| The detection of the earth’s gravity field is of great significance to the study of geophysical information.GOCE satellite,as the equipment with the highest measurement accuracy at present,is expected to make a greater breakthrough in the detection of the earth’s gravity field.In order to ensure the accuracy of measurement,GOCE satellite requires very high noise level of thruster in the frequency range of 0.001-10 Hz.As an alternative thruster for future gravity field detection in China,the noise characteristics of HEMPT need to be studied urgently.At present,the international research on thrust noise of HEMPT has not been carried out: no systematic measurement of thrust noise;no discussion on the performance requirements of power supply,flowmeter and other equipment carried by HEMPT;no knowledge of the source of thrust noise of HEMPT;and no noise suppression scheme for HEMPT.In view of the above research blank background topics,this paper carries out the following aspects of work:At present,there are two main measurement schemes for thrust noise in the world,which are thrust bracket measurement and beam voltage and beam current calculation.This paper proposes a combination scheme of Faraday probe and RPA probe in thruster plume area to measure thrust noise,and verifies its feasibility.For the thruster standard demarcated by gravity field detection,the output resolution and fluctuation task requirement of the thruster for power supply and flowmeter are analyzed.The experimental data show that the thrust noise of HEMPT under all working conditions meets the requirement task requirement.The control accuracy of towless control system is directly related to the measurement accuracy,so genetic algorithm is used to search and optimize the PID control parameters.The simulation results show that adding noise fluctuation to the thrust output of the towless control system can reduce the control accuracy and increase the displacement error from 500 to 1000 nm.The influence of gross error,system error and random error in the removal of thrust noise is discussed.For HEMPT thruster noise source analysis,it is considered that the main influencing factors are thruster drift,anode voltage fluctuation and mass flow fluctuation,and thrust noise is in the form of Gauss white noise.On this basis,the model between thrust noise and anode voltage and mass flow is established.Finally,in order to suppress the thrust noise of HEMPT,the oscillation of power supply and flowmeter is suppressed separately.The experiment proves that the twostage filter circuit designed for power supply can effectively reduce the thrust noise,and the closed-loop feedback control established for the thrust system can effectively reduce the thrust noise. |
