基于磁通门和时域数字鉴频的磁场锁定系统

Magnetic Field Locking System Based on Fluxgate and Time Domain Digital Frequency Discrimination

永磁型磁共振仪器的磁体易受温度和其他环境磁场干扰,造成主磁场波动,进而影响仪器测量的重复性和准确性.本文讨论了两种解决磁场波动的锁定方法:一方面,通过磁通门传感器对环境波动引起的瞬态磁场进行高灵敏探测,然后采用现场可编程门阵列进行实时处理并计算磁场补偿量;另一方面,针对环境温度变化引起的缓慢磁场偏移,则采用时域数字鉴频锁场方法,在对锁样品进行射频激发后,将磁共振信号通过混频变换到较低的频率范围,再转换为方波,然后直接送入现场可编程门阵列进行周期测量,并计算磁场补偿量.将两种方法获得的磁场补偿量叠加后,再转换为电流信号驱动安装在磁体上的B_(0)补偿线圈,并研制了一套磁场锁定系统,以实现对磁场的锁定.在0.5 T食品快检磁共振分析仪上进行测试验证,结果显示当受到瞬态干扰时,可将磁场稳定在±4 Hz(对应磁场为±0.0939μT)范围内,同时也可以精准测量温度造成的磁场偏移,该结果验证了本文磁场锁定方法的可行性.

The magnet of permanent magnet magnetic resonance instrument is susceptible to the interference of temperature and other environmental magnetic fields,resulting in fluctuations of the main magnetic field,which affects the repeatability and accuracy of the measurement.In this paper,two locking methods to solve the magnetic field fluctuation are discussed.On the one hand,the transient magnetic field caused by environmental fluctuation is detected by fluxgate sensor with high sensitivity,and then the field programmable gate array is used for real-time processing and calculation of the magnetic field compensation.On the other hand,the time-domain digital frequency discrimination locking method is employed for the slow magnetic field drift caused by changes in ambient temperature.After the locked sample being excited by radio frequency,nuclear magnetic resonance(NMR)signal is converted to a lower frequency range through frequency mixing,converted into a square wave signal,and directly sent to the field programmable gate array for periodic measurement.The magnetic field compensation amount is also obtained by calculation.The magnetic field compensation obtained by the two methods is superimposed,and then converted into current signal to drive B_(0) compensation coil mounted on the magnet,thereby a magnetic field locking system is developed to realize the locking of the magnetic field.The test is carried out on a 0.5 T food rapid detection magnetic resonance analyzer.When subjected to transient interference,the magnetic field can be stabilized within the range of±4 Hz(corresponding to±0.0939μT),and the magnetic field drift caused by temperature can also be accurately measured,which verifies the feasibility of the magnetic field locking method presented in this paper.