mm was used. The diameter of the shaft <NUM> was set as <NUM> mm. When performing the shot peening, the shot media was injected while rotating the shaft <NUM> at a low speed (e.g., <NUM> rpm).

Firstly, <FIG> shows error measuring result when using a steel shot media having the Vickers hardness of <NUM> HV and setting injection pressure as <NUM> MPa and <NUM> MPa. As shown in <FIG>, the total error is increased to not less than <NUM>% when using the shot media having the Vickers hardness of <NUM> HV. The error (hysteresis error and angle error) shown in <FIG> represents the maximum value including temperature characteristics (measured under conditions of ambient temperature from - <NUM>°C to <NUM>°C). The horizontal axis of <FIG> represents the injection time of the shot media, and the injection time of the shot media is represented by dividing the total injection time by the length of the shaft <NUM>. That is, the injection time of the shot media represents the injection time per <NUM> cm in the axial direction of the shaft <NUM> (hereinafter, simply referred to as the injection time).

Next, <FIG> shows error measuring result when using a steel shot media having the Vickers hardness of <NUM> HV and setting injection pressure as <NUM> MPa and <NUM> MPa. In the meantime, since there is a variation in hardness of the shot media, the shot media may have the Vickers Hardness of not less than <NUM> HV and not more than <NUM> HV. As shown in <FIG>, when using the shot media having the Vickers hardness of <NUM> HV, the hysteresis error decreases as compared with the case using the shot media having the Vickers hardness of <NUM> HV. Meanwhile, the angle error increases. As a result, the total error increases.

In the present embodiment, the shot media having the particle diameter of not less than <NUM> mm was used, and the injection pressure was set as not less than <NUM> MPa and not more than <NUM> MPa. As clearly understood from the result shown in <FIG>, when using the shot media having the particle diameter of <NUM> mm, it is preferable to set the shot condition with the injection pressure of <NUM> MPa and the injection time of not less than <NUM> sec/cm and not more than <NUM> sec/cm, in order to reduce the total error. In the present embodiment, the shot peening was performed under the shot condition with the injection pressure of <NUM> MPa and the injection time of <NUM> sec/cm. As shown in <FIG>, the sensor sensitivity of <NUM> mV/Nm before shot peening can be improved to <NUM> mV/Nm by shot peening under the above-mentioned shot condition. The sensor sensitivity of not less than <NUM> mV/Nm that is set as the target value was obtained. Also, as shown in <FIG>, the hysteresis error of <NUM>%FS before shot peening was reduced to <NUM>% FS, and the hysteresis error of not more than<NUM>%FS that is set as the target value was achieved.

As described above, it was found that the total error cannot be reduced only by increasing the hardness of the shot media. Therefore, the inventors studied the reason why the angle error increases. As the result of study, the inventors have found that the angle error is largely increased by two factors.

The first factor is that the retained austenite remaining in the shot peening step is not uniformly distributed in the circumferential direction of the shaft <NUM>. As described above, the retained austenite is transformed into martensite by deformation inducing and a non-magnetic region is magnetized, and at the same time, the retained austenite region which is easily plastically deformed can be reduced to achieve high hardness of the surface by shot peening. As a result, improvement of the sensor sensitivity and reduction of the hysteresis error can be achieved at the same time. However, the retained austenite is not completely removed, and the retained austenite remains on the surface of the shaft member after shot peening, as shown by the dashed line in <FIG>.

Here, as shown by the dashed line in <FIG>, when measuring the amount of retained austenite by each angular position of the shaft member after shot peening, it was found that the amount of retained austenite varies from <NUM> % to <NUM> %. Since the retained austenite is non-magnetic, the angle error increases when the distribution of retained austenite varies in the circumferential direction in the measuring part. It is believed that such variations in retained austenite cannot be sufficiently eliminated only by shot peening.

The second factor is that Fe<NUM>B is generated on the surface of the shaft member by the effect ot boron contained in the shot media. According to the claimed invention, boron is added to the steel shot media having the Vickers hardness of <NUM> HV to increase the hardness of the shot media. Thus, Fe<NUM>B is formed on the surface of the shaft member by using such the shot media. Since Fe<NUM>B is considered to have a large angle error due to the large crystal magnetic anisotropy, as shown by the dashed line in <FIG>, it was confirmed that Fe<NUM>B is formed on the surface of the shaft member when analyzing the surface of the shaft member after shot peening by using the X-ray diffraction apparatus.

Therefore, in the present embodiment, the surface polishing step for surface polishing the shaft member after shot peening was performed in order to eliminate these two factors and to improve the angle error. In the surface polishing step, the surface of the shaft member may be polished by using, e.g., a polishing paper referred to as water paper or water-resistant paper. The arithmetic average roughness Ra of the surface of the substrate after polishing was <NUM> µm, and the maximum height Rz was <NUM> µm.

The stress in surface polishing accelerates the martensitic transformation by surface polishing the shaft member after shot peening. And thus, the distribution of the retained austenite can be made uniform. The distribution of the retained austenite after polishing is shown by solid lines in <FIG> and <NUM>B. As shown by the solid line in <FIG> and <NUM>B, it will be understood that the amount of the retained austenite was reduced by surface polishing the shaft member after shot peening, and the variation in the amount of the retained austenite by the angular position is suppressed to approximately <NUM>% (<NUM> to <NUM> %).

Further, Fe<NUM>B formed on the surface of the shaft member in shot peening is removed by surface polishing the shaft member after shot peening. The results of analysis of the shaft member after surface polishing by the X-ray diffraction device are shown by the solid line in <FIG>. As shown by the solid line in <FIG>, it will be understood that the surface of the shaft member after surface polishing has low Fe<NUM>B (volume ratio decreases).

In the surface polishing step, surface polishing may be performed in such a manner that the arithmetic average roughness Ra of the surface of the shaft member is not more than <NUM> µm and the maximum height Rz is not more than <NUM> µm. This is because when the arithmetic average roughness Ra of the surface of the shaft member is greater than <NUM> µm or the maximum height Rz is greater than <NUM> µm, the surface polishing is not sufficient, and uniformization of the distribution of retained austenite and the removal of Fe<NUM>B may not be sufficiently performed.

In this way, the uniformization of the distribution of the retained austenite and the removal of Fe<NUM>B formed on the surface of the shaft member can be achieved at the same time by surface polishing the shaft member after shot peening. As a result, the angle error can be greatly reduced. As shown in <FIG>, in the shaft <NUM> after surface polishing, the angle error is significantly reduced as compared to the shaft member before surface polishing (after shot peening).

Specifically, the shaft <NUM> can have the hysteresis error of approximately <NUM>%, the angle error of approximately <NUM>%, and the total error of not more than <NUM>%. Even though the circuit error caused by the detection circuit is included, it is possible to provide the sum of the errors of not more than <NUM>%, and to achieve the torque sensor <NUM> having a significantly small error. <FIG> shows using the steel shot media having the particle diameter of <NUM> mm and the Vickers hardness of <NUM> HV and shot peening under the injection pressure of <NUM> MPa and the injection time of <NUM> sec/cm.

The non-magnetic retained austenite can decrease only on the surface of the shaft <NUM> by providing the shot peening step, and thus the deterioration of the toughness of the shaft <NUM> can be suppressed. The hysteresis error can decrease and the sensor sensitivity can be improved by performing shot peening step with using a shot media having the high hardness of <NUM> HV. However, the angle error may increase due to the influence of boron included in the shot media. According to the claimed invention, by surface polishing the shaft member after shot peening, the distribution in the circumferential direction of the retained austenite is uniformized, and Fe<NUM>B formed on the surface of the shaft member due to the boron included in the shot media can be removed, so that the angle error can decrease. As a result, it is possible to achieve the torque sensor <NUM> having high sensor sensitivity while securing toughness and having the total error of not more than <NUM>%.

Although the embodiments of the invention have been described, the invention according to claims is not to be limited to the embodiments. In addition, please note that all combinations of the features described in the embodiments are not necessary to solve the problem of the invention.

Claim 1:
A method for manufacturing a magnetostrictive torque sensor shaft (<NUM>) to which a sensor portion (<NUM>) of a magnetostrictive torque sensor (<NUM>) is to be attached,
the method comprising:
a heat treatment step of subjecting an iron-based shaft member to a carburizing, quenching, and tempering process;
a shot peening step of performing shot peening at least in a position on the shaft member, after the heat treatment step, to which the sensor portion (<NUM>) is to be attached; and
a surface polishing step of subjecting the shaft member after the shot peening to surface polishing,
characterized in that
a steel shot media containing boron and having a Vickers hardness at least equal to <NUM> HV and at most equal to <NUM> HV is used in the shot peening step, and
the surface polishing step removes Fe<NUM>B formed on a surface of the shaft member in the shot peening.