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In terms of food safety,mixture of contaminants in food is a serious problem for not only consumers but also manufacturers. In general, the target size of the metallic contaminant to be removed is 0.5 mm. However, it is a difficult task for manufacturers to achieve this target, because of lower system sensitivity. Therefore, we developed a food contaminant detection system based on high-Tc RF superconducting quantum interference devices (SQUIDs), which are highly sensitive magnetic sensors. This study aims to improve the signal to noise ratio (SNR) of the system and detect a 0.5 mm diameter steel ball. Using a real time digital signal processing technique along with analog band-pass filters, we improved the SNR of the system. Owing to the improved SNR, a steel ball with a diameter as small as 0.3 mm, with stand-off distance of 117 mm was successfully detected. These results suggest that the proposed system is a promising candidate for the detection of metallic contaminants in food products.

IntroductionMixture of metallic contaminants to food is serious problems not only for consumers but manufactures . To ensure the food safety, finding small metallic contaminants is important . A metal contaminant may come from a machine in the process of food and it contaminates food . Finding small metallic contaminants is important for food safety . Food manufactures are installing inspection systems such as eddy current detectors and X-ray imaging . The eddy current metal detector is widely used in a food factory . However, the sensitivity (threshold level) is not stable and is highly influenced by the conductivity of the material . The X-ray imaging is a useful technique and is getting popular in food factories and other industries . However, the lower detection limit for practical X-ray usage is in the order of 1 mm . Moreover, X-ray radiation sometimes causes ionization of the food, which may often change a taste of the food . We have been proposing a detection system using SQUID magnetic sensors to circumvent the difficulties outlined above [1][2][3][4] . We have developed a detection system that inspect a food package with a height of 100 mm, which is the size required for practical food inspection . Because the signal is reduced if the distance between the sensor and the target (stand-off distance) becomes large, improvement of the signal to noise ratio (SNR) in the practical system is required . Therefore, a program of real time digital filter was developed and applied to the system . The target size of the metallic contaminant in food is Φ0 .5 mm . Experimental Setup and Measurement The system consists of a permanent magnet, conveyors, a mu-metal magnetic shield box, an aluminum electro-magnetic shield box and three High-Tc RF SQUIDs magnetometer . The controlling and data processing software was exclusively developed for the system . A block diagram of the detection system is shown in Fig .1 . Detection technique is based on recording the remnant magnetic field of a magnetic contaminant using SQUIDs magnetometer . The packaged food is conveyed on a belt conveyor and passes the gate of the magnet . The SQUID sensor senses remnant magnetic field of metal contaminant of food . The SQUID and its driving electronics were manufactured by Juelicher SQUID GmbH (JSQ) . The noise of each RF SQUIDs magnetometer in the system is 300-600fT/Hz 1/2 at 10Hz, and 70-170fT/Hz 1/2 at the noise floor (>1kHz) . The signals of SQUIDs output were filtered in an analog high pass filter (-6dB/oct, f c =0 .4Hz) and a low pass filter (-24db/oct, f c = 11Hz), but SNR was not enough to detect the signal of steel ball (SUJ-2) Φ0 .5mm . Therefore we considered the introduction of a real-time moving average digital filter in addition to the analog filters . The program (C++) for the digital filter was developed and evaluated using the system . The experimental conditions are as follows; conveyer speed: 20 m/m, Stand-off (distance from SQUID to the test sample): 117 mm, test samples: steel ball Φ0 .3-0 .8 mm, sampling rate: 20-1000 H...