The Study Of Control Characteristics About The Cusped Filed Thruster

Earth gravity field research has always been one of the significant goals in the field of geodesy.GOCE satellite is expected to make a major breakthrough in this field because of its higher accuracy of measurement.As one of the most important technologies of GOCE satellites,drag-free control technology is becoming increasingly vital.As a result of carrying out the commands from the control system,the accuracy of the electric thruster directly determines the accuracy of the measurement of satellite.In this paper,the control characteristics of HEMPT(high efficiency multi-stage plasma thruster)which may be one of the choices used in the gravity field measurement satellite will be studied deeply.First,the author designed the drag-free controller by PID method.After calculation,the controller has a better performance in predicting the drag coefficient,displacement and speed difference between test mass and satellite body.Meanwhile,the controller show a strong stability when the satellite encounters the emergencies.On the basis of the controller,the feasibility of the application about HEMPT used in drag-free satllate is discussed,including the thrust range,the thrust resolution and the delay time.The calculation results show that the HEMPT meets the requirements initially.The performance of HEMPT is measured experimentally during the thrust range of the result of drag-free controller.And three different adjustment paths are designed for three characteristics of the thruster: the low power,high efficiency,and fast response.The results show that the low power mode is in the high-flow and low-voltage working area,and the high efficiency mode cross the high-flow-low-flow and middle-voltage-high-flow conditions,but these two paths can’t avoid the high shock of the current.The integrated path is in the high voltage and flow area,which noise may be low,so the controller can get a more accurate thrust.Since the thrust resolution of the HEMPT can’t be measured by the three-wire pendulum for the the μN level,the paper confirms the feasibility of calculating the thrust by the double probes and then compares the calculated resuluts with the typical working conditions.According to the method,the thrust resolutions of some integrated path conditions are measured.The results show that the datas are very similar to the theoretical calculation values,so we can replaced the value calculated by double probes with theoretical values and then fed back to the drag-free control system.The results show that the resolutions are below the engineering requirements and make the controller performance more accurate.Finally,the actual thrust noise power spectral density and delay time of the HEMPT are measured by experiments.The experimental results show that the increase of flow and voltage of the HEMPT will cause the thrust fluctuations’ increase,but the effect of voltage on the thrust fluctuation is bigger than that caused by flow.The noise power spectrum calculated according to these experiments is lower than the 1.5mN/√Hz,which is given by the standard engineering value.The values of the delay time,which order is 1ms to 2ms,is related to the voltage value.However,the delay time is about 1s when the flow is changed.When the results are fed back to the drag-free control system,it is found that the inaccurate caused by the open-loop-control thrust,also the slow response caused by the flow regulation,would lead to an unfavorable effect on the accuracy of the control system.