Drag-Free Control Based On Cusped Hall Electric Propulsion

As a scientific research field with fierce international competition,space precision measurement science has great scientific and application value.In order to reduce the influence of space environment disturbance on the measurement process as much as possible,the drag-free control technology is used in most precision measurement missions,that is,the thrust of propulsion system is used to counteract disturbing forces of environment on the satellite in real time.Therefore,the dynamic characteristics of the propulsion system will seriously affect the design process and control accuracy of the drag-free control system.As a result,these precision measurement tasks put forward the requirements of the propulsion system with a wide adjustable thrust range,low thrust noise and fast thrust response.Among the existing types of propulsion systems,a variety of propulsion systems such as cold gas propulsion,colloid electric propulsion,and cusped hall electric propulsion have been proven to be applicable to precision measurement tasks.Among them,due to the characteristics of the wide thrust adjustable range,stable discharge process,and long life,the cusped hall electric propulsion has become one type of the propulsion systems that are currently focused on research and development.Therefore,the research of drag-free control based on the cusped hall electric propulsion is significant to the development of space precision measurement science.The Earth’s gravitational field measurement and space gravitational wave detection are the most representative research fields in the current space precision measurement science.Phase 2 and Phase 3 of China’s Tianqin plan are devoted to completing the above two tasks,respectively.Although both of them require the use of the drag-free control technology,differences in the principle of detection and the orbit of two systems result in differences in the main non-conservative disturbance forces and the mode of the drag-free control system,and so are their corresponding thrust levels,which are milli-newton and micro-newton,respectively.Therefore,although the drag-free control system and the propulsion system involved in these two tasks in the Tianqin plan are similar in terms of research ideas,there are differences in specific implementation details.Therefore,targeted research work should be carried out for the two tasks,respectively.The testing and modeling of thrust dynamic characteristics are prerequisite for the realization of drag-free control system design,thrust performance evaluation and optimization.However,the thrust noise and thrust dynamic response process of the current cusped hall propulsion system have not been tested in public reports,and their impacts on the drag-free control system are also unclear.Based on those mentioned above,this article has carried out experimental tests on the thrust noise and thrust dynamic response processes of the milli-newton and micro-newton cusped hall propulsion systems.The results show that the thrust noise of the milli-newton propulsion system has reached the noise index for the gravitational field measurement task,while the thrust noise of the micro-newton propulsion system has not yet met the index requirements for the gravitational wave detection mission.In addition,main causes of thrust noise in different frequency bands are also analyzed.Results of thrust step experiment show that the dynamic response of the thruster itself has little effects on the dynamic process of thrust,while the dynamic response process of the flow system has a large impact,which ultimately leads to a slower thrust response speed of the propulsion system.Based on the experimental data,this paper proposes a component-level propulsion system modeling method and establishes a mathematical model that can reflect the actual thrust dynamic characteristics of the two propulsion systems,which provides model support for the subsequent research work of the drag-free control systems of the gravitational field measurement and space gravitational wave detection tasks,respectively.Based on the established mathematic model of the milli-newton propulsion system,this paper firstly carried out research work on the drag-free control system of the gravity field measurement satellite.The results show that the main disturbing force of the gravity field measurement satellite is the atmospheric drag,while the thrust noise of the milli-newton propulsion system has little effects on the satellite motion.Based on the above disturbance characteristics and thrust dynamic characteristics,this paper designs a drag-free controller based on the extended state observer,and builds a satellite control/propulsion co-simulation system.The simulation results show that the thrust response speed of the propulsion system is so slow that it limits the bandwidth of the control system,resulting in the control accuracy not meeting the mission requirements.To solve this problem,a thrust response speed optimization method based on coordinated control of input parameters of propulsion system is proposed.The results show that the thrust response speed is improved and the bandwidth of the control system is increased after the coordinated control of the flow rate and the voltage,which makes the residual accelerations of the satellite meet the drag-free control target of the gravity field measurement mission.Then,based on the established mathematical model of the micro-newton propulsion system,research on the drag-free control system of gravitational wave detection satellite is carried out.The results of satellite disturbance analysis show that thrust noise is the main disturbance forces in the concerned frequency bands.According to the obtained disturbance characteristics,a H_∞compound drag-free controller with an expanded state observer is designed.The satellite control/propulsion co-simulation results show that due to the excessive thrust noise of the current propulsion system,even if input parameters of the propulsion system are coordinated,the drag-free control system still cannot provide enough bandwidth to suppress thrust noise.Therefore,the control index of the gravitational wave detection task cannot be satisfied.In order to solve those problems,this paper proposes a noise suppression method based on the closed-loop thrust control within the propulsion system.The results show that this method can effectively suppress thrust noise in a wide range.Combined with research results of the thrust closed-loop control,this paper redesigned the drag-free controller for the gravitational wave detection satellite,and finally formed a cascade control system with inner-loop thrust control and outer-loop drag-free control.The joint simulation results of the satellite control/propulsion system show that the noise-optimized micro-newton cusped hall electric propulsion system and the newly designed drag-free controller can cooperate to complete the drag-free control task of gravitational wave detection satellite,so they are possible to be applied to the gravitational wave detection task of Tianqin.