A Portable Halbach NMR Sensor for Detecting the Moisture Content of Soybeans

We report a low-field (160 mT) and lightweight (2.45 kg) Halbach nuclear magnetic resonance (NMR) sensor for non-destructive measurement of soybean moisture content (MC) on site. The sensor was composed of Halbach magnets and a solenoid radio frequency (RF) coil. A uniform design (UD) method was applied to enhance the efficiency of optimizing structural parameters of magnets by selecting a reasonable combination of parameters for simulation. An RF coil structure was then optimized with the goal of meeting the requirement of maximizing signal-to-noise ratio (SNR). Third, the portable sensor prototype with the dimensions of
$\Phi 11.5$
cm
$\times $
15 cm was utilized to measure soybeans with different MCs using Carr–Purcell–Meiboom–Gill (CPMG) sequence to obtain
$^{1}\text{H}$
transverse relaxation decay curves. The 1-D
$T_{2}$
spectrum distributions of soybeans with different MCs were then postprocessed by applying inverse Laplace transform. The experimental results illustrate that
$T_{2}$
of soybeans increases with the increase in soybean MC. The sensitivity of Halbach sensor is 1.2241 normalized peak area/%, which is increased by 28.85%. The maximum nonlinearity and resolution are 6.388% and 1% in the moisture ranging from 5% to 60%. Besides, the weight of Halbach sensor is 63.97% lighter than the previous mobile sensor.