Passive wireless pulse current partial discharge sensor

Abstract

The invention discloses a passive wireless pulse current partial discharge sensor, and relates to the technical field of partial discharge sensors, the passive wireless pulse current partial discharge sensor comprises a monitor, two sides of the monitor are fixedly provided with angle irons, the passive wireless pulse current partial discharge sensor also comprises a bottom plate, and the bottom plate is connected with the monitor through a multi-angle adjusting structure; the multi-angle adjusting structure enables the monitor to be flexibly adjusted in angle and to be installed in a required area for current measurement. The device has the advantages that the detection surface of the monitor is dynamically adjusted along with the direction of a partial discharge signal source, the capturing precision of weak pulse current is remarkably improved, meanwhile, the foldable shielding cover and the monitor are synchronously unfolded during angle adjustment, and the metal shielding plate and the honeycomb electromagnetic wave absorbing layer form an electric-magnetic dual-shielding system; the distortion influence of interference signals on current measurement is avoided, the opening degree is automatically adjusted along with the angle in cooperation with a swing blade in a shielding heat dissipation structure, it is ensured that accurate collection of current signals is achieved under the stable working condition of equipment, and the reliability of partial discharge monitoring is comprehensively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of partial discharge sensors, and in particular to a passive wireless pulse current partial discharge sensor. Background Art

[0002] When partial discharge occurs in power equipment, a pulse current is usually generated in its grounding down conductor or other ground potential connection line. The pulse current signal flowing through the insulator is monitored by a pulse current partial discharge monitoring device to achieve live detection of partial discharge in power equipment.

[0003] Existing passive wireless pulse current partial discharge sensors have significant technical bottlenecks in practical applications: their installation structure lacks the flexibility of angle adjustment and is difficult to adapt to the complex spatial layout of power equipment, resulting in a significantly weakened ability to capture weak discharge signals and a greatly increased risk of missed detection of early insulation defects; the electromagnetic shielding and heat dissipation system functions of the equipment hinder each other, and the fixed protective structure cannot be adaptively adjusted with the installation angle. The hidden danger of dust accumulation is prominent in vertical scenarios, and electromagnetic interference takes advantage of the opportunity to enter in horizontal layouts, causing the signal quality and equipment temperature rise to deteriorate simultaneously; more importantly, angle adjustment and functional component calibration during on-site operation and maintenance need to be performed step by step. The process is cumbersome and time-consuming, and manual adjustment errors are difficult to avoid, which directly leads to a significant reduction in detection accuracy and can no longer meet the technical requirements of smart grids for accurate monitoring of equipment status. Summary of the Invention

[0004] The purpose of the present invention is to solve the problem in the prior art that the installation angle is difficult to adjust, and to propose a passive wireless pulse current partial discharge sensor.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A passive wireless pulse current partial discharge sensor includes a monitor, angle irons are fixedly provided on both sides of the monitor, and further includes:

[0007] The base plate is connected to the monitor through a multi-angle adjustment structure. The multi-angle adjustment structure allows the monitor to be flexibly adjusted in angle to be installed in a desired area for current measurement. Heat dissipation structures are provided on both sides of the monitor. The heat dissipation structures change accordingly with the adjustment of the multi-section adjustment structure, so that the ventilation angle is adapted to the current use angle. A foldable shielding cover is provided on the top of the monitor. The foldable shielding cover is synchronously unfolded with the angle change of the multi-section adjustment structure to enhance the shielding effect against electromagnetic interference.

[0008] In the above-mentioned passive wireless pulse current partial discharge sensor, two mounting holes are provided on each of the two angle irons, and the mounting holes are elliptical. Two fixed angle plates are fixedly provided at the four corners of the base plate, and a universal shaft is rotatably installed between the two fixed angle plates. A spring column is fixedly provided on each of the universal shafts, and the upper end of each spring column passes through the corresponding mounting hole and is fixedly installed with a baffle. A tension spring is sleeved and installed on the outside of each spring column, and the lower end of the tension spring is fixedly provided on the corresponding universal shaft.

[0009] In the above-mentioned passive wireless pulse current partial discharge sensor, a connecting frame is fixedly installed on the base plate, a circular groove and a square groove are provided on the connecting frame, and the circular groove is connected to the square groove, a limiting guide block is slidably provided in the square groove, a torsion bar is fixedly provided on one side of the limiting guide block, a fixing ring is fixedly provided in the circular groove, a connecting rod is fixedly installed on the fixing ring, and a card slot is provided at one end of the connecting rod for cooperating with the limiting guide block.

[0010] In the above-mentioned passive wireless pulse current partial discharge sensor, the shape of the square groove is the same as that of the limiting guide block, the axial length of the circular groove and the card slot is greater than the axial length of the limiting guide block on the torsion bar, and the diameter of the torsion bar is smaller than the diameter of the circular groove.

[0011] In the above-mentioned passive wireless pulse current partial discharge sensor, a slide groove is provided on the base plate, and two fixed sleeves are fixedly installed in the slide groove. The two fixed sleeves are commonly threadedly installed with a threaded rod, and a sliding seat is rotatably installed on the threaded rod, and the sliding seat is slidably set in the slide groove.

[0012] In the above-mentioned passive wireless pulse current partial discharge sensor, the multi-angle adjustment structure includes a rotating bracket rotatably arranged at one end of the connecting rod, the rotating bracket is fixedly connected to the bottom of the monitor with a fixed plate, a universal rod is rotatably arranged on the rotating bracket, a slip ring is fixedly arranged at one end of the universal rod, an arc guide rail is fixedly arranged on the sliding seat, and the slip ring is slidably arranged on the arc guide rail, and a fixing screw is threadedly installed on the slip ring.

[0013] In the above-mentioned passive wireless pulse current partial discharge sensor, the foldable shielding cover is composed of multiple metal shielding plates connected by hinges, one end of the foldable shielding cover is hinged to the top of the monitor, and the other end is connected to the rotating bracket through a linkage rod; when the rotating bracket rotates with the multi-angle adjustment structure, the linkage rod drives the foldable shielding cover to expand or fold, so that the shielding surface of the foldable shielding cover always corresponds to the detection surface of the monitor, so as to enhance the shielding effect against electromagnetic interference.

[0014] In the above-mentioned passive wireless pulse current partial discharge sensor, an electromagnetic absorbing layer is provided on the inner side of the foldable shielding cover, and the electromagnetic absorbing layer adopts a honeycomb structure; the linkage rod includes an inner rod and an outer rod that are mutually nested, and a return spring is provided between the inner rod and the outer rod. The return spring is used to provide a buffering force when the rotating bracket rotates, so that the foldable shielding cover can be smoothly unfolded or folded, thereby achieving shielding of electromagnetic interference at different angles.

[0015] In the above-mentioned passive wireless pulse current partial discharge sensor, the heat dissipation structure includes multiple air inlets opened on the monitor, multiple mounting plates are fixedly provided on one side of the monitor, and a rotating shaft is installed between the corresponding two mounting plates so as to rotate together. A swing blade is fixedly provided on each rotating shaft, and an incomplete gear is fixedly installed on each rotating shaft.

[0016] In the above-mentioned passive wireless pulse current partial discharge sensor, a through hole is opened on one side of the monitor, and two universal balls are fixedly arranged on the base plate. A gear rod is rotatably arranged on both of the universal balls, and the gear rod slides in the through hole and engages with multiple incomplete gears for use.

[0017] Compared with the existing technology, the advantages of the present invention are: the detection surface of the monitor is dynamically adjusted along with the direction of the partial discharge signal source, the signal attenuation blind spot of the fixed installation is eliminated from the physical level, and the capture accuracy of weak pulse current is significantly improved. At the same time, the foldable shielding cover is synchronously unfolded with the angle adjustment of the monitor, and the metal shielding plate and the honeycomb electromagnetic absorbing layer constitute an "electromagnetic" double shielding system, which forms an omnidirectional barrier to the complex electromagnetic interference of the substation, avoiding the distortion effect of the interference signal on the current measurement, and the swing blades in the shielded heat dissipation structure automatically adjust the opening according to the angle, ensuring that the equipment can realize accurate collection of current signals under stable working conditions, and comprehensively improving the reliability of partial discharge monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic structural diagram of a passive wireless pulse current partial discharge sensor proposed by the present invention;

[0019] Figure 2 It is a structural schematic diagram of another perspective of the present invention;

[0020] Figure 3 Schematic diagram of the structure of the bottom plate of the present invention;

[0021] Figure 4 Schematic diagram of the structure of the monitor in the present invention;

[0022] Figure 5 For the present invention Figure 4 A top view of

[0023] Figure 6For the present invention Figure 5 Structural cross-sectional view along the AA direction;

[0024] Figure 7 For the present invention Figure 6 A schematic diagram of the structure of part a is enlarged;

[0025] Figure 8 Schematic diagram of the structure of the incomplete gear in the present invention;

[0026] Figure 9 A top view of the rotating bracket of the present invention;

[0027] Figure 10 For the present invention Figure 9 Structural cross-sectional view along the BB direction.

[0028] In the figure: 1. Monitor; 2. Angle iron; 3. Base plate; 4. Fixed angle plate; 5. Baffle; 6. Tension spring; 7. Spring column; 8. Connecting frame; 9. Torsion bar; 10. Universal joint; 11. Fixed screw; 12. Slip ring; 13. Arc guide rail; 14. Slide groove; 15. Fixed sleeve; 16. Threaded rod; 17. Universal rod; 18. Sliding seat; 19. Fixed plate; 20. Rotating bracket; 21. Universal ball; 22. Gear rod; 23. Mounting plate; 24. Rotating shaft; 25. Swing blade; 26. Mounting hole; 27. Incomplete gear; 28. Air inlet; 29. ​​Fixed ring; 30. Circular groove; 31. Square groove; 32. Limit guide block. DETAILED DESCRIPTION

[0029] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] Reference Figure 1-Figure 3 as well as Figure 9-10 A passive wireless pulse current partial discharge sensor includes a monitor 1, angle irons 2 are fixedly provided on both sides of the monitor 1, two mounting holes 26 are provided on the two angle irons 2, and the mounting holes 26 are elliptical. Two fixed angle plates 4 are fixedly provided at the four corners of the bottom plate 3, and a universal shaft 10 is rotatably installed between the two fixed angle plates 4. A spring column 7 is fixedly provided on each universal shaft 10, and the upper end of each spring column 7 passes through the corresponding mounting hole 26 and is fixedly installed with a baffle 5. A tension spring 6 is sleeved on the outer side of each spring column 7, and the lower end of the tension spring 6 is fixed on the corresponding universal shaft 10. When the angle of the monitor 1 changes, multiple tension springs 6 cooperate with the spring column 7 to achieve stable adjustment of the monitor 1.

[0031] The monitor 1 is mounted at a desired position through the base plate 3, the base plate 3 is connected to the monitor 1 through a multi-angle adjustment structure, a connecting frame 8 is fixedly mounted on the base plate 3, a circular groove 30 and a square groove 31 are provided on the connecting frame 8, and the circular groove 30 is connected to the square groove 31, a limited guide block 32 is slidably provided in the square groove 31, a torsion bar 9 is fixedly provided on one side of the limited guide block 32, a fixing ring 29 is fixedly provided in the circular groove 30, a connecting rod is fixedly mounted on the fixing ring 29, and one end of the connecting rod is opened. A slot is provided for use with the limiting guide block 32. The shape of the square slot 31 is the same as that of the limiting guide block 32. The axial length of the circular slot 30 and the slot is greater than the axial length of the limiting guide block 32 on the torsion bar 9. The diameter of the torsion bar 9 is smaller than the diameter of the circular slot 30. A slide 14 is provided on the bottom plate 3. Two fixing sleeves 15 are fixedly installed in the slide 14. The two fixing sleeves 15 are threadedly mounted with a threaded rod 16. A sliding seat 18 is rotatably mounted on the threaded rod 16, and the sliding seat 18 is slidably set in the slide 14. The multi-angle adjustment structure allows the monitor 1 to be flexibly adjusted in angle and installed in the desired area for current measurement. The multi-angle adjustment structure includes a rotating bracket 20 rotatably set at one end of the connecting rod. The rotating bracket 20 is fixedly connected to the bottom of the monitor 1 with a fixed plate 19.

[0032] When the angle of the monitor 1 is adjusted, the torsion bar 9 is pushed in the direction of the rotating bracket 20, so that the limit guide block 32 on the torsion bar 9 is engaged in the card slot, and the torsion bar 9 rotates in the square slot 31. At this time, the torsion bar 9 is rotated, and a dial is set on the outer side of the rotating torsion bar 9 on the connecting frame 8. A protrusion is set on the torsion bar 9. By observing the angle formed between the protrusion and the dial, the required angle of the monitor 1 is adjusted to achieve precise adjustment. At the same time, under the cooperation of the limit guide block 32 and the card slot, the torsion bar 9 drives the rotating bracket 2 through the connecting rod. 0 rotates, and the rotating bracket 20 drives the monitor 1 to rotate in the X-axis direction through the fixed plate 19 (this direction is set as the X-axis, and the plane perpendicular to it is set as the Y-axis). When the angle adjustment of the monitor 1 is completed, the torsion bar 9 is pulled in the opposite direction, so that the limiting guide block 32 is partially located in the card slot and the other part is located between the square slot 31 and the circular slot 30, thereby achieving the positioning after the angle adjustment and ensuring the stability of the position of the monitor 1. In the process of angle adjustment, the multi-tension spring 6 and the spring column 7 are deformed accordingly to maintain the stability of the monitor 1.

[0033] A universal rod 17 is rotatably mounted on the rotating bracket 20. A slip ring 12 is fixedly mounted on one end of the universal rod 17. An arc-shaped guide rail 13 is fixedly mounted on the sliding seat 18. The slip ring 12 is slidably mounted on the arc-shaped guide rail 13. A fixed screw 11 is threadedly mounted on the slip ring 12. By twisting the fixed screw 11, the slip ring 12 and the arc-shaped guide rail 13 are kept in a sliding state. The arc-shaped guide rail 13 is also provided with a scale. By pulling the slip ring 12 to rotate on the arc-shaped guide rail 13, the angle of the monitor 1 in the Y-axis direction is adjusted. When the angle adjustment is completed, the fixed screw 11 is twisted in the opposite direction to keep the slip ring 12 and the arc-shaped guide rail 13 relatively fixed. An arc-shaped groove is provided on the outer periphery of the arc-shaped guide rail 13. A threaded groove is provided at the front end of the fixed screw 11. The combination of the threaded groove and the arc-shaped groove can increase the friction between the slip ring 12 and the arc-shaped guide rail 13, thereby maintaining the stability of the monitor 1.

[0034] The following parts are not shown in the figure. A foldable shielding cover is provided on the top of the monitor 1. The foldable shielding cover is composed of multiple metal shielding plates connected by hinges. One end of the foldable shielding cover is hinged to the top of the monitor 1, and the other end is connected to the rotating bracket 20 through a linkage rod; when the rotating bracket 20 rotates with the multi-angle adjustment structure, the linkage rod drives the foldable shielding cover to unfold or fold, so that the shielding surface of the foldable shielding cover always corresponds to the detection surface of the monitor 1, so as to enhance the shielding effect against electromagnetic interference.

[0035] An electromagnetic absorbing layer is provided on the inner side of the foldable shielding cover, and the electromagnetic absorbing layer adopts a honeycomb structure; the linkage rod includes an inner rod and an outer rod which are mutually nested, and a reset spring is provided between the inner rod and the outer rod. The reset spring is used to provide a buffering force when the rotating bracket 20 rotates, so that the foldable shielding cover can be smoothly unfolded or folded, thereby achieving shielding of electromagnetic interference at different angles, while avoiding affecting the detection signal of the monitor 1.

[0036] The foldable shielding cover is mechanically coupled to the rotating bracket 20 via a linkage rod. When the monitor 1 rotates with the multi-angle adjustment structure, the shielding cover is synchronously expanded / folded, and its shielding surface always maintains a fixed angle relative to the detection surface. Compared with traditional fixed shielding covers, it improves the attenuation capability of electromagnetic interference (such as switch cabinet operation pulses and substation radio frequency noise), improves the signal-to-noise ratio of partial discharge signals, and prevents interference from drowning out the real discharge signal. The honeycomb electromagnetic absorbing layer (such as a ferrite honeycomb structure) on the inside of the shielding cover uses the principle of multi-cavity resonance to enhance the absorption rate of high-frequency interference in the 200MHz to 3GHz range. When the interference signal penetrates the metal shielding plate, the absorbing layer further converts it into heat energy, solving the problem of "reflected interference causing secondary coupling" in traditional metal shielding. It is particularly suitable for suppressing narrowband interference generated by high-frequency switching operations in substations. The metal shielding plate provides electrostatic shielding (attenuating low-frequency interference), and the honeycomb absorbing layer is responsible for magnetic shielding (attenuating high-frequency interference), forming an "electromagnetic" dual shielding system.

[0037] Reference Figure 3-Figure 8 Both sides of the monitor 1 are provided with heat dissipation structures. The heat dissipation structures change accordingly with the adjustment of the multi-section adjustment structure, so that the electromagnetic shielding and ventilation angles are adapted to the current use angle. The heat dissipation structure includes multiple air inlets 28 opened on the monitor 1. Multiple mounting plates 23 are fixedly provided on one side of the monitor 1. A rotating shaft 24 is mounted between the corresponding two mounting plates 23 so as to rotate together. A swing blade 25 is fixedly provided on each rotating shaft 24. An incomplete gear 27 is fixedly installed on each rotating shaft 24. A through hole is opened on one side of the monitor 1. Two universal balls 21 are fixedly provided on the bottom plate 3. A gear rod 22 is rotatably provided on each of the two universal balls 21. The gear rod 22 slides in the through hole and engages with the multiple incomplete gears 27 for use.

[0038] The arrangement of the universal ball 21 allows the gear rod 22 to rotate along the length of the monitor 1. If multiple sets of pendulum blades 25 are placed on the front and rear sides of the monitor 1, the gear rod 22 rotates on the universal ball 21 along the width of the monitor 1. When the angle of the monitor 1 changes, the gear rod 22 rotates on the universal ball 21, changing with the angle of the monitor 1. This causes the gear rod 22 on the lower side to slide upward relative to the multiple incomplete gears 27, driving the rotation of the shaft 24. The rotation of the multiple shafts 24 drives the multiple pendulum blades 25 to relatively seal the air inlet 28. The opposite occurs on the side with a higher horizontal height after the angle of the monitor 1 is adjusted. When the pendulum blades 25 rotate synchronously, the pendulum blades 25 on the lower side close to form a continuous shielding surface, improving the attenuation of electromagnetic interference (such as ground loop pulse noise) from the bottom of the device. The pendulum blades 25 on the higher side maintain their basic opening, balancing heat dissipation and shielding.

[0039] It is further explained that the above-mentioned fixed connection should be understood in a broad sense unless otherwise clearly specified and limited. For example, it can be welding, gluing, or one-piece molding, etc., which are common means well known to those skilled in the art.

[0040] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A passive wireless pulse current partial discharge sensor, comprising a monitor (1), characterized in that: Angle irons (2) are fixedly provided on both sides of the monitor (1), and the monitor (1) further comprises: A base plate (3) is connected to a monitor (1) via a multi-angle adjustment structure. The multi-angle adjustment structure allows the monitor (1) to be flexibly adjusted in angle so as to be installed in a desired area for current measurement. Heat dissipation structures are provided on both sides of the monitor (1). The heat dissipation structures change accordingly with the adjustment of the multi-section adjustment structure so that the ventilation angle is adapted to the current angle. A foldable shielding cover is provided on the top of the monitor (1). The foldable shielding cover is synchronously unfolded with the angle change of the multi-section adjustment structure to enhance the shielding effect against electromagnetic interference.

2. A passive wireless pulse current partial discharge sensor according to claim 1, characterized in that: Two mounting holes (26) are provided on each of the two angle irons (2), and the mounting holes (26) are elliptical. Two fixed angle plates (4) are fixedly provided at the four corners of the base plate (3), and a universal shaft (10) is rotatably installed between the two fixed angle plates (4). A spring column (7) is fixedly provided on each of the universal shafts (10), and the upper end of each of the spring columns (7) passes through the corresponding mounting hole (26) and is fixedly provided with a baffle (5). A tension spring (6) is sleeved and installed on the outer side of each of the spring columns (7), and the lower end of the tension spring (6) is fixedly provided on the corresponding universal shaft (10).

3. A passive wireless pulse current partial discharge sensor according to claim 1, characterized in that: A connecting frame (8) is fixedly mounted on the bottom plate (3), a circular groove (30) and a square groove (31) are provided on the connecting frame (8), and the circular groove (30) is communicated with the square groove (31), a limiting guide block (32) is slidably arranged in the square groove (31), a torsion bar (9) is fixedly arranged on one side of the limiting guide block (32), a fixing ring (29) is fixedly arranged in the circular groove (30), a connecting rod is fixedly mounted on the fixing ring (29), and a card slot for matching the limiting guide block (32) is provided at one end of the connecting rod.

4. A passive wireless pulse current partial discharge sensor according to claim 3, characterized in that: The shape of the square groove (31) is the same as that of the limiting guide block (32), the axial length of the circular groove (30) and the clamping groove is greater than the axial length of the limiting guide block (32) on the torsion bar (9), and the diameter of the torsion bar (9) is smaller than the diameter of the circular groove (30).

5. A passive wireless pulse current partial discharge sensor according to claim 4, characterized in that: A slide groove (14) is provided on the bottom plate (3), two fixing sleeves (15) are fixedly installed in the slide groove (14), a threaded rod (16) is commonly threadedly installed on the two fixing sleeves (15), a sliding seat (18) is rotatably installed on the threaded rod (16), and the sliding seat (18) is slidably arranged in the slide groove (14).

6. A passive wireless pulse current partial discharge sensor according to claim 5, characterized in that: The multi-angle adjustment structure includes a rotating bracket (20) rotatably arranged at one end of a connecting rod, the rotating bracket (20) is fixedly connected to a fixed plate (19) at the bottom of the monitor (1), a universal rod (17) is rotatably arranged on the rotating bracket (20), a slip ring (12) is fixedly arranged at one end of the universal rod (17), an arc guide rail (13) is fixedly arranged on the sliding seat (18), and the slip ring (12) is slidably arranged on the arc guide rail (13), and a fixing screw (11) is threadedly installed on the slip ring (12).

7. A passive wireless pulse current partial discharge sensor according to claim 6, characterized in that: The foldable shielding cover is composed of a plurality of metal shielding plates connected by hinges, one end of the foldable shielding cover is hinged to the top of the monitor (1), and the other end is connected to the rotating bracket (20) through a linkage rod; when the rotating bracket (20) rotates with the multi-angle adjustment structure, the linkage rod drives the foldable shielding cover to unfold or fold, so that the shielding surface of the foldable shielding cover always corresponds to the detection surface of the monitor (1), so as to enhance the shielding effect against electromagnetic interference.

8. The passive wireless pulse current partial discharge sensor according to claim 7, characterized in that: An electromagnetic wave absorbing layer is provided on the inner side of the foldable shielding cover, and the electromagnetic wave absorbing layer adopts a honeycomb structure; the linkage rod comprises an inner rod and an outer rod which are sleeved with each other, and a return spring is provided between the inner rod and the outer rod, and the return spring is used to provide a buffering force when the rotating bracket (20) rotates, so that the foldable shielding cover can be smoothly unfolded or folded, thereby achieving shielding of electromagnetic interference at different angles.

9. The passive wireless pulse current partial discharge sensor according to claim 1, characterized in that: The heat dissipation structure includes a plurality of air inlets (28) provided on the monitor (1), a plurality of mounting plates (23) being fixedly provided on one side of the monitor (1), a rotating shaft (24) being mounted between two corresponding mounting plates (23) for common rotation, a swing blade (25) being fixedly provided on each rotating shaft (24), and an incomplete gear (27) being fixedly provided on each rotating shaft (24).

10. The passive wireless pulse current partial discharge sensor according to claim 9, characterized in that: A through hole is provided on one side of the monitor (1), and two universal balls (21) are fixedly provided on the base plate (3). A gear rod (22) is rotatably provided on each of the two universal balls (21), and the gear rod (22) slides in the through hole to engage with a plurality of incomplete gears (27) for use.

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