Magnetic encoder and production method therefor

A magnetic encoder having a plurality of rows of magnetic tracks and capable of detecting an absolute angle is easily producible with higher accuracy. The magnetic encoder includes: a core member of annular shape having a bending plate portion that bends and extends from an edge of a track formation surface; and two or more rows of magnetic tracks arranged adjacent to each other on a magnetic member provided on the track formation surface, each track having N poles and S poles alternately magnetized thereon. The magnetic tracks include a main track that has a largest number of magnetic poles and is used for calculating an angle, and a sub track used for calculating a phase difference from the main track. The main track is located on a side closer to the bending plate portion than the sub track.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a magnetic encoder used for detecting a rotation speed or a rotation position, and a production method therefor. In particular, the present invention relates to a technique applicable to: a magnetic encoder having a plurality of rows of magnetic tracks that are used for detecting an absolute angle; and a production method therefor.

Description of Related Art

Patent Document 1 proposes a magnetic encoder in which, when magnetization of a plurality of circumferential rows of magnetic encoder tracks is performed, flow of a magnetic flux to the rows of magnetic encoder tracks other than a magnetization target row is shielded by use of a magnetic shield.

RELATED DOCUMENT

Patent Document

SUMMARY OF THE INVENTION

In the magnetic encoder described in Patent Document 1, a difference of one pole pair is provided between the respective magnetic tracks. Thus, the magnetic encoder can be used for detecting an absolute angle. However, the magnetic track (main track) that serves as a reference for angle detection is required to have a high magnetization accuracy. For example, in a case where an absolute angle is detected by using two rows of magnetic tracks respectively magnetized with 32 pole pairs and 31 pole pairs, an angle per pole pair, on the 32-pole-pairs side, is 11.25° (360/32). In order to determine the present phase position, a magnetization accuracy of not more than 0.35° (11.25/32), or, for the sake of safety, a magnetization accuracy within +0.1°, is required. If the number of magnetic poles is increased to be, for example, 64 pole pairs and 63 pole pairs, the required accuracy becomes higher. For example, a magnetization accuracy within +0.04° is required. Therefore, it is difficult to produce a magnetic encoder that satisfies required accuracy.

Therefore, an object of the present invention is to provide: a magnetic encoder, having a plurality of rows of magnetic tracks and capable of detecting an absolute angle, which can be easily produced with higher accuracy; and a method for producing the magnetic encoder.

A magnetic encoder according to the present invention includes: a core member of annular shape having a track formation surface and a bending plate portion that bends and extends from an edge of the track formation surface; and two or more rows of magnetic tracks arranged adjacent to each other on a magnetic member provided on the track formation surface of the core member, each track having N poles and S poles alternately magnetized thereon. The two or more rows of magnetic tracks include a main track that has a largest number of magnetic poles and is used for calculating an angle, and a sub track used for calculating a phase difference from the main track. The main track is located on a side closer to the bending plate portion than the sub track. The magnetic member may be a single member used for both the main track and the sub track, or a plurality of magnetic members may be individually provided for the respective magnetic tracks.

A part, of the track formation surface of the core member, which is close to the bending plate portion is formed by bending the core member and therefore has high rigidity and less rotational deflection. Since the main track as a magnetic track, which is required to have high accuracy and has the larger number of magnetic poles, is disposed on the part having less rotational deflection, improvement and stabilization of the accuracy of a detected angle are expected. In production, by simply adjusting the arrangement of the main track and the sub track, the magnetic encoder can be easily produced while achieving improvement of accuracy.

In the magnetic encoder of the present invention, the core member may have: a cylindrical portion having an outer peripheral surface that serves as the track formation surface; the bending plate portion that bends from the cylindrical portion toward an inner diameter side; and an attachment portion of cylindrical shape that extends from an inner-diameter-side edge of the bending plate portion to a side opposite to the cylindrical portion, concentrically with the cylindrical portion. That is, the magnetic encoder may be a radial type. Such a radial type magnetic encoder can also be easily produced while achieving improvement of accuracy. Since the bending plate portion is a portion for connecting the track formation surface and the attachment portion and is not intended to be used for improving the rigidity, the structure of the core member is unlikely to be complicated for improvement of the rigidity.

In the magnetic encoder of the present invention, the core member may have: a plate portion of annular shape having one surface that serves as the track formation surface; and the bending plate portion that bends and extends from an inner-diameter-side edge of the plate portion to a side opposite to the track formation surface and that serves as an attachment portion of cylindrical shape. That is, the magnetic encoder may be an axial type. Such an axial type magnetic encoder can also be easily produced while achieving improvement of accuracy. Since the bending plate portion is a portion for connecting the attachment portion to a shaft or the like and is not intended to be used for improving the rigidity, the structure of the core member is unlikely to be complicated for improvement of the rigidity.

In the magnetic encoder of the present invention, accuracy of pitch of magnetic poles may be higher in the main track than in the sub track. The “accuracy” is a difference between an actual pitch and a theoretical pitch. For example, assuming that a magnetic track is magnetized with 32 pole pairs, an angle per pole pair is theoretically 11.25°. Then, if the angle of a certain pole pair is 11.3° in actuality, the actual pitch is 11.3° whereas the theoretical pitch is 11.25°. Generally, as for a magnetic encoder, an un-magnetized magnetic encoder is produced in advance, and thereafter, magnetization is performed on the un-magnetized magnetic encoder. In this case, a plurality of magnetic tracks are sequentially magnetized. A magnetic track that has been magnetized first is assumed to be reduced in accuracy due to leakage of a magnetic flux when a subsequent magnetic track is magnetized. Therefore, it is difficult to magnetize, with high accuracy, all the magnetic tracks arranged adjacent to each other.

Therefore, in the magnetic encoder of the present invention, a magnetic track, whose accuracy of pitch of magnetic poles is assumed to be reduced, is regarded as a sub track. Since the sub track is a magnetic track used for calculating a phase difference from the main track, influence of the accuracy of the magnetization pitch thereof becomes relatively little by adopting the aforementioned magnetization order. Therefore, if the accuracy of the magnetization pitch of the main track, which has the larger number of magnetic poles and is used for calculating an angle, is made higher than that of the sub track, the magnetic encoder becomes able to detect an absolute angle with high accuracy within a limited range of accuracy in production. Preferably, the number of magnetic poles of the main track is larger by one than the number of magnetic poles of the sub track. It is noted that this magnetic encoder is applicable not only to a magnetic encoder in which magnetizations for the respective magnetic tracks are successively performed but also to general magnetic encoders in which a difference in accuracy occurs between magnetic tracks.

A magnetic encoder production method according to the present invention is a method for producing a magnetic encoder having any of the aforementioned configurations. The method includes: producing an un-magnetized magnetic encoder in which the magnetic member is provided on an outer periphery of the core member; and sequentially magnetizing the respective rows of magnetic tracks in such a manner that, during the magnetization, N poles and S poles are alternately magnetized one by one while shielding, with a magnetic shield member, a magnetic track or a portion to be a magnetic track, which is not currently being magnetized.

As described above, since the N poles and the S poles are alternately magnetized one by one while shielding, with the magnetic shield, a portion to be a magnetic track on a side that is not currently being magnetized, influence of leakage of a magnetic flux is minimized, whereby magnetization with relatively high accuracy can be performed. Therefore, it is possible to produce, with higher accuracy, the magnetic encoder of the present invention in which the main track is formed on a portion, of the core member, having high rigidity, and which can detect an absolute angle with high accuracy. In addition, the magnetic encoder can be produced by simple modification of an existing production method with configurations mentioned above.

In the magnetic encoder production method of the present invention, the main track may be magnetized after the sub track has been magnetized. Although degradation in accuracy may be caused by leakage of a magnetic flux during magnetization of a magnetic track as described above, since the main track having the larger number of magnetic pole pairs that affect the angular accuracy is magnetized last in the magnetization order, degradation in accuracy of the main track is inhibited, and an absolute angle can be detected with high accuracy.

DESCRIPTION OF EMBODIMENTS

A first embodiment of the present invention will be described with reference toFIG. 1toFIG. 6. The first embodiment is an example in which the present invention is applied to a radial type magnetic encoder.FIG. 1is a longitudinal-sectional view of the magnetic encoder. InFIG. 2, chart (a) shows magnetization patterns of magnetic tracks developed in the circumferential direction, charts (b) and (c) show detection signals corresponding to respective magnetic pole pairs in the magnetization patterns, and chart (d) shows a phase difference between the detection signals.

The magnetic encoder1is produced as follows. A rubber material, in which a magnetic powder is kneaded, is put in a mold together with a core member2of annual shape which may be a metal ring, and is bonded through vulcanization to the outer peripheral surface of the core member2to form an annular magnetic member3. Alternatively, a core member2and a mixture of a plastic material and a magnetic powder are integrally molded to form an annular magnetic member3on the outer peripheral surface of the core member2. Then, a plurality of rows (two rows in this embodiment) of magnetic tracks4having different numbers of magnetic pole pairs are formed on the surface of the magnetic member3that has not been magnetized.

The core member2is formed through press-molding of an iron-based rolled steel plate. The core member2has: a cylindrical portion2A having an outer peripheral surface that serves as a track formation surface2Aa; a bending plate portion2B that bends from the cylindrical portion2A toward the inner diameter side; and an attachment portion2C of cylindrical shape that extends from an inner-diameter-side edge of the bending plate portion2B to a side opposite to the cylindrical portion2A, concentrically with the cylindrical portion2A. A rotary shaft (not shown) is fixed to the attachment portion2C by press-fitting or the like.

The magnetic member3is magnetized with, for example, 32 pole pairs, with the magnetic track4on the side close to the bending plate portion2B being a main track5, while the magnetic member3is magnetized with, for example, 31 pole pairs, with the magnetic track4on the side distant from the bending plate portion2B being a sub track6. This magnetic encoder1is used for detection of an absolute angle of a rotary shaft by utilizing the fact that a difference of one pole pair is generated per rotation.

For example, as magnetic sensors for absolute angle detection, magnetic sensors31and32are disposed so as to oppose the main track5and the sub track6of the magnetic encoder1, respectively, and the magnetic encoder1is rotated around the center-of-annulus O. In this case, the detection signal shown in chart (b) ofFIG. 2is outputted from the magnetic sensor31on the main track5side while the detection signal shown in chart (c) ofFIG. 2is outputted from the magnetic sensor32on the sub track6side. Each detection signal is a signal in which one pair of an N pole and an S pole corresponds to a cycle from 0° to 360° in phase. By taking a difference between these detection signals, as shown in chart (d) ofFIG. 2, a phase difference signal that linearly change of waveform is obtained with rotation of the magnetic encoder1. In this case, with one rotation from 0° to 360° of the magnetic encoder1, the phase difference signal indicates a waveform of one period.

In detecting an absolute angle by the magnetic encoder, an angle is calculated with high accuracy on the basis of the main track5, and an absolute angle can be detected while recognizing the position of the main track based on the difference of the phase between the main track5and the sub track6. It is noted that an absolute angle detection device is composed of: the magnetic encoder1; the magnetic sensors31and32; and an operation software or hardware (not shown) such as an electronic circuit that performs calculation of the absolute angle from the detection signals of the magnetic sensors31and32.

Examples of magnetization methods include: a method of magnetizing the magnetic tracks4(5and6) in a predetermined order while rotating the magnetic encoder1, by using an index magnetization device that magnetizes N poles and S poles alternately one by one; and one-shot magnetization in which both the magnetic tracks4(5and6) are simultaneously magnetized. Either method may be used. However, the one-shot magnetization complicates the structure of the magnetizing yoke and causes magnetic interference between the magnetic tracks4(5and6) during magnetization, which makes magnetization with high accuracy difficult. Therefore, the magnetization using the index magnetization device is preferred when the magnetic encoder1has a plurality of rows of magnetic tracks4.

For example, in a case where an absolute angle is detected by using two rows of magnetic tracks4(5and6) that are magnetized with 32 pole pairs and 31 pole pairs, respectively (in this case, the number of the magnetic poles of the main track5is larger by one than the number of the magnetic poles of the sub track6), an angle per pole pair on the 32 pole pairs side (main track5side) is 11.25° (360/32). In order to determine the present phase position, a magnetization accuracy of not more than 0.35° corresponding to one 32th of 11.25° (11.25/32), or, for the sake of safety, a magnetization accuracy within +0.1°, is required. If the number of magnetic poles is increased to be, for example, 64 pole pairs and 63 pole pairs, the required accuracy becomes stricter. For example, a magnetization accuracy within +0.04° is required.

In order to improve the accuracy of the main track5that affects the angular accuracy, it is preferable to inhibit rotational deflection of the magnetic encoder1and maintain high rigidity. Therefore, in this embodiment, a magnetic track4on a side close to the bending plate portion2B, of the magnetic track formation surface2Aa of the core member2, which is formed by bending the core member2and therefore has high rigidity due to continuing to the bending plate portion2B, is rendered as a main track5, whereby improvement of the angular accuracy is expected.

In a case where the main track5having the larger number of magnetic pole pairs to be used for calculation of an angle is magnetized first, when the sub track6is magnetized thereafter, leakage of a magnetic flux may affect the accuracy of the main track5, e.g., a pitch error (pitch accuracy) or an accumulated pitch error (accumulated pitch accuracy) of the magnetic poles. In this case, the angular accuracy is reduced.

Each of the pitch error and the accumulated pitch error is an index indicating the accuracy of the magnetized track. For example, assuming that a magnetic track is magnetized with 32 pole pairs, an angle per pole pair is theoretically 11.25°. Then, if the angle of a certain pole pair is 11.3° in actuality, this pole pair has a pitch error of +0.05°. The accumulated pitch error is obtained by accumulating the pitch errors of all the pole pairs, and is represented by the maximum value (amplitude) thereof.

Therefore, the main track5having the larger number of magnetic pole pairs that affect the angular accuracy is magnetized last. Thus, degradation in accuracy of the main track5is inhibited, and an absolute angle can be detected with high accuracy. That is, since the aforementioned magnetization order is adopted, degradation in accuracy of the main track5is inhibited, the main track5is formed with higher pitch accuracy and higher accumulated pitch accuracy of the magnetic poles than those of the sub track6. In this case, when the main track5is magnetized, this magnetization may affect the accuracy of the sub track6that has been magnetized first. However, since the sub track6is used for recognizing the phase relationship with the main track5, the accuracy thereof need not be taken into much consideration.

FIG. 3shows a magnetization device.FIG. 4is a cross-sectional view taken along a line IV-IV inFIG. 3. The magnetization device7of the magnetic encoder includes: a spindle9configured to cause a chuck8that holds an un-magnetized magnetic encoder1as a magnetization target to rotate, with the center-of-annulus O coinciding with the rotation axis RO; a motor10configured to rotate the spindle9; a magnetizing yoke11; a positioning mechanism12configured to position the magnetizing yoke11in three axial directions; a magnetization power source13; and a controller14. The motor10has a highly accurate encoder device24which is a detection device for detecting a rotation angle. The magnetization device7further includes a magnetic sensor15configured to measure magnetization accuracy when magnetization of the magnetic encoder1held by the chuck8is finished. The magnetic sensor15is fixed to a positioning mechanism16capable of positioning the magnetic sensor15in three axial directions. The motor10and the positioning mechanism12of the magnetizing yoke11form a positioning device29configured to position a tip portion19of the magnetizing yoke11relative to the un-magnetized magnetic encoder1.

The controller14is implemented as a computer or the like. The controller14controls, through numerical control or the like, the magnetization power source13, and the positioning mechanism12and the motor10of the positioning device29such that individual magnetic tracks4of the un-magnetized magnetic encoder1are sequentially magnetized, such that the main track5is magnetized after the sub track6with this order, and such that N magnetic poles and S magnetic poles are alternately arranged.

The magnetizing yoke11has a pair of opposed end portions (also referred to as tip portions)19and20that are magnetically opposed to each other across a magnetic gap. The magnetizing yoke11magnetizes the magnetic tracks4of the un-magnetized magnetic encoder1disposed at a predetermined position and in a predetermined attitude with respect to the opposed end portions19and20. Specifically, the magnetizing yoke11is composed of a U-shaped magnetizing yoke body17, an exciting coil18, and a first tip portion19and a second tip portion20respectively provided at one end and the other end of the magnetizing yoke body17. The exciting coil18is wound around the outer periphery of the magnetizing yoke body17. The magnetizing yoke11causes a magnetic flux a (seeFIG. 4), for magnetization, to penetrate the magnetic encoder1. The first tip portion19of the magnetizing yoke11has a pointed end. During magnetization, the first tip portion19is opposed to the surface of the magnetic encoder1(i.e., the magnetic track4). The second tip portion20is opposed to the chuck8with a gap therebetween, and a magnetic loop, which extends from the first tip portion19to the second tip portion20through the magnetic encoder1and the chuck8, is formed. The second tip portion20may be omitted.

A magnetic shield member21has a rectangular hole22that has a tapered vertical cross section along the axis RO, and the first tip portion19is disposed with respective gaps above and below the hole22. The magnetic shield member21and the first tip portion19, each opposing the magnetic encoder1, are positioned with a predetermined gap, e.g., about 0.1 mm, with respect to the un-magnetized magnetic track4.

The magnetic shield member21is fixed to an end portion of a support base23that is fixed at a position close to the second tip portion20of the magnetizing yoke body17. Of magnetic fluxes generated from the first tip portion19, a magnetic flux that affects the other magnetic track4not to be magnetized is guided to the magnetic shield member21so as to be alleviated toward the second tip portion20on the opposite side from the first tip portion19that opposes the magnetic encoder1. The magnetic shield member21and the support base23are formed of a magnetic body, e.g., a low-carbon steel material. In magnetizing the magnetic encoder1having the plurality of rows of magnetic tracks, the magnetic shield member21can be opposed to the magnetic track4so as to shield the flow of the magnetic flux to the magnetic track other than the magnetization target.

FIG. 5shows the position where the first tip portion19of the magnetizing yoke11is disposed when the two rows of magnetic tracks4(5and6) are magnetized to the magnetic member3of the un-magnetized magnetic encoder1.FIG. 6shows an example of a magnetization pattern of the magnetic encoder1magnetized in the two rows.

Specifically, chart (a) ofFIG. 5shows arrangement of the first tip portion19of the magnetizing yoke11and the magnetic shield member21in a case where the upper half of the magnetic member3of the magnetic encoder1is magnetized as the magnetic track4to be the sub track6. In this case, the surface of the magnetic member3, on which the other magnetic track4(main track5) is to be formed, is covered with the magnetic shield member21to prevent the magnetic flux, which flows from the first tip portion19, from flowing to the other magnetic track4(main track5).

Meanwhile, chart (b) ofFIG. 5shows arrangement of the first tip portion19of the magnetizing yoke11and the magnetic shield member21in a case where the lower half of the magnetic member3of the magnetic encoder1is magnetized as the magnetic track4to be the main track5. At this time, the surface of the magnetic member3, on which the magnetic track4as the sub track6magnetized first has been formed, is covered with the magnetic shield member21to prevent the magnetic flux, which flows from the first tip portion19, from flowing to the magnetic track4(sub track6).

When magnetization is performed in an order such that the sub track6(magnetic track4) is formed in the process shown in chart (a) ofFIG. 5and the main track5(magnetic track4) is formed last in the process shown in chart (b) ofFIG. 5, degradation in accuracy of the main track5is inhibited, whereby an absolute angle can be detected with high accuracy.

According to the present embodiment, as described above, the magnetic track4on the side close to the bending plate portion2B, which is formed by bending the core member2and therefore has high rigidity and less rotational deflection, serves as the main track5which is required to have high accuracy and has the larger number of magnetic pole pairs, whereby it contributes to improvement and stability of angular accuracy.

Further, in magnetizing the magnetic encoder1having the plurality of rows of magnetic tracks4, the main track5as the magnetic track4for calculating an angle is magnetized last, whereby degradation in accuracy of the main track5is inhibited, and an absolute angle can be detected with high accuracy.

FIG. 7toFIG. 9show a second embodiment of the present invention. In this embodiment, the present invention is applied to an axial type magnetic encoder1. The second embodiment is the same as the first embodiment described with reference toFIG. 1toFIG. 6, except the features to be specifically described below. In this embodiment, the core member2has: a plate portion2D of annular shape having one surface that serves as a track formation surface2Da; and a bending plate portion2E that bends and extends from an inner-diameter-side edge of the plate portion2D to a side opposite to the track formation surface2Da, and that serves as an attachment portion of cylindrical shape.

The magnetic member3is provided on the track formation surface2Da, and a plurality of rows of magnetic tracks4are provided on the magnetic member3. In this case, one of the magnetic tracks4, on the innermost peripheral side close to the bending plate portion2E, serves as the main track5while the other magnetic track4servers as the sub track6. The main track5and the sub track6are sequentially magnetized on the un-magnetized magnetic encoder1such that the sub track6is magnetized first and the main track5is magnetized last.

A device used for the above magnetization is basically the same as the magnetization device shown inFIG. 3except that the direction in which the magnetic track4of the magnetic encoder1faces is the axial direction whereas the direction is the radial direction inFIG. 3. According to this difference, as shown inFIG. 9, the tip portion19of the magnetization yoke11and the magnetic shield member21face in the direction of the center-of-annulus O of the magnetic encoder1(FIG. 7), that is, in the direction of the rotary shaft axis RO (see charts (a) and (b) ofFIG. 9). In addition, the direction, in which the magnetization yoke11and the magnetic shield member21, used for switching the magnetic track4to be magnetized, move relative to the magnetic encoder1, is the radial direction of the magnetic encoder1as shown in charts (a) and (b) ofFIG. 9which show the respective positioning states.

Also in this configuration, the magnetic track4on the side close to the bending plate portion2E, which is formed by bending the core member2and therefore has high rigidity and less rotational deflection, serves as the main track5which is required to have high accuracy and has the larger number of magnetic pole pairs, thereby contributing to improvement and stability of angular accuracy. Further, in magnetizing the magnetic encoder1having the plurality of rows of magnetic tracks4, the main track5as the magnetic track4for calculating an angle is magnetized last, whereby degradation in accuracy of the main track5is inhibited, and an absolute angle can be detected with high accuracy.

Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, numerous additions, changes, or deletions can be made without departing from the gist of the present invention. Therefore, such additions, changes, and deletions are also construed as included within the scope of the present invention.

REFERENCE NUMERALS