Accuracy criteria and finite element study of a highly saturated magnetic device with a large air-gap.

摘要

The Finite Element Method has been frequently used to obtain a reliable virtual prototype of a device. The accuracy of the device model gains considerable importance when measured data are not available for comparison. This thesis proposes to use Ampere's circuital law to obtain a global accuracy criterion, Ampere's law error, which measures the accuracy of the FE model and solutions.; The C-shaped magnet of an open-concept Magnetic Resonance Imaging system is used as the focus for this thesis. Being three-dimensional, highly saturated, and having a large air-gap, the open-concept MRI magnet represents an interesting modeling challenge. The objective of the thesis is thus to use this device as a vehicle to explore accuracy issues when modeling saturated, 3D, non-symmetric magnetostatic devices.; A stable 3D modeling approach is developed that encloses the MRI magnet in a spherical FE volume, the exterior surface of which can represent the true infinite boundary condition. A FE model of the MRI magnet is thus developed and studied. It is concluded that the average and standard deviation of Ampere's law errors provide a valid global accuracy measure for this class of FE solutions.; A selection study is also necessary to obtain the most accurate FE model of a device. This thesis describes (a) a selection strategy and (b) selection criteria. A selection strategy incorporates two methods that simplify the search for better FE models of the device. Both methods have proven to be practical and constructive. The selection criteria include Ampere's law error, the energy content of the model, the energy content in specific regions of the model, and the average of the magnetic field data at specific regions of the magnet. These selection criteria have proven to be feasible for selecting the best FE model of the device.; An alternative design of the MRI magnet is introduced. The performance of the alternative design is shown to be superior to the original design of the MRI magnet.; In addition, the thesis introduces two two-dimensional modeling approaches to simplify the FE study of unsymmetrical 3D devices. One of the 2D modeling approaches is shown to be applicable for calculating the magnet data.