What Is a Metal Detector?
A metal detector is a detector that checks for the presence of foreign metal objects in a sample.
They are widely used in the food and pharmaceutical industries. Metal detectors can detect metal objects that are too large to be visually inspected. It can detect foreign objects more reliably than visual inspection.
Typical food items that can be inspected with metal detectors are dry foods such as noodles and confectionery, wet processed foods such as raw meat and fish, and foods packaged in aluminum vapor deposition such as frozen foods and snack foods. However, it should be noted that the moisture content, salt content, and temperature of the object may affect the results.
Uses of Metal Detectors
Metal detectors are often used to inspect products that enter the body, such as food and pharmaceuticals. In particular, they are used to ensure food safety under the HACCP (Hazard Analysis Critical Control Point). The use of metal detectors in food factories is increasing as sanitation control methods become fully mandatory under the HACCP.
In addition to metal detectors, X-ray foreign matter detectors are available as devices for detecting foreign matter in food products. Since metal detectors are specialized for detecting metals, they are easy to introduce because of their relatively simple structure, low price, and compact size.
In contrast, X-selective foreign matter detectors can detect glass and resin in addition to metals, but they are relatively expensive and sometimes difficult to introduce due to their complex structure and large size. Therefore, depending on the object and budget, they are used individually or in combination.
Principle of Metal Detectors
Metal detectors are classified into AC type and DC type according to their detection principle.
1. AC Type Metal Detectors
The main feature of AC metal detectors is that they can detect both magnetic and non-magnetic metals. The AC type metal detector consists of a transmitter coil that emits an AC magnetic field and two differential receiving coils installed opposite each other. A sample is then placed in the space between this transmitter coil and the two receiver coils for metal inspection.
If there is no metal in the sample between the transmitter and receiver coils, the two receiver coils receive magnetic field lines of equal strength from the transmitter coil. However, if there is a metal with magnetism, the alternating magnetic field emitted from the transmitter coil magnetizes the metal, attracting the magnetic field lines to the metal side and creating a difference in the magnetic field lines received by the two receiver coils, which allows detection of the metal. Metals with magnetic properties include iron, cobalt, and nickel.
If there is a non-magnetic metal that has no magnetism, the AC magnetic field from the transmitter coil generates eddy currents around the non-magnetic metal. The magnetic field near the nonmagnetic metal, which changes due to the eddy currents, can be detected by the receiving coil. Nonmagnetic metals include aluminum, copper, and stainless steel.
In addition, some AC type metal detectors transmit magnetic field lines of multiple frequencies from the transmitter coil. This is because the optimum frequency for detection varies depending on the type and shape of the metal. By sequentially emitting magnetic field lines of multiple frequencies, optimum detection can be achieved.
2. DC Type Metal Detector
The main feature of DC-type metal detectors is that they can detect magnetic metals mixed in retort pouch foods and snack foods that use aluminum as the packaging material. The DC metal detector uses a permanent magnet to apply a magnetic field to the sample.
If the sample contains a magnetic metal, the metal is magnetized and its internal spins are aligned in one direction. When the metal spins in the sample are aligned and pass between two coils, called pickup coils, an electromotive force is generated by Faraday’s law of electromagnetic induction, and this is detected to determine the presence of metal.
Note that aluminum is a non-magnetic metal, so it is not magnetized, and only the magnetic metals in the sample are detected.
Other Information on Metal Detectors
Calibration of Metal Detectors
Metal detectors are calibrated using a test piece, typically made of iron or stainless steel, to quantitatively check performance and characteristics. Rather than calibrating only the test piece by placing it over the metal detector, it is better to place the test piece in the same container as the actual sample or to attach it to the sample for more accurate calibration.
When fixing the test piece to the sample, depending on the shape of the sample, it may not always be fixed stably. Since any change in the fixation of the test piece will affect the calibration, various methods are employed to fix the test piece to the sample.
One example is to place the test piece in the pocket of a plastic bag with a pocket and cover the sample. In this method, the sample and test piece are wrapped together in the plastic bag, and the test piece can be fixed in the desired position. In addition, the vinyl material is less susceptible to magnetic fields, making the method using a vinyl bag suitable for calibration.
Another method is to use rubber bands to secure the test piece to the sample. Rubber bands are also suitable for calibration because they are less likely to be affected by magnetic fields. Another method is to attach the test piece to a basket made of a resin material that does not easily affect the magnetic field, and place the sample in the basket for calibration, which is used when the sample has many irregularities.
In the method of fixing the test piece to a special jig, a special jig is created to support a product that easily falls over, such as a stand pouch, and the test piece is fixed to this jig.