Actively Controlled Mgnetic Bearing Momentum Wheel and Its Application to Satellite Attitude Control

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For the high-accuracy attitude control of spacecraft, Magnetic Bearing MomentumWheel (MBMW) has been being investigated as an actuator because of its merits of low disturbancecharacteristics and "gimballability" when a suitable active controller is used. The active control of MBMW in this context means to make the rotor spin around its principal axis of inertia to achieve low disturbance purpose while being able to tilt the principal axis of inertia for attitudecontrol of spacecraft, and this has to be done with adequate nutation damping.In previous literature with respect to the active control of MBMW, considering the non-linearity of electromagnets, Nam et al. [1] applied the H∞ robust control method to the active control of MBMW, but only a fixed rotational speed is considered. Without thinking of the non-linearity of electromagnets, Bichler [2] proposed a Model Following Control (MFC) method and indicatedthat this method has superior capability of active-vibration suppression for a large range ofwheel rotational speed. However, this method is based on a special observer to realize activevibrationsuppression, thus its response to fast externally exerted torque and movements is slow.If externally exerted disturbance torque, including those due to eddy current loss, ohmic loss and those exerted from the spacecraft body on which the physical space-environmental torque acts exist, large overshoot and small nutation of the wheel rotor will be excited when wheel rotationalspeed is low. Therefore, to realize the active control of MBMW with non-linear electromagnets, an Adaptive Model Following Control (AMFC) method is proposed in this report.In this new method, first an adaptive mechanism is inserted in the above model following control method. This adaptive mechanism can adaptively regulate the bandwidth of a special observer based on on-line identification of state characteristics so as to improve system dynamic response if needed and to make compromise between fast dynamic response and active-vibrationsuppression at harmonic frequencies of the wheel rotation. Second, logical derivative [3] is applied to the controller design to reduce overshoot further. Third, a new nutation damping method that can automatically adjust the gain of nutation damping according to wheel speed is introduced in order to obtain consistent damping rate for different wheel speed. Finally, a non-linearity compensator is put forward to overcome electromagnetic non-linearity. The advantage of this method is that it is easy to make compromise between nutation damping and disturbance suppression at harmonic frequencies of the wheel rotation. Since the closed-loop system is nonlinear and time-varying system, the stability and robustness of this method is analyzed from the engineering viewpoint.A large amount of simulation has been done. The simulation results show that the AMFCmethod has great capability of disturbance suppression, good dynamic response and satisfactory nutation damping for a large range of wheel speed. Therefore it is possible to realize high accuracy attitude control by using the MBMW controlled by the AMFC method as actuator.How to apply the MBMW controlled by the AMFC method to satellite attitude control is alsostudied in this report. As an example, Solar-A Satellite is selected to examine the effectiveness of this method. The report is organized as follows. In Chapter 1, the active control of magnetic bearing includingMBMW is reviewed. In Chapter 2, MBMW including structure and its mathematical model is introduced. In Chapter 3, the adaptive model following control method is presented and its performance, stability and robustness are analyzed in Chapter 4. To verify the effectiveness of this new method digital simulation is carried out in Chapter 5, in which, AMFC, MFC and H∞ method are compared. In Chapter 6, the possible application of the AMFC method to the activecontrol of MBMW for spacecraft attitude control is discussed. Finally, some conclusion is given in Chapter 7.
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Actively Controlled Mgnetic Bearing Momentum Wheel and Its Application to Satellite Attitude Control

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