Magnetic drum and magnetic encoder having the same

Abstract

The present invention provides a magnetic drum and a magnetic encoder having the same, wherein the magnetic drum comprises a support ring and a first permanent magnetic ring and a second permanent magnetic ring mounted on the outer circumferential side of the support ring, the first permanent magnetic ring and the second permanent magnetic ring are sequentially arranged along the axial direction of the support ring, the first permanent magnetic ring and the second permanent magnetic ring are both unipolar magnetic rings with opposite polarities, the outer circumferential wall of the first permanent magnetic ring is constructed with a first protrusion and a first recess alternating along its circumferential direction, and the outer circumferential wall of the second permanent magnetic ring is constructed with a second protrusion and a second recess alternating along its circumferential direction. According to the present invention, an N / S alternating magnetic signal can be directly obtained at the circumferential matching gap between the first permanent magnetic ring and the second permanent magnetic ring, or an N / S alternating magnetic signal can be obtained after data processing of the unipolar magnetic signal obtained from the first permanent magnetic ring and the second permanent magnetic ring, so that the magnetic signal types output by the magnetic drum are more abundant and the signal accuracy is higher.

Description

Technical Field

[0001] The invention belongs to the technical field of magnetic encoder design, and in particular relates to a magnetic drum and a magnetic encoder having the same. Background Art

[0002] Magnetic encoder is an angle or displacement measuring device composed of magnetoresistive sensor, magnetic drum and signal processing circuit. Magnetic encoder has many advantages such as small size, high precision, high resolution, no contact and no wear, high vibration resistance, simple installation, long service life and multiple interface forms. Therefore, it is widely used in industrial control, machinery manufacturing, shipbuilding, textile, printing, aviation, aerospace, radar, communication, military industry and other fields.

[0003] The number of magnetic poles on the drum determines the resolution of the magnetic encoder, the uniformity of the magnetic poles on the drum determines the quality of the output signal of the magnetic encoder, and the magnetic field strength and action distance of the magnetic poles on the drum determine the structure and volume of the magnetic encoder. In short, the drum has a very important influence on the various performance parameters of the magnetic encoder, and many performances of the drum are inseparable from the structure of the magnetic signal source on its circumference.

[0004] like Figure 1 As shown in the figure, the magnetic encoder's magnetic drum is mainly composed of a permanent magnet ring a as a magnetic signal source, a support ring b and a rotating shaft c. If the permanent magnet ring on the outer circumference of the magnetic drum is unfolded, the arrangement of the magnetic poles on its surface is as follows: Figure 2 As shown, the cross-sectional magnetic pole structure of the permanent magnet expansion diagram on the outer circumference of the magnetic drum is as follows Figure 3 As shown, the characteristics of the magnetic drum with this structure are: the magnetic signal source comes from the N poles and S poles of the permanent magnets spaced on the circumference of the magnetic drum. The shortcomings of the magnetic drum with this structure are: first, with the increase in the number of magnetic poles, the proportion of magnetic lines of force forming a closed loop between adjacent magnetic poles increases significantly, causing the effective distance of the magnetic field to decrease rapidly, the magnetic signal received by the magnetoresistive sensor to weaken, and the output signal quality of the magnetic encoder to decrease. The number of magnetic poles on the magnetic drum has an important influence on the resolution of the magnetic encoder; second, there is a non-magnetic area between the N pole and the S pole on the magnetic drum. The uniformity, width, and shape of the non-magnetic area directly determine the uniformity, width, and shape of the magnetic signal, and then determine the quality of the output signal of the magnetic encoder. At present, the uniformity of the magnetic signal source of the magnetic drum can only be controlled by the magnetization method. The manufacturing process of high-quality magnetic drums is very complicated and the consistency is difficult to ensure. At present, the uniformity of the magnetic signal source of the magnetic drum can only be controlled by the magnetization method. The manufacturing process of high-quality magnetic drums is very complicated and the consistency is difficult to ensure. The following are several defects that are easy to occur in the magnetic drum, which is currently commonly used by magnetizing the N / S poles formed by the permanent magnet ring as the magnetic signal source:

[0005] The first type is that the boundary of the non-magnetic zone between the N / S poles is fuzzy, such as Figure 4As shown;

[0006] The second type is the deformation of the boundary of the non-magnetic zone between the N / S poles, such as Figure 5 As shown;

[0007] The third is that the local defects in the magnet cause the magnetic signal in that area to suddenly decrease or even disappear, such as Figure 6 shown.

[0008] However, the magnetic drum in the magnetic encoder in the related art mostly uses only one magnetic ring to form a magnetic signal source, the output magnetic signal type of the magnetic signal source is relatively single, and the corresponding control accuracy (signal accuracy) is low. Summary of the invention

[0009] Therefore, the present invention provides a magnetic drum and a magnetic encoder having the same, so as to overcome the shortcomings of the related art that the magnetic drum has only one magnetic ring as a magnetic signal source, the output magnetic signal type is relatively single, and the signal accuracy is low.

[0010] In order to solve the above problems, the present invention provides a magnetic drum, including a support ring and a first permanent magnet ring and a second permanent magnet ring mounted on the outer circumferential side of the support ring, the first permanent magnet ring and the second permanent magnet ring are arranged in sequence along the axial direction of the support ring, the first permanent magnet ring and the second permanent magnet ring are both unipolar magnetic rings with opposite polarities, the outer circumferential wall of the first permanent magnet ring is constructed with a first protrusion and a first recess alternating along its circumferential direction, and the outer circumferential wall of the second permanent magnet ring is constructed with a second protrusion and a second recess alternating along its circumferential direction.

[0011] In some embodiments, the magnetic signal probe of the magnetic encoder is arranged relative to the circumferential matching gap formed between the first permanent magnetic ring and the second permanent magnetic ring, so as to obtain the N / S alternating magnetic signal formed by the first permanent magnetic ring and the second permanent magnetic ring; or, the magnetic signal probe of the magnetic encoder is arranged relative to the outer circumferential wall of the first permanent magnetic ring and the second permanent magnetic ring, so as to obtain the N-pole magnetic signal and the S-pole magnetic signal formed by the first permanent magnetic ring and the second permanent magnetic ring, respectively, and data processing is performed based on the obtained N-pole magnetic signal and S-pole magnetic signal to obtain the N / S alternating magnetic signal.

[0012] In some embodiments, along the axial direction of the magnetic drum, the first protrusions and the second recesses are alternately arranged, and the first recesses and the second protrusions are alternately arranged.

[0013] In some embodiments, the first protrusion, the first recess, the second protrusion, and the second recess are all formed by machining.

[0014] In some embodiments, a magnetic isolation ring is further mounted on the support ring, and the first permanent magnetic ring and the second permanent magnetic ring clamp the magnetic isolation ring.

[0015] In some embodiments, the magnetic signal probe of the magnetic encoder is arranged opposite to the magnetic isolation ring so as to acquire the N / S alternating magnetic signal formed by the first permanent magnetic ring and the second permanent magnetic ring.

[0016] In some embodiments, the axial height of the magnetic isolation ring is Da, the axial height of the first permanent magnet ring is Db, the axial height of the second permanent magnet ring is Dc, the radial thickness of the first permanent magnet ring and the second permanent magnet ring are equal and both are △R, Da<3(Db+Dc+△R).

[0017] In some embodiments, Da<2(Db+Dc+ΔR).

[0018] In some embodiments, the magnetic drum further includes a third permanent magnet ring, and an outer ring wall of the third permanent magnet ring has N poles and S poles alternating along its axial direction.

[0019] The present invention also provides a magnetic encoder, comprising the magnetic drum mentioned above.

[0020] The present invention provides a magnetic drum and a magnetic encoder having the same, wherein the support ring has at least a first permanent magnet ring and a second permanent magnet ring with opposite polarities on the outer ring wall, so that the magnetic signal source of the magnetic drum is improved from a single one in the prior art to at least two, thereby making the magnetic signal types output by the magnetic drum more abundant and the signal accuracy higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the internal structure of a magnetic drum in the prior art;

[0022] Figure 2 It is a development diagram of the outer circumference of a radially magnetized magnetic ring (multi-pole magnetic ring) used in a magnetic drum in the prior art;

[0023] Figure 3 for Figure 2 A cross-sectional view (radial cross-section) of a radially magnetized magnetic ring (multipolar magnetic ring);

[0024] Figure 4 It is a schematic diagram of the fuzzy boundary defect of the non-magnetic area of ​​the permanent magnet ring in the prior art;

[0025] Figure 5 It is a schematic diagram of the deformation defect of the boundary of the non-magnetic area of ​​the permanent magnet ring in the prior art;

[0026] Figure 6 It is a schematic diagram of magnetic signal deformation caused by defects in the non-magnetic area of ​​the permanent magnet ring in the prior art;

[0027] Figure 7: is a structural schematic diagram of the first permanent magnet ring or the second permanent magnet in the magnetic drum of an embodiment of the invention;

[0028] Figure 8 for Figure 7 The outer circumferential surface development diagram when the outer ring wall of the permanent magnetic ring is a single N pole;

[0029] Fig. 9 for Figure 8 A cross-sectional view of the permanent magnet ring in (radial cross-section);

[0030] Fig.10 Another structural schematic diagram of the first permanent magnet ring or the second permanent magnet in the magnetic drum of an embodiment of the invention;

[0031] Fig.11 Schematic diagram of the structure of the first permanent magnet ring and the second permanent magnet ring after circumferential expansion in the magnetic drum of the embodiment of the present invention, in which the black rectangle shows the protruding position and the white rectangle shows the concave position;

[0032] Fig.12 for Fig.11 The N / S alternating magnetic signals obtained at the outer circumferential surface of the magnetic drum (the circumferential matching gap formed by the first permanent magnet ring and the second permanent magnet ring) are shown;

[0033] Fig.13 It is a schematic structural diagram of a first permanent magnet ring and a second permanent magnet ring in a magnetic drum after circumferential expansion in another embodiment of the present invention, wherein an axial interval exists between the first permanent magnet ring and the second permanent magnet ring;

[0034] Fig.14 Shows Fig.13 Two groups of magnetic signals corresponding to the first permanent magnet ring and the second permanent magnet ring are obtained at the outer circumferential surface of the magnetic drum shown in the figure, and the two are processed to obtain a group of magnetic signals with N / S alternating arrangement;

[0035] Fig.15 This is a schematic structural diagram of a first permanent magnet ring and a second permanent magnet ring in a magnetic drum after circumferential expansion according to another embodiment of the present invention, wherein a magnetic isolation ring is provided between the first permanent magnet ring and the second permanent magnet ring.

[0036] Fig.16 It shows a set of N / S alternating magnetic signals obtained by the magnetic probe detecting the position of the magnetic isolation ring;

[0037] Fig.17 It is a schematic structural diagram of a first permanent magnet ring and a second permanent magnet ring in a magnetic drum after circumferential expansion in another embodiment of the present invention;

[0038] Fig.18 It is shown that two first permanent magnet rings and one second permanent magnet ring are sequentially mounted on the outer circumferential surface of the support ring along its axial direction.

[0039] The reference numerals are:

[0040] 1. First permanent magnetic ring; 11. First protrusion; 12. First recess; 2. Second permanent magnetic ring; 21. Second protrusion; 22. Second recess; 3. Magnetic isolation ring; 4. Third permanent magnetic ring. DETAILED DESCRIPTION

[0041] See also Figures 1 to 18 As shown, according to an embodiment of the present invention, a magnetic drum is provided, comprising a support ring and a first permanent magnetic ring 1 and a second permanent magnetic ring 2 mounted on the outer circumferential side of the support ring, wherein the first permanent magnetic ring 1 and the second permanent magnetic ring 2 are sequentially arranged along the axial direction of the support ring, the first permanent magnetic ring 1 and the second permanent magnetic ring 2 are both unipolar magnetic rings with opposite polarities, the outer circumferential wall of the first permanent magnetic ring 1 is configured with a first protrusion 11 and a first recess 12 alternating along its circumferential direction, and the outer circumferential wall of the second permanent magnetic ring 2 is configured with a second protrusion 21 and a second recess 22 alternating along its circumferential direction. In this technical solution, the support ring has at least the first permanent magnetic ring 1 and the second permanent magnetic ring 2 with opposite polarities on the outer ring wall, so that the magnetic signal source of the magnetic drum is improved from a single one in the prior art to at least two, so that the magnetic signal types output by the magnetic drum are more abundant and the signal accuracy is higher. It can be understood that when the outer ring wall of the first permanent magnet ring 1 is an N pole, its inner ring wall is an S pole, and when the outer ring wall of the second permanent magnet ring 2 is an S pole, the inner ring wall is an N pole.

[0042] The N / S alternating magnetic signals of the currently common magnetic encoders are all obtained from a single magnetic ring, that is, the single magnetic ring is magnetized into N / S alternating magnetic poles. The N / S alternating magnetic signals of the magnetic encoder provided by the present invention can be obtained in two ways: the first way is to directly obtain the N / S alternating magnetic signal by detecting the circumferential junction of the first permanent magnetic ring 1 and the second permanent magnetic ring 2; the second way is to perform data processing on the unipolar magnetic signals obtained from the first permanent magnetic ring 1 and the second permanent magnetic ring 2 to obtain the N / S alternating magnetic signal. Correspondingly, the magnetic signal probe of the magnetic encoder is arranged relative to the circumferential matching gap formed between the first permanent magnetic ring 1 and the second permanent magnetic ring 2, so as to be able to obtain the N / S alternating magnetic signal (such as Fig.11 and Fig.12 As shown); or, the magnetic signal probes of the magnetic encoder are respectively arranged relative to the outer circumferential walls of the first permanent magnetic ring 1 and the second permanent magnetic ring 2, so as to be able to respectively obtain the N-pole magnetic signal and the S-pole magnetic signal formed by the first permanent magnetic ring 1 and the second permanent magnetic ring 2, and perform data processing based on the obtained N-pole magnetic signal and the S-pole magnetic signal to obtain the N / S alternating magnetic signal (as shown Fig.13 and Fig.14As shown). It should be pointed out that another major difference between the present invention and the currently commonly used magnetic encoder is that the uniformity and accuracy of the N / S alternating magnetic signal of the currently commonly used magnetic encoder is achieved by controlling the accuracy of the magnetizing coil, the inner and outer diameter tolerances of the magnetic ring, the gap between the magnetic ring and the magnetizing coil during magnetization, and many other factors. Not only are there many variable factors, but they are also difficult to control. The uniformity and accuracy of the N / S alternating magnetic signal of the magnetic encoder of the present invention is achieved by controlling the machining accuracy of the protrusions and depressions of the magnetic ring during the mechanical processing. It is obvious that the accuracy and consistency of the mechanical processing are very easy to control. The accuracy of the magnetic signal plays a decisive role in the accuracy of the magnetic encoder, so the present invention can significantly improve the accuracy of the magnetic encoder.

[0043] In some embodiments, along the axial direction of the magnetic drum, the first protrusions 11 and the second depressions 22 are alternately formed, and the first depressions 12 and the second protrusions 21 are alternately formed. This can prevent the magnetic field strength at the axially adjacent protrusions or depressions from being zero due to the corresponding first protrusions 11 and the second protrusions, and the corresponding first depressions 12 and the second depressions in the same axial direction, and failing to provide the magnetic signal required by the magnetic encoder. It can be understood that after deviating from the middle area between the first permanent magnet ring 1 and the second permanent magnet ring 2, only the magnetic signal of one magnetic pole can be detected. Specifically, when deviating from the magnetic ring with an N-pole outer ring wall, the N-pole magnetic signal is detected, and when deviating from the magnetic ring with an S-pole outer ring wall, the S-pole magnetic signal is detected. The magnetic signal distributed at the interval between the N-pole and the S-pole cannot be detected. In order to obtain the magnetic signal of the interval between the N-pole and the S-pole in the middle position of the two magnetic rings (that is, the first permanent magnet ring 1 and the second permanent magnet ring 2), the protrusions or depressions of the two magnetic rings must be staggered by an angle. The signal strengths of the N-pole and S-pole obtained by the different staggered angles are different. When the protrusion of the N-pole magnetic ring just corresponds to the depression of the S-pole magnetic ring (that is, the aforementioned technical solution), the symmetry of the magnetic signal of the interval between the N-pole and the S-pole is the best. As for the value of the staggered angle, it is related to the number of raised parts on the magnetic ring (similar to the number of teeth on a gear). The fewer the raised and recessed parts on the outer wall of the magnetic ring, the larger the staggered angle, and vice versa. In theory, the more magnetic poles there are on the outer circumference of the magnetic encoder drum, the higher the accuracy of the magnetic encoder. Therefore, the number of magnetic poles on the magnetic encoder drum of the same diameter is one of the important parameters for measuring the accuracy of the magnetic encoder.

[0044] In some embodiments, the maximum circumferential width of the first recess 12 is L, the radial height of the first protrusion 11 is H, L>0.1H; and / or, the maximum circumferential width of the second recess 22 is L, the radial height of the second protrusion 21 is H, L>0.1H, further, L>0.5H, so as to ensure that the difference in magnetic field intensity between the magnetic drum and the corresponding positions of the protrusion 13 and the recess 14 can obtain the best detection resolution, which is conducive to detection and utilization. It can be understood that the width L of the first protrusion 11, the first recess 12, the second protrusion 21, and the second recess 22 can be equal or unequal, and the depth H of the recess can be equal or unequal, which can be designed according to actual needs.

[0045] In some embodiments, the first protrusion 11, the first recess 12 and the second protrusion 21, the second recess 22 are all formed by machining, specifically by laser processing, so as to ensure the dimensional accuracy of each protrusion and groove, and can ensure the uniform distribution of the magnetic signal source in 360°, which can effectively overcome the defects of blurring, deformation and inconsistent width of the non-magnetic area between the N pole and the S pole that are easily caused during the magnetization process of the magnetic ring, greatly improve the quality of the magnetic signal source of the magnetic drum, and ensure the accuracy of the magnetic encoder.

[0046] If two magnetic rings are placed close together, unless the detection probe (i.e., magnetic signal probe) is placed exactly at the contact gap between the two magnetic rings, otherwise, as long as there is a slight deviation, the magnetic signal detected will be either S is extremely strong and N is extremely weak, or N is extremely strong and S is extremely weak. Therefore, in some embodiments, a magnetic isolation ring 3 is also mounted on the support ring, and the first permanent magnetic ring 1 and the second permanent magnetic ring 2 clamp the magnetic isolation ring 3, so that the deviation amplitude of the magnetic signal caused by the position deviation of the magnetic signal probe can be greatly reduced. The material of the magnetic isolation ring 3 must be a poor magnetic conductor, such as copper, aluminum, plastic, rubber, etc. At this time, the magnetic signal probe of the magnetic encoder is arranged relative to the magnetic isolation ring 3, so as to be able to obtain the N / S alternating magnetic signal formed by the first permanent magnetic ring 1 and the second permanent magnetic ring 2.

[0047] Research has found that the axial height Da of the magnetic isolation ring 3 is related to both the thickness of the magnetic ring in the radial direction (i.e., the radial thickness ΔR) and the height of the magnetic ring in the axial direction (i.e., the axial height Dc of the first permanent magnet ring 1Db and the second permanent magnet ring 2). However, the larger the axial height Da of the magnetic isolation ring 3, the weaker the magnetic signal detected at the magnetic isolation ring 3. Therefore, the axial height of the magnetic isolation ring 3 cannot be too large. However, as mentioned earlier, if the position of the magnetic probe can be guaranteed to be absolutely correct, it is possible even without the magnetic isolation ring 3. In fact, in the actual implementation process, this position is difficult to be reliably guaranteed. Therefore, in some embodiments, the radial thicknesses of the first permanent magnet ring 1 and the second permanent magnet ring 2 are designed to be equal and both are △R, Da<3(Db+Dc+△R), and the best is Da<2(Db+Dc+△R). This can ensure the convenient determination of the position of the magnetic probe and the accuracy and reliability of the detection signal. If Da>3(Db+Dc+△R), the magnetic lines of force of each magnetic ring may form an independent loop. In this case, either the N / S alternating magnetic signal cannot be detected between the two magnetic rings or the detected N / S alternating magnetic signal is not strong enough, resulting in reduced accuracy of the magnetic encoder.

[0048] In some embodiments, the magnetic drum further includes a third permanent magnet ring 4 (such as Fig.17 As shown in the figure, the outer ring wall of the third permanent magnet ring 4 has N poles and S poles alternating along its axial direction, which can further enrich the output magnetic signal types of the magnetic drum, and is conducive to further improving the control accuracy after the magnetic encoder chip comprehensively processes the signal. It can be understood that more permanent magnet rings can be mounted on the outer circumferential surface of the support ring to further enrich the output types of the magnetic drum magnetic signal.

[0049] The inner ring wall of the support ring is connected to the rotating shaft. At this time, the inner ring wall of the support ring can be processed to form necessary connection structures, such as keyways (which can be splines or flat keys), so that the magnetic drum can be disassembled more conveniently.

[0050] In some embodiments, when projected on any radial plane of the support ring, the shape of the first protrusion 11 and / or the second protrusion 21 is a rectangle, an isosceles trapezoid, an isosceles triangle, or an arc. It should be noted that the specific shapes of the first protrusion 11 and the second protrusion 21 are not particularly limited in the present invention. However, no matter what specific shape is adopted, the plurality of first protrusions 11 and the second protrusions 21 should be centrally symmetrical about the axis of the magnetic drum to meet the requirement of serving as a magnetic signal source of an encoder.

[0051] In some embodiments, the permanent magnet ring body 1 is a radially oriented magnetic ring or an isotropic magnetic ring with a monopole magnetization characteristic.

[0052] In some embodiments, the radiation-oriented magnetic ring is made of at least one of neodymium iron boron, samarium cobalt, aluminum nickel cobalt, and ferrite, that is, the radiation-oriented magnetic ring can be made of a single material or a composite of multiple materials. The isotropic magnetic ring with a single-pole magnetization feature is made of at least one of neodymium iron boron and samarium cobalt permanent magnets.

[0053] According to an embodiment of the present invention, there is also provided a magnetic encoder, comprising the magnetic drum described above. It is understandable that the magnetic encoder also includes the magnetoresistive sensor and a corresponding signal processing circuit board (i.e., a magnetic encoder chip), which is a conventional technology of the magnetic encoder and will not be described in detail here.

[0054] It is easy for those skilled in the art to understand that, under the premise of no conflict, the above-mentioned advantageous methods can be freely combined and superimposed.

[0055] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention shall be included in the protection scope of the present invention. The above are only preferred embodiments of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and variations can be made without departing from the technical principles of the present invention, and these improvements and variations should also be regarded as the protection scope of the present invention.

Claims

1. A magnetic drum, characterized in that: The invention comprises a support ring and a first permanent magnetic ring (1) and a second permanent magnetic ring (2) which are mounted on the outer circumferential side of the support ring. The first permanent magnetic ring (1) and the second permanent magnetic ring (2) are arranged in sequence along the axial direction of the support ring. The first permanent magnetic ring (1) and the second permanent magnetic ring (2) are both unipolar magnetic rings with opposite polarities. The outer circumferential wall of the first permanent magnetic ring (1) is provided with a first protrusion (11) and a first recess (12) which are alternately arranged along its circumferential direction. The outer circumferential wall of the second permanent magnetic ring (2) is provided with a second protrusion (21) and a second recess (22) which are alternately arranged along its circumferential direction. The support ring is also provided with a magnetic isolation ring (3). The first permanent magnetic ring (1) and the second permanent magnetic ring (2) clamp the magnetic isolation ring (3). The magnetic signal probe of the magnetic encoder is arranged relative to the magnetic isolation ring (3) so as to obtain the N / S alternating magnetic signal formed by the first permanent magnetic ring (1) and the second permanent magnetic ring (2).

2. The magnetic drum according to claim 1, characterized in that Along the axial direction of the magnetic drum, the first protrusions (11) and the second recesses (22) are alternately arranged, and the first recesses (12) and the second protrusions (21) are alternately arranged.

3. The magnetic drum according to claim 1, characterized in that The first protrusion (11), the first depression (12) and the second protrusion (21), the second depression (22) are all formed by machining.

4. The magnetic drum according to claim 1, characterized in that The axial height of the magnetic isolation ring (3) is Da, the axial height of the first permanent magnetic ring (1) is Db, the axial height of the second permanent magnetic ring (2) is Dc, the radial thickness of the first permanent magnetic ring (1) and the second permanent magnetic ring (2) are equal and both are △R, Da<3(Db+Dc+△R).

5. The magnetic drum according to claim 4, characterized in that Da<2(Db+Dc+△R).

6. The magnetic drum according to claim 1, characterized in that It also comprises a third permanent magnetic ring (4), the outer ring wall of the third permanent magnetic ring (4) having N poles and S poles alternating along its axial direction.

7. A magnetic encoder comprising a magnetic drum, characterized in that: The magnetic drum is the magnetic drum according to any one of claims 1 to 6.

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