Binocular image stabilization assembly and image stabilization telescope
By employing a sandwich structure and a closed-loop feedback control dual-sided magnet design in the binoculars, the imaging blurring problem of traditional telescopes in shaky scenarios is solved, achieving high-precision and low-cost optical image stabilization.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-19
- Publication Date
- 2026-03-31
AI Technical Summary
Traditional binoculars are easily affected by external shaking when handheld or in motion, resulting in blurred images. Existing optical image stabilization technologies suffer from problems such as complex structure, large size, high assembly cost, and insufficient control precision.
The binocular image stabilization component adopts a sandwich structure. By installing three sets of coils in the middle and magnets on both sides, it achieves smooth movement using a ball bearing isolation structure. It also combines Hall sensors and gyroscopes for closed-loop feedback control, which simplifies the structure of the magnet component and reduces the number of parts and assembly complexity.
It achieves high-precision optical image stabilization, reduces the size and assembly cost of the image stabilization module, improves assembly convenience and cost-effectiveness, is suitable for Porro prism telescopes, and has strong expandability.
Smart Images
Figure CN224067076U_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The utility model relates to optical anti-shake technical field, concretely relates to a binocular image-stabilized assembly and image-stabilized telescope. BACKGROUND
[0002] Binocular telescope is susceptible to external shaking under handheld or motion scene, leading to imaging blur. Traditional optical anti-shake technology compensates for optical path deviation by moving lens or prism, but has problems of complex structure, large volume, high assembly cost and insufficient control precision. In conventional lens anti-shake assembly, multiple U-shaped yokes are used to drive magnetic steel assembly, which requires independent installation of multiple magnetic steels and coils, resulting in large number of components and complicated assembly steps. Moreover, the dispersed layout of driving assembly leads to large overall size. SUMMARY
[0003] Therefore, in order to solve the problems of complex structure, large volume and high assembly cost of traditional binocular anti-shake device, the utility model provides a binocular image-stabilized assembly and image-stabilized telescope. Three coils are installed on the middle moving assembly in a sandwich structure, and the magnetic steels on both sides of the coils are installed on the fixed assemblies on both sides. The moving assembly and the fixed assemblies are isolated by balls and move smoothly. The fixed assemblies on both sides and the moving assembly in the middle are designed as a whole. During assembly, the corresponding components are placed in order. The utility model has the advantages of simple structure, convenient assembly, easy debugging, high reliability and high cost performance. The structure of the magnetic steel assembly is optimized, the number of components and the cost are reduced, the assembly and debugging steps are reduced, the size of the image-stabilized module is reduced, and the utility model has the advantages of high image-stabilized precision, high integration, simple structure, convenient production and high cost performance. The utility model can be installed in a Paul prism type binocular telescope to realize optical image stabilization.
[0004] To achieve the above-mentioned purpose, the utility model provides the following technical scheme:
[0005] Based on the above purpose, in the first aspect, the utility model provides a binocular image-stabilized assembly, which comprises a front fixed assembly, a moving assembly, a rear fixed assembly and a control circuit.
[0006] The front fixed assembly and the rear fixed assembly are fixedly connected by fasteners, and the moving assembly is clamped between the front fixed assembly and the rear fixed assembly to form a sandwich structure.
[0007] Three groups of driving magnetic steels are arranged on the front fixed assembly and the rear fixed assembly respectively, two groups of driving magnetic steels are arranged horizontally, and the other group of driving magnetic steels is arranged vertically to the first two groups.
[0008] Three groups of driving coils are arranged on the moving assembly correspondingly, each group of driving coils is arranged between the corresponding driving magnetic steels of the front fixed assembly and the rear fixed assembly, and a double-sided magnetic steel type voice coil motor is formed.
[0009] The moving component is also equipped with three sets of sensing magnets, and the corresponding position of the rear fixed component is equipped with three Hall sensors to form a displacement detection unit. The moving component is also equipped with two anti-shake lenses, which are set in the mounting holes on the moving component. The control circuit is fixed to the rear fixed component and is electrically connected to the drive coil on the moving component through a flexible ribbon cable.
[0010] In this embodiment, a clamping mechanism is provided between the front fixed component and the moving component. The clamping mechanism uses a tension spring or a magnetic attraction structure to achieve the pre-tightening force between the components.
[0011] In this embodiment, a ball bearing isolation structure is provided between the front fixing component and the rear fixing component and the clamped moving component. The moving component is isolated and moves smoothly through the ball bearing isolation structure. The ball bearing isolation structure contains multiple rolling contact balls. The moving component and the front fixing component are connected together by the tension spring or magnet of the clamping mechanism, so that the moving component can move with low resistance on the plane through the tight contact of the balls.
[0012] In this embodiment, the control circuit integrates a gyroscope, a microprocessor, and a drive module, which are electrically connected to the drive coil via a flexible cable.
[0013] In this embodiment, the control circuit directly serves as the structural substrate of the rear fixing component, driving the magnet to be attached to a designated position on the circuit board. The gyroscope and microprocessor are integrated on the circuit board to generate anti-shake compensation target values based on gyroscope data and control the movement of the voice coil motor through feedback from the Hall sensor.
[0014] In this embodiment, each set of driving magnets is a magnet magnetized in the left and right directions, or is composed of two magnets magnetized in the top and bottom directions.
[0015] In this embodiment, the displacement detection unit includes three sets of sensing magnets on the moving component and three Hall sensors on the rear fixed component, which respectively detect the relative positions of left-side up-down movement, right-side up-down movement, and left-right movement.
[0016] In this embodiment, the displacement detection unit has two sets of Hall sensors that detect vertical displacement, and a third Hall sensor that detects horizontal displacement.
[0017] Secondly, this utility model also provides an image-stabilized telescope, which uses the above-mentioned binocular image-stabilized components, including a dual objective lens group, a dual Porro prism group, a focusing mechanism, a locking mechanism, a battery compartment, a telescope housing, and a binocular eyepiece group.
[0018] The objective lens group is optically connected to the front end of the binocular image stabilization assembly, and the Porro prism group is connected to the rear of the image stabilization lens of the moving assembly;
[0019] The focusing mechanism is mechanically linked to the objective lens group or image stabilization assembly;
[0020] The locking mechanism can switch between fixing and releasing the moving component;
[0021] The battery compartment is integrated into the telescope housing and supplies power to the control circuitry.
[0022] The binocular eyepiece group is optically connected to the output end of the Porro prism group.
[0023] In this embodiment, the Porro prism group adopts an anti-Porro structure, and the image stabilization component is installed in the optical path between the objective lens group and the Porro prism group.
[0024] In this embodiment, the driving magnets on the front and rear fixing components are fitted with iron plates as magnetic yokes on the outside of the relative coils.
[0025] In this embodiment, two CMOS sensors are replaced on the moving component to form a stereo camera's image stabilization module.
[0026] During operation, the image-stabilized telescope of this invention uses a microprocessor to process the jitter data of the gyroscope in real time to generate a compensation target value, synchronously compares the vertical displacement feedback of the Hall sensor, and drives two sets of vertical voice coil motors through a PID control algorithm to achieve deflection-free translation; and independently controls the horizontal voice coil motor to compensate for lateral jitter.
[0027] Compared with the prior art, the binocular image stabilization component and image stabilization telescope proposed in this utility model have the following beneficial effects:
[0028] This invention relates to a binocular image stabilization assembly and telescope. The sandwich layout and dual-sided magnet design reduce the number of components. Assembly only requires sequentially stacking the components and fixing them with screws. The optimized magnet assembly structure reduces the number of components and cost, decreases assembly and debugging steps, and reduces the size of the image stabilization module. Employing a Hall effect closed-loop feedback combined with a synchronous drive strategy, it achieves high displacement control accuracy and low deflection error, resulting in high-precision image stabilization. Furthermore, the magnets and coils reuse dual-sided magnetic fields, reducing material costs and fitting into the compact space of Porro prism telescopes. The movable component can be replaced with a CMOS sensor or lens group, making it suitable for various optical devices such as cameras and microscopes. It can be installed inside a Porro prism binocular telescope to achieve optical image stabilization, offering strong expandability. This invention, through optimized magnet layout, simplified drive structure, and integrated closed-loop feedback control, achieves high-precision image stabilization while reducing size, cost, and assembly complexity.
[0029] These or other aspects of this application will become more apparent from the following description of embodiments. It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the application. Attached Figure Description
[0030] To more clearly illustrate the technical solutions in the embodiments of this utility model or related technologies, the accompanying drawings used in the description of the exemplary embodiments or related technologies will be briefly introduced below. The drawings are used to provide a further understanding of this utility model and constitute a part of the specification. They are used together with the embodiments of this utility model to explain this utility model and do not constitute a limitation on this utility model. In the drawings:
[0031] Figure 1 This is a schematic diagram of the structure of the binocular image stabilization component provided by this utility model.
[0032] Figure 2 This is a front view of the binocular image stabilization assembly provided by this utility model.
[0033] Figure 3 This is a schematic diagram of the image-stabilized telescope provided by this utility model.
[0034] Figure 4 This is an exploded schematic diagram of the image-stabilized telescope provided by this utility model.
[0035] Figure label:
[0036] 100-Binocular image stabilization assembly, 1-Front fixed assembly, 2-Moving assembly, 3-Rear fixed assembly, 11-Front drive magnet, 21-Drive coil, 22-Sensing magnet, 31-Rear drive magnet, 32-Hall sensor, 4-Ball isolation structure, 41-Ball, 5-Objective lens group, 51-Objective lens tube, 52-Objective lens, 6-Binocular eyepiece group, 61-Adjustment plate, 7-Adjustment shaft, 8-Telescope housing, 81-Hatch cover, 82-Hatch body, 83-Cover, 9-Porro prism group. Detailed Implementation
[0037] The present application will now be further described in conjunction with the accompanying drawings and specific embodiments. It should be noted that, without conflict, the various embodiments or technical features described below can be arbitrarily combined to form new embodiments.
[0038] To make the objectives, technical solutions, and advantages of this utility model clearer, the embodiments of this utility model are further described in detail below with reference to specific examples and the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit this application.
[0039] It should be noted that all uses of the terms "first" and "second" in the embodiments of this utility model are for the purpose of distinguishing two different entities or different parameters with the same name. Therefore, "first" and "second" are merely for convenience of expression and should not be construed as limiting the embodiments of this utility model. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion, such as other steps or units inherent in a process, method, system, product, or device that includes a series of steps or units.
[0040] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0041] The flowchart shown in the attached diagram is for illustrative purposes only and does not necessarily include all content and operations / steps, nor does it necessarily have to be performed in the order described. For example, some operations / steps can be broken down, combined, or partially merged, so the actual execution order may change depending on the actual situation.
[0042] The following detailed description of some embodiments of this application is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.
[0043] Conventional lens-based image stabilization assemblies often employ multiple drive magnets with U-shaped yokes to improve power supply efficiency, resulting in complex structures, cumbersome assembly, increased costs, and larger size and weight. Therefore, this invention provides a binocular image stabilization assembly and a stabilized telescope, offering advantages such as high stabilization accuracy, high integration, simple structure, convenient manufacturing, and high cost-effectiveness. It can be installed inside a Porro prism binocular telescope to achieve optical image stabilization. Compared to existing technologies, the structure of the magnet assembly is optimized, reducing the number of components and costs, simplifying assembly and debugging steps, and decreasing the size of the image stabilization module.
[0044] See Figure 1 and Figure 2As shown, an embodiment of this utility model provides a binocular image stabilization component, including a front fixing component 1, a moving component 2, a rear fixing component 3, and a control circuit. The front fixing component 1 and the rear fixing component 3 are fixed together by screws as fasteners. The moving component 2 is sandwiched between the front fixing component 1 and the rear fixing component 3 to form a sandwich structure. Each of the front fixing component 1 and the rear fixing component 3 is provided with three sets of driving magnets, two sets of driving magnets are arranged horizontally, and the other set of driving magnets is arranged perpendicular to the first two sets. The front fixing component 1 is provided with three sets of front driving magnets 11, and the rear fixing component 3 is provided with three sets of rear driving magnets. The moving component 2 is provided with three sets of drive coils 21, each set of drive coils 21 is arranged between the corresponding drive magnets of the front fixed component 1 and the rear fixed component 3, forming a dual-sided magnet voice coil motor; the moving component 2 is also provided with three sets of sensing magnets 22, and the rear fixed component 3 is provided with three Hall sensors 32 at corresponding positions to form a displacement detection unit; the moving component 2 is also equipped with two anti-shake lenses, which are set in the mounting holes on the moving component 2; the control circuit is fixed to the rear fixed component 3 and electrically connected to the drive coils 21 on the moving component 2 through a flexible ribbon cable.
[0045] In this embodiment, a clamping mechanism is provided between the front fixing component 1 and the moving component 2. The clamping mechanism uses a tension spring or a magnetic attraction structure to achieve pre-tightening force between the components. Furthermore, a ball bearing isolation structure 4 is provided between the front fixing component 1, the rear fixing component 3, and the clamped moving component 2. The moving component 2 is isolated and moves smoothly through the ball bearing isolation structure 4, which contains multiple rolling contact balls 41. The moving component 2 and the front fixing component 1 are connected by the tension spring or magnet of the clamping mechanism, allowing the moving component 2 to move with low resistance on a plane through close contact with the balls.
[0046] The binocular image stabilization component of this embodiment adopts a sandwich structure, with three sets of coils installed on the middle movable component 2, and magnets on both sides of the coils installed on the front and rear fixed components 3 on both sides respectively. The movable component 2 and the front and rear fixed components 3 are isolated by ball bearings and move smoothly. This utility model adopts an integrated design for the front and rear fixed components and the movable component 2 sandwiched in the middle. During assembly, the corresponding components can be placed in sequence. It has the advantages of simple structure, convenient assembly, convenient debugging, high reliability and high cost performance.
[0047] In this embodiment, the control circuit integrates a gyroscope, a microprocessor, and a drive module, and is electrically connected to the drive coil 21 via a flexible ribbon cable. The control circuit directly serves as the structural substrate of the rear fixing component 3. The drive magnet is mounted on a designated position on the circuit board. The gyroscope and microprocessor are integrated on this circuit board, used to generate anti-shake compensation target values based on gyroscope data, and to control the voice coil motor movement via feedback from the Hall sensor 32.
[0048] In this embodiment, each set of driving magnets is a magnet magnetized in the left and right directions, or is composed of two magnets magnetized in the top and bottom directions.
[0049] In this embodiment, the displacement detection unit includes three sets of sensing magnets 22 mounted on the moving component 2 and three Hall sensors 32 mounted on the rear fixing component 3, which respectively detect the relative positions of left-side up-down movement, right-side up-down movement, and left-right movement. Specifically, two sets of Hall sensors 32 in the displacement detection unit detect the vertical displacement, and the third Hall sensor 32 detects the horizontal displacement.
[0050] See Figures 1 to 4 As shown, an embodiment of this utility model also provides an image-stabilized telescope, which uses the above-mentioned binocular image stabilization component 100 and further includes a dual objective lens group 5, a dual Porro prism group 9, a focusing mechanism, a locking mechanism, a battery compartment, a telescope housing 8, and a binocular eyepiece group 6. Objective lens group 5 is optically connected to the front end of binocular image stabilization assembly 100. Objective lens group 5 consists of objective lens tube 51 and objective lens 52 disposed inside objective lens tube 51. A cover 81 is also provided at the front end of objective lens tube 51. The cover 81 is connected to the body 82. The rear end of the body 82 is connected to the protective cover 83 on the outside of binocular eyepiece group 6. The cover 81, body 82 and protective cover 83 constitute the telescope housing 8. Porro prism group 9 is connected to the rear of the image stabilization lens of moving assembly 2. The focusing mechanism is an adjustment shaft 7. The focusing mechanism is mechanically linked with objective lens group 5 or image stabilization assembly. The locking mechanism can switch between fixing and releasing moving assembly 2. The battery compartment is integrated into the telescope housing 8 and provides power to the control circuit. Binocular eyepiece group 6 is optically connected to the output end of Porro prism group 9. The binocular eyepiece group 6 is also provided with a distance adjustment plate 61.
[0051] In this embodiment, the Porro prism group 9 adopts an anti-Porro structure, and the image stabilization component is installed in the optical path between the objective lens group 5 and the Porro prism group 9.
[0052] In this embodiment, the driving magnets on the front fixing component 1 and the rear fixing component 3 are fitted with iron plates as magnetic yokes on the outside of the relative coils.
[0053] In this embodiment, two CMOS sensors are replaced on the moving component 2 to form a stereo camera's image stabilization module, which can be used in a stereo camera; two image stabilization lenses can also be placed on the moving component 2, which can be used in a telescope.
[0054] The control method for the binocular image stabilization assembly and image-stabilized telescope in this embodiment is as follows: Each set of voice coil motors corresponds to a set of Hall elements and a detection magnet as the motion position feedback quantity. The left and right pairs of voice coil motors drive up and down simultaneously, and through synchronous control, the moving component moves up and down without deflection; the middle voice coil motor drives the moving component 2 to move left and right. Specifically, for example, the microcontroller is a GD32F103, the gyroscope is an ICM20602, and the Hall element is an SS49E. A microcontroller control program is developed to read gyroscope data to obtain the telescope's jitter information, calculate the compensation target values in the up and down and left and right directions, read Hall element data to obtain the offset distance of the moving part relative to the fixed part, and run a feedback control algorithm to drive coil 21, pushing the corresponding magnet to move, so that the motion of the moving part follows the compensation target value brought by the jitter. Among them, the Hall elements A and B arranged on the left and right simultaneously detect the motion in the up and down direction, and drive the corresponding two sets of coils A and B through feedback control to push the moving part. The position data of the two Hall elements A and B change simultaneously and remain consistent, realizing the synchronous up and down movement of the two lenses and avoiding the phenomenon of deflection and tilting.
[0055] The telescope uses a reverse Porro prism architecture. The image stabilization component is installed between the left and right sets of Porro prisms and the five objective lens groups. The fixing parts of the image stabilization component are fixed inside the telescope housing 8. Focusing is achieved by controlling the forward and backward movement of the five objective lens groups and / or the image stabilization component via a focusing handwheel. In addition to the image stabilization component, the telescope also includes two sets of objective lenses, two sets of Porro prisms, two sets of eyepieces, a focusing mechanism, housing structural components, a battery compartment, and power switches.
[0056] During control operation, each servo structure includes a set of motion mechanisms (coil and driving magnet) and a displacement sensing mechanism (Hall sensor and sensing magnet 22) corresponding to its motion direction; the servo components on the left and right sides are used for up and down movement, and the two driving components implement up and down movement synchronously, which can eliminate the deflection motion around the optical axis; another set of servo components is used for left and right movement; the microcontroller obtains the jitter data of the external device through the gyroscope, and calculates the motion target value of the three sets of servo mechanisms according to the anti-shake control ratio; the motion feedback of the corresponding motion mechanism is obtained through the Hall sensor 32, and the microcontroller runs the three-way feedback control algorithm to realize the precise motion control of the three servo mechanisms, track the motion target value, and realize the anti-shake function of the optical path.
[0057] In this embodiment, during operation, the microprocessor processes the jitter data of the gyroscope in real time to generate a compensation target value, synchronously compares the vertical displacement feedback of the Hall sensor 32, and drives two sets of vertical voice coil motors through a PID control algorithm to achieve deflection-free translation; and independently controls the horizontal voice coil motor to compensate for lateral jitter.
[0058] The above are exemplary embodiments disclosed in this utility model. However, it should be noted that various changes and modifications can be made without departing from the scope of the embodiments of this utility model as defined by the claims. The functions, steps, and / or actions of the methods according to the disclosed embodiments described herein do not need to be performed in any particular order. Furthermore, although the elements disclosed in the embodiments of this utility model may be described or claimed individually, they may be understood as multiple unless explicitly limited to a singular number.
[0059] It should be understood that, as used herein, the singular form "a" is intended to include the plural form as well, unless the context clearly supports an exception. It should also be understood that, as used herein, "and / or" refers to any and all possible combinations of one or more of the associatedly listed items. The embodiment numbers disclosed above are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.
[0060] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the present invention (including the claims) is limited to these examples. Within the framework of the present invention, technical features of the above embodiments or different embodiments can also be combined, and many other variations of different aspects of the present invention exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.
Claims
1. A binocular image-stabilization assembly, comprising: The front fixing assembly, the moving assembly, the rear fixing assembly and the control circuit are included. The front fixing assembly and the rear fixing assembly are fixed by fasteners, and the moving assembly is clamped between the front fixing assembly and the rear fixing assembly to form a sandwich structure. Three groups of driving magnetic steels are arranged on the front fixing assembly and the rear fixing assembly respectively, two groups of driving magnetic steels are arranged horizontally, and the other group of driving magnetic steels is arranged in a vertical direction to the first two groups. Three groups of driving coils are arranged on the moving assembly correspondingly, each group of driving coils is arranged between the corresponding driving magnetic steels of the front fixing assembly and the rear fixing assembly, and a double-sided magnetic steel type voice coil motor is formed. Three groups of sensing magnetic steels are arranged on the moving assembly, three Hall sensors are arranged at the corresponding positions of the rear fixing assembly to form a displacement detection unit, and two anti-shake lenses are installed on the moving assembly and arranged in the mounting holes on the moving assembly.
2. The binocular stabilizing assembly of claim 1, wherein, The control circuit is fixed on the rear fixing assembly and electrically connected with the driving coils on the moving assembly through a flexible flat cable.
3. The binocular stabilizing assembly of claim 2, wherein, A compression mechanism is arranged between the front fixing assembly and the moving assembly, and the compression mechanism adopts a tension spring or a magnetic attraction structure to realize the pre-tightening force between the components.
4. The binocular stabilizing assembly of claim 1, wherein, A ball isolation structure is further arranged between the front fixing assembly, the rear fixing assembly and the clamped moving assembly, the moving assembly is isolated and smoothly moved through the ball isolation structure, and the ball isolation structure contains a plurality of rolling contact balls.
5. The binocular stabilizing assembly of claim 4, wherein, The control circuit is integrated with a gyroscope, a microprocessor and a driving module and is electrically connected with the driving coils through a flexible flat cable.
6. The binocular stabilizing assembly of claim 5, wherein, The control circuit directly serves as a structure substrate of the rear fixing assembly, the driving magnetic steels are attached to the specified positions of the circuit board, and the gyroscope and the microprocessor are integrated on the circuit board.
7. The binocular stabilizing assembly of claim 6, wherein, Each group of driving magnetic steels is left-right magnetized magnetic steel or is spliced by two pieces of up-down magnetized magnetic steels.
8. A stabilized telescope, characterized by, The two groups of Hall sensors in the displacement detection unit correspond to detect the vertical displacement, and the third Hall sensor detects the horizontal displacement. The binocular image stabilization assembly is applied to the binocular image stabilization component according to any one of claims 1-7, and the image stabilization telescope further includes a binocular objective lens group, a binocular Paul prism group, a focusing mechanism, a locking mechanism, a battery compartment, a telescope shell and a binocular eyepiece group. The objective lens group is optically connected to the front end of the binocular image stabilization assembly, and the Paul prism group is connected behind the image stabilization lens of the moving assembly. The focusing mechanism is mechanically linked with the objective lens group or the image stabilization assembly. The locking mechanism can switch to fix / release the moving assembly. The battery compartment is integrated in the telescope shell to supply power for the control circuit.
9. The image stabilized telescope of claim 8 wherein, The binocular eyepiece group is optically connected to the output end of the Paul prism group.
10. The image stabilized telescope of claim 8 wherein, The Paul prism group adopts a reverse Paul structure, and the image stabilization assembly is installed in the optical path between the objective lens group and the Paul prism group. The driving magnetic steels on the front fixing assembly and the rear fixing assembly are provided with iron sheets as magnetic yokes on the outer sides of the relative coils.