| The drag-free spacecraft is a key platform for implementing missions such as gravitational wave detection and gravity field measurement.To meet the requirements of space gravitational wave detection missions,the thrust resolution of the propulsion system needs to reach the sub-μN level,which puts high demands on the design methods and working performance of the thruster and control system.In this paper,from the perspective of China’s future space gravitational wave detection mission,a fundamental study on the design method,mechanism and operating characteristics of the Micronewton-level piezoelectric cold gas thruster and system control method of the drag-free spacecraft is conducted to provide technical guidance and theoretical support for the development of the drag-free spacecraft in China.Firstly,the design of structure and piezoelectric drive method is carried out around the piezoelectric cold gas thruster of the Micro-newton stage,and the simulation analysis of the multi-scale thin flow mechanism in the micro-nozzle of the thruster is carried out,and the thrust model of the micro-thruster is established based on the thrust calculation results.The small-deflection bending theory is adopted to establish a mathematical model of the electro-mechanical characteristics of single-chip piezoelectric wafers considering the intrinsic relationship between piezoelectric materials and elastic base materials,and the analytical relationship between the driving voltage and the deflection of piezoelectric wafers is obtained.Drawing on the principle of ant step crossing,a piezoelectric actuator with multiple single crystal wafers connected in series and warp amplification is proposed.This new driver can achieve the desired displacement by increasing the number of piezoelectric wafers according to the actual displacement output demand.The problems of large diameter,high voltage and low reliability of single piezoelectric wafer driver are solved.The driver module designed in this study can output a displacement of 51.997 μm at a drive voltage of 120 V.The micro-thruster controlled by this driver can achieve a wide range of thrust adjustment from 1 to 300 μN.Scanning electron microscopy and roughness measurements were used to obtain the microscopic morphological structure and roughness characteristics of the flow channel walls inside the micro-jet tube.By idealizing the rough wall surface as a sinusoidal wave shape and introducing the Ishboard model to describe the collisional deflection process of nitrogen molecules with the sinusoidal wave wall surface,and introducing the model of the solid atomic potential well action at the wall surface to describe the refraction effect of gas molecules near the wall surface,we finally established the improved CLL model is finally established.On this basis,a DSMC mathematical model of the flow process in the micro-jet of the micro-thruster is established.The computational performance of the flow model is analyzed by comparing the results of micro-jet tube thrust tests.By calculating the flow in the microinjector with different needle valve strokes,inlet temperatures and pressures,three aspects are analyzed: 1)the multi-scale thin flow characteristics existing in the microinjector and its relationship with the microinjector needle valve stroke;2)the thin flow mechanism in the microinjector under tiny stroke(<1%);3)the sensitivity of inlet temperature and pressure on the flow characteristics in the Micro-newton stage thruster;and 4)the relationship between obtaining the microinjector thrust with MAP of inlet pressure,stroke degree or local Kn number of the throat,and establish the thrust model.It is found that the flow mechanism inside the micro-jet tube is significantly affected by the needle valve stroke,and when the needle valve stroke is reduced from 1% to 0.25%,the transition flow start position gradually moved up from the region about 4 times the flow path diameter downstream of the needle valve tip to the needle valve tip region.Then a one-dimensional simulation model of the piezoelectric cold gas propulsion system at the Micro-newton-level is established based on AMESim software.The influence law of system parameters on filling characteristics is analyzed by adjusting cylinder pressure,mass block mass of pressure reducing valve,spring stiffness of pressure reducing valve and pipeline volume.The flow and thrust characteristics of the system are analyzed by changing the pressure of the pressure storage chamber,medium temperature,spool cone angle angle,valve port diameter,valve body volume,spool assembly and system damping taking values.By introducing the piezoelectric drive voltage excitation,the influence law of excitation on the needle valve displacement and thrust fluctuation is investigated.The one-dimensional system model established in this part of the study provides input parameter boundaries for the closed-loop control study and provides a basis for the optimized design of the microfluidic module structure and control strategy.And then,the flow control strategy based on data-driven PID is proposed for the problem that fixed parameter PID control is difficult to meet the demand of fast response and high accuracy performance in wide flow range.Using the historical control data,a PID parameter adjustment method based on the most rapid descent method is proposed,and a PID parameter database adapted to a wide flow range is improved.The mechanism of hysteresis,creep and vibration nonlinearity on control performance is analyzed,and the MPI-based hysteresis sub-model is established by using sweep frequency excitation to identify creep and vibration sub-models,variable frequency excitation and particle swarm optimization methods.The feedforward/feedback composite control method based on inverse model and data-driven PID is proposed by doing feedback through data-driven PID to suppress the effects of unmodeled dynamics and external disturbances.Finally,the flow experimental system and thrust experimental test system of piezoelectric cold gas thruster of Micro-newton-level were built,and the experiments were set up and tested with the scientific problems faced by Micro-newton-level piezoelectric cold gas thruster,including voltage-displacement dynamic test,temperature effect on stroke voltage test,stroke-flow-thrust test and very small thrust experimental study under thin flow,and compared with the simulation results.The accuracy of the calculation results of the coupled model of flow model and one-dimensional thrust system is verified.A coupled Matlab/Simulink control/AMESim one-dimensional thrust system model is developed and the oscillation delay characteristics of the system are simulated and analyzed.The results show that the data-driven PID control can effectively eliminate external disturbances and improve the system robustness;the response speed of this control method is significantly improved compared with that of the data-driven PID controller. |
