The mechanical property analysis of the large supportive stage with super smooth surface

With the rapid development of satellite astronomy technique, In order to ensure the effective operation of the spacecraft in orbit, the spacecraft requires full ground physical simulation tests. So the construction of the ground simulation test system is an important guarantee for the development of space technology [1]. In the system of full physical simulation test for lunar orbit rendezvous and docking, the large supportive stage with super smooth surface is one of the most important large-scale precision test equipment, this stage can provide a high precision horizontal support surface in the range of 40m*30m for the test load, Each test load is usually supported by three circular air cushion, whose diameter is 200mm~400mm. High pressure gas is stored in the load equipment, and is released into the air through the air holes on the air cushion, A thin layer of air film is formed between the air cushion and the stage, the thickness of gas film is 10μm~15μm, The load equipment can freely move on the surface of a large stage with the gas film. There are strict requirements for the deformation of the system, Since the floating height is 10 μm and the test load is 3T. So the altitude difference of neighboring platforms is in range of 10μm. In order to guarantee the experiment effects, the paper analyses mechanical properties of the stage, distribute index of 10μm, and provide theoretical support for subsystem [2], [3].


Introduction
In order to ensure construction requirements of the GNC laboratory, and consider great difference of the Mechanics environment between ground and orbit, the microgravity simulation system has been developed.The spacecraft performance of Mechanics, machinery, navigation and control can be tested and verified [4].A supportive surface of high-precision and highlevelness can be provided on the large supportive stage with super smooth surface [5].Each test load is usually supported by three circular air cushion, whose diameter is 200mm ~ 400mm, The air cushions are distributed on the circumference of Diameter is 1~3m,As shown in Fig. 1. High pressure gas is stored in the load equipment , and is released into the air through the air holes on the air cushion, A thin layer of air film is formed between the air cushion and the stage, the thickness of gas film is 10μm ~15μm, The load equipment can freely move on the surface of a large stage with the gas film.As shown in Fig. 2. The weight of each test load is 3t, multiple loads can move freely on surface of the stage with the gas film Under the condition of no friction almost [6].As shown in Fig. 3.

Working plane
Adjustment support The large stage is stitched together by 200 block "jinan green" Marble, whose size is 3m*2m*0.4m.Because a plane can be determined by three points, the Supporting method of single block platform is Three main supports and three auxiliary supports [7].asshown in Fig. 4. The main supports are 1,3,5, the auxiliary supports are 2,4,6.

The index decomposition of altitude difference
The altitude difference of neighboring platforms is in range of 10μm when the weight of load is 3t.the altitude difference comes from two points as follows: (1) The adjustment error of mechanical drive is defined as H c .(2) the elastic deformation of overall system is defined as H k .
In the following, these two parts above would be analysed, as shown in Fig. 5, and the equation 1 is true.

C K H H H
(1)

The analysis of adjustment error
The adjustment error of mechanical drive caused by the two parts: one is the accuracy of integrated control subsystem, denoted by H cp , the other is the Straightness of platform, denoted by H cl , as shown in Fig. 6, and The equations 2 is true.(2)

The analysis of adjustment accuracy
The adjusting methods is that a platform is adjusted without load.so this adjustment accuracy is the accuracy without load.
Based on 3 points leveling method, the distance between support and the edge of platform is small.The speed of adjusting platform is slow, the Height adjustment of support is approximately equal to the lifting height of neighboring platforms.Basis of analysis and research, the adjustment accuracy of servo motor can reach 0.03 .With pienty of experiments and data analysis, the adjustment accuracy of altitude difference can reach 0.2μm.The following equation 3 is established.

The analysis of platform straightness
Considering the impact of machining grinding deformation and temperature.Because the impact of straightness, the altitude difference of neighbouring platforms is random.the straightness is got by statistical method usually.
In general, the overall profile is concave when the grinding of platform is finished.as shown in Fig. 7.Because of the higher technical targets of altitude difference, the straightness of platform must be no greater than 4μm, and satisfy consistence of neighboring platforms.

The analysis of stiffness
the stiffness of system has effects on altitude difference, include platform stiffness, support stiffness and foundation stiffness.asshown in Fig. 8, and the equation 6 is established.

The analysis of platform mechanical characteristics
The elasticity deformation of platform is different when the position of air bearing pad is different.As shown in figure 9, the worst condition can be got by calculation and analysis.In the worst condition, the 3t load can result in maximum deformation of platform.The deformation could be calculated by using finite element method [8], as shown in Fig. 10.

The mechanical characteristics analysis of foundation subsystem
In general, the weight of whole system and the 3t load would result in the deformation of foundation subsystem.The altitude difference is the most important, the paper would analyze foundation deformation, which is caused by 3t load.In the process of moving load, the maximum load variation of supporting mechanism can be got by a lot of calculation, this maximum value is the input condition of analyzing foundation deformation.The variation of support 5 is the maximum value,as shown in Fig. 9. support and design method are provided in this paper, which is based on objective and reasonable considerations.

Figure 1 .
Figure 1.The schematic diagram of test load structure

Figure 2 .Figure 3 .
Figure 2. The side view of the large stage

Figure 4 .
Figure 4.The diagram of supporting method

Figure 7 .
Figure 7.The diagram of platform overall profile Considering the shape of the overall profile and the effects of temperature, the straightness of platform can reach 3.3μm by test in constant temperature room.The following equation 4 is established.
above analysis, the altitude difference of neighbouring platforms, importing data into equation 2, and the equation 5 is true.

Figure 8 .
Figure 8.The diagram of index decomposition because of stiffness

Figure 9 .
Figure 9.The diagram of the worst condition

Figure 11 .
Figure 11.The diagram of the foundation deformation without steel plateUsing finite element method, the maximum variation value of support 5 is 12076.6N.The maximum elastic deformation of foundation subsystem is not more than