Purpose: Therefore a detection method of small contaminants in food is important. We developed a practical food contaminant detection system based on ultra-sensitive SQUID magnetic sensors.
Methods: The method is based on the detection of the remnant magnetic field from small metallic contaminant in food. After magnetization, the remnant magnetic field from any metallic contaminants is detected by the ultra-sensitive SQUID sensors when the food passes below the sensor. The SQUIDs are cooled at 77 K by liquid nitrogen, which can be supplied automatically from a reservoir by a pump. The system is covered with waterproof stainless steel plates and acceptable to HACCP (Hazard Analysis Critical Control Point) program. The acceptable object size is W150 mm x H100 mm, which is large enough for practical inspection. All the system is controlled by PC. This system employs tri-layered permeable metal magnetic shield box with thickness of 1 mm and an aluminum electromagnetic shield box for preventing from magnetic and radio frequency interferences. The shielding factor of the magnetic shield is -60 dB at 0.01Hz, which is good enough to operate the system in a factory. A digital filtering technique has been newly introduced to reduce noise.
Results: The signal-to-noise ratio (SNR) was dramatically improved, and we were able to robustly detect a steel ball as small as 0.3 mm in diameter.
Significance: This system is the first product using rf-SQUID sensor in the world.