Classification of metal detector and its application in food industry

The people take food as the day, and food takes peace as the first. As consumers pay more attention to food safety, there are more and more complaints about foreign objects in food. According to statistics, about 50% of food quality complaints are foreign body complaints, and others taste. In order to detect foreign objects in food, large domestic food companies are basically equipped with gold inspection machines or X-ray machines. The X-ray foreign object detector for food uses the equipment to generate X-rays and apply the penetration ability of X-rays to detect metal foreign objects mixed in products and non-metallic foreign objects with higher density. In addition, the food X-ray machine can also perform product missing detection, broken packaging detection, and weight detection, but its cheapest price is more than 100,000. For small and medium food enterprises, especially small food enterprises, it is difficult to bear. The domestic price of the gold inspection machine is about 15,000-30,000, and the price of the foreign gold inspection machine is only 120,000 or 20,000. Relatively speaking, small and medium-sized food companies can basically afford it. 

1 Working principle of metal detector 

The gold detector is the abbreviation of metal detector, which is used to detect metal foreign objects mixed in the production process of food, medicine, cosmetics, and textiles. At present, 95% of the market is a balanced coil metal detector. The principle is a central transmitting coil and two equal receiving coils. These three coils are installed in a detection head. The oscillator emits a high-frequency magnetic field through the middle transmitting coil, which is connected to the two receiving coils, but the polarity is reversed. When the magnetic field is not interfered with by the outside world, the voltage output signals generated by them cancel each other. Once the metal impurities enter the magnetic field area, this balance will be destroyed, and the metal detector can detect the presence of the metal. 

The lower the operating frequency of the metal detector (25kHz--800KHz), the lower the sensitivity of the equipment; the higher the frequency, the higher the sensitivity of the equipment, this phenomenon is very obvious for non-ferrous metals. Low frequency is used for wet products and metal film packaging products, and high frequency is used for dry products. When the single frequency operation metal detector is used for monitoring dry products or wet products, the sensitivity is reduced. 

In order to improve the detection rate of foreign objects, some commercially available metal detectors use a dual-frequency magnetic field detection method, which can detect iron / non-iron with high sensitivity. Dual-frequency means that both low frequency and high-frequency signals are used for simultaneous detection. According to the correlation between detection sensitivity and frequency, low-frequency signals are easier to detect ferromagnetic metals, and high-frequency signals are easier to detect non-ferromagnetic metals. The method can simultaneously detect different kinds of metal substances with high accuracy. The actual detection accuracy of iron and stainless steel can reach below 0.8mm. At the same time, the system can automatically modify the parameters such as phase and gain according to the specific environment of the test to suppress product effects and improve the test results 

After the low-frequency and high-frequency signals are superimposed, a dual-frequency alternating magnetic field is formed in the detection area (channel) by the transmitting coil, and an induced electromotive force of the same frequency, reverse, and equal amplitude is generated in the two differentially connected receiving coils. This is the system is in a receiving balanced state. When the detected object passes, a voltage signal is generated because the induced potentials of the two receiving coils are not equal. According to the phase and amplitude characteristics of the metal substance in the unsteady magnetic field, the impedance value of the transmitting coil is changed, so that the electromagnetic field in the detection area has a transient phase and intensity, so the phase and amplitude of the generated voltage signal are also Changes. After the voltage signal passes through the band-pass filter unit, it performs low-frequency and high-frequency detection and A / D sampling (analog signal to digital signal), and then the processing center makes a judgment. It can be seen from the above principle that the performance of the dual-frequency gold detector is better than that of the single-frequency metal detector. Of course, the price is higher. 

2 Classification of metal detector

 Divided by the conveying method of the detected items, metal detectors are usually divided into channel type, falling type, and pipeline type.

 (1) Channel type. At present, the most common metal detection equipment on the market is a channel-type metal detector. The channel of the detector is square, generally equipped with a conveyor belt mechanism, with an automatic rejection device, or providing an alarm signal. When the items on the conveyor belt past the detector, they will automatically reject or stop the conveyance once there is metal. Mainly aimed at the online inspection of finished products and semi-finished products, providing final inspection before shipment.

There are two types of channel metal detectors, one is a split-channel metal detector and the other is an integrated channel metal detector. ① Split type: The advantage of the split channel metal detector is that the belt is easy to install. Especially when the channel metal detector is installed on large equipment, the split type is not limited by the belt installation or installation method. ②Integrated: The integrated channel gold detector is connected to the article conveyor belt. It is connected to the automatic rejection device and can be used in production lines, such as bagged materials, boxed products, etc.

(2) Falling metal detector. Falling metal detectors are generally equipped with automatic rejection devices, so it is used to call falling metal detectors. The packaging requirements of the metal detector for the product must not contain metal, but considering the high requirements of sealing and light-shielding, the metal composite film must be used for packaging. The metal composite film is itself a metal, so if the channel-type metal detector is used, the detection sensitivity will be greatly deviated, and even unable to detect. For the above reasons, you can choose to test before packaging. The falling metal detector is developed for the above situation and is mainly used for the detection of tablets, capsules, granules (plastic particles, etc.) and powdery particles. When these items fall through the falling metal detector, once the metal impurities are detected, the system immediately activates the separation mechanism to eliminate suspicious items. It has the characteristics of simple installation, high sensitivity, convenient maintenance, high efficiency, stability, and reliability.

（3）Pipeline. General metal detectors cannot fully monitor the entire production process of fluid products, such as ham sausage paste, chewing gum, oral liquid, sauces, etc. The pipeline-type metal detector can remove metal impurities online in real-time to ensure the safe delivery of products to the next process.

Divided by function: ①All metal detector: can detect all metals such as iron, stainless steel, copper, aluminum, and so on. The detection accuracy and sensitivity are relatively high, stable and reliable. ②Ferrous metal detector: only ferrous metals can be detected, commonly known as needle detector. The detection accuracy and sensitivity are low, and it is easy to interfere. ③Aluminum foil metal detection machine: It can only detect ferrous metals, but when detecting products with aluminum foil packaging, its detection accuracy and sensitivity are still high. 

3 Sources of foreign objects in food 

(1) Human factors bring in: such as buttons, pens, jewelry, coins, keys, hairpins, pins, paper clips, etc. 

(2) Bringing in the raw materials, the foreign materials that the raw materials purchased by the food company are brought in due to incomplete cleaning. 

(3) Cleaning the metal wire, welding slag, iron shavings generated during cutting and repairing, cutting and cutting of wire segments, etc.

 (4) Foreign objects generated during stamping, mixing, stirring, slicing, etc.

 4 Factors affecting accuracy 

The opening size of the metal detector. The larger the size opening, the larger the range beyond the product packaging, and the worse the detection effect. In general, the detection range should be controlled to the minimum value as possible. For high-frequency sensitive products, the detector channel size should match the product size. The adjustment of the detection sensitivity should be determined with reference to the center of the detection coil, and the center position has the lowest induction.

Metal type, metal shape, and direction. Balls have repeatability and the smallest surface area is also the most difficult to detect for metal detectors. The ball can be used as a reference sample for detection sensitivity. For non-spherical metals, the detection sensitivity depends largely on the location of the metal. Different locations have different cross-sectional areas, and the detection effect is different. For example, when passing longitudinally, iron is more sensitive; while high carbon steel and non-iron are less sensitive.

Operating frequency. In the food industry, systems often use higher operating frequencies. For food such as cheese, due to its inherent high-frequency induction performance, it will proportionally increase the response of high-frequency signals. In short, food companies can choose the appropriate gold inspection machine based on their specific products.

As one of the leading domestic metal detector companies, Guangdong Kenwei Intellectualized Machinery Co., Ltd. has developed many types of metal detector products with years of technical strength, including metal inspection machines, aluminum foil metal detector, and metal detector with check weigher., products are widely used in food, pharmaceutical, chemical, textile, clothing, toys, rubber, and other industries, is the food industry HACCP certification and pharmaceutical industry GMP certification of the first choice for gold inspection equipment.