Universal pneumatic self-centering gravity unloading device

Abstract

The application provides a universal pneumatic self-centering gravity unloading device, which comprises a bottom plate, a lining plate, a cylinder, a support column, a guide column, a displacement sensor, a lifting motor and a gas supply control cabinet, the bottom plate supports the lining plate, and the cylinder, the support column, the guide column, the displacement sensor and the lifting motor are installed on the bottom plate; the cylinder is connected with the gas control system through a gas circuit and is used for providing unloading support force; the three guide columns are uniformly distributed at the edge of the bottom plate with a separation of 120 degrees, so as to limit the position of the lining plate; the displacement sensor and the lifting motor are uniformly distributed on the bottom plate with a separation of 120 degrees, and the position of the lifting motor is consistent with the position of the mirror nest; the support column is used for supporting the lining plate on the bottom plate, the lining plate supports the mirror, and the lining plate is processed with through holes corresponding to the positions of the cylinder, the guide column, the displacement sensor and the lifting motor; the application meets the vertical detection requirement of the optical axis of the mirror with an aperture of 4m, the unloading force is accurately controllable, the centering speed of the mirror on the device is fast, the accuracy is high, and the detection efficiency is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a gravity unloading technology in the field of optical processing detection, in particular to a gravity unloading device for large-aperture mirrors in the surface shape precision detection in the polishing stage. BACKGROUND

[0002] The space large-aperture reflecting primary mirror is easily formed with non-manufacturing deformation errors of the mirror surface under the influence of factors such as gravity and stress on the ground, thereby causing aberrations such as spherical aberration, coma, and astigmatism, which will make the mirror surface of the reflecting mirror deviate from the design and processing requirements of the optical system, and directly affect the imaging effect of the space camera. In order to keep the consistency of the detection precision of the reflecting mirror on the ground and the optical precision in the space use environment, and to realize the effective evaluation of the processing quality, the gravity unloading technology of the space large-aperture reflecting primary mirror needs to be researched.

[0003] In the process of ground processing, detection and adjustment, the space large-aperture reflecting primary mirror is usually in two states of optical axis horizontal and optical axis vertical, and the corresponding gravity unloading is also divided into two categories of axial support and radial support, which are respectively used to bear the gravity of the space large-aperture reflecting primary mirror in the optical axis vertical and horizontal states. According to whether the support force can be actively adjusted, the support can be divided into passive support and active support. The support point distribution of the passive support is static and easy to calculate, and the support force distribution can only obey some simple laws without adjustment. The support force of the active support can be adjusted, but the support unit structure is complex. In the past design, in order to realize high-precision unloading, the active form is usually selected, and the unloading mechanism needs to be customized according to the geometric parameters and structural characteristics of the reflecting mirror, which has a narrow application range and is not universal, and the development cost is expensive. Therefore, it is urgent to develop a high-precision universal gravity unloading device to save cost and shorten the model development cycle. SUMMARY

[0004] The technical problem to be solved by the present application is to overcome the shortcomings of the prior art and provide a universal pneumatic self-centering gravity unloading device which meets the optical axis vertical detection requirements of reflecting mirrors with an aperture of 4m or less and has accurately controllable unloading force.

[0005] The technical solution of the present application is: a general type of pneumatic self-centering gravity unloading device, comprising a bottom plate, a lining plate, N air cylinders, M support columns, 3 guide columns, 3 displacement sensors, 3 lifting motors and a gas supply control cabinet, N>1, M>1; the air cylinder, the support column, the guide column, the displacement sensor and the lifting motor are all installed on the bottom plate, the support column is located between the bottom plate and the lining plate and is used to support the lining plate, the mirror is placed on the lining plate, the top of the lifting motor is designed with a contact disc, the lifting motor passes through the through hole on the lining plate and contacts the back of the mirror through the contact disc, the air cylinder is connected with the gas supply control cabinet after gas circuit collection, the gas supply control cabinet supplies gas to each gas circuit, and is used for unloading the gravity of the mirror; the displacement sensor is used to measure the specific height of the mirror lifted away from the lining plate.

[0006] The 3 guide columns are uniformly distributed on the edge circumference of the bottom plate, and constrain the position of the lining plate, so that the lining plate is coaxial with the bottom plate.

[0007] The 3 lifting motors are uniformly distributed on the other circumference of the bottom plate, when the mirror is placed, the attitude of the mirror is adjusted by adjusting the height of the three lifting motors, so that the axis of the mirror and the axis of the bottom plate coincide, the coaxiality between the mirror and the bottom plate is ensured, and the mirror is quickly and accurately centered on the device.

[0008] Further, the air cylinder, the support column, the guide column, the displacement sensor and the lifting motor are installed on the bottom plate by screwing.

[0009] Further, the number of the air cylinders is consistent with the number of support points required when the mirror is unloaded, the positions of the air cylinders and the support points correspond one by one, the number of support points required when the mirror is unloaded and the support force corresponding to each support point are determined through theoretical simulation calculation, and the method is:

[0010] S1, a mirror simulation analysis model is established, and the positions of the support points are selected at the entire mirror support ribs, under the action of the support points, the mirror surface unloading deformation and unloading force under 1g gravity are calculated through a simulation software Hypermesh, the coordinate data and deformation data of each node in the simulation result are exported and converted into a standard surface shape file of MetroPro software, and MetroPro software outputs specific unloading surface shape precision values.

[0011] S2, when the unloading surface shape precision does not meet the design requirement, the positions and number of the unloading support points are changed, the process of step S1 is repeated, and after the selected unloading support points meet the design requirement of the unloading surface shape precision, step S3 is entered;

[0012] S3, when the unloading surface shape precision meets the design requirements, the gas circuit is collected according to the size of the unloading force of each support point, starting from the minimum value of the unloading force, taking 1N as the interval of the gas circuit collection, collecting all the support points into different gas circuits, then applying the unloading force values of the collected gas circuits to each gas circuit of the mirror model for unloading force review calculation to evaluate whether the unloading surface shape precision meets the requirements;

[0013] S4, if the unloading surface shape precision of step 3 does not meet the requirements, taking 0.5N as the interval of the gas circuit collection, collecting all the support points into different gas circuits, re-collecting the gas circuit, performing unloading force review calculation, and evaluating whether the unloading surface shape precision meets the requirements; if the unloading surface shape precision still does not meet the requirements, the position and number of the unloading support points are reselected, the unloading force and the unloading surface shape precision values are obtained by using the simulation software, and steps S2-S4 are repeated until the unloading surface shape precision meets the requirements, and the unloading support points and the corresponding unloading force are determined.

[0014] Further, the method of gas circuit collection is that the gas cylinders in the same gas circuit are connected in series; all the gas cylinders are collected into H gas circuits and connected to the gas supply control cabinet, the gas supply control cabinet supplies gas to each gas circuit, and the unloading support force provided by the gas cylinders in the same gas circuit is the arithmetic mean of all the unloading support forces in the gas circuit, and H>1.

[0015] Further, the gas cylinders in the same gas circuit are connected in series through the gas pipes and T-shaped three-way valves, and the gas circuit connection adopts the quick plug-in and quick plug-out mode: each gas cylinder is connected to the middle valve port of each T-shaped three-way valve through a gas pipe, the left valve port of each T-shaped three-way valve is connected to the right valve port of the left adjacent three-way valve through a gas pipe, and the right valve port of each T-shaped three-way valve is connected to the left valve port of the right adjacent three-way valve through a gas pipe, so that the gas cylinders are connected in series; when the loop is formed, an additional T-shaped three-way valve is connected to the gas supply control cabinet: the middle valve port of the additional T-shaped three-way valve is connected to the gas supply control cabinet through a gas pipe, the left valve port is connected to the right valve port of the left adjacent three-way valve, and the right valve port is connected to the left valve port of the right adjacent three-way valve.

[0016] Further, the liner plate is processed with through holes corresponding to the positions of the gas cylinders, guide columns, displacement sensors and lifting motors; during the surface shape test, the upper end of the gas cylinder penetrates through the through hole of the liner plate 2 to contact the back of the mirror, and the mirror is supported to be separated from the liner plate to realize gravity unloading.

[0017] Further, the front surface of the liner plate is pasted with a rubber pad, the back surface of the liner plate is a plane, and the front surface of the liner plate is designed in the same shape as the supported mirror to ensure that the liner plate is closely attached to the back surface of the mirror.

[0018] Further, when the liner plate is placed, the three through holes on the liner plate matched with the guide columns are embedded with the three guide columns one by one to ensure the coaxiality between the liner plate and the bottom plate.

[0019] The gravity unloading device and the method for quickly and accurately centering a mirror according to the present application, when the back of the mirror is open, comprise the following steps:

[0020] S1. The back of the mirror has mounting hole positions, and three nests are selected and mounted on the same circumference at intervals of 120°, the positions of the nests correspond to the positions of the lifting motors mounted on the base plate, and the three nests are located at the vertices of an inscribed equilateral triangle in the circumference, thereby achieving three-point automatic centering; the determined center of the mirror coincides with the center of the base plate, and the center axes of the mirror and the base plate coincide, thereby ensuring the coaxiality of the mirror and the base plate.

[0021] S2. The lifting motor is raised, the contact disc on the lifting motor is attached to the nest on the back of the mirror, and then the lifting motor is slowly lowered, so that the mirror falls on the backing plate.

[0022] The gravity unloading device and the method for quickly and accurately centering a mirror according to the present application, when the back of the mirror is closed, comprise the following steps:

[0023] S1. A laser tracker is used in combination with a two-dimensional adjustment platform to adjust: the gravity unloading device is placed on the two-dimensional adjustment platform, within the irradiation range of the laser tracker, P target ball seats are attached to the cylindrical surface of the mirror at intervals, Q target ball seats are attached to the cylindrical surface of the base plate at intervals, there are target balls on the target ball seats, P < 10, and Q > 10.

[0024] S2. In the state of hoisting the mirror, the center axes of the mirror and the base plate are tested by using the laser tracker, the position of the gravity unloading device is adjusted by adjusting the two-dimensional platform, so that the center axes of the mirror and the base plate coincide.

[0025] S3. The lifting motor is raised, the contact disc on the lifting motor is attached to the back of the mirror, and the entire mirror is supported, at which time the hoisting tool is removed, and then the lifting motor is slowly lowered, so that the mirror falls on the backing plate.

[0026] Compared with the prior art, the present application has the following advantages:

[0027] 1. In the present application, the cylinder, the support column, the guide column, the displacement sensor and the lifting motor are universal for mirrors of different diameters, which realizes universal design of the mirror gravity unloading device, can meet the vertical detection requirements of mirrors with diameters of 4 m or less, and effectively shortens the development cycle and cost.

[0028] 2) The gravity unloading device designed in the application has accurate controllable unloading force, three evenly distributed guide columns with a separation of 120° can achieve high repeated positioning accuracy, and through selection and optimization of the number and position of unloading support points, the design requirement that the unloading surface shape accuracy is better than 1 / 100λ can be met, and the unloading accuracy is high.

[0029] 3) The application is simple to install, and all air path connections adopt a quick plug-in and quick pull-out mode, so the installation efficiency is high.

[0030] 4) Two schemes for correctly placing the mirror on the backing plate according to whether there are mounting holes on the back of the mirror are designed, which realizes fast centering speed and high centering accuracy of the mirror on the application, and improves the detection efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 It is a structure diagram of a general type pneumatic self-centering gravity unloading device of the embodiment

[0032] Figure 2 It is a design flow chart of mirror mechanical simulation analysis of the embodiment

[0033] Figure 3 It is an overall appearance diagram of a general type pneumatic self-centering gravity unloading device of the embodiment

[0034] Figure 4 It is a schematic diagram of the self-adaptive principle of the tangency of the conical surface and the spherical surface of the embodiment

[0035] Figure 5 It is a centering scheme schematic diagram of a back closed type mirror of the embodiment DETAILED DESCRIPTION

[0036] The application will be further described in combination with the drawings: the specific structure of a general type pneumatic self-centering gravity unloading device is shown in Figure 1 , which comprises a bottom plate 1, a backing plate 2, a gas cylinder 3, a support column 4, three guide columns 5, three displacement sensors 6, three lifting motors 7 and a gas supply control cabinet, and the gas cylinder 3, the support column 4, the guide column 5, the displacement sensor 6 and the lifting motor 7 are all installed on the bottom plate 1 by screwing.

[0037] The gas cylinder 3 is a unit for providing unloading support force, the position and number of the gas cylinder 3 correspond to the position and number of the support points required during unloading of the mirror, considering the engineering practicability, if there are dozens or even hundreds of unloading support points, it is impossible to control all the gas cylinders individually, so gas path collection must be performed; after the gas path collection of the gas cylinder 3, it is connected with the gas supply control cabinet, the gas supply control cabinet supplies gas to each gas path, and the gas cylinders in the same gas path provide the same support force.

[0038] As shown in Figure 2 , the determination method of the support points required during unloading of the mirror and the corresponding unloading force is:

[0039] S1, a mirror simulation analysis model is established, and support points are uniformly selected at the positions of the entire mirror support ribs according to experience. At this time, the number of selected support points is small, and is approximately ten to several tens. Under the action of the support points fixed support, the mirror surface unloading deformation and unloading force under 1g gravity are calculated by the simulation software Hypermesh. The coordinate data and deformation data of each node of the simulation result are exported and converted into a standard surface shape file of MetroPro software. The MetroPro software outputs specific unloading surface shape precision values.

[0040] S2, when the unloading surface shape precision does not meet the design requirements, the positions and number of unloading support points are changed, and the process of step 1 is repeated until the selected unloading support points can meet the design requirements of the unloading surface shape precision.

[0041] When the unloading surface shape precision meets the design requirements, the gas circuits of the gas cylinder 3 are collected according to the size of the unloading force of each support point. Starting from the minimum value of the unloading force, the unloading force is collected into different gas circuits with 1N as the separation interval of the gas circuit collection. After the collection of the gas circuit, each unloading force value is applied to each gas circuit of the mirror model for unloading force review calculation, and whether the unloading surface shape precision meets the requirements is evaluated.

[0042] S4, if the unloading surface shape precision of step 3 does not meet the requirements, the support points are collected into different gas circuits with 0.5N as the separation interval of the gas circuit collection, the gas circuit collection is re-performed, the unloading force review calculation is performed, and whether the unloading surface shape precision meets the requirements is evaluated. If the unloading surface shape precision still does not meet the requirements, the positions and number of unloading support points are re-selected, the simulation software is used to obtain the unloading force and the unloading surface shape precision value, steps S2-S4 are repeated, and the unloading surface shape precision is determined until the unloading surface shape precision meets the requirements, and the unloading support points and the corresponding unloading force are determined.

[0043] The selection and optimization of the support points aim to meet the required unloading requirements using the least unloading support points.

[0044] After the positions and number of the unloading support points are obtained, the installation number and installation position of the gas cylinder 3 are determined.

[0045] In the gas circuit collection, the air cylinders 3 in the same gas circuit are connected in series through the air pipes and the T-shaped three-way valves. In the embodiment, the selected T-shaped three-way valve is T-shaped three-way PE-6mm; the gas circuit connection adopts the quick plug-in and quick pull-out mode: each air cylinder 3 is connected with the middle valve port of each T-shaped three-way valve through the air pipe, the left valve port of each T-shaped three-way valve is connected with the right valve port of the left adjacent three-way valve through the air pipe, and the right valve port of each T-shaped three-way valve is connected with the left valve port of the right adjacent three-way valve through the air pipe, so as to realize the series connection between the air cylinders 3; when the loop is formed, a T-shaped three-way valve needs to be added and connected to the gas supply control cabinet: the middle valve port of the added T-shaped three-way valve is connected to the gas supply control cabinet through the air pipe, the left valve port is connected to the right valve port of the left adjacent three-way valve, and the right valve port is connected to the left valve port of the right adjacent three-way valve.

[0046] According to the specific position of the unloading support point and the size of the mirror, the production bottom plate 1 and the lining plate 2 can be designed.

[0047] The three guide columns 5 are uniformly distributed on the same circumference of the edge of the bottom plate 1 at intervals of 120°, the hole positions matched with the guide columns 5 have been processed in advance on the lining plate 2, and the guide columns 5 fix the lining plate 2 to the bottom plate 1; through the machining precision, the positioning precision error between the lining plate 2 and the bottom plate 1 can be ensured to be within ±0.5mm each time. The three displacement sensors 6 are uniformly distributed at intervals of 120° on another circumference of the bottom plate 1 and are used to measure the specific height of the mirror lifted away from the lining plate. The three lifting motors 7 are also uniformly distributed at intervals of 120° on a certain circumference of the bottom plate 1, and the height between the mirror and the bottom plate 1 is determined by the lifting motor 7. When the mirror is placed, the attitude of the mirror is adjusted by adjusting the height of the three lifting motors 7, so that the axis of the mirror and the axis of the bottom plate 1 coincide, the coaxiality between the mirror and the bottom plate 1 is ensured, and the mirror is quickly and accurately centered on the device. The support column 4 realizes the connection between the lining plate 2 and the bottom plate 1, the lining plate 2 is placed on the support column 4, the number of the support column 4 is selected according to the weight and size of the mirror, the position of the support column 4 can be any position on the bottom plate 1 without installed components, and the position of the support column 4 is preferably uniformly distributed.

[0048] The back of the backing plate 2 is a plane, and the front is designed into a corresponding shape according to the supported mirror: a plane or a curved surface, to ensure good fit with the back of the mirror; according to the simulation unloading point position, corresponding holes are arranged on the backing plate, and the air cylinder 3 can contact the back of the mirror through the holes during surface testing, so that the mirror is supported to separate from the backing plate, and gravity unloading is realized. In addition, the front of the backing plate 2 is attached with a rubber pad, which serves as a buffer when the mirror is placed. The backing plate 2 can also be used in the machining process as a tool for supporting the mirror, and the backing plate is hoisted and placed on the machining platform together, to ensure the safety and reliability of the mirror during hoisting. The backing plate 2 is also processed with through holes corresponding to the guide columns 5, displacement sensors 6 and lifting motors 7; each hole position has three.

[0049] Figure 3 It is an overall appearance view of the gravity unloading device of the application. When the backing plate 2 is placed, the three through holes on the backing plate 2 matched with the guide columns 5 and the three guide columns 5 are embedded one by one, to ensure the coaxiality between the backing plate 2 and the bottom plate 1.

[0050] The mirror is placed on the backing plate 2. How to correctly place the mirror on the backing plate 2 to ensure the coaxiality between the mirror and the bottom plate 1 and realize the rapid centering of the mirror is completed by the lifting motor 7. Two schemes are given according to whether the back of the mirror has a nested hole:

[0051] Scheme one, the back of the mirror is open, the back of the mirror has a mounting hole position, and the mirror centering steps are as follows:

[0052] S1, the back of the mirror has a mounting hole position, three holes on the same circumference are selected to be equipped with a nested hole, the position of the nested hole corresponds to the position of the lifting motor 7 mounted on the bottom plate 1, the three nested holes are located at the vertices of the inscribed equilateral triangle in the circumference, to realize three-point automatic centering; the determined center of the mirror coincides with the center of the bottom plate 1, and then the center axis of the mirror coincides with that of the bottom plate 1, to ensure the coaxiality of the mirror and the bottom plate 1;

[0053] S2, the lifting motor 7 is raised, the contact disc on the lifting motor 7 is attached to the nested hole on the back of the mirror, and then the lifting motor 7 is slowly lowered, so that the mirror falls on the backing plate 2.

[0054] As shown in Figure 4 In this embodiment, the nested hole mounted on the back of the mirror and the contact disc mounted on the lifting motor 7 satisfy the tangent self-adapting principle of the conical surface and the spherical surface when designed, specifically: the mating interface on the nested hole and the lifting motor 7 is designed as a conical surface, and the contact disc on the lifting motor 7 matched with the nested hole is designed as a spherical surface, so that the nested hole and the contact disc are tangent at the contact surface, to enhance the stability of the attachment.

[0055] Scheme two, the back of the mirror is closed, cannot install the nest, the mirror centering steps are as follows:

[0056] S1, use laser tracker to adjust two-dimensional adjustment platform: the whole gravity unloading device is placed on the two-dimensional adjustment platform, in the laser tracker irradiation surface range, 20 target balls and target ball seats are spaced and adhered on the cylindrical surface of the mirror, 20 target ball seats are spaced and adhered on the cylindrical surface of the bottom plate 1;

[0057] S2, in the state of hoisting the mirror, use the laser tracker to test the center axis of the mirror and the bottom plate 1 respectively, adjust the two-dimensional platform to adjust the position of the gravity unloading device, so that the center axes of the mirror and the bottom plate 1 coincide;

[0058] S3, at this time, the lifting motor 7 is raised, the contact disc on the lifting motor 7 is in contact with the back of the mirror, supporting the whole mirror, at this time, the hoisting tool is removed, and the lifting motor 7 is slowly lowered, so that the mirror falls on the lining plate 2.

[0059] At this time, when the contact disc of the lifting motor 7 is designed, the upper surface of the contact disc is the same as the shape of the back of the mirror, so that the lifting motor 7 can be attached to the mirror through the contact disc.

[0060] Finally, according to the unloading support force value required by each gas path given by the simulation calculation, the gas supply control cabinet is used to supply gas to each gas path to reach the required force value for unloading, so that the mirror is separated from the lining plate 2, the surface unloading is realized, and the displacement sensor 6 can monitor the specific height of the mirror rising. It should be noted that before use, the performance calibration test of the gas cylinder 3 is carried out: through the mechanical sensor and the pressure sensor, the force-gas pressure, force-position relationship curve of each gas cylinder 3 is tested, in order to reduce the error, the gas cylinders with the difference of the force-gas pressure, force-position curve coefficient within 0.1 are placed in the same gas path.

[0061] In the gravity unloading device, the gas cylinder 3, the support column 4, the guide column 5, the displacement sensor 6 and the lifting motor 7 are universal for different caliber mirrors and can be repeatedly used. Only the bottom plate and the lining plate need to be designed according to the corresponding mirror, according to the simulation analysis of the specific shape of the mirror and the unloading point, the design of the specific appearance shape and size of the connecting hole of the bottom plate 1 and the lining plate 2 is carried out.

Claims

1. A universal pneumatic self-centering gravity unloading device, characterized by: The device comprises a bottom plate (1), a backing plate (2), N cylinders (3), M support columns (4), 3 guide columns (5), 3 displacement sensors (6), 3 lifting motors (7) and a gas supply control cabinet, N>1, M>1; The cylinder (3), the support column (4), the guide column (5), the displacement sensor (6) and the lifting motor (7) are all installed on the bottom plate (1), the support column (4) is located between the bottom plate (1) and the backing plate (2) and is used for supporting the backing plate (2), a mirror is placed on the backing plate (2), the top of the lifting motor (7) is designed with a contact disc, the lifting motor (7) passes through a through hole on the backing plate (2) and contacts the back of the mirror through the contact disc, the cylinder (3) is connected with the gas supply control cabinet after gas circuit collection, the gas supply control cabinet supplies gas to each gas circuit and is used for unloading the gravity of the mirror; the displacement sensor (6) is used for measuring the specific height of the mirror lifted away from the backing plate; The 3 guide columns (5) are uniformly distributed on the edge circumference of the bottom plate (1) and constrain the position of the backing plate (2), so that the backing plate (2) is coaxial with the bottom plate (1); The 3 lifting motors (7) are uniformly distributed on another circumference of the bottom plate (1), when the mirror is placed, the height of the three lifting motors (7) is adjusted, the posture of the mirror is adjusted so that the axis of the mirror and the axis of the bottom plate (1) coincide, the coaxiality between the mirror and the bottom plate (1) is ensured, and the mirror is quickly and accurately centered on the device.

2. A universal pneumatic self-centering gravity unloading device according to claim 1, characterized in that: The cylinder (3), the support column (4), the guide column (5), the displacement sensor (6) and the lifting motor (7) are installed on the bottom plate (1) by screwing.

3. A universal pneumatic self-centering gravity unloading device according to claim 1, characterized in that: The number of the cylinder (3) is consistent with the number of support points required when the mirror is unloaded, the position of the cylinder (3) and the position of the support point correspond one by one, the number of support points required when the mirror is unloaded and the support force corresponding to each support point are determined through theoretical simulation calculation, and the method is as follows: S1, a mirror simulation analysis model is established, the positions of support points are selected at the whole mirror support ribs, under the action of the support points, the mirror surface unloading deformation and unloading force under 1g gravity are calculated through a simulation software Hypermesh, the coordinate data and deformation data of each node in the simulation result are exported and converted into a standard surface shape file of MetroPro software, and specific unloading surface shape precision values are output by MetroPro software; S2, when the unloading surface shape precision does not meet the design requirement, the positions and number of unloading support points are changed, the process of step S1 is repeated, and after the selected unloading support points can meet the design requirement of the unloading surface shape precision, step S3 is entered; S3, when the unloading surface shape precision meets the design requirement, the cylinder (3) is subjected to gas circuit collection according to the size of the unloading force of each support point, starting from the minimum value of the unloading force, taking 1N as the separation interval of the gas circuit collection, collecting all the support points into different gas circuits, and then applying the unloading force values of the collected gas circuits to each gas circuit of the mirror model for unloading force review calculation, and evaluating whether the unloading surface shape precision meets the requirement. S4, if the unloading surface shape precision of step 3 does not meet the requirements, all support points are collected into different gas circuits with 0.5N as the interval of gas circuit collection, the gas circuit collection is performed again, the unloading force is recalculated, and whether the unloading surface shape precision meets the requirements is evaluated; If the unloading surface shape precision still does not meet the requirements, the positions and number of unloading support points are selected again, the unloading force and unloading surface shape precision values are obtained by using the simulation software, steps S2-S4 are repeated until the unloading surface shape precision meets the requirements, and the unloading support points and corresponding unloading force are determined.

4. A universal pneumatic self-centering gravity unloading device according to claim 3, characterized in that: The method for collecting the gas circuits is that the cylinders (3) in the same gas circuit are sequentially connected in series; all the cylinders (3) are collected into H gas circuits and connected to a gas supply control cabinet, the gas supply control cabinet supplies gas to each gas circuit, the unloading support force provided by the cylinders (3) in the same gas circuit is the arithmetic mean of all the unloading support forces of the gas circuit, and H>1.

5. A universal pneumatic self-centering gravity unloading device according to claim 4, characterized in that: The cylinders (3) in the same gas circuit are sequentially connected in series through air pipes and T-shaped three-way valves, and the gas circuit connection adopts a quick plug-in and quick plug-out mode: each cylinder (3) is connected to the middle valve port of each T-shaped three-way valve through an air pipe, the left valve port of each T-shaped three-way valve is connected to the right valve port of the left adjacent three-way valve through an air pipe, the right valve port of each T-shaped three-way valve is connected to the left valve port of the right adjacent three-way valve through an air pipe, and the series connection between the cylinders (3) is realized; when the loop is formed, an added T-shaped three-way valve is connected to the gas supply control cabinet: the middle valve port of the added T-shaped three-way valve is connected to the gas supply control cabinet through an air pipe, the left valve port is connected to the right valve port of the left adjacent three-way valve, and the right valve port is connected to the left valve port of the right adjacent three-way valve.

6. A universal pneumatic self-centering gravity unloading device according to claim 1, characterized in that: The liner plate (2) is processed with through holes corresponding to the positions of the cylinders (3), guide columns (5), displacement sensors (6) and lifting motors (7); during surface shape testing, the upper end of the cylinder (3) penetrates through the through hole of the liner plate (2) to contact the back of the mirror, and the mirror is supported to be separated from the liner plate (2) to realize gravity unloading.

7. A universal pneumatic self-centering gravity unloading device according to claim 1, characterized in that: The front surface of the liner plate (2) is pasted with a rubber pad, the back surface of the liner plate (2) is a plane, the front surface of the liner plate (2) is designed in the same shape according to the supported mirror to ensure that the liner plate (2) is closely attached to the back surface of the mirror.

8. A universal pneumatic self-centering gravity unloading device according to claim 6, characterized in that: When the liner plate (2) is placed, the three through holes on the liner plate (2) matched with the guide columns (5) are embedded with the three guide columns (5) one by one to ensure the coaxiality between the liner plate (2) and the bottom plate (1).

9. A method for fast and high-precision centering of a mirror, using the gravity unloading device of claim 1, characterized in that the back of the mirror is open. The method comprises the following steps: S1, the back of the mirror has mounting hole positions, 3 nests are mounted on the same circumference with a separation of 120°, the positions of the nests correspond to the positions of the lifting motors (7) mounted on the bottom plate (1) one by one, the 3 nests are located at the vertices of an inscribed equilateral triangle in the circumference to realize 3-point automatic centering; the determined mirror center is coincident with the center of the bottom plate (1), and then the center axis of the mirror is coincident with the center axis of the bottom plate (1) to ensure the coaxiality of the mirror and the bottom plate (1); S2, the lifting motor (7) is raised to make the contact disc on the lifting motor (7) attached to the nest on the back of the mirror, and then the lifting motor (7) is slowly lowered to make the mirror fall on the liner plate (2).

10. A method for fast and high-precision centering of a mirror, using the gravity unloading device of claim 1, when the back of the mirror is closed, characterized in that The method comprises the following steps: S1, using laser tracker with two-dimensional adjustment platform to adjust: the whole gravity unloading device is placed on the two-dimensional adjustment platform, within the irradiation range of the laser tracker, P target ball seats are spaced and adhered on the cylindrical surface of the reflector, Q target ball seats are spaced and adhered on the cylindrical surface of the bottom plate (1), the target ball seats have target balls, P < 10, Q > 10; S2, in the state of hoisting the reflector, the central axes of the reflector and the bottom plate (1) are respectively tested by using the laser tracker, the two-dimensional platform is adjusted to adjust the position of the gravity unloading device, so that the central axes of the reflector and the bottom plate (1) coincide; S3, the lifting motor (7) is lifted, the contact disc on the lifting motor (7) is in contact with the back surface of the reflector, and the whole reflector is supported, at this time, the hoisting tool is removed, and then the lifting motor (7) is slowly lowered, so that the reflector falls on the backing plate (2).

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