Development of a detection method of foreign materials in agri-food products by electrical impedance tomography

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A measurement system of electrical impedance tomography, EIT for non-destructive and non-invasive detecting of foreign materials in foods and agricultural and livestock products was developed. It was tested for the application of the imaging and position-detection of foreign materials (metals and plastics) and voids embedded in meat products.As basic material properties for EIT measurement, volume resistivity of tested meat product was measured at 130-180 Ω-cm and of tested bread dough at 100-1000Ω-cm respectively which values were largely different from those of the above foreign materials. Therefore, EIT has an advantage for detecting those foreign materials in foods.An EIT system is composed of data collection sub-system and image restructure sub-system.The data collection probe with 16 electrodes equally-spaced on the annular plastic base was developed. Electrical alternate-current injection and voltage measurement through the electrodes were carried out in the neighboring method. An image-reconstruction method based on FEM approximation and Newton-Raphson method were adopted. A developed EIT system provided no acceptable images of foreign materials in a static image, however the detection of foreign materials and voids could be possible in a difference image taken from a reference frame including no foreign materials. Sensitivity of foreign material imaging is strongly influenced by electrical current density in a material. The influences of material thickness and an electrode layout between a plain type and a lateral type were evaluated in the experiments.