Development of a Vibration Control System for the Coil Drum of MRI equipment

核磁共振成像设备产生的高水平噪声一直是其在医学诊断中广泛应用的主要障碍。该噪声是由MRI中用于支撑梯度磁场线圈的圆柱壳的振动引起的。采用聚合物薄膜和压电薄膜制备了缩小尺寸的卷筒模型。在壳体振动控制中，采用两部分压电薄膜作为弯曲致动器，不需要外加致动器。由于壳体受预定力的激励，采用了扰动消除方法。讨论了执行器的尺寸、位置和输出幅度与控制效果的关系。在三种主要模态的共振频率下对壳体进行了激励实验和仿真。结果表明，在不同模态下，控制振动的最小振幅是不同的。其次，提出了一种控制圆柱壳系统振动的方法。圆柱壳系统由中间的聚合物膜和两侧的压电膜组成。压电薄膜被分成几个部分，其中一些部分起分布式致动器的作用。采用模态展开法对周期性激励下的壳体振动进行了分析。基于H_*控制理论和模态分析结果，设计了一种控制器。利用所设计的控制器进行了数值仿真和实验，结果吻合较好，证明了该方法对受迫振动的抑制效果。最后，利用分布式压电传感器和作动器对圆柱壳的振动控制进行了研究。采用模态展开法建立了系统的状态方程。采用H_*控制理论和mu综合方法设计了满足给定控制性能和鲁棒稳定性的控制器。在受控模态共振频率下，对壳体进行了仿真和实验。仿真结果与实验结果吻合较好，表明采用分布式传感器和执行器、H_*控制理论和mu-综合方法的控制方法是有效的。少

High level noise induced in MRI equipments has been a main obstacle for their wide application in medical diagnosis. The noise is caused by the vibration of the cylindrical shell used to support the gradient magnetic field coils in MRI.A model of the coil drum with reduced size is made from polymer films and piezoelectric films. In the control of the shell vibration, two parts of the piezoelectric films is used as bending actuators so that no external actuators are needed. The disturbance cancellation method is used since the shell is excited by pre-determined forces. The relation between the size, position and output magnitude of the actuators and control effect is also discussed. Experiment and simulation were carried out when the shell was excited at resonance frequencies of three main modes. The results shows that the minimum amplitude of controlled vibration varies for different modes.Next, a method was proposed to control the vibration of a cylindrical shell system. The cylindric … More al shell system consists of a polymeric film in the middle and one piezoelectric film on each side. The piezoelectric films are divided into several parts, some of which function as distributed actuators.Analysis of the shell vibration under periodic excitation is performed by means of the modal expansion method. A controller is designed based on H_* control theory and the results of modal analysis. Numerical simulation and experiment were carried out using the designed controller and the results show good agreement, as well as the effectiveness of this method in suppression of forced vibration.At last, a study on vibration control of a cylindrical shell using distributed piezoelectric sensors and actuators which are parts of the shell wall. Modal expansion method is used in estabishment of the state equation of the system. The robust control including H_* control theory and mu synthesis method are applied to the design of the controller which satisfies given control performance and robust stability. Simulation and experiment are carried out when the shell is excited at the resonance frequency of a controlled mode. The results shows good agreement between simulation and experiment, as well as the effectiveness of the method using distributed sensors and actuators, and H_* control theory and mu-synthesis method. Less