Optimized Design Of Shimming Power Of Magnetic Resonance Imaging System

Shimming power is an important part of MRI system,and it provides power to the shimming coil to produce a rapidly changing output gradient current,which compensates the main magnetic field.The resonance between shimming coil and the LC output filter is easy to cause the system instability in the traditional full-bridge structure.The stability of the system can be improved by the passive damping method,but the damping loss is caused by damping resistance.Therefore,it is of great significance to optimize the parameters of shimming power to reduce damping loss and improve system stability.Based on the full-bridge structure,the working principle and mathematical model are analyzed in detail.On this basis,a solution is designed to optimize the parameters of the filter and control loop circuit.According to the constraints of resonant frequency and damping coefficient,the value range of filter inductance and filter capacitance are obtained.The expression is derived by establishing the equivalent circuit model of filter and load inductance.In order to reduce the damping loss,the filter parameters are optimized in the interval.Furtherly,the control loop parameters are optimized to improve the system stability.The simulation and experimental results verify the validity of the theoretical analysis.In order to furtherly reduce the damping loss,from the perspective of the system topology,a double Buck converter is adopted to replace the full-bridge converter as the structure.The working principle and mathematical model are analyzed in detail.The parameters of filter and control loop are optimized by using the proposal proposed in the previous article.Compared with the shimming power based on the full-bridge structure,the filter parameters and damping loss of the dual Buck converter are smaller.Also,the scheme reduces the ripple of the output current and improves the precision of the current.The simulation and experimental results verify the validity of the theoretical analysis.In low power situations,the damping loss has a great influence on the efficiency of shimming power.From the perspective of the control,an active damping control scheme with output current feedback compensation as the inner loop is designed to eliminate the damping loss,the working principle and mathematical model are analyzed in detail.Based on the reasons of system instability,a first-order filter compensator and a second-order filter compensator are designed respectively in the inner loop circuit of the scheme.Compared with the passive damping control based on the double Buck converter structure,the proposed solution with two different filter compensator removes the damping loss and reduces the ripple of output current.Compared with the first-order filter compensator in the inner loop control,the output current ripple of the second-order filter compensator is smaller.The simulation results verify the validity of the theoretical analysis.