Research On FPGA-based Multi-channel Magnetic Field Detection System

Transcranial magnetic stimulation(TMS)is a non-invasive,painless and non-invasive treatment.It has important application value in clinical medicine,biomagnetism,basic brain research and engineering application.TMS generates pulsed magnetic field through magnetic field generator,which acts on the brain of organisms and generates induced electric field in brain tissues.TMS generater causes a series of complex physiological and biochemical reactions,so as to achieve the purpose of treating mental diseases and studying brain physiological reactions.However,there are few studies on the detection and evaluation of the actual magnetic field generated by the TMS magnetic field generator.In order to improve the safety of TMS in biological experiments,it is necessary to pre-detect the magnetic field before carrying out TMS experiments on organisms,and to fully understand the distribution of magnetic field before carrying out clinical application.In this dissertarion,a multi-channel magnetic field detection system based on field programmable gate array(FPGA)is designed to detect the magnetic field of TMS.In this thesis,coil array is used to detect the magnetic field generated by the TMS stimulus coil.The induction electromotive force generated by the detection coil is analyzed according to the pulse width,frequency and amplitude of the signal.The sampling rate and other related design parameters are determined.At the same time,in order to synchronously detect multi-channel signals,according to the characteristics of high-speed parallel of FPGA,this dissertation uses the design ideas of FPGA,analog-to-digital converter(ADC)and analog switch switching.The scheme of circuit design and the printed circuit board of important signals are introduced in detail.The design principle can not only ensure the synchronization of sampling to a certain extent,but also ensure the speed and resolution of sampling,and also ensure the stability of the acquisition system.This dissertation introduces the hardware circuit design of the whole detection system in detail.The main circuit platform includes induction coil array,data acquisition and host computer software.The acquisition card includes analog signal processing circuit,multiplexer,ADC and FPGA.It has the idea of high-speed data transmission,parallel acquisition and channel switching.The system can sample at a rate of 500 Ksps per channel with 14-bit resolution and can collect 12 channels at the same time.The acquisition card transmits the collected data to the computer through USB module,and then restores the three-dimensional magnetic field by Matlab.At the same time,the three-dimensional magnetic field information is presented in the way of three-dimensional vector and two-dimensional waveform.The designed data acquisition card has good signal to noise and distortion ratio and channel rejection ratio of 88.35 dB and-79.69 dB,respectively.Compared with NI DAQ acquisition card,the relative error of the system is less than 1.81%and the mean square error is less than 2.89*10-6,which verifies the good performance of the proposed detection system.The multi-channel magnetic field detection system designed in this thesis provides an important platform for the research of TMS in medicine,engineering and other fields.