Short-focus projection lens

Abstract

The utility model belongs to the technical field of optical projection lenses, and particularly relates to a short-focus projection lens, which comprises a first lens, a second lens and a third lens which are coaxially arranged along the reverse direction of light propagation, wherein the first lens is a convex-concave spherical lens with positive focal power, the second lens is a convex-concave aspheric lens with negative focal power, and the third lens is a concave-convex aspheric lens with positive focal power; the first lens and the short-focus projection lens meet the following relational expressions: 0.2 lt; f1 / flt; 0.8; the first lens, the second lens and the short-focus projection lens meet the following relational expression: 2lt; f12 / flt; 6; the third lens and the short-focus projection lens meet the following relational expressions: 0.7 lt; f3 / flt; and 1.5. According to the utility model, the focal length of the lens is greatly reduced on the premise that the number of lenses used in the lens is not increased, and the projection ratio of the lens is achieved; meanwhile, during focal length adjustment, the lens does not incline greatly, the accuracy of focal length adjustment is guaranteed, and pictures are displayed more clearly.

Description

Technical Field

[0001] This utility model belongs to the field of optical projection lens technology, specifically relating to a short-throw projection lens. Background Technology

[0002] As projectors become increasingly popular, the market demands higher quality projection lenses. Currently, projectors on the market are mainly divided into two types: DLP projectors and LCD projectors. DLP projectors are small in size, offer good color, and generally use short-throw lenses, but their overall cost is high, resulting in a low market share. LCD projectors have dominated the market in recent years, offering good performance, but their size is larger than DLP projectors. They generally use long-throw lenses (throw ratio 1.2~1.4), have fewer lens elements, and their overall lens design and manufacturing costs are lower.

[0003] Currently, due to the large size of LCDs, the manufacturing difficulty and cost of lens components are very high. If a short-throw lens is required, the number of lenses used increases significantly, making cost control difficult. Furthermore, because LCD projection lenses are too large to be interchangeable with common DLP micro-projection lenses, and the market demand for short-throw lenses is becoming increasingly strong, there is an urgent need to develop a short-throw optical projection lens that also has a cost advantage. Utility Model Content

[0004] The purpose of this invention is to provide a short-throw projection lens that can significantly reduce the focal length without increasing the number of lenses used in the lens, achieving a projection ratio of <0.9. At the same time, the lens will not tilt significantly when adjusting the focal length, ensuring the accuracy of the focal length adjustment and presenting a clearer image.

[0005] The specific technical solution adopted by this utility model is as follows:

[0006] A short-throw projection lens includes a first lens, a second lens, and a third lens coaxially arranged in opposite directions of light propagation;

[0007] Wherein, the first lens is a spherical lens with positive optical power, the second lens is an aspherical lens with negative optical power, and the third lens is an aspherical lens with positive optical power.

[0008] The first lens and the short-throw projection lens satisfy the following relationship: 0.2 < |f1 / f| < 0.8, where f1 is the focal length of the first lens and f is the focal length of the optical system of the short-throw projection lens;

[0009] The first lens, the second lens, and the short-throw projection lens satisfy the following relationship: 2 < |f12 / f| < 6, where f12 is the combined focal length of the first lens and the second lens, and f is the optical system focal length of the short-throw projection lens;

[0010] The third lens and the short-throw projection lens satisfy the following relationship: 0.7 < |f3 / f| < 1.5, where f3 is the focal length of the third lens and f is the focal length of the optical system of the short-throw projection lens.

[0011] As a preferred embodiment, the first lens has a positive focal length, the second lens has a negative focal length, and the third lens has a positive focal length.

[0012] As a preferred embodiment, the first lens is made of glass, the second lens is made of plastic, and the third lens is made of plastic.

[0013] As a preferred embodiment, the first lens has an Abbe number greater than 35 and a refractive index greater than 1.6, the second lens has an Abbe number less than 35, and the third lens has an Abbe number greater than 50.

[0014] As a preferred embodiment, the system further includes a front movable support frame for mounting the first lens and the second lens, and a rear movable support frame for mounting the third lens. A support housing assembly for adjusting the focal length between the third lens, the second lens, and the first lens is also provided between the front movable support frame and the rear movable support frame.

[0015] As a preferred embodiment, the support housing assembly includes a front housing and a rear housing fixed to the projector body. A front track frame and a rear track frame are fixedly installed between the front housing and the rear housing. A front push frame is rotatably connected to the front housing and rotatably sleeved on the front track frame. A rear push frame is rotatably connected inside the rear housing and rotatably sleeved on the rear track frame.

[0016] As a preferred embodiment, the outer surface of the front movable support frame is integrally formed with a first positioning leg and is surrounded by a first positioning frame. A gap is left between the outer surface of the first positioning leg and the inner surface of the first positioning frame, and a magnetic sheet of the same polarity is attached between their opposing surfaces. The front track frame is provided with a first track groove for limiting the sliding of the front movable support frame. The first positioning frame is slidably embedded in the first track groove. A first magnetic positioning groove is provided on the side wall of the first track groove. A first limiting magnetic metal is fixedly embedded in the outer surface of the first positioning frame and the first magnetic positioning groove. A first spiral pushing groove is provided on the inner wall of the front push frame for pushing the front movable support frame to move. The top surface of the first positioning leg is integrally formed with a columnar body that is slidably embedded in the first spiral pushing groove.

[0017] As a preferred embodiment, the outer surface of the rear movable support frame is integrally formed with a second positioning leg and a second positioning frame is sleeved around it. A gap is left between the outer surface of the second positioning leg and the inner surface of the second positioning frame, and a magnetic sheet of the same polarity is attached between their opposing surfaces. The rear track frame is provided with a second track groove for limiting the sliding of the rear movable support frame. The second positioning frame is slidably embedded in the second track groove. The side wall of the rear track frame is provided with a second magnetic positioning groove. The outer surface of the second positioning frame is adapted to and fixedly embedded with a second limiting magnetic metal. The inner wall of the rear push frame is provided with a second spiral pushing groove for pushing the rear movable support frame to move. The top surface of the second positioning leg is integrally formed with a columnar body that is slidably embedded in the second spiral pushing groove.

[0018] The technical effects achieved by this utility model are as follows:

[0019] This invention utilizes a combination of a first lens, a second lens, and a third lens, employing a mixture of glass spherical mirrors and plastic aspherical mirrors. Through the rational combination and optimization of materials, the focal length is reduced, achieving a projection ratio of approximately 1:1. This is about 1% less than the focal length of fixed-focus lenses in the prior art, resulting in an excellent experience of projecting a large image from a short distance while ensuring that the resolution meets imaging requirements.

[0020] This invention, by setting up a first positioning frame, a second positioning frame, and a first positioning foot in conjunction with the second positioning foot, utilizes the principle of repulsion between like poles of magnets to apply the same repulsive force around the front and rear movable support frames. When adjusting the relative focal length of the first, second, and third lenses, if the front and rear movable support frames tilt at an angle, the change in the repulsive force will readjust them to a balanced state. This ensures that the first, second, and third lenses remain perpendicular during adjustment, thereby guaranteeing the accuracy of focal length adjustment, reducing the impact of lens tilt on image presentation, and resulting in a clearer image. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the lens structure in an embodiment of this utility model;

[0022] Figure 2 This is a schematic diagram of the light refraction structure of the lens in an embodiment of this utility model;

[0023] Figure 3 This is a structural schematic diagram of the lens and related components in an embodiment of this utility model;

[0024] Figure 4 This is a utility model Figure 3 Side sectional view;

[0025] Figure 5 This is a utility model Figure 3 Exploded view;

[0026] Figure 6 This is a schematic diagram of the combined structure of the front movable support frame and the rear movable support frame in an embodiment of this utility model;

[0027] Figure 7 This is a utility model Figure 6 Exploded view;

[0028] Figure 8 This is a utility model Figure 6 Top view;

[0029] Figure 9 This is a utility model Figure 8 Cross-sectional view of section AA.

[0030] The attached diagram lists the components represented by each number as follows:

[0031] 1. First lens; 2. Second lens; 3. Third lens; 4. Front housing; 41. Positioning knob; 5. Rear housing; 6. Front push frame; 61. First spiral push groove; 7. Rear push frame; 71. Second spiral push groove; 72. Push rod; 8. Rear cover; 9. Front track frame; 91. First track groove; 92. First magnetic positioning groove; 10. Rear track frame; 101. Second track groove; 102. Second magnetic positioning groove; 11. Front movable support frame; 111. First positioning foot; 112. First positioning frame; 113. First limiting magnetic metal; 12. Rear movable support frame; 121. Second positioning foot; 122. Second positioning frame; 123. Second limiting magnetic metal; 13. Aperture; 14. Fresnel lens; 15. LCD display screen. Detailed Implementation

[0032] To make the objectives and advantages of this utility model clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the following text is merely used to describe one or more specific embodiments of this utility model and does not strictly limit the scope of protection specifically claimed by this utility model.

[0033] like Figures 1-9 As shown, a short-throw projection lens includes a first lens 1, a second lens 2, and a third lens 3 arranged coaxially in the opposite direction of light propagation. By combining the three lens groups, the focal length of the lens can be greatly reduced without increasing the number of lenses used in the lens, achieving a projection ratio of <0.9.

[0034] See attached document Figure 1The first lens 1 is a spherical lens with positive optical power, the second lens 2 is an aspherical lens with negative optical power, and the third lens 3 is an aspherical lens with positive optical power. The first lens 1 is made of glass, the second lens 2 is made of plastic, and the third lens 3 is made of plastic. The glass spherical lens can accurately transmit light, providing clearer and sharper image quality and reducing projection distortion and blurring. The plastic aspherical lens has a high degree of shape freedom, which can combine the mechanical components and optical components of the system, reducing the number of parts. By combining these features, the use of lenses can be minimized, and the focal length can be reduced.

[0035] Secondly, it should be noted that the focal length of the first lens 1 is positive, the focal length of the second lens 2 is negative, and the focal length of the third lens 3 is positive. The image side of the first lens 1 can be concave, convex, or flat. The object side of the second lens 2 can be convex, concave, or flat. The object side of the third lens 3 can be concave, convex, or flat.

[0036] In this embodiment, the image side of the first lens 1 is concave, the object side of the second lens 2 is convex, and the object side of the third lens 3 is concave. The side closer to the Fresnel lens 14 and the LCD display screen 15 is the image side, and the other side is the object side.

[0037] Based on the above structure, it can be understood that optical power is equal to the difference between the convergence of the image-side beam and the convergence of the object-side beam; it characterizes the ability of an optical system to deflect light rays. The larger the absolute value of the optical power, the stronger the bending ability of light rays; the smaller the absolute value, the weaker the bending ability. When the optical power is positive, the refraction of light rays is converging; when the optical power is negative, the refraction of light rays is diverging. Optical power can be used to characterize a single refractive surface of a lens, a single lens, or a system formed by multiple lenses.

[0038] See attached document Figures 6-7 In this embodiment, by fixing the first lens 1 and the second lens 2 in the front movable support frame 11, and fixing the third lens 3 in the rear movable support frame 12, and sliding the front movable support frame 11 and the rear movable support frame 12 in the front track frame 9 and the rear track frame 10 respectively, and fixing the front track frame 9 and the rear track frame 10 together to form a lens barrel, the imaging effect of the short-throw projection lens can be better by reasonably distributing the optical power of the lens.

[0039] Secondly, an aperture stop 13 is provided between the image side of the second lens 2 and the object side of the third lens 3, and the aperture stop 13 is fixedly installed between the front track frame 9 and the rear track frame 10 to limit the light beam passing through the projection lens, thereby controlling the direction and range of light propagation, which helps to ensure that only suitable light can enter the projection system to form a clear and accurate image.

[0040] See attached document Figures 4-5 In order to adjust the focal length between the third lens 3, the second lens 2, and the first lens 1, a front housing 4 and a rear housing 5 are fixed to the projector body. At the same time, a front track frame 9 and a rear track frame 10 are fixedly installed between the front housing 4 and the rear housing 5. A front push frame 6 is rotatably connected to the front housing 4 and rotatably sleeved on the front track frame 9. A rear push frame 7 is rotatably connected inside the rear housing 5 and rotatably sleeved on the rear track frame 10. By rotating the front push frame 6, the front movable support frame 11 can be pushed to slide relative to the front track frame 9. At the same time, by rotating the rear push frame 7, the rear movable support frame 12 can be pushed to slide relative to the rear track frame 10. This allows for the relative adjustment of the distance between the first lens 1, the second lens 2, and the third lens 3 installed on them, thereby adjusting the focal length.

[0041] See attached document Figures 4-9 The outer surface of the front movable support frame 11 is integrally formed with a first positioning foot 111. At the same time, the front track frame 9 is provided with a first track groove 91 for limiting the sliding of the front movable support frame 11. By sliding the first positioning foot 111 into the first track groove 91, and providing a first spiral push groove 61 for pushing the front movable support frame 11 to move on the inner wall of the front push frame 6, the top surface of the first positioning foot 111 is integrally formed with a columnar body that slides into the first spiral push groove 61. When the front push frame 6 is rotated, the first spiral push groove 61 can be used to push the columnar body to move along the groove inside, thereby pushing the front movable support frame 11 to slide along the first track groove 91, thereby driving the first lens 1 and the second lens 2 to move synchronously, so as to change the distance between the first lens 1, the second lens 2 and the third lens 3, and adjust the focal length. The front housing 4 is threadedly connected with a positioning knob 41. After the focal length is adjusted, the positioning knob 41 can be rotated to press against the front push frame 6 to position the adjusted components.

[0042] In this embodiment, three sets of the first spiral push groove 61, the first positioning support 111, and the first track groove 91 are arranged opposite each other to ensure stability during the lens displacement process; of course, in other embodiments, at least two sets of the first spiral push groove 61, the first positioning support 111, and the first track groove 91 are arranged opposite each other.

[0043] Furthermore, by circumferentially fitting a first positioning frame 112 onto the first positioning foot 111, and leaving a gap between the outer surface of the first positioning foot 111 and the inner surface of the first positioning frame 112, and attaching like-pole magnetic sheets between their opposing surfaces, the same repulsive force can be generated. Simultaneously, the first positioning frame 112 is slidably embedded into the first track groove 91, and a first magnetic positioning groove 92 is formed on the side wall of the first track groove 91. A first limiting magnetic metal 113 is fixedly embedded on the outer surface of the first positioning frame 112, matching and fitting the first magnetic positioning groove 92. The magnetic attraction between the first limiting magnetic metal 113 and the first magnetic positioning groove 92 allows the first positioning frame 112 to stably move within the first track groove 91, ensuring... To ensure the stability of focus adjustment, when the first positioning frame 112 tilts during adjustment (i.e., when the lens tilts), the gap between the outer surface of the first positioning foot 111 and the inner surface of the first positioning frame 112 changes, and the magnetic repulsion force in that direction changes, breaking the balance between the two sides. When the repulsion force on one side increases, it will be pushed in the opposite direction, eventually pushing both sides back to balance, thus maintaining the relative balance between the first positioning foot 111 and the first positioning frame 112. This ensures that the first lens 1 and the second lens 2 remain vertical during adjustment, thereby guaranteeing the accuracy of focus adjustment, reducing the impact of lens tilt on image presentation, and resulting in a clearer image.

[0044] Please refer to the appendix again. Figures 4-9 The rear movable support frame 12 has a second positioning foot 121 integrally formed on its outer surface. At the same time, the rear track frame 10 has a second track groove 101 for limiting the sliding of the rear movable support frame 12, and the second positioning foot 121 is slidably embedded in the second track groove 101. The rear push frame 7 has a second spiral push groove 71 for pushing the rear movable support frame 12 to move, and a columnar body is integrally formed on the top surface of the second positioning foot 121 and slidably embedded in the second spiral push groove 71. By rotating the rear push frame 7 using the push rod 72 fixedly installed on the rear push frame 7, the columnar body can be pushed to move along the groove inside by the second spiral push groove 71, thereby pushing the rear movable support frame 12 to slide along the second track groove 101, thereby driving the third lens 3 to move synchronously, so as to change the distance between the first lens 1, the second lens 2 and the third lens 3, and adjust the focal length.

[0045] In this embodiment, three sets of the second spiral push groove 71, the second positioning support 121, and the second track groove 101 are arranged opposite each other to ensure stability during the lens displacement process; of course, in other embodiments, at least two sets of the second spiral push groove 71, the second positioning support 121, and the second track groove 101 are arranged opposite each other.

[0046] Furthermore, by having a second positioning frame 122 encircled by the rear movable support frame 12, and leaving a gap between the outer surface of the second positioning leg 121 and the inner surface of the second positioning frame 122, and attaching like-pole magnetic sheets between their opposing surfaces, the same repulsive force can be generated. Simultaneously, the second positioning frame 122 is slidably embedded into the second track groove 101, and a second magnetic positioning groove 102 is formed on the side wall of the rear track frame 10. A second limiting magnetic metal 123 is fixedly embedded in the outer surface of the second positioning frame 122 and the second magnetic positioning groove 102, and the magnetic attraction between the second limiting magnetic metal 123 and the second magnetic positioning groove 102 allows the second positioning frame 122 to stably position itself in the second track groove 101. The movement ensures the stability of the focus adjustment. During adjustment, when the second positioning frame 122 tilts (i.e., when the lens tilts), the gap between the outer surface of the second positioning foot 121 and the inner surface of the second positioning frame 122 changes, causing a change in the magnetic repulsion force in that direction. This disrupts the balance between the two sides. When the repulsion force on one side increases, it pushes the lens in the opposite direction, eventually restoring balance to both sides. This maintains the relative balance between the second positioning foot 121 and the second positioning frame 122, ensuring the third lens 3 remains vertical during adjustment. This guarantees the accuracy of the focus adjustment, reduces the impact of lens tilt on image presentation, and results in a clearer image.

[0047] Based on the above structure, when needed, the distance between the first lens 1, the second lens 2, and the third lens 3 can be adjusted relative to each other by rotating the front push frame 6 or the rear push frame 7. At the same time, the distance between the first lens 1, the second lens 2, the third lens 3, and the aperture 13 can be adjusted, thereby adjusting the projection size and sharpness to suit different usage scenarios and improve the flexibility of the projection system.

[0048] Secondly, during the focus adjustment process, the principle of repulsion between like poles of magnets is utilized. The same repulsive force is applied around the front movable support frame 11 and the rear movable support frame 12. When the relative focus of the first lens 1, the second lens 2, and the third lens 3 is adjusted, the change in the repulsive force can be used to readjust them to a balanced state when the front movable support frame 11 and the rear movable support frame 12 are tilted at an angle. This keeps the first lens 1, the second lens 2, and the third lens 3 in a vertical state during the adjustment process, thereby ensuring the accuracy of focus adjustment, reducing the impact of lens tilt on image presentation, and making the image clearer.

[0049] See attached document Figures 3-4The rear track frame 10 extends to one end of the rear housing 5 and is detachably fitted with a rear cover 8, which is integrated with the rear housing 5 to ensure the integrity of the entire projection lens housing and to provide complete protection for the internal lens, preventing external dust or impurities from entering the projection lens and affecting the image presentation.

[0050] See attached document Figures 1-2 The first lens 1 and the short-throw projection lens satisfy the following relationship: 0.2 < |f1 / f| < 0.8, where f1 is the focal length of the first lens 1 and f is the focal length of the optical system of the short-throw projection lens; the first lens 1 is used to collect light and is suitable for lenses with a large field of view. At the same time, since the shape of the lens has an important influence on distortion and imaging effect, in order to improve the imaging effect, the design value of the first lens 1 needs to satisfy 4.5 < |CT1 / ET1| < 6, where CT1 is the center thickness of the first lens 1 and ET1 is the edge thickness of the first lens 1 along the axial direction.

[0051] Furthermore, the first lens 1, the second lens 2, and the short-throw projection lens satisfy the following relationship: 2 < |f12 / f| < 6, where f12 is the combined focal length of the first lens 1 and the second lens 2, and f is the focal length of the optical system of the short-throw projection lens. Within this range, aberrations such as spherical aberration, coma, astigmatism, and field curvature can be well corrected. The second lens 2 is a plastic aspherical lens. In order to ensure that the second lens 2 has good manufacturability, it is required that 1.8 < |ET2 / CT2| < 3, where CT2 is the center thickness of the second lens 2, and ET2 is the edge thickness of the second lens 2 along the axial direction.

[0052] Furthermore, the third lens 3 and the short-throw projection lens satisfy the following relationship: 0.7 < |f3 / f| < 1.5, where f3 is the focal length of the third lens 3 and f is the focal length of the optical system of the short-throw projection lens, in order to correct residual aberrations.

[0053] In this embodiment, the Abbe number of the first lens 1 is greater than 35 and the refractive index is greater than 1.6, the Abbe number of the second lens 2 is less than 35, and the Abbe number of the third lens 3 is greater than 50, in order to reduce chromatic dispersion and thus improve image quality; of course, in other embodiments, the selection can be made according to specific usage requirements.

[0054] The above description is merely a preferred embodiment of this utility model. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principle of this utility model, and these improvements and modifications should also be considered within the scope of protection of this utility model. Structures, devices, and operating methods not specifically described or explained in this utility model, unless otherwise specified or limited, shall be implemented using conventional methods in the field.

Claims

1. A short-throw projection lens, characterized in that, It includes a first lens (1), a second lens (2), and a third lens (3) arranged coaxially in the opposite direction of light propagation; Wherein, the first lens (1) is a spherical lens with positive optical power, the second lens (2) is an aspherical lens with negative optical power, and the third lens (3) is an aspherical lens with positive optical power. The first lens (1) and the short-throw projection lens satisfy the following relationship: 0.2 < |f1 / f| < 0.8, where f1 is the focal length of the first lens (1) and f is the focal length of the optical system of the short-throw projection lens; The first lens (1), the second lens (2) and the short-throw projection lens satisfy the following relationship: 2<|f12 / f|<6, where f12 is the combined focal length of the first lens (1) and the second lens (2), and f is the optical system focal length of the short-throw projection lens; The third lens (3) and the short-throw projection lens satisfy the following relationship: 0.7 < |f3 / f| < 1.5, where f3 is the focal length of the third lens (3) and f is the focal length of the optical system of the short-throw projection lens.

2. A short-throw projection lens according to claim 1, characterized in that: The focal length of the first lens (1) is positive, the focal length of the second lens (2) is negative, and the focal length of the third lens (3) is positive.

3. A short-throw projection lens according to claim 1, characterized in that: The first lens (1) is made of glass, the second lens (2) is made of plastic, and the third lens (3) is made of plastic.

4. A short-throw projection lens according to claim 1, characterized in that: The first lens (1) has an Abbe number greater than 35 and a refractive index greater than 1.6, the second lens (2) has an Abbe number less than 35, and the third lens (3) has an Abbe number greater than 50.

5. A short-throw projection lens according to claim 1, characterized in that: It also includes a front movable support frame (11) for mounting the first lens (1) and the second lens (2) and a rear movable support frame (12) for mounting the third lens (3). A support housing assembly for adjusting the focal length between the third lens (3), the second lens (2) and the first lens (1) is also provided between the front movable support frame (11) and the rear movable support frame (12).

6. A short-throw projection lens according to claim 5, characterized in that: The supporting housing assembly includes a front housing (4) and a rear housing (5) fixed to the projector body. A front track frame (9) and a rear track frame (10) are fixedly installed between the front housing (4) and the rear housing (5). A front push frame (6) is rotatably connected to the front housing (4) and rotatably sleeved on the front track frame (9). A rear push frame (7) is rotatably connected inside the rear housing (5) and rotatably sleeved on the rear track frame (10).

7. A short-throw projection lens according to claim 6, characterized in that: The outer surface of the front movable support frame (11) is integrally formed with a first positioning foot (111) and a first positioning frame (112) is sleeved around it. There is a gap between the outer surface of the first positioning foot (111) and the inner surface of the first positioning frame (112), and a magnetic sheet of the same polarity is attached between the two opposing surfaces. The front track frame (9) is provided with a first track groove (91) for limiting the sliding of the front movable support frame (11). The first positioning frame (112) is slidably embedded in the first track groove (91). The side wall of the first track groove (91) is provided with a first magnetic positioning groove (92). The outer surface of the first positioning frame (112) is adapted to and fixedly embedded with the first magnetic positioning groove (92) with a first limiting magnetic metal (113). The inner wall of the front push frame (6) is provided with a first spiral push groove (61) for pushing the front movable support frame (11) to move. The top surface of the first positioning foot (111) is integrally formed with a columnar body that is slidably embedded in the first spiral push groove (61).

8. A short-throw projection lens according to claim 6, characterized in that: The outer surface of the rear movable support frame (12) is integrally formed with a second positioning foot (121), and a second positioning frame (122) is sleeved around it. A gap is left between the outer surface of the second positioning foot (121) and the inner surface of the second positioning frame (122), and a magnetic sheet of the same polarity is attached between their opposing surfaces. The rear track frame (10) is provided with a second track groove (101) for limiting the sliding of the rear movable support frame (12), and the second positioning frame (122) is slidably embedded in the second track groove (101). 01) Inside, the side wall of the rear track frame (10) is provided with a second magnetic positioning groove (102), the outer surface of the second positioning frame (122) is adapted to and fixedly embedded with the second magnetic positioning groove (102) and a second limiting magnetic metal (123). The inner wall of the rear push frame (7) is provided with a second spiral push groove (71) for pushing the rear movable support frame (12) to move. The top surface of the second positioning foot (121) is integrally formed with a columnar body that slides and is embedded in the second spiral push groove (71).

Images