First Practical High Tc SQUID System for the Detection of Magnetic Contaminants in Commercial Products

The first practical High Tc SQUID system for the detection of magnetic contaminants in commercial products was developed. The system consists of three SQUIDs cooled by liquid nitrogen, a permalloy magnetic shield, a conveyer, and a magnetization unit. The SQUIDs are driven by modulated FLL circuits, which are controlled by a microchip computer operated by a PC through a USB interface. A LabView-based operating software was developed for SQUID tuning, data acquisition, and identification of the presence of contaminants. The developed system can inspect objects having cross sectional areas of up to 200 mm in width and 80 mm in height. The system performance was evaluated using stainless steel balls. The system could detect a stainless steel ball having diameter as small as 0.3 mm. The sensitivity of the system is practically preferable to that of metal detectors. Moreover, the SQUID system could detect stainless steel fragments down to several hundred microns that could not be detected by an X-ray detector on an actual factory line. Two systems have already been installed in food processing plants and have been functioning reliably and successfully detecting magnetic contaminants for over half a year.