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

研制了一种用于食品和农畜产品中异物无损检测的电阻抗成像（EIT）测量系统。作为EIT测量的基本材料特性，被测肉制品的体积电阻率为130-180 Ω·cm，面包面团的体积电阻率为100-1000Ω·cm，与上述异物的体积电阻率有很大差异。EIT系统由数据采集子系统和图像重建子系统组成，研制了16个电极等间距分布在环形塑料底座上的数据采集探头。在相邻方法中进行通过电极的电交流注入和电压测量。采用基于有限元逼近和牛顿-拉夫逊法的图像重建方法。所开发的EIT系统在静态图像中没有提供可接受的异物图像，然而，在从不包括异物的参考系拍摄的差异图像中可以检测到异物和空隙。异物成像的灵敏度受材料中的电流密度的强烈影响。在实验中评估了材料厚度和平面型和横向型之间的电极布局的影响。

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.