Detection of small impurities in bulk material by MMW radar

During the production of high-quality bulk materials it may happen that unwanted metallic and/or non-metallic impurities lower the quality of the final product. In case of food materials this may even generate health problems which cannot be tolerated. In case of granulated intermediate products as for instance in plastics industry, the performance of a final structural element may suffer considerably. The major problem of detecting such impurities is the fact that many bulk materials are moved as a dense material agglomeration during production, so that the detection within the bulk cannot be performed by optical or infrared means due to insufficient penetration capability. Ultra-sound methods as well may be of limited use because of uncontrolled disturbing production noise. X-ray techniques mostly are forbidden because of harmful operation for the environment. Furthermore a suitable detection system usually must fit into existing production infrastructure to keep the additional cost as low as possible. Now, microwave and millimeter-wave imaging technology offers an attractive method to solve previously described issues. In a former study the use of millimeter-waves for detecting very small typical impurity particles in the order of around one to few millimeters size in a heap of granulated sugar was investigated and it could be shown that sufficient response is available. Now it is considered to use synthetic aperture radar (SAR) technology to perform in-situ detection during the conventional material flow of the production process. Starting with a review on the available response for typical impurities, the experimental setup for multi-static SAR experiments is outlined and first results are shown and discussed.