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

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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