Measuring dielectric properties for sensing foreign material in peanuts

Authors: JULRAT, SAKOL - Oak Ridge Institute For Science And Education (ORISE); Trabelsi, Samir

Submitted to: IEEE Sensors Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2018
Publication Date: 3/1/2019
Citation: Julrat, S., Trabelsi, S. 2019. Measuring dielectric properties for sensing foreign material in peanuts. IEEE Sensors Journal. 19(5):1756-1766.

Interpretive Summary: When peanut farmers deliver their freshly harvested peanuts to the buying points, samples are taken from the wagon or semitrailer loads for grading to determine peanut Kernel moisture content, trash content and other grading factors that are important in determine the selling price. If moisture content exceeds a certain level,10.49 % on the wet basis, the peanut loads must be sent to drying bays to be dried before they can be sold. Also, if the trash content of the sample exceeds a certain level, 6% to 10%, the load will be directed to a cleaning site for trash removal from the load. In either case, moisture or trash levels too high, the sale is delayed, and the load has to comes back for regrading. If the moisture content and trash content could be determined before the grading procedure is started, considerable time and expense could be saved at the buying points. Research has shown that kernel moisture content can be rapidly and reliably determined with a suitably designed microwave moisture meter. So new research has been conducted to learn whether trash content in samples from the peanut loads can also be suitably determined with a rapid test. This has involved measurements of the microwave dielectric properties of the peanuts and various forms of trash commonly found in freshly harvested peanuts, and studying possibilities for measuring the trash content of the peanut samples with microwave techniques. Kinds of trash from peanuts delivered to drying points in Georgia included sticks (from peanut vines), leaves, peanut shells, peanut raisins (shriveled and immature peanuts) and stones. Microwave dielectric properties of these kinds of trash and those of clean peanuts of various moisture contents were determined by microwave measurements. A new microwave measurement system was designed for the research and implemented with the cabinet of a microwave moisture meter already described. This meter used a sample holder for the unshelled peanuts 22 cm by 22 cm by 12 cm and measured the attenuation and phase shift for microwaves traversing the 12-cm sample dimension, from which the dielectric properties were determined. The new design included four pairs of low-cost printed-circuit board Yagi-Uda antennas, so that separate, simultaneous measurements could be taken on each of the four quadrants of the sample holder, thus providing improved sampling of the peanut sample for the measurements. Measurements on clean peanuts and on clean peanuts with different amounts of foreign material mixed in showed good potential for free-space measurement with four pairs of Yagi-Uda printed antennas for real-time nondestructive sensing of quantities of foreign materials in unshelled peanuts. Moisture contents with this system were also predicted with 0.5% accuracy. Such an instrument for measuring both moisture content and trash content would be very useful in the peanut industry.

Technical Abstract: Free-space dielectric properties measurements at 10 GHz with four pairs of low-cost printed Yagi-Uda antennas are proposed for detecting foreign material in unshelled peanuts, including sticks, peanut shells, peanut raisins and stones. The four-element array design of Yagi-Uda antenna allowed better averaging over the entire sample volume and improved the measurement resolution. The antennas gain is about 9.7 dBi. The beam width of each antenna is about 18 degrees and 38 degrees for E-plane and H-plane at 3dB, respectively. The attenuation and phase shift were measured for each pair of antennas by controlling automatically the microwave switches. Measurement repeatability of the system was confirmed by using different thicknesses of polyethylene dielectric slab samples. The standard deviation was less than 0.5 dB for the attenuation and less than 5.0 degrees for phase measurements. Sensitivity of each antenna pair for unshelled peanuts and different foreign materials, each loaded in a small volume (4.4 cm x 4.4 cm x 12.1 cm), was tested by using a partitioned sample container. It was shown that difference in attenuation was more than 1.5 dB for different types of foreign materials and also for unshelled peanut samples with 1% moisture content difference. Prediction of moisture content in cleaned unshelled peanut samples agreed well with the standard oven-drying method (SEC = 0.51%). The dielectric properties of foreign materials mixed with cleaned unshelled peanut samples were measured through the attenuation and phase shift. The measurement repeatability of each antenna and all four pairs of antennas are presented.