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At present, 95% of metal detectors machines use the principle of balanced coils. The advantage is that they can not only detect iron and stainless steel 304, which are both magnetic and conductive, but also detect non-magnetic metals such as copper, aluminum and stainless steel 316. , But when there are product effects, there are weaknesses such as reduced sensitivity and large interference from mechanical vibration.
Metal detection has become a key control point for food processing
For the food processing process, there is inevitably the risk of mixing metal foreign objects. Once people eat food containing metal foreign bodies, it will seriously damage the digestive system such as the mouth and esophagus.
At present, some countries have listed HACCP, or Hazard Analysis Critical Control Point (Hazard Analysis Critical Control Point), as a mandatory implementation of the regulations. The inspection and customs clearance of some imported aquatic products has become the product quality. Critical control point. Once metal foreign objects are detected, all goods will be returned or destroyed. In order to obtain green permits for entering the international market, enterprises are increasingly paying attention to the implementation of the HACCP guarantee system. The high-efficiency and stable metal detector becomes a solid barrier for the flow of canned meat, poultry, vegetables and seafood containing metal impurities to the market, which effectively guarantees food safety.
Principles of metal detection
Metal detection generally uses two methods: permanent magnets detect magnetic field changes, and balance coils detect electromagnetic field changes. The permanent magnet method can only detect magnetic metals: iron and stainless steel 304 metals, but non-metals such as stainless steel 316 and copper and aluminum are commonly used in food plants; therefore, the principle of balanced coils is generally used to detect all types of metals. The inside of the detection head for the balance coil to detect changes in the electromagnetic field is composed of three sets of coils, including the transmitting coil in the middle and the receiving coils at equal distances on both sides. Its working principle is: the electromagnetic wave generator generates a high-frequency electromagnetic field through the transmitting coil in the middle, and the receiving coils on both sides convert the induced electromagnetic field changes into voltage changes. When the magnetic metal approaches the receiving coil, the electromagnetic field increases. When the magnetic metal passes through the two receiving coils, the voltage changes from high to low; when the non-magnetic metal approaches the receiving coil, the electromagnetic field weakens, and the voltage when the non-magnetic metal passes through the two receiving coils. Change from low to high, according to the voltage change and detection algorithm to determine whether it contains metal foreign bodies.
Because the use of high-frequency alternating magnetic fields is susceptible to electromagnetic interference caused by other equipment such as frequency converters and vibrations, some "damp" products or products with inherent conductivity (ie product effects) interfere with the detection of small metals. It is necessary to quickly and accurately detect small metals. Metal must use special filtering methods. More than 20 years ago, GoringKerr in the United Kingdom first developed DSP (Dynamic Filtering) technology, which loaded the synthetic digital filter into the signal processor to become a special integrated circuit processor, which can effectively overcome the effects of product effects, bulk material effects and interference noise. To commemorate the development of DSP technology by GoringKerr, SYNDAR's previous metal detection products are named DSP series.
The signal processed by DSP technology also needs to adopt a certain algorithm to determine whether the product contains metal foreign matter. Two algorithms are usually used: amplitude detection and narrow area detection (zero crossing) to determine. Amplitude detection is when the received signal exceeds the positive and negative thresholds, it is considered that there is a metal foreign body. Its disadvantage is that it can only detect larger metal particles and cannot accurately determine the position of the metal foreign body; narrow zone detection or zero crossing is when the foreign body crosses In the narrow area of the zero axis (transmitting coil), when the receiving voltage signal has two positive and negative cross voltages, it is considered that there is a metal foreign body. The advantage is that small metals can be found and the position of the metal foreign body can be accurately determined. The disadvantage is that the small metal does not follow the large metal closely. Two voltages of positive and negative crossover will occur and are ignored. SYNDAR's DSP technology can automatically switch between these two detection algorithms to find the corresponding small metals.
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