Double-magnet array rotary electromagnetic energy collector based on variable reluctance

Abstract

The invention discloses a variable reluctance-based double-magnet array rotary electromagnetic energy collector, which comprises a stator assembly and a rotor assembly, the stator assembly is provided with periodically arranged stator teeth, each stator tooth is provided with a coil, and the coils are connected in series with an external matching load or an external matching circuit combined with a capacitor. Two modes of connecting a pure resistor with a load and connecting a matching capacitor with the load are realized; the rotor assembly is composed of magnets and rotor teeth which are periodically arranged, and the magnets and the rotor teeth are alternately arranged; a variable air gap exists between the stator assembly and the rotor assembly, the rotor assembly enables magnetic resistance of the variable air gap to change periodically in the rotating process, a coil in the stator assembly generates induction voltage, and therefore mechanical energy in the rotating occasion is converted into electric energy to be output. The magnetic flux density gradient passing through the stator teeth in the magnetic circuit is enhanced, and the magnetic flux change of unit angular displacement is improved, so that the induced voltage is amplified according to the Faraday's law of electromagnetic induction, and higher power output is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of energy collection, and mainly relates to a double-magnet array rotary electromagnetic energy collector based on variable magnetic resistance. BACKGROUND

[0002] With the rapid development of Internet of Things technology, the construction of wireless sensor network is crucial in the automated industrial environment, and the sustainable power supply problem of sensor nodes has become the research focus. Since the sensor device is often embedded inside the running machine, it requires long-term operation stability. The traditional battery power supply scheme is limited in application due to the problems of environmental pollution risk, high maintenance cost, and limited endurance capacity. The self-powered technology based on environmental energy collection realizes electric energy conversion by capturing distributed energy such as mechanical vibration, light radiation, or thermal gradient, providing an innovative solution to this problem. Rotating parts (such as bearings, gears, cams, etc.) widely exist in industrial scenes, which contain rich kinetic energy resources. The application of energy collection to the condition monitoring of key rotating parts can improve the reliability in changing working environments, and the self-powered monitoring technology based on rotating mechanical energy collection has become an important research direction. By detecting the early failure of rotating parts such as shafts, bearings, and gears, timely maintenance can be carried out to reduce downtime and economic losses. The current mainstream energy conversion mechanisms include piezoelectric, magnetoelectric, and electromagnetic energy collection technologies.

[0003] Electromagnetic energy harvesters, typically based on Faraday's law of electromagnetic induction, consist of structures including coils and magnets. Their operation is non-contact, offering advantages such as simple structure, high durability, and high power output. In recent years, an increasing number of researchers have optimized magnetic flux gradients using Halbache arrays or alternating magnetic pole arrays. Among existing technologies, a bearing motion electromagnetic energy harvester based on a circular Halbache array, proposed by Zhang et al. in 2019 (Zhang Y, Cao J, Zhu H, et al. Design, modeling and experimental verification of circular Halbache electromagnetic energy harvesting from bearing motion[J]. Energy conversion and management, 2019, 180: 811-821.), can provide an average power output of 2.79 to 4.59 volts and 50.8 to 131.1 milliwatts at 600 to 1000 rpm. Meanwhile, the variable reluctance principle, initially applied to sensor devices, has gained attention in the field of electromagnetic energy harvesting in recent years due to its ease of integration into rotating machinery and its high power density. Variable reluctance energy harvesters generate changes in magnetic flux by altering the reluctance of the air gap in the magnetic circuit. In 2021, Gong et al. proposed an N-shape variable reluctance electromagnetic energy harvester based on a cylindrical Halbach arc magnet array (Gong Y, Wang S, Xie Z, et al. Design, modeling and optimization of an N-shape electromagnetic energy harvester for smart bearing of high speed train[J]. Smart Materials and Structures, 2021, 30(7): 075026.). By optimizing the concentrated closed magnetic flux path through a magnetoelectric coupling model, they achieved an output power of 4.26 watts under conditions of 1200 rpm and a 2 mm air gap.

[0004] Despite these advancements, most applications remain limited to relatively high rotational speeds because the output power of variable reluctance energy harvesters is quadratic with rotational speed. Therefore, optimizing performance at low speeds is crucial. Considering the compact layout of rotating components in industrial environments, improving volumetric power density at low speeds is essential for enabling real-time condition monitoring. Summary of the Invention

[0005] This invention addresses the limitation of power density in low-speed applications in existing technologies. To enhance the magnetic flux change in the coil pickup area at low speeds, it provides a rotating electromagnetic energy harvester based on a dual-magnet array with variable reluctance. The harvester includes a stator assembly and a rotor assembly. The stator assembly has periodically arranged stator teeth, each with a coil. The coil is connected in series with an external matching load or an external matching circuit incorporating a capacitor, allowing for both purely resistive and capacitor-connected load configurations. The rotor assembly consists of periodically arranged magnets and rotor teeth, alternating in arrangement. A variable air gap exists between the stator and rotor assemblies. During rotation, the rotor assembly causes a periodic change in the reluctance of this air gap, inducing a voltage in the coil within the stator assembly, thereby converting mechanical energy into electrical energy output during rotation. This invention enhances the magnetic flux density gradient through the stator teeth in the magnetic circuit, increasing the magnetic flux change per unit angular displacement. This amplifies the induced voltage according to Faraday's law of electromagnetic induction, achieving higher power output.

[0006] To achieve the above objectives, the technical solution adopted by this invention is: a rotating electromagnetic energy harvester based on a dual-magnet array with variable reluctance, comprising a stator assembly and a rotor assembly. The stator assembly is provided with periodically arranged stator teeth and coils; the rotor assembly is provided with periodically staggered magnets and rotor teeth; a variable air gap is formed between the stator assembly and the rotor assembly, and the reluctance of this air gap changes periodically during the rotation of the rotor assembly. The coils wound on the stator teeth are used to generate an induced voltage when the air gap reluctance changes; the coils are connected in series with an external matching load or an external matching circuit combined with a capacitor to achieve maximum power output of the device under low-speed rotation conditions. The air gap between the stator and rotor is maintained within a preset range to ensure that the desired magnetic flux change rate and output voltage are obtained at a given rotational speed.

[0007] As an improvement of the present invention, the magnet of the rotor assembly includes an internal magnet and an external magnet, and the external magnet and the internal magnet are arranged alternately in opposite directions to enhance the amplitude of magnetic flux variation inside the stator coil pickup unit; the magnet assembly is consistent and all use magnets of grade N35 with a residual magnetic flux density of about 1.21 T.

[0008] As an improvement of the present invention, both the stator teeth and the rotor teeth are made of silicon steel, which is processed by wire cutting technology and then bonded together; the cross-sectional dimensions of the silicon steel teeth along the magnetic circuit direction are 8.8 mm × 9.8 mm.

[0009] As another improvement of the present invention, the coil winding is fixed on the stator silicon steel tooth assembly by a coil frame. The coil frame is made of nylon with an inner circle and an outer square structure and is manufactured by 3D printing to ensure a stable connection between the coil pickup unit and the silicon steel tooth assembly. The coil winding has 320 turns, a wire diameter of about 0.15 mm, an inner diameter of 16 mm, and an outer diameter of 25 mm.

[0010] As another improvement of the present invention, both the stator assembly and the rotor assembly are provided with 12 units arranged periodically, and the variable air gap also includes a lifting platform device to adjust the air gap accuracy and alignment during assembly or maintenance.

[0011] As another improvement of the present invention, the root mean square value of the output voltage of the coil is related to the length of the external magnet. The root mean square value of the output voltage first increases and then decreases as the length of the external magnet increases. When the length of the external magnet is set to around 4mm, the root mean square value of the voltage reaches a peak point.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] (1) This invention provides a rotating electromagnetic energy harvester based on a dual magnet array with variable reluctance. It adopts a periodically staggered dual magnet array and a toothed stator-rotor structure to enhance the change of magnetic flux and optimize space utilization. It solves the problem of enhancing the change of magnetic flux in the coil pickup area under low speed conditions and the power density limitation under low speed conditions.

[0014] (2) The dual-magnet array rotating electromagnetic energy harvester of the present invention enhances the magnetomotive force in the magnetic circuit, thereby increasing the change in magnetic flux density and achieving optimal power output. Unlike the previous partial tooth structure design, this design adopts a fully matched stator and rotor tooth structure to maximize volume utilization.

[0015] (3) The present invention arranges a magnet array on the rotor, which optimizes the intensity of the change in magnetic flux. The staggered arrangement of the magnet array effectively restricts the double magnetic flux loops and concentrates them to the center pole of the stator. During the periodic rotation, the magnetic flux through the central region reverses direction at each alignment position, thereby enhancing the change in magnetic flux intercepted by the coils wound around the stator teeth.

[0016] (4) This invention analyzes the key factors of the structure of the dual-magnet array rotating electromagnetic energy harvester using system parameters and evaluates the influence of the magnet length inside the rotor on the local magnetic saturation of the ferromagnetic components. Considering the influence of magnetic saturation and magnetic reluctance changes, the specific impact of the magnet length in the magnetic circuit on the output performance is analyzed to optimize the energy output performance. Attached Figure Description

[0017] Figure 1This is a schematic diagram of the rotating electromagnetic energy harvester based on a dual magnet array with variable magnetoresistance according to the present invention.

[0018] Figure 2 This is a schematic diagram of the unit structure parameters of the rotating electromagnetic energy harvester based on a dual magnet array with variable magnetoresistance according to the present invention.

[0019] Figure 3 This is a comparison of the root mean square voltage response at different rotation speeds between the rotating electromagnetic energy harvester based on a dual magnet array with variable reluctance and the structure without external magnets in Embodiment 2 of the present invention.

[0020] Figure 4 This is a comparison of the maximum power curves of the rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array and the structure without external magnets in Embodiment 2 of the present invention, under pure resistance matching and different rotation speeds.

[0021] Figure 5 This is a comparison chart showing the relationship between the maximum output power of the external load and the external matching capacitor at a speed of 600 rpm under a capacitor-matched load, and the structure of the dual-magnet array rotary electromagnetic energy harvester based on variable reluctance and the structure without external magnets in Embodiment 3 of the present invention.

[0022] Figure 6 This is a schematic diagram comparing the relationship between the output root mean square voltage of the rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array and the length of its external magnet in Embodiment 4 of the present invention. Detailed Implementation

[0023] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. It should be noted that the terms "front," "rear," "left," "right," "up," and "down" used in the following description refer to directions in the accompanying drawings, and the terms "inner" and "outer" refer to directions toward or away from the geometric center of a specific component, respectively.

[0024] Example 1

[0025] A rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array features a periodic alternating dual-magnet array design. This design enhances the magnetic flux density gradient passing through the stator teeth in the magnetic circuit, increasing the change in magnetic flux per unit angular displacement. This amplifies the induced voltage according to Faraday's law of electromagnetic induction, achieving higher power output. The induced voltage generated in the coil wound around each stator tooth can be expressed as:

[0026]

[0027] in, This represents the instantaneous voltage of a multi-turn coil. It is the number of coil turns fixed on each stator tooth. This represents the time-varying magnetic flux through each coil pickup unit. Represents the mechanical angular displacement of the rotor. The rotational frequency of the rotor (Hertz).

[0028] In this embodiment, the rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array mainly consists of a stator assembly and a rotor assembly. Each stator assembly comprises 12 components, arranged periodically. The stator assembly has 12 periodically arranged stator teeth and coils, while the rotor assembly has periodically alternating magnets and rotor teeth. The magnets are divided into external magnets and internal magnets. Therefore, in this embodiment, the rotor assembly consists of 12 external magnets, 12 internal magnets, and 12 rotor teeth arranged alternately. Figure 1 As shown, the stator assembly, as the core component for energy capture, has identical coils mounted on each stator tooth. To facilitate the movement of the rotor assembly, a necessary air gap is introduced between the stator and rotor assemblies as a mechanical clearance. When the rotor rotates to the aligned position, the magnetic reluctance of the magnetic circuit is minimized, thus achieving maximum magnetic flux coupling in the windings. As the rotor rotates progressively, periodic reluctance modulation is introduced due to the change in the air gap. The reluctance reaches its maximum at the position where the stator and rotor teeth are completely misaligned, resulting in a minimum absolute magnetic flux in the stator windings. Furthermore, subsequent realignment is controlled by a geometrically interlaced magnet configuration. As the rotor rotates further to the next aligned position, the interlaced distribution of the magnets causes a reversal of magnetic field polarity, which in turn causes a reversal of magnetic flux. Therefore, there is a π-phase difference in the induced voltage in adjacent stator tooth coils.

[0029] In traditional M-shaped variable reluctance energy harvesting devices, the magnet is mounted on the stator assembly. However, the electromagnetic energy harvester based on variable reluctance of this invention places the magnet on the rotor assembly. During the rotation of the rotor assembly, the magnetic flux direction of each fixed coil reverses at consecutive alignment positions, increasing the amplitude of magnetic flux variation and ultimately improving energy harvesting efficiency.

[0030] In this embodiment, the coil pickup unit of the rotary electromagnetic energy harvester based on a variable reluctance dual-magnet array is fixed to the stator teeth using a 3D-printed black nylon clamp. The novel clamp design with an outer circle and inner square ensures the stability of the structure during operation, conforms to the shape characteristics of the stator teeth and coil, and does not introduce external magnetic interference. The rotor structure is secured by a CNC-manufactured aluminum alloy clamp, ensuring its reliability. The double-groove design and the introduction of external magnets significantly improve the stability of the structure, greatly reducing the deformation and detachment of silicon steel sheets compared to traditional variable reluctance structures.

[0031] The described rotary electromagnetic energy harvester based on a dual-magnet array with variable reluctance utilizes the change in reluctance caused by the air gap during the relative motion between the rotor and stator assemblies. Because the reluctance of the air gap is irregular and influenced by multiple geometric parameters, such as… Figure 2 As shown. The magnetic circuit distribution on the twelve teeth exhibits periodic symmetry, therefore a representative unit is chosen to represent it. The magnetic pole directions of the internal and external magnets of the rotor assembly are arranged alternately. and These represent the radii of the outer and inner rings of the stator, respectively. In this embodiment of the invention, 83mm and 75mm are used. and These represent the outer and inner radii of the rotor, respectively. In this embodiment of the invention, 48 mm and 32.5 mm are used. and The lengths of the internal and external magnets in the magnetic circuit are respectively represented by 10 mm and 3 mm in this embodiment of the invention. The height of the rotor teeth is 5mm in this embodiment of the invention, and the total length of the rotor teeth and the external magnet is 8mm. The width of the internal magnet is 16 mm in this embodiment of the invention, which is determined by the geometric relationship of the internal magnet as a centrally symmetric cuboid. The width of the magnetic teeth is indicated. A feature of this invention is that the stator magnetic teeth and rotor magnetic teeth have the same width. In this embodiment, 9.8 mm is used.

[0032] Example 2

[0033] The output performance of the rotary electromagnetic energy harvester based on a variable reluctance dual-magnet array described in Example 1 was tested. This embodiment included a comparative experimental structure: a dual-magnet structure with external magnets (the present invention) and a structure without external magnets. The rotor silicon steel teeth height of the structure without external magnets was set to 8 mm, and the total height of the rotor silicon steel teeth and external magnets in the dual-magnet structure with external magnets was also 8 mm. Both rotor structures maintained the same air gap with the stator. The device is driven by an adjustable-speed servo motor; the voltage response of the coil pickup unit is acquired by an oscilloscope; the lifting platform is responsible for adjusting the centering of the device; the stator and rotor clamps are made of non-magnetic 6061 aluminum alloy using CNC machining, and the coil frame is made of black nylon using 3D printing, both serving as fixed supports to ensure the stability of the connection between components; the stator and rotor silicon steel teeth are made of Nsc-B35A300 material using wire cutting technology, and then 28 pieces of 0.35mm silicon steel sheets are bonded together; the external magnet and internal magnet of the rotor are both made of N35-NdFeB, with a remanence of 1.21T. To compare the effect of different speeds on the voltage output response, the motor was adjusted from 150rpm, increasing by 50rpm each time, up to 600rpm, and the output response was compared.Figure 3 The root mean square voltage response of 12 coils connected in series is given, increasing from 150 rpm to 600 rpm. It can be seen that as the rotational speed increases, the magnetic flux changes more rapidly, the frequency of the induced voltage increases, and the voltage amplitude also increases. Figure 3 As shown.

[0034] The twelve coils of the rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array are all identically configured to ensure periodic symmetry. In this embodiment, each coil has 320 turns and a wire diameter of 0.35mm. The pure resistance and inductance of the 12 coils connected in series on the stator teeth were measured using a bridge meter (LCR-VC4080) to match the corresponding load. When using a dual-magnet structure with an external magnet, the resistance of the 12 coils connected in series was approximately 50.22 ohms, and the inductance was 90.23 mH. When the external magnet was removed and replaced with silicon steel teeth, the resistance of the 12 coils connected in series, measured by the bridge meter, was approximately 50.28 ohms, and the inductance was 94.91 mH. We calculated the total impedance at different speeds to perform load matching, using the following formula:

[0035]

[0036] in, It is the resistance value of 12 coils connected in series. It is the motor speed. This is the inductance value of 12 coils connected in series. Based on the measured RMS voltage and resistance values, the estimated maximum power reaches the watt level; therefore, a cement resistor is used as the load. The maximum power that the cement resistor can withstand is 10W~20W.

[0037] A multimeter was used to help adjust the load resistance during each test. Tests were conducted using loads with resistances ranging from 31 ohms to 82.5 ohms, and the maximum power curves at different speeds are shown below. Figure 4 As shown, the output power has a quadratic relationship with the rotational speed. Under purely resistive conditions, this invention achieves 0.6598 watts at 150 rpm and 8.6318 watts at 600 rpm. Notably, within the 150 to 600 rpm range, the enhanced configuration of the dual magnets in this invention achieves a power increase of 47.52-55.28% compared to the reference configuration without external magnets.

[0038] Example 3

[0039] The difference between this embodiment and Embodiment 2 is that Embodiment 2 uses a pure resistance matching scheme to test power, while this embodiment uses an external resistor and a matching capacitor connected in series to test power. Because the coil inductance of the variable reluctance dual-magnet array rotary electromagnetic energy harvester gradually increases with rotational speed, its impact on output power becomes increasingly significant. Therefore, a matching capacitor scheme can also be used. By connecting the matching capacitor in series with the coil inductance, the reactive power of the circuit is reduced, while the active power is increased, thus increasing the effective output power of the coil. At 600 rpm, the corresponding coil induced voltage frequency is 60 Hz, according to the following formula:

[0040]

[0041] in, It is the frequency of the induced voltage. It is the inductance value of 12 coils connected in series. It is the matching capacitor value.

[0042] For structures with external magnets, the matching capacitor value calculated based on the statically measured coil inductance is around 78uF. Since the coil inductance remains constant in the static state, but the magnetic flux through the stator silicon steel tooth core changes continuously due to the periodic variation of magnetic reluctance during rotor rotation, the coil inductance is actually variable. Therefore, the calculated matching capacitor value based on the statically measured inductance can only be used as a reference and cannot be used for precise selection. Five sets of capacitors (68uF, 82uF, 100uF, 120uF, and 150uF) were tested. It was found that the power increase was more significant when matching with 100uF, 120uF, and 150uF capacitors compared to when connected to a purely resistive load. Twelve coils were connected in series, then in series with the matching capacitors, and then external resistors ranging from 45.0 ohms to 75.0 ohms were connected in series sequentially to measure the average power across the external resistors. Tests showed that under capacitor-matched load conditions, the output power at 600 rpm increased from 8.6318 watts under purely resistive conditions to 9.3750 watts. Figure 5 As shown.

[0043] Example 4

[0044] The geometric parameters of the external and internal magnets of the rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array affect the magnetic flux saturation degree of the silicon steel teeth made of ferromagnetic material. Simulation analysis can be performed using the COMSOL rotating machinery and magnetic finite element simulation platform to obtain the relationship between the root-mean-square voltage output response of the coils connected in series on the 12 stator teeth and the length of the external magnet. Figure 6As shown. While a longer external magnet increases the magnetic flux density (MMF), it also increases the magnetic reluctance per unit magnetic circuit, thus hindering magnetic field flow. Finite element analysis revealed a correlation between the root mean square (RMS) value of the output voltage of the 12 series-connected coils and the length of the external magnet. Figure 6 As shown, the voltage response exhibits a trend of first increasing and then decreasing, approximately at the length of the external magnet. The peak point appeared at that time.

[0045] To maintain the same air gap, we fixed other dimensional parameters, setting the total length of the external magnet and the height of the rotor silicon steel teeth to 8mm. Therefore, as the length of the external magnet increases, the height of the silicon steel teeth on the rotor decreases, causing the magnetic flux through the ferromagnetic material to reach saturation prematurely. This saturation phenomenon is due to the limited permeability of magnetic materials like silicon steel and soft iron. After the magnetic flux reaches a certain value, the material's permeability decreases, preventing the flux from continuing to pass effectively through the material, and causing some to leak into the external air region. For current variable reluctance structures, the reduction in leakage flux and MMF is unavoidable in reality, and the degree of magnetic saturation is closely related to the geometry of the external magnet, internal magnet, and silicon steel teeth. Therefore, it is necessary to design and optimize the simulated magnetic circuit based on this saturation phenomenon.

[0046] Example 5

[0047] The difference between this embodiment and the previous implementation case is that after the 12 coil pickup units are connected in series, a bridge rectifier circuit is needed. The positive voltage output by the rectifier circuit is then connected in parallel with the energy storage unit. The energy storage unit in this embodiment includes eight 4700uF energy storage capacitors connected in parallel. The energy storage unit is then connected in parallel with a Zener diode to output a 3.3V voltage to the f103c6t6 microcontroller module. The microcontroller module is used to build a LIS3DSH triaxial accelerometer to monitor the self-powered acceleration of rotating machinery.

[0048] It should be noted that the above content merely illustrates the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. For those skilled in the art, various improvements and modifications can be made without departing from the principle of the present invention, and all such improvements and modifications fall within the scope of protection of the claims of the present invention.

Claims

1. A rotating electromagnetic energy harvester based on a dual-magnet array with variable reluctance, characterized in that... Includes stator assembly and rotor assembly. The stator assembly has periodically arranged stator teeth, and each stator tooth has a coil. The coil is connected in series with an external matching load or an external matching circuit with a capacitor, so as to realize two methods: pure resistance and matching capacitor connection to the load. The rotor assembly consists of periodically arranged magnets and rotor teeth, with the magnets and rotor teeth arranged alternately. A variable air gap exists between the stator assembly and the rotor assembly. During the rotation of the rotor assembly, the magnetic reluctance of the variable air gap changes periodically, and the coils in the stator assembly generate an induced voltage, thereby converting the mechanical energy generated by the rotor assembly during rotation into electrical energy output.

2. The rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array as described in claim 1, characterized in that: The magnets in the rotor assembly include internal magnets and external magnets, with the external and internal magnets arranged alternately in opposite directions, and both using N35 permanent magnets.

3. The rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array as described in claim 2, characterized in that: Both the stator teeth and the rotor teeth are made of silicon steel, and the cross-sectional dimensions of the silicon steel teeth along the magnetic circuit direction are 8.8 mm × 9.8 mm.

4. The rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array according to claim 2, characterized in that: The stator assembly also includes a coil pickup unit, which is connected to the stator teeth via a nylon coil skeleton with an inner circle and an outer square. The coil pickup unit and the stator teeth form a coil winding with 320 turns, a wire diameter of approximately 0.15 mm, an inner diameter of 16 mm, and an outer diameter of 25 mm.

5. The rotating electromagnetic energy harvester based on a variable reluctance dual-magnet array according to claim 2, characterized in that: Both the stator assembly and the rotor assembly are provided with 12 units, arranged periodically. The variable air gap also includes a lifting platform device to adjust the air gap accuracy and alignment.

6. The rotating electromagnetic energy harvester based on a dual-magnet array with variable reluctance as described in any one of claims 3-5, characterized in that: The root mean square value of the coil's output voltage is related to the length of the external magnet. The root mean square value of the output voltage first increases and then decreases as the length of the external magnet increases. When the length of the external magnet is set to 4mm, the root mean square value of the voltage reaches a peak.

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