The Trick of Finding Contamination

日本語

In Tanaka’s laboratory, a method is being studied in which strong magnetism is applied to food to magnetize the metal fragments inside, so that these metals can then be detected by sensing their magnetic fields using a high-sensitivity sensor, or SQUID (Superconducting Quantum Interference Device)¹. A functional system of detecting contaminants with this method has already been completed, and has shown excellent metal detection ability in food factories. The researchers in Tanaka’s laboratory are currently working to improve the performance of this system so that even small metal fragments can be detected.

The key to improving performance is to more effectively differentiate between actual signals and noise. Metal fragments are not the only sources of magnetic fields, rather space is filled with many magnetic fields generated from different sources. For example, the Earth is a giant magnet, and it emits geomagnetism. In addition, if electricity is flowing nearby, a magnetic field is generated. The aforementioned high-sensitivity sensor device requires a strong magnet to be placed close to the sensor in order to magnetize the metal fragments.

Magnetic fields that originate from sources other than the metal fragments are called noise. The fields of large metal fragments can be identified over such noise, but those of smaller fragments are masked by the noise and are thus difficult to detect. Even strongly magnetized metal fragments will have small magnetic fields if the fragments are small in size.

To reduce the impact of noise as much as possible, the sensor is placed inside a square metallic box designed so that food can be tested as it passes through this box. The box is made of 2 mm iron-nickel alloy plates. Magnetic fields have strong affinities to this iron-nickel alloy. Thus, magnetic fields around the sensor are concentrated on the walls of this box. In contrast, magnetic fields that pass through the box are decreased significantly. Furthermore, at Tanaka’s laboratory, this box is surrounded by another box made of the same iron-nickel alloy, which is itself inside of another box, in order to maintain the high sensitivity of the sensor.²

The external magnetic fields primarily converge toward the outer box, and even if a weak magnetic field extends inside the first box, it is converged by the second box. Similarly, if remnants of the magnetic field exist inside the second box, they are further converged by the innermost box. In this system, the high-sensitivity magnetic field sensor is installed in the space in which the external magnetic fields are reduced as much as possible. If highly magnetized metallic fragments enter this box, even small fragments could be identified with very high probability even if they were contained in cheese or wrapped in aluminum foil.

To accurately detect even smaller metallic fragments, digital filters have also been used. Magnetic fields detected by the sensor are expressed as fine waveforms, but if the sensitivity is increased, signals from metallic fragments become mixed with noise, making them difficult to identify. A digital filter can be used to accentuate these signals.

The digital filter used in this method is a computational program rather than a physical device. It is applied much like a filter and has the effect of sharpening the blurry outlines. Using a technology called “moving-average processing,” it detects signals that would otherwise be masked by noise. However, in food factories, foodstuff is transferred at a speed of 20 m/min on a conveyer belt; computing systems that can perform real-time calculations are necessary to keep up with this speed. This technique is possible since computers with such capabilities have become available.

The fundamental mechanism of the high-sensitivity magnetism sensor, SQUID uses the property of superconductivity. Since the sensor does not work unless it is in a superconducting state³, it must be kept at a very low temperature at all times; thus, it is equipped with a device that provides a constant stream of liquid nitrogen. Its principal is quite interesting on its own, but it will not be presented it in this article due to the limited space.

By the way, when watching a magic trick involving a coin on a variety show on television, even if one stares closely, one cannot figure out the trick. Modern magic cannot be performed with a single trick but is instead achieved through multiple layers of tricks. Similarly, this technology, which can accurately identify small metal fragments of about 300 microns (invisible to human eyes) in cheese that is passing through at a speed of 20 m/min, also involves a careful combination of multiple “tricks” of modern technology, such as a magnetic sensor, a triple-layered box, a strong magnet, and a digital filter. In tests of this equipment for industrial purposes that use the same principle, its sensitivity has proven to be so high that metal fragments less than the width of two human hairs could be detected.

Researchers with a wide range of knowledge and technology determine what combinations of “tricks” can achieve the desired results, thanks to their superhuman creativity. Because of their achievements, television stations can safely broadcast magic shows and food factories can safely ship their products. There are no elements that can interfere with our happiness. Therefore, the eyes of Toyohashi Tech researchers still shine today, with confidence and the sense of fulfillment.

Technological Remarks by Dr. Saburo Tanaka

• 1 When microscopic metal fragments in food are magnetized by a powerful and permanent magnet with a magnetic flux density of 0.3 T (Tesla) or higher, the magnetic domain in the metal grows and expands, lowering the slope of the magnetization curve, and leading to the saturated state. A magnetic domain that is grown in this manner does not return to its original conditions easily even if the magnetic field is removed; instead it remains in the metal as residual magnetization. The residual magnetization is weak, at several pT (picotesla, 10^-12), but it can be detected with a high-sensitivity magnetic sensor: SQUID (Superconducting Quantum Interference Device).
• 2 In magnetic shield technology, materials with high permeabilities are used.
• 3 Superconducting state: a state in which electrical resistance is reduced to zero by sufficiently lowering the temperature. Technically, three phenomena are known to occur in a superconducting state: 1) “perfect conduction,” in which the electrical resistance becomes zero, 2) the “Meissner effect” (perfect diamagnetism) that prevents magnetic flux from entering the inside of superconductor, and 3) “quantization of magnetic flux” in which only magnetic fluxes that are integer multiples of the flux quantum (Φ₀ = h/2e: 2.07 × 10^-15 Wb) can exist inside of the superconducting ring. In the high-sensitivity magnetic sensor SQUID, the third phenomenon, “quantization of magnetic flux,” is utilized. When a SQUID is placed inside of a weak magnetic field, magnetic flux attempts to enter the thin-film superconducting ring that constitutes the SQUID. However, because of the quantization of magnetic flux, only magnetic fluxes that are integer multiple of the magnetic flux quantum can exist; thus, to prevent this flux from entering, the superconducting ring generates a shielding current. A gate on the superconducting ring of SQUID that is called the Josephson junction controls the current. If the current is greater than the designated current (the critical current of the junction), the gate generates a voltage. This gate converts the shielding current into a voltage, enabling the measurement of weak magnetic fields. In other words, conversions occur in the order of changes in magnetic field → changes in shielding current → changes in voltage. However, the SQUID ring detects magnetic fields as zero when it is cooled and becomes superconducting; thus, it cannot be used to obtain accurate absolute measurements. Instead, it only measures magnetic field changes. Its sensitivity is remarkable: magnetism that is 1/100 million to 1/1 billion of geomagnetism can be measured. Superconductivity was discovered by Dr. Kamerlingh Onnes of Leiden University, Netherlands, in 1911. Initially, this phenomenon could only be confirmed when materials were cooled down to 4.2 K, which is close to absolute zero. However, since then, materials have been discovered that become superconductors at about 90 K. With the device used in this study, a high-temperature superconductor is used, which becomes a superconductor at 90 K or higher.