Blade driving method and driving structure of multi-blade collimator

Abstract

The invention belongs to the technical field of multi-leaf collimators, and particularly relates to a multi-leaf collimator blade driving method and a driving structure. The driving structure comprises a constant-torque fixed-rotating-speed motor, a plurality of transmission subsystems and a plurality of blades; the plurality of transmission subsystems are in one-to-one correspondence with the plurality of blades of the collimator; a single constant-torque fixed-rotating-speed motor is used as a power source, power is transmitted to collimator blades through transmission subsystems in one-to-one correspondence with a plurality of blades of the collimator, and the collimator blades are driven to complete linear motion; the power input ends of the transmission subsystems are connected in parallel. The driving blades only need a single high-power motor and are greatly different from a traditional multi-blade collimator, and the number of the blades of the traditional multi-blade collimator is the same as that of the motors for driving the blades. According to the driving structure, the moment of motion of the blades can be increased in a larger range, the motion speed is higher, the requirement for the size of the motor is greatly lowered, the requirement for the performance of the motor is lowered, and a large power change range is not needed.

Description

Technical Field

[0001] The invention relates to a multi-leaf collimator blade driving method and a driving structure, belonging to the technical field of multi-leaf collimators. Background Art

[0002] The design of the multi-leaf collimator includes two symmetrically distributed blade boxes, each box is equipped with a certain number of blades, and the common configuration is 40 to 80 blades per box, so the total number of blades ranges from 80 to 160. Existing multi-leaf collimators mostly use independent blade drive, that is, one motor drives the blade of a multi-leaf collimator, so each set of multi-leaf collimators requires a large number of motors, and the blades are heavy, and the movement speed and accuracy requirements are high. Limited by the size of the multi-leaf collimator, the smaller the size of the motor, the better (which is conducive to expanding the number of blades), and the greater the power, the better (which is conducive to driving the movement of the blades).

[0003] The technical solution of the existing patent [A drive system] announcement number CN103489497B discloses a drive system. The above patent adopts a gear set as a primary power transmission structure. This method requires multiple gears in series, and multiple gears in series for power transmission will cause obvious power loss. For example, an MLC with 60 pairs of blades has 60 blades in a single blade box. Using the method of the above patent, a primary transmission gear set requires 120 gears. Assuming a transmission efficiency of 90% (considering the small size of the gears, the fixing and connection of the gears cannot use bearings, etc., and there is no effective way to improve the gear transmission efficiency), when the power of the drive motor is transmitted to the 120th gear, only 90% of the power is left 119=0.00036%. Unless the transmission efficiency can reach the maximum value of the gear transmission efficiency of 99%, when the power is transmitted to the 120th gear, 99% of the power is left 119=30.2%, which can be barely used, but is obviously difficult to achieve. In addition, due to static friction and inertia, the force required to start the blade movement is large, and the force required after starting is small (at this time, driving force is not required but movement speed is required). The only way is to increase the torque by reducing the motor speed through a variable speed motor, or to increase the motor speed by reducing the torque. However, after the motor speed increases and the torque decreases, due to the transmission efficiency problem of the above system, it is more difficult to drive some blades that have just started; and after the motor torque increases and the speed decreases, it will affect the blade transmission speed. Furthermore, the above patent uses a motion converter, which is a common existing method, that is, the blade is connected to the lead screw, and the channel connected to the blade has a matching thread. The blade is controlled to move forward and backward by turning the lead screw, which is slow and easy to damage. Summary of the invention

[0004] In view of the above shortcomings in the prior art, the technical problem to be solved by the present invention is to provide a multi-leaf collimator blade driving method and driving structure, which improves the shortcomings of the existing multi-leaf collimator motors, which have a large number of motors and require high precision motor encoders. The use of a high-power motor connected to the transmission shaft, and the transmission shaft is connected to multiple drive combinations in parallel, can easily expand the number of blades, and a small speed-changing structure is designed at the end of the drive combination, which can accurately adjust the speed of a single blade and provide different torques to a single blade.

[0005] The multi-leaf collimator blade driving method described in the present invention is a method for driving several blades of the multi-leaf collimator to perform linear motion, using a single constant torque constant speed motor as a power source, and transmitting power to the collimator blades through transmission subsystems corresponding to the several blades of the collimator one by one, thereby driving the collimator blades to complete linear motion; the power input ends of the several transmission subsystems are connected in parallel.

[0006] The multiple transmission subsystems of this driving mode are connected in parallel. Each transmission subsystem is relatively independent, there is no energy transfer, no energy attenuation, and a high-power motor with constant torque and constant speed is used to ensure the driving force for the blade movement, and there will be no problem of insufficient power.

[0007] Among them, the power input end of the transmission subsystem is a constant gear fixed on the drive shaft, the constant gear is concentric with the drive shaft, and several constant gears are arranged in parallel along the length direction of the drive shaft. The drive shaft is connected to a constant torque and constant speed motor.

[0008] In addition, the torque and speed of the power output of several transmission subsystems are adjustable.

[0009] A multi-leaf collimator blade driving structure of the present invention comprises:

[0010] A constant torque and constant speed motor, a plurality of transmission subsystems, and a plurality of blades, wherein the plurality of transmission subsystems correspond to the plurality of blades of the collimator in a one-to-one manner;

[0011] A constant torque and constant speed motor is connected to the drive shaft;

[0012] The transmission subsystem includes a permanent gear fixed on the driving shaft, the permanent gear is concentric with the driving shaft, and a plurality of permanent gears are arranged in parallel along the length direction of the driving shaft;

[0013] It also includes a shaft sleeve and a belt coupler sleeved on the support shaft, the support shaft is parallel to the drive shaft, the support shaft is fixed, the shaft sleeve and the belt coupler are fixedly connected, and the shaft sleeve can rotate on the support shaft but cannot move along the support shaft;

[0014] It also includes a driven gear sleeved on the shaft sleeve, which can move along the shaft sleeve but cannot rotate relative to the shaft sleeve; the driven gear is equipped with a pushing device that can make it move along the shaft sleeve.

[0015] It also includes a transmission, the input end of the transmission is connected to the belt coupler through a transmission belt, and the output end of the transmission is provided with a driving wheel. The teeth on the driving wheel are meshed with the blade coupling teeth, and the blade is driven to move linearly through the rotation of the driving wheel.

[0016] Among them, the transmission is a speed-changing cone cylinder, which includes a cone cylinder A and a cone cylinder B. Both cone cylinder A and cone cylinder B include a small end face and a large end face. A plurality of wires are connected between the small end face and the large end face. The wires are all along the cone cylinder generatrix direction, and the plurality of wires are distributed in a circular array outside the cone cylinder; the small end faces of cone cylinder A and cone cylinder B are composed of a plurality of radially distributed support rods, the two end faces of cone cylinder A are connected by a central support column, and the two end faces of cone cylinder B are connected by a central sleeve, and a plurality of slots are provided on the sleeve wall. The slots are strip-shaped, and along the length direction of the sleeve, the slots correspond one by one to the plurality of support rods constituting the small end faces. The small end faces of cone cylinder A and cone cylinder B are cross-connected, and the cross depth is adjustable; the transmission belt is connected to the cross contact surface of cone cylinder A and cone cylinder B. The two ends of the speed change cone cylinder are rotatably arranged between a pair of brackets, and a micro bidirectional cylinder is also arranged between the pair of brackets. The micro bidirectional cylinder can adjust the crossing depth of cone cylinder A and cone cylinder B, so as to change the contact surface diameter of the transmission belt and the speed change cone cylinder, thereby achieving the effect of speed change.

[0017] When the blade is stationary, the intersection degree between cone A and cone B is the largest (the distance between the two is the shortest), which can provide the maximum torque and provide a larger starting acceleration for the blade. When the blade is moving, the intersection degree between cone A and cone B becomes smaller (the distance between the two becomes farther), which can provide a higher rotation speed and provide a higher movement speed for the blade.

[0018] The thickness of the driving wheel is smaller than the width of the blade, so that the driving wheel can mesh with the coupling teeth of the blade no matter how it moves.

[0019] The driven gear in the present invention is made of permanent magnetic material, and the driving device is a strong magnetic coil. The strong magnetic coil is located on one side of the driven gear, and a brake disc fixed on the support shaft is provided between the strong magnetic coil and the driven gear. When the strong magnetic coil is energized, it pushes the driven gear to move a certain distance along the drive shaft, thereby meshing with the normally moving gear to transmit power. Braking function: When the strong magnetic coil is not energized, it fits with the brake disc and remains stationary, and brakes the blades through the blade drive structure. When the equipment is powered off or there is no signal, the driven gear fits with the brake disc to ensure that the blades do not move unexpectedly.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The multi-leaf collimator blade driving method and driving structure described in the present invention only require a single high-power motor to drive the blades, which is quite different from the traditional multi-leaf collimator, in which the number of blades in the traditional multi-leaf collimator is the same as the number of motors driving the blades. The driving structure of the present patent can increase the torque of the blade movement in a larger range, and the movement speed is faster, the requirements for the motor size are greatly reduced, the requirements for the motor performance are reduced, and a large power variation range is not required.

[0022] Reduce the number of motors and improve efficiency: By using a single constant torque and constant speed motor as the power source and connecting multiple transmission subsystems in parallel, the number of motors required is effectively reduced, reducing the complexity and cost of the system. At the same time, since each transmission subsystem is relatively independent, there is no problem of energy transfer and attenuation, so the power can be transferred to each blade more efficiently, and the power attenuation of the first-stage transmission gear set in the existing patent will not occur.

[0023] Enhanced driving force and flexibility: The use of high-power constant torque and constant speed motors ensures sufficient driving force for blade movement and avoids the problem of insufficient power. In addition, by designing a small speed-changing structure (speed-changing cone) at the end of the transmission subsystem, the speed of a single blade can be accurately adjusted and different torques can be provided to meet different movement requirements.

[0024] Optimized start-up and motion performance: The design of the variable speed cone enables the blade to obtain a larger start-up acceleration when stationary and provide a higher speed when moving, thereby optimizing the start-up and motion performance of the blade. This design not only improves the motion efficiency of the blade, but also reduces energy loss.

[0025] Improve safety and reliability: The driven gear is made of permanent magnetic material, and the design of strong magnetic coil and brake disc realizes precise control and braking of the blades. In the case of power failure or no signal, the driven gear will fit the brake disc to ensure that the blades will not move unexpectedly, improving the safety and reliability of the system.

[0026] Easy to expand and maintain: The driving structure and driving method of the present invention allow the number of blades to be easily expanded to meet the needs of multi-leaf collimators of different sizes. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is a schematic diagram of the leaf distribution structure of a multi-leaf collimator;

[0028] Figure 2 It is one of the transmission principle schematic diagrams of the present invention;

[0029] Figure 3 This is the second schematic diagram of the transmission principle of the present invention;

[0030] Figure 4It is a schematic diagram of the belt transmission in the present invention;

[0031] Figure 5 is a schematic cross-sectional view of a transmission belt;

[0032] Figure 6 It is a schematic diagram of the overall structure of the speed change cone;

[0033] Figure 7 It is one of the schematic diagrams of the frame structure of the speed change cone;

[0034] Figure 8 This is the second schematic diagram of the frame structure of the speed change cone;

[0035] Fig. 9 This is the third schematic diagram of the frame structure of the speed change cone;

[0036] Fig.10 is a side view of cone A;

[0037] Fig.11 It is a schematic diagram of the coordination relationship between the blade and the driving wheel.

[0038] In the figure: 1. blade; 2. drive shaft; 3. constant gear; 4. support shaft; 5. driven gear; 6. strong magnetic coil; 7. brake disc; 8. bushing; 9. belt coupler; 10. transmission belt; 11. speed change cone cylinder; 12. drive wheel; 13. tensioner; 14. cone cylinder A; 15. cone cylinder B; 16. bracket; 17. micro bidirectional cylinder; 18. support column; 19. small end face; 20. large end face; 21. sleeve; 22. slot; 23. wire; 24. blade coupling teeth; 25. guide wheel; 26. limit block. DETAILED DESCRIPTION

[0039] The present invention is further described below in conjunction with specific embodiments.

[0040] However, the description of the present invention is merely an embodiment of structural and even functional description, and the scope of rights of the present invention is not limited by the embodiments described in the text.

[0041] For example, multiple embodiments may have various modifications and various forms, and it should be understood that the scope of rights of the present invention includes equivalents that can implement the technical concept.

[0042] like Figures 1 to 11 As shown, the multi-leaf collimator blade driving method:

[0043] It is a method for driving several blades of a multi-leaf collimator to perform linear motion, using a single constant torque constant speed motor as a power source, and transmitting power to the collimator blades through a transmission subsystem corresponding to the several blades of the collimator, driving the collimator blades to complete linear motion; the power input ends of several transmission subsystems are connected in parallel: the power input end of the transmission subsystem is a constant gear 3 fixed on the drive shaft 2, the constant gear 3 is concentric with the drive shaft 2, and several constant gears 3 are arranged in parallel along the length direction of the drive shaft 2. The torque and speed of the power output ends of the several transmission subsystems are adjustable.

[0044] A multi-leaf collimator leaf driving structure, comprising:

[0045] A constant torque constant speed motor, a plurality of transmission subsystems (for example, a multi-leaf collimator suitable for 60 pairs of blades requires 120 transmission subsystems), a plurality of blades 1, and the plurality of transmission subsystems correspond one to one with the plurality of blades 1 of the collimator; a plurality of guide wheels 25 guide the blades 1 to move, and a limit block 26 is provided on the blade 1 to limit the forward and backward movement.

[0046] The constant torque constant speed motor is connected to the drive shaft 2; the transmission subsystem includes a constant moving gear 3 fixed on the drive shaft 2, the constant moving gear 3 is concentric with the drive shaft 2, and a plurality of constant moving gears 3 are arranged in parallel along the length direction of the drive shaft 2;

[0047] It also includes a sleeve 8 and a belt coupler 9 sleeved on the support shaft 4, the support shaft 4 is parallel to the drive shaft 2, the support shaft 4 is fixed, the sleeve 8 and the belt coupler 9 are fixedly connected, and the sleeve 8 can rotate on the support shaft 4 but cannot move along the support shaft 4;

[0048] It also includes a driven gear 5 sleeved on the shaft sleeve 8, which can move along the shaft sleeve 8 but cannot rotate relative to the shaft sleeve 8; the driven gear 5 is equipped with a driving device that can move it along the shaft sleeve 8. The driving device is a strong magnetic coil 6, the driven gear 5 is made of permanent magnetic material, the strong magnetic coil 6 is located on one side of the driven gear 5, and a brake disc 7 fixed on the support shaft 4 is provided between the strong magnetic coil 6 and the driven gear 5.

[0049] It also includes a transmission, the input end of which is connected to the belt coupler 9 through a transmission belt 10, and the output end of the transmission is provided with a drive wheel 12, the teeth on the drive wheel 12 are meshed with the blade coupling teeth 24, and the blade 1 is driven to move linearly through the rotation of the drive wheel 12. The transmission is a speed change cone cylinder 11, and the speed change cone cylinder 11 includes a cone cylinder A14 and a cone cylinder B15. The cone cylinder A14 and the cone cylinder B15 both include a small end face 19 and a large end face 20, and a plurality of wires 23 are connected between the small end face 19 and the large end face 20. The wires 23 are all along the cone cylinder generatrix direction, and the plurality of wires 23 are distributed in a circular array outside the cone cylinder; the small end faces 19 of the cone cylinder A14 and the cone cylinder B15 are both composed of a plurality of radially distributed support rods, and the two ends of the cone cylinder A14 are connected to the cone cylinder B15. The end faces of the cone cylinder A14 and cone cylinder B15 are connected by a central support column 18, and the two end faces of the cone cylinder B15 are connected by a central sleeve 21. The wall of the sleeve 21 is provided with a plurality of slots 22, which are strip-shaped. Along the length direction of the sleeve 21, the slots 22 correspond to a plurality of support rods constituting the small end face 19 one by one. The small end faces of the cone cylinder A14 and cone cylinder B15 are cross-connected, and the cross-depth is adjustable; the transmission belt 10 is connected to the cross-contact surface of the cone cylinder A14 and cone cylinder B15. The two ends of the speed change cone cylinder 11 are rotatably arranged between a pair of brackets 16, and a micro bidirectional cylinder 17 is also arranged between the pair of brackets 16. The micro bidirectional cylinder 17 can adjust the cross-depth of the cone cylinder A14 and cone cylinder B15. The thickness of the driving wheel 12 is less than the width of the blade 1.

[0050] The power of the driving structure comes from the driving shaft, which is connected to a high-power motor with constant torque and constant speed (a multi-leaf collimator has two blade boxes and two high-power motors, one in each box). The motor is connected to the driving shaft 2, and 60 constant-moving gears 3 are installed on each driving shaft 2, and the constant-moving gears 3 can mesh with the driven gears 5. The driven gears 5 can drive the belt coupler 9 (equivalent to the pulley) and transmit power to the transmission (speed-changing cone 11) through the transmission belt 10. The speed-changing cone 11 is connected to the driving wheel 12, and the driving wheel 12 meshes with the blade coupling teeth 24. 4) The constant-moving gears are installed on the driving shaft, and the driving shaft is connected to the high-power motor.

[0051] Principle of the speed-changing cone cylinder combination structure: cone cylinder A14 is cross-connected with cone cylinder B15, and cone cylinder A14 and cone cylinder B15 are hollow conical structures without tips. Support column 18 (or sleeve 21) connects small end face 19 and large end face 20, and multiple small holes are evenly distributed on the outer edges of small end face 19 and large end face 20. High-strength wire 23 passes through the small holes adjacent to the two end faces (not shown in the figure) and is woven into the outer shell of a single cone cylinder. Cone cylinder A14 and cone cylinder B15 have the same appearance size, and cone cylinder A14 and cone cylinder B15 are nested structures. Cone cylinder A14 and cone cylinder B15 are connected by support column 18 and sleeve 21, and support column 18 is inserted into sleeve 21. The wires 23 on the outer surfaces of cone cylinder A14 and cone cylinder B15 are staggered. The micro bidirectional cylinder 17 can push the speed change cone 11 from both sides. When the intersection distance between cone A14 and cone B15 increases, the diameter of the contact surface of the two cones increases, the transmission torque of the drive wheel 12 connected thereto increases, and the blade speed decreases. When the intersection distance between cone A14 and cone B15 decreases, the diameter of the contact surface of the two cones decreases, the transmission torque of the drive wheel 12 connected thereto decreases, and the blade speed increases.

[0052] The contact surface of the two speed-changing cones is installed with a transmission belt 10. The angle of the lower end surface of the cross section of the transmission belt 10 is the same as the angle of the contact surface of the two cones, and the transmission belt 10 moves in close contact. The other end of the transmission belt 10 is connected to the belt coupler 9. The contact surface of the belt coupler 9 and the transmission belt 10 has the same shape as the lower end surface of the transmission belt 10, and the transmission belt 10 moves in close contact. The transmission belt 10 is equipped with a tensioner 13. The tensioner 13 has a certain pulling force, and the transmission belt 10 is always stretched outward, so that the transmission belt 10 is always in a tensioned state.

[0053] The driven gear 5 is made of high-strength permanent magnetic material. When the strong magnetic coil 6 is energized, it pushes the driven gear 5 to move a certain distance along the shaft sleeve 8, thereby meshing with the permanent gear 3 to transmit power. When the strong magnetic coil 6 is not energized, it fits with the brake disc 7 and remains stationary, and brakes the blade 1 through the blade drive structure. When the equipment is powered off or there is no signal, the driven gear 5 fits the brake disc 7 to ensure that the blade 1 will not move unexpectedly. The micro bidirectional cylinder 17 that controls the speed change cone 11 is linked with the strong magnetic coil 6 under the control system. When the strong magnetic coil 6 is energized, the micro bidirectional cylinder 17 extends, the diameter of the contact surface of the two cones gradually decreases, and the movement speed of the blade 1 gradually increases; when the strong magnetic coil 6 loses power, the micro bidirectional cylinder 17 shrinks, and the diameter of the contact surface of the two cones gradually increases, waiting for the next start. This allows the blade to obtain a larger starting acceleration when it is stationary, and to provide a higher speed when it is moving, thereby optimizing the starting and movement performance of the blade.

[0054] Of course, the above contents are only preferred embodiments of the present invention and cannot be considered to limit the scope of the embodiments of the present invention. The present invention is not limited to the above examples, and equal changes and improvements made by ordinary technicians in the technical field within the essential scope of the present invention should all fall within the scope of the patent coverage of the present invention.

Claims

1. A method for driving blades of a multi-leaf collimator, which is a method for driving several blades of a multi-leaf collimator to perform linear motion, characterized in that: A single constant torque and constant speed motor is used as a power source, and power is transmitted to the collimator blades through transmission subsystems corresponding to the collimator blades one by one, driving the collimator blades to complete linear motion; the power input ends of the transmission subsystems are connected in parallel.

2. The multi-leaf collimator blade driving method according to claim 1, characterized in that: The power input end of the transmission subsystem is a permanent gear (3) fixed on the drive shaft (2); the permanent gear (3) is concentric with the drive shaft (2); and a plurality of permanent gears (3) are arranged in parallel along the length direction of the drive shaft (2).

3. The multi-leaf collimator blade driving method according to claim 1, characterized in that: The torque and speed of the power output of several transmission subsystems are adjustable.

4. A multi-leaf collimator blade driving structure, characterized in that: include: A constant torque constant speed motor, a plurality of transmission subsystems, and a plurality of blades (1), wherein the plurality of transmission subsystems correspond one to one with the plurality of blades (1) of the collimator; The constant torque and constant speed motor is connected to the drive shaft (2); The transmission subsystem comprises a permanent gear (3) fixed on the driving shaft (2), the permanent gear (3) is concentric with the driving shaft (2), and a plurality of permanent gears (3) are arranged in parallel along the length direction of the driving shaft (2); It also includes a shaft sleeve (8) and a belt coupler (9) sleeved on the support shaft (4), the support shaft (4) is parallel to the drive shaft (2), the support shaft (4) is fixed, the shaft sleeve (8) and the belt coupler (9) are fixedly connected, and the shaft sleeve (8) can rotate on the support shaft (4) but cannot move along the support shaft (4); It also includes a driven gear (5) sleeved on the shaft sleeve (8), wherein the driven gear (5) can move along the shaft sleeve (8) but cannot rotate relative to the shaft sleeve (8); The driven gear (5) is provided with a pushing device which enables it to move along the shaft sleeve (8). The invention also comprises a transmission, wherein the transmission input end is connected to the belt coupler (9) via a transmission belt (10), and the transmission output end is provided with a driving wheel (12), the teeth on the driving wheel (12) are meshed with the blade coupling teeth (24), and the blade (1) is driven to move linearly by the rotation of the driving wheel (12).

5. The multi-leaf collimator blade driving structure according to claim 4, characterized in that: The transmission is a speed change cone cylinder (11), the speed change cone cylinder (11) comprises a cone cylinder A (14) and a cone cylinder B (15), the cone cylinder A (14) and the cone cylinder B (15) both comprise a small end surface (19) and a large end surface (20), a plurality of wires (23) are connected between the small end surface (19) and the large end surface (20), the wires (23) are all along the cone cylinder generatrix direction, and the plurality of wires (23) are distributed in a circumferential array outside the cone cylinder; the small end surfaces (19) of the cone cylinder A (14) and the cone cylinder B (15) are both composed of a plurality of radially distributed support rods, and the cone cylinder A (14) and the cone cylinder B (15) are connected to each other by a plurality of wires (23) arranged in a circular array outside the cone cylinder. The two end surfaces of the cone cylinder A (14) are connected by a central support column (18), and the two end surfaces of the cone cylinder B (15) are connected by a central sleeve (21). A plurality of slots (22) are arranged on the wall of the sleeve (21). The slots (22) are strip-shaped. Along the length direction of the sleeve (21), the slots (22) correspond to a plurality of support rods constituting the small end surface (19) one by one. The small end surfaces of the cone cylinder A (14) and the cone cylinder B (15) are cross-connected, and the cross-depth is adjustable. The transmission belt (10) is connected to the cross-contact surface of the cone cylinder A (14) and the cone cylinder B (15).

6. The multi-leaf collimator blade driving structure according to claim 5, characterized in that: The two ends of the speed change cone (11) are rotatably arranged between a pair of brackets (16), and a micro bidirectional cylinder (17) is also arranged between the pair of brackets (16). The micro bidirectional cylinder (17) can adjust the crossing depth of the cone A (14) and the cone B (15).

7. The multi-leaf collimator blade driving structure according to claim 5, characterized in that: The thickness of the driving wheel (12) is smaller than the width of the blade (1).

8. The multi-leaf collimator blade driving structure according to claim 4, characterized in that: The driven gear (5) is made of permanent magnetic material. The driving device is a strong magnetic coil (6). The strong magnetic coil (6) is located on one side of the driven gear (5). A brake disc (7) fixed on the support shaft (4) is provided between the strong magnetic coil (6) and the driven gear (5).

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