Research On Flexible Technology Of Center Of Gravity Measurement For Large-scale Spacecraft

The test of mass properties parameters is an important part of the ground test of aircraft,in which mass and center of gravity are the necessary parameters.However,the existing measurement technology cannot meet the measurement requirements of large-scale aircraft with high precision and versatility.Based on this,a flexible measurement method is proposed in this paper according to the characteristics of the large-scale aircraft,which can measure mass at the same time.This paper focuses on establishment of measurement model,optimization of the structure used for load cells based on Kelvin coupling,design of flexible center of gravity measurement system,and error separation and error correction from main error sources.The main research work is as follows.To solve the problem that the traditional measurement method needs to measure the large-scale aircraft in a specific attitude,the measurement accuracy is low,and the mechanical positioning limits the improvement of measurement accuracy,a flexible measurement method of center of gravity is proposed.This method is based on the principle of static balance,the principle of static moment balance,and the characteristic that the line of gravity always passes through the center of mass.With the help of high-precision coordinate measuring equipment,the mounting and attitude conversion of the measured part are more flexible.The proposed method can be compatible with various types of tested parts and has good versatility.Analyzing the data fusion method of two sets of subsystems,proposing the constructing method of gravity action line,then by solving the intersection of two or more gravity action lines,the three-dimensional center of gravity coordinates of the measured part are measured.The measurement uncertainty of this method was analyzed by Monte Carlo method,for the tested part with a mass of 4000 kg and a length of 10000 mm,measurement uncertainty along X axis is 0.3 mm,along Y axis is 0.2 mm and along Z axis is 0.3 mm.Side-force is the main factor restricting the further improvement of measurement accuracy.To eliminate the side-force,an improved structure based on Kelvin coupling was studied.First,an improved structure based on Kelvin coupling was proposed to solve the problem of side-force loading on load cell.The structure makes the position of the supporting point of the load cell unique,and the load cell can only bear the normal load to ensure the measurement repeatability.The mathematical model of the coupling is established and analyzed by using principle of moment balance.Secondly,the method of correcting the mathematical model of the coupled support structure in the presence of friction is analyzed,which can beused to analyze the automatic repeatability of the improved structure.Finally,the Hertz contact theory is adopted to analyze the contact area size of the improved structure,and the factors influencing the contact area size are studied.Simulation results show that the proposed structure can avoid side-force on the load cell.The deformation of the supporting structure is related to the normal load,Poisson’s ratio,elastic modulus,and curvature of the supporting structure,and will affect the repeatability of automatic alignment,that is to say,it will affect the measurement results.Therefore,the reasonable design of the support structure is of great significance to improve the measurement accuracy.Aiming at the measurement demand of a large-scale rotary aircraft,a flexible measuring system of center of gravity was developed,which can measure a body with a length of 10 meters,mass of the body can be measured at the same time.Based on analyzing the automatic leveling algorithm of two sets of subsystems and the mechanical system,electrical system and software system were analyzed and designed respectively.Standard parts and samples were measured by two subsystems respectively,and a body with a length of 10 meters was measured,average of mass measurement result is 2858.1 kg,standard deviation of mass results is 0.2 kg,average of center of gravity measurement results along X axis,along Y axis and along Z axis are 6392.3mm、32.7mm and-2.2mm respectively,standard deviations of the results are 0.3 mm,0.1mm and 0.1 mm respectively.Comparing flexible measurement method with other method,we can find that difference of center of gravity measurement result along X axis of the two methods is within 1.0mm,and along Y axis is within 0.5 mm.The error separation and compensation technique for the measurement results of center of gravity was studied.To solve the problem that the coordinate of the actual force point of the load cell does not coincide with the calibrated coordinate,the method of correcting the coordinate of the load cell by the total least square method was proposed.Experimental results showed that error of center of gravity measurement result is within 0.1 mm after correction.The influence of installation eccentricity and inclination on measurement results was studied,and the error separation was carried out by reverse method.Then analyzing the error source of the combined measurement method and its influence on the measurement result,the measurement result of correcting the center of gravity along X axis by multi-step method and correcting the center of gravity along Y axis and Z axis by reverse method was put forward.Finally,the proposed coordinate correction method and error separation method were verified by experiments.The results show that the load cell coordinate correction method can improve the measurement accuracy,the multi-step method can separate the measurement error of center of gravity along X axis,but there is a residual of constant term error,while the reverse method canseparate the error caused by eccentricity and position between two sets of subsystems.