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Since the "balanced coil" technology was first registered in the 19th century, metal detectors have seen considerable development. Even major advances have occurred in recent years: the replacement of analog detectors for digital detectors and the adoption of powerful microprocessor technology are the most important developments. They provide greater performance, higher sensitivity and processing capabilities, and now even in extremely demanding applications, metal detectors can still be developed and used as customer needs.
How does a metal detector work?
Industrial metal detectors can be installed in two ways. A 'plate' coil can be mounted above or below the conveyed product on the conveyor. However, a more sensitive detector should allow the product to pass through the coil and install the coil in a metal shell. This kind of more sensitive 'coil' type, which usually contains 3 coils (see Figure 1, Figure 2) - 1 central transmitter coil and 2 opposite receiver coils are housed in a single probe. The oscillator connected to the center coil generates a high frequency magnetic field. The two opposing receiver coils are connected to each other so as to cancel out the induced voltages generated by them when the magnetic field is not disturbed. 
Can detect multiple sizes of metal objects?
The level of sensitivity depends on two factors: the size of the aperture and the operating frequency. The smaller the aperture size, the higher the sensitivity of the probe. For example, a hole with a height of 50 mm will be able to detect metal objects of only 0.5 mm. Second, the higher the frequency, the higher the sensitivity. However, increasing the frequency does not automatically mean that the detection capability is improved because it may cause the food itself to produce a strong signal to the detector, known as the 'product effect'. Therefore, our goal is to determine the highest frequency that will not cause a malfunction. In this way, the detector detects both contaminants and false alarms, thereby avoiding product waste. For best results, specific detection parameters should be determined for each product to be tested and automatically recorded and retained.
The industry standard for verifying the performance of metal detectors is the size of various metal balls detected by the detector. In fact, the shape of the contaminants may be debris from the wire or sheet metal (see Figure 3). Depending on their position in the detector aperture, they generate different signals to the detector, known as the 'azimuth effect'. 
Conductive, magnetic metals such as iron, chromium steel, carbon steel, and tungsten carbide are the most easily detected metals because they have a significant effect on the magnetic field of the metal detector. The same applies to other low-resistive non-ferrous metals such as copper, aluminum, lead, brass, and bronze. Although they are not magnetic, they are conductors and are easier to detect. Only pure, high-resistance and non-magnetic metals, such as 304 and 316 in stainless steel, pose the greatest challenge for metal detectors. They are poorly conductive and difficult to detect but are often used in food and packaging machinery. However, as long as the detectors perform well, they can still be reliably identified.
How can a metal detector be integrated with a production line?
In order to obtain the best detection effect of a metal detector, it is just as important to select the best machine for it as possible. Although metal detection systems are often considered when planning to install new production lines, metal detectors for existing production lines can also be easily installed in existing production lines.
Most food and drug manufacturers place metal detectors at the end of the production line to conduct final inspections of the goods before they are delivered to retailers and customers. However, from the cost point of view, the earlier detection of the contaminants is more valuable because only the low-cost ingredients are wasted rather than the finished products that have been packaged. The ideal mounting position for metal detection systems varies with product and industry, and varies from factory to factory. For most manufacturers, if the metal detectors are installed in the relatively front-facing process within the production line, the limited space is often a problem. However, some major manufacturers have already noticed this problem. For instance, Rock Lock Company has developed a metal detector named “Waferthinâ€. Its compact volume can be completely accommodated in a limited space, for example, in a bag making machine/packaging. Above the machine. Most manufacturers can configure the system according to the customer's space requirements.
Liquid products, such as soups or fillings, can be detected on-line by pipeline detectors. Pipes connected to the processing equipment carry the product through the detector. Metal detector manufacturers should be able to provide accessories that connect the equipment to any processing equipment. The type of pipe required depends on temperature, flow rate, and viscosity of the product, but must be made of non-metallic materials.
Many food manufacturers install metal detectors prior to heavy and fragile equipment, as this protects the downstream machinery from damage and avoids production interruptions, thereby directly benefiting the economy. The detectors can then act as guards for fragile transfer pumps, press rolls and valves, preventing them from being damaged by metallic contaminants. For example, in the production of cookies, the detector is usually formed before the dough is formed into the forming drum. The patterned brass roller and the forming roller are very expensive and easy to scratch, and will encounter more serious damage to metal impurities. Installing metal detectors before molding the rollers protects them from hard metals in the biscuits. This also ensures that the contaminants are not further crushed, making detection more difficult.
The strict hygiene regulations for the clean-handling food industry have always been the primary considerations for metal detector manufacturers. The equipment must be easily disassembled for cleaning to avoid production interruptions. High-quality metal detectors are made of stainless steel and have a degree of protection in accordance with the IP56/66 standard, which facilitates thorough flushing at the end of each shift. If there is a higher requirement for cleaning, and there are corrosive or high-temperature products in the food industry, then it is more necessary to have such characteristics.
The important role of detecting and rejecting metal detectors is to provide reliable protection of the product through sensitive and reliable detection of harmful metal fines. However, this protection can only be achieved when the contaminated product is reliably removed from the production process and the safe product is delivered to the customer. Therefore, the performance of the automatic reject system is as important as the detector itself.
When a contaminant is found, the detector will generate a reject signal to drive the reject device after an adjustable time delay. It ensures that the contaminated product is rejected into the reject box. Excluding devices are mostly pneumatic devices. In conveyor systems, they can be push plates, flap doors or simple air blowout rejection. Some products may require certain parts of the belt to droop or tilt in order to separate the contaminated product from the good product. For example, in biscuit production, the product passes through a return zone with a recycling bin below. If a metal object is found, an entire row of cookies is removed to ensure complete product protection. On the other hand, for broth, a diverter valve is used to introduce contaminants into the second pipe, thereby removing the contaminated product without interrupting production.
Because the reject device adopts the fail-safe mode, and the electronic sensor can be used to check that the contaminated product is indeed removed from the reject box and not enter the production line, the product's safety is maximally guaranteed.
Conclusion Today's generation of metal detectors proves that innovations in the metal detection industry provide security for companies' products from contamination. At the same time, however, there is a gap between physics and process requirements, and even the most sensitive metal detector cannot guarantee the detection of every metal particle passing through it. However, the manufacturers’ expectations of the challenges faced by the production and testing equipment indicate that they will see some remarkable developments in the coming years.
Keywords: metal detectors, food industry, balance coils were invented as needed, and metal detectors have now become a major part of many food processing lines in Asia. However, how many times do we think: What is in this stainless steel shell, how do metal detectors work, and what problems do they have in the production process every day? For best sensitivity, the aperture size of the metal detector should match the specific product size. Too large a hole will risk 'weakening' the signal; if the hole is too small, it will not be convenient for the product to pass on the conveyor belt. The sensitivity of the detector is usually measured from the geometric center of the aperture—the least sensitive geometric point. When metal contaminated products pass, the hidden metal objects interfere with the detector's magnetic field, and the output signals of the two receiving coils cannot cancel each other, thereby detecting the presence of metal impurities. Which metals are not easy to detect and why?