| Nuclear Magnetic Resonance(NMR) has become a powerful measurement technique in medical diagnostics and material research et al. In spite of this, the huge device, the high cost and the enclosed structure limited the maximum size of the test object. In practice, for non-destructive measurement of the big object in engineering field, the NMR device has to be redesigned, and its structure should be fully open and portable.Aiming at such requirement, this work is based on a new NMR technology: portable and fully open NMR. The features of this technology are the portable device, the open structure and the test object is outside the sensor which means the big object can be measured non-destructively. However, research on this technology faces many problems. Comparing to traditional closed NMR, portable and fully open magnet structure are at the expense of the strength and uniformity of the magnetic field. Therefore, the SNR of the signal detected by portable and fully open NMR is low and the transverse relaxation is very fast, which increases the difficulty of the signal analysis. For different test object in engineering, the structures of the magnet and RF coil must be different which are the challenges of electromagnetic design. Meanwhile, the complicated environment of the engineering field might influence the measurement result. Above all, the research on portable and fully open NMR is a theory and practice tight coupling area. So far, there is not a mature and flexible way for the sensor design of the portable and fully open NMR, and the signal analysis method are mostly learned from traditional NMR with uniform magnetic field. Therefore, for the special structure of protable and fully open NMR sensor, the research on the above problems has important theoretical and practical value.From the perspective of the electromagnetic field, a set of design and optimization methods of the magnet and the RF coil structures of portable and fully open NMR sensors are proposed in this paper. The corresponding signal analysis method was studied as well. Those laid the foundation of the development of portable and fully open NMR technology. The research content includes following parts:1) The research on portable and fully open NMR sensor. This kind of sensor consists of the magnet and RF coil. This stdudy is expanded by the design of the magnet structure which is separated in two parts: Firstly, based on the equivalent surface current law and Biot-Savart Law, the strength and gradient of the magnetic field of the permanent magnet was calculated. Aiming at minimizing the gradient of the magnetic field in the measurement volume, the basic structure of the magnet can be comfirmed quickly. The the magnet structure is modified by the simulation employing finite element method. For the RF coil structure, based on the suppose of quasi stable magnetic field, the magnetic field of the RF coil is calculated by Biot-Savart Law and the optical structure of the coil is determined by the magnetic field distribution. Since the measurement volume of the existing protable and fully open NMR sensor is very close to the surface of the magnet, the object can not be detected deeply. To overcome this problem, this work presents a three parts magnet structure. A new 1800 fully open sensor and a new 3600 fully open sensor are designed by employing the above method, measurements are done for different applications.2) Analytical method for NMR signals from inhomogeneous magnetic field. Due to the low and inhomogeneous magnetic field, the signal to noise ratio of portable and fully open NMR measurements is low. The existing signal analysis method is not suitable in this case. To solve this problem, combined with the advantages of truncated singular value decomposition(TSVD) and BRD regularization algorithm, this part presents a hybrid TSVD-BRD algorithm for signal processing in fully open NMR. To find the optimal value of the regularization factor, the BFGS of quasi-Newton method is employed for iterating. Application examples demonstrated that the algorithm can give a reliable result of the inverse Laplace transformation for portable and fully open NMR signals.3) Applications for portable and fully open NMR. A series of sample measurements has been carried out on the platform of fully open NMR sensors developed in this work. The 1800 open NMR sensor sensor has been also employed to measure the NMR parameters of the artificial cores and real cores, and the results show that this sensor is suitable for core analysis. The 1800 open NMR sensor has been employed to evaluate the aging status for the turbine oils, and relationship between the degree of aging of the turbine oils and the transverse and longitudinal relaxation times was analyzed. The 3600 open NMR sensor has been employed for measuring moisture status in the wood, and the distribution of free water and bound water of which can be distinguished very well. The 3600 open NMR sensor has been also employed for the measurements of porous media but not cores, and it demonstrated that the porous media with different pore sizes can be differentiated.The research on the method of sensor design, signal inverse algorithm and practical applications for portable and fully open NMR has been done in this work which leads a comprehensive study of this technology. These work has laid a foundation for advancing portable and fully open NMR in engineering fields. |
PDF Full Text Request