Reaction flywheel is a significant actuator for satellites＇ attitude control. To improve output torque and rotational speed accuracy for reaction flywheel, this paper reviews the modeling and control approaches of DC-DC converters and presents an application of the variable structure system theory with associated sliding regimes. Firstly, the topology of reaction flywheel is constructed. The small signal linearization process for a buck converter is illustrated. Then, based on the state averaging models and reaching qualification expressed by the Lee derivative, the general results of the sliding mode control （SMC） are analyzed. The analytical equivalent control laws for reaction flywheel are deduced detailedly by selecting various sliding surfaces at electromotion, energy consumption braking, reverse connection braking stages. Finally, numerical and experimental examples are presented for illustrative purposes. The results demonstrate that favorable agreement is established between the simulations and experiments. The proposed control strategy achieves preferable rotational speed regulation, strong rejection of modest disturbances, and high-precision output torque and rotational speed tracking abilities.
研究了采用双框架控制力矩陀螺(Double Gimbaled Control Momentum Gyroscope,DGCMG)的敏捷卫星姿态/角动量联合控制问题,针对DGCMG的饱和奇异问题,提出了基于Lyapunov的姿态/角动量联合控制方法。首先,建立了采用两个平行构型DGCMG的卫星姿态动力学模型,然后根据陀螺的力矩方程,通过可视化分析得出该构型只有内部隐奇异和饱和奇异两类奇异。隐奇异可以通过操纵律进行避免,而饱和奇异只能通过卸载方式来解决。为了避免采用推力器或磁力矩器等卸载方式带来的问题,设计了连续管理角动量的姿态/角动量联合控制器。此外,为了缩短系统的稳定时间,采用Sigmoid函数对控制器的参数选取进行了改进。该控制器完成敏捷卫星快速机动快速稳定任务的同时,还能连续调节角动量,达到姿态控制和角动量管理的折中。数值仿真结果验证了控制器的有效性。
A method of measuring in-situ magnetic field gradient is proposed in this paper. The magnetic shield is widely used in the atomic magnetometer. However, there is magnetic field gradient in the magnetic shield, which would lead to additional gradient broadening. It is impossible to use an ex-situ magnetometer to measure magnetic field gradient in the region of a cell, whose length of side is several centimeters. The method demonstrated in this paper can realize the in-situ measurement of the magnetic field gradient inside the cell, which is significant for the spin relaxation study. The magnetic field gradients along the longitudinal axis of the magnetic shield are measured by a spin-exchange relaxation-free （SERF） magnetometer by adding a magnetic field modulation in the probe beam＇s direction. The transmissivity of the cell for the probe beam is always inhomogeneous along the pump beam direction, and the method proposed in this paper is independent of the intensity of the probe beam, which means that the method is independent of the cell＇s transmissivity. This feature makes the method more practical experimentally. Moreover, the AC-Stark shift can seriously degrade and affect the precision of the magnetic field gradient measurement. The AC-Stark shift is suppressed by locking the pump beam to the resonance of potassium＇s D1 line. Furthermore, the residual magnetic fields are measured with σ＋- and σ--polarized pump beams, which can further suppress the effect of the AC-Stark shift. The method of measuring in-situ magnetic field gradient has achieved a magnetic field gradient precision of better than 30 pT/mm.
