INSTITUTE OF PHYSICS PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGYSupercond. Sci. Technol. 19 (2006) S280–S283

doi:10.1088/0953-2048/19/5/S24

A food contaminant detection system
based on high-Tc SQUIDs
Saburo Tanaka1 , H Fujita1 , Y Hatsukade1, T Nagaishi2 , K Nishi2 ,H Ota2 , T Otani3 and S Suzuki3
1Toyohashi University of Technology, 1-1 Hibarigaoka Tempaku-cho Toyohashi Aichi
441-8580, Japan
2
Sumitomo Electric Hightechs Co., Ltd, Itami, Hyogo 664-0016, Japan3Advance Food Technology Co., Ltd, Toyohashi, Aichi 441-8113, Japan

Received 20 September 2005, in final form 22 November 2005
Published 6 March 2006
Online at stacks.iop.org/SUST/19/S280
Abstract
We have designed and constructed a computer controlled food contaminantdetection system for practical use, based on high-Tc SQUID detectors. Thesystem, which features waterproof stainless steel construction, is acceptable
under the HACCP (Hazard Analysis and Critical Control Point) programme
guidelines. The outer dimensions of the system are
1500 mm length × 477 mm width × 1445 mm height, and it can accept
objects up to 200 mm wide × 80 mm high. An automatic liquid nitrogen
filling system was installed in the standard model. This system employed adouble-layered permeable metallic shield with a thickness of 1 mm as amagnetically shielded box. The distribution of the magnetic field in the box
was simulated by FEM; the gap between each shield layer was optimized
before fabrication. A shielding factor of 732 in the Z -component wasachieved. This value is high enough to safely operate the system in a
non-laboratory environment, i.e., a factory. During testing, we successfully
detected a steel contaminant as small as 0.3 mm in diameter at a distance of75 mm.

1. Introduction
The consumption of processed foods is a common featureof daily life. Although great efforts are made to prevent
contamination during food processing, the possibility remainsthat contaminants may be accidentally mixed with foodproducts. Examples of these contaminants include small metal
chips or fine wires from a strainer element of a processing
machine, and broken syringe needles from immunizations
or hormone injections that may remain in the meat of foodanimals. Because of the increase in international concern
regarding food safety, there is a clear need to develop ahighly sensitive metal detector to ensure food and drug
safety. Several detection methods currently exist, such as
eddy current detectors, x-ray imaging, and systems basedon superconducting quantum interference devices (SQUIDs).
The eddy current method is widely used; however, thesensitivity is highly affected by the conductivity of thecontaminants and the food itself. Since wires of strainer
elements or syringe needles are made of stainless steel,their conductivities are lower than carbon steel, making0953-2048/06/050280+04$30.00 © 2006 IOP Publishing Ltdthem difficult to detect using the eddy current method.
The x-ray method is a useful technique and is increasingly
popular in food factories, and it can detect both metal andnonmetallic contaminants. However, maintenance costs for
x-ray equipment are high, and the lower detection limit forpractical x-ray usage is of the order of 1 mm. Alongwith others, the authors have proposed the developmentof a detection system using a SQUID magnetic sensor tocircumvent the difficulties outlined above [1–5]. The detection
technique is based on recording the remanent magnetic fieldof contaminant by SQUID sensors. Here we describe acontaminant detection system for food contaminants with arobust magnetically shielded box.

2. Principle
A block diagram of the detection system is shown in figure 1.
It consists of a permanent magnet, a conveyor, a magneticallyshielded box, and a SQUID sensor. All of the samples movefrom left to right and pass through the magnet tunnel before

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A food contaminant detection system based on high-Tc SQUIDs

Figure 1. Block diagram of food contaminant detection system.
The system consists of a permanent magnet, a conveyor, amagnetically shielded box, and SQUIDs.

detection. An austenitic stainless steel material is originallynonmagnetic. However, it shows properties similar to thoseof a ferromagnetic material after martensitic transformation
during its fabrication [6]. Therefore the magnetization prior
to the detection is also effective for austenitic stainless steel
contaminants. The remanent magnetic field from a metallic
contaminant in food is detected by the SQUID magnetometerswhen it passes below the sensor.

Figure 2. Appearance of the complete system. The size of thesystem (L × W × H) is 1500 mm × 477 mm × 1445 mm, and it iscovered with stainless steel, which is approvable under the HACCP(Hazard Analysis and Critical Control Point) guidelines.

3. System
3.1. General specification
The size of the whole system (length × width × height) is1500 mm × 477 mm × 1445 mm; it is covered with stainlesssteel, and approvable under HACCP (Hazard Analysis andCritical Control Point) guidelines. The magnet is made of Ndbase alloy and its magnetic field is 0.1 T. The liquid nitrogen(LN2 ) cryostat used for maintaining the temperature of theSQUIDs at 77 K consists of three separated glass Dewars. Thesize of each Dewar is 70 mm O.D. × 50 mm I.D. × 300 mm
in length. The total volume of the cryostat is 1.5 l and theliquid nitrogen can be maintained for 12 h without filling.
An automatic LN2 supply system is attached. Three highTc SQUIDs are installed in this system. The SQUID and
its driving electronics employed here were manufactured bySumitomo Electric Hightechs [7]. The size of the pickup loop
is 10 mm × 10 mm square and it is of the high-Tc directly
coupled type. The sensitivity of the SQUID is nominally
300 fT Hz−1/2 at 10 Hz. The SQUID driving electronics areof modulation type with a bandwidth of 10 kHz. The signal
is passed through a low-pass filter (LPF) at a cutoff frequencyof 5 Hz. The system is totally controlled by a PC and can be
operated by touching the display panel in front of the system.
All the electronics except for the PC and the glass Dewars withSQUIDs are surrounded by aluminium shield panels which
prevent electromagnetic disturbance outside the system. The
appearance of the system is shown in figure 2 [8].
3.2. Magnetic shield design
Since the magnetic shield covering the sensors is the crucialpart of the system for practical use, special attention wasgiven to its design. Prior to system design, 3D analyses ofthe magnetic field distribution inside the magnetic shield werecarried out.

Figure 3. Simple tri-layered cylindrical magnetic shield model for
finite element method (FEM) simulation. The shield dimensions are
φ850 mm × φ750 mm × φ650 mm × 1600L mm. Thickness of thematerial is 2 mm.

First, a finite element model (FEM) simulation wasperformed on a PC for the simple tri-layered cylindricalmagnetic shield model shown in figure 3. The modeldimensions were φ850 mm × φ750 mm × φ650 mm ×1600L mm. The thickness of the material was 2 mm. The
simulation software Maxwell (Ansoft Corporation, Japan) wasused. A dc magnetic field of 5.3 µT was applied along theZ -direction and the field distribution inside the shield wascalculated as a function of the relative permeability µs of thematerial. The shielding factor (SF) was calculated as a ratioof the imposed external magnetic induction to the value ofthe magnetic induction at the centre of the internal region ofthe shield. Figure 4 shows the dependence of the SF on therelative permeability. It indicates that the SF increases linearlywith relative permeability µs . An actual tri-layered cylindricalmagnetic shield, having the same dimensions as the model
described above, was then built using permalloy. The SF ofthis shield was obtained by applying a dc field of 5.3 µT; theSF was found to be 1250 at the centre of the cylinder. We thus
estimate the relative permeability of the material to be about
20 000 by comparing the value shown in figure 4.
Second, a rectangular shape model as shown in figure 5,
which is more realistic and suitable for our system, wasS281

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Figure 4. Dependence of the SF on the relative permeability. The
SF was found to increase linearly along the relative permeability µs .

Figure 6. Shielding factor as a function of layer separation. The SF
presents a nonlinear behaviour with a maximum at around the layer
separation of 100–200 mm.

4. Characterization of the system

Figure 5. Rectangular shape model for FEM simulation. Thismodel is more realistic and suitable for the system. Two shieldlayers are used. The size of the inner shield box is fixed(1000 mm × 252 mm × 482 mm, L × W × H); the spacing between
inner and outer boxes is changed from 10 to 400 mm.

employed for the simulation. Two shield layers were used.
The same software was used for the FEM simulation. A
magnetic field of 50 µT was applied along the Y -direction withan angle of 10◦ . The thickness of the permalloy layer and therelative permeability were assumed to be 1 mm and 20 000,respectively. The size of the inner shield box was fixed at1000 mm in length, 252 mm in width, and 482 mm in height;
the spacing between the inner and outer boxes was changed
from 10 to 400 mm [9]. The shielding factor SF was calculated
in the same manner as for the cylindrical case. Figure 6 shows
the shielding factor calculated as the ratio between the Z component of the magnetic field outside and at the centre of
the shield (z = 140 mm from the bottom of the inner shieldbox). The shielding factor presents a nonlinear behaviour anda maximum at around the layer separation of 100–200 mm.
Due to material cost considerations, a separation distance of
100 mm was selected for the shield box design. In this modela shielding factor SF of 634 was obtained.
The shielding factor was then analysed in the same manner
when the length Y of the inner shield was shortened to 700 mm
and the height was extended to 532 mm; an SF of 523 wasobtained, which is high enough for practical use. The final
dimensions of the inner shield (L × W × H) were set at
700 mm × 252 mm × 532H mm, while the outer shield was
902 mm × 454 mm × 734 mm.
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The double-layered shield box described above was fabricated.
Then the actual shielding factor of the shield box was evaluatedby applying a dc magnetic field. An SF value of 732 wasobtained, which was 40% larger than the calculated value. Thissuggests that the relative permeability of the shield materials
was higher than the value we expected because it is dependent
on the annealing conditions.
The performance of the total system was tested. Steelballs and stainless steel balls with different diameters wereprepared as test samples. The samples were put on theconveyer and magnetized. Then they were passed below theSQUIDs with a speed of 14 m min−1 . Figure 7 shows thetime traces of the signals from a SQUID located above themiddle of the conveyor. The signals were recorded wheneach stainless steel ball (sus304) with a diameter of 0.3 and0.6 mm passed through the device just under the SQUID witha separation of 75 mm. A conversion factor (volt to tesla)of 1.2 nT V−1 was used. Clear signals with signal-to-noise(S/N) ratios of more than 4 were obtained. The signal was
not affected by either electromagnetic radiation of a nearby
mobile phone or the motion of a bulky steel cart near thesystem. Figure 8 shows the dependence of the SQUID output
peak signal on the sample diameter. For steel balls (solidcircle), the signals scaled well with the cube of the diameter.
This suggests that the signal is proportional to the volume ofeach sample. In contrast, stainless steel balls (open circles),which are nominally austenitic stainless, produced signals
which deviated from expected values because their magnetism
depends on the martensitic phase content which is affected by
the mechanical history of the material [5].
It was demonstrated that the system can successfully find astainless steel ball as small as 0.3 mm in diameter. However, wenote that it may be difficult to find a contaminant in food whichcontains large amounts of iron, such as blueberries, seaweeds,
and chocolates, because the magnetic signal of the contaminantbecomes comparable to that of the food.

5. Conclusion
We have designed and constructed a food contaminantdetection system using high-Tc SQUID magnetometers. TheA food contaminant detection system based on high-Tc SQUIDs

Figure 7. Time traces of the signals from a SQUID located in the centre. They were recorded when each stainless steel ball (sus304) with adiameter of 0.3 mm and 0.6 mm passed through the device just under the SQUID with a separation of 75 mm. The conversion factor of volt
to tesla is 1.2 nT V−1 .

as small as 0.3 mm in diameter were successfully detected withan S/N ratio higher than 4. The system was robust and was notaffected by the disturbance of electromagnetic waves generated
by a mobile phone or the motion of nearby iron objects.

Acknowledgments
This work was supported in part by the Cooperation ofInnovative Technology and Advanced Research in Evolutional
Area, and by the 21st Century COE Program ‘EcologicalEngineering for Homeostatic Human Activities’ from theMinistry of Education, Culture, Sports, Science, andTechnology.
Figure 8. Dependence of the SQUID output peak signal on thesample diameter. For steel balls (solid circles), the signals scaled
well with the cube of the diameter. In contrast, the signals obtained
from stainless steel balls (open circles), deviated from expectedvalues.

detection technique is based on recording the remanent
magnetic field of a contaminant using SQUID sensors. The
magnetic shield which covers the SQUID sensors was designed
carefully because it is a crucial part of the system for practicaluse. After the FEM simulation, a double-layered rectangularmagnetic shield using permalloy metal was made with ashielding factor SF of 732. Spherical stainless steel test balls

References[1] Tanaka S 2005 IEICE Trans. Electron. C 88 175–9[2] Tanaka S 2004 Supercond. Sci. Technol. 17 620–3[3] Bick M 2003 ISEC03: 9th Int. Superconductive Electronics
Conf. PTh06 (Extended abstracts)[4] Donaldson G B, Cochran A and McKirdy D 1996
Fundamentals and Applications ed H Weinstock (New York:Kluwer–Academic) p 599[5] Krause H 2005 IEEE Trans. Appl. Supercond. 15 745–8[6] Huang H 1996 Mater. Sci. Eng. A 216 178–84[7] Available online http://www.shs.co.jp/squid/[8] Available online http://www.aftweb.co.jp/squid/index.htm[9] Fauri M 1990 IEEE Trans. Magn. 26 364–7

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