金属探测仪是如何工作的

How does a metal detector work

食品在生产加工的过程中，经常使用的食品金属探测器是如何工作的？能给我说说吗？

In the process of food production and processing, how does the commonly used food metal detector work? Can you tell me something about it?

为提高食品的安全性，各种食品安全检测无所不用，而食品金属探测器则在这众多的检测环节检测设备中成为一不可忽略的检测设备。

In order to improve the safety of food, all kinds of food safety detection are used everywhere, and food metal detector has become a non-negligible detection equipment in many detection links.

金属探测器是如何工作的？

How does the metal detector work?

工业金属探测器可有两种方式安装。可以在传送带上输送的产品上面或下面安装板式线圈。但更灵敏的探测器则应该让产品穿过线圈并将线圈安装于金属壳内。这种更灵敏的线圈型，其中通常装有3个线圈——1个中心发射线圈和2个绕向相反的接收线圈装在一个探头内。与中心线圈相连的振荡器产生一个高频磁场。而两个绕向相反接收线圈相互连接，以便在磁场未受干扰时可将它们所产生的感应电压自相抵消。

Industrial metal detectors can be installed in two ways. Plate coils can be installed on or below the conveyor belt. But more sensitive detectors should allow the product to pass through the coil and install the coil in the metal shell. This more sensitive coil type is usually equipped with three coils - one central transmitting coil and two opposite receiving coils in one probe. The oscillator connected to the central coil generates a high frequency magnetic field. The two opposite receiving coils are connected to each other so that the induced voltage generated by them can be self-cancelled when the magnetic field is not disturbed.

为了获得最佳灵敏度，金属探测器的孔径尺寸应该与特定的产品大小相匹配。孔径太大，会有„削弱‟信号的危险；而孔径太小会不便于产品在传送带上通行。探测器的灵敏度通常是从由孔径的几何中心测量得来的 —— 最不敏感的几何点。当被金属污染的产品通过时，隐含的金属物对探测器的磁场产生干扰现象，两个接收线圈的输出信号不能相互抵消，从而探测出金属杂质的存在。

In order to obtain the best sensitivity, the aperture size of metal detector should match the specific product size. If the aperture is too large, it will be dangerous to weaken the signal; and if the aperture is too small, it will not be easy 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 through, the hidden metal objects interfere with the magnetic field of the detector. The output signals of the two receiving coils can not cancel each other, thus detecting the presence of metal impurities.

灵敏度与什么有关系？

What does sensitivity have to do with it?

金属探测器的灵敏度

Sensitivity of metal detectors

灵敏度的高低取决于两方面的因素：孔径的尺寸与工作频率。孔径的尺寸越小，探头的灵敏度就越高。其次，频率越高，灵敏度则也越高。不过，提高频率并不自动意味着保证探测能力的提高，因为它可能会使食品本身对探测器产生一个很强的信号，也就是通常所说的产品效应。因此，确定一个不致出现误动作的最高频率使探测器既探测了污染物、又不会发出错误警报，从而避免了产品的浪费。为了得到最佳的结果，应该对每种待检验的产品确定特定的探测参数并自动予以记录和保留。

Sensitivity depends on two factors: aperture size and working frequency. The smaller the aperture, the higher the sensitivity of the probe. Secondly, the higher the frequency, the higher the sensitivity. However, increasing the frequency does not automatically mean improving the detection capability, because it may cause the food itself to produce a strong signal to the detector, which is commonly referred to as the product effect. Therefore, the determination of a maximum frequency without misoperation enables the detector to detect pollutants without giving false alarms, thus avoiding waste of products. In order to obtain the best results, specific detection parameters should be determined for each product to be tested and automatically recorded and retained.

检验金属探测器性能的行业标准，是通过探测器检测到的各种金属球的尺寸。但事实上，污染物的形状可能是金属丝或金属片的碎屑。它们依自己在探测器孔径位置的不同而对探测器会产生不同的信号，即通常所说的方位效应。

The industry standard for testing the performance of metal detectors is the size of various metal balls detected by detectors. But in fact, the contaminants may be in the shape of scraps of wire or metal fragments. Depending on their position in the aperture of the detector, they will produce different signals to the detector, which is commonly called azimuth effect.

哪些金属不易探测?

Which metals are not easy to detect?

不易检测到的金属

Hard-to-detect metals

导电的、有磁性的金属——如铁，铬钢，碳钢和碳化钨等为最容易被探测到的金属，因为它们对金属探测器的磁场具有显著的效应。其它低电阻的有色金属也是如此——如紫铜，铝，铅，黄铜，青铜等。虽然它们并非具有磁性，但它们却属导体，也较容易探测。唯有那些纯净的、高电阻且无磁性的金属——如不锈钢类中的304和316，对金属探测器构成最大的挑战。它们的导电性差，较难探测但在食品和包装机械中却经常使用。可是只要探测器性能好，它们依然可以被可靠地识别。

Conductive and magnetic metals, such as iron, chromium steel, carbon steel and tungsten carbide, are the most easily detected metals because they have significant effects on the magnetic field of metal detectors. The same is true of other low-resistance non-ferrous metals, such as copper, aluminium, lead, brass, bronze, etc. Although they are not magnetic, they are conductors and easy to detect. Only pure, high resistance and non-magnetic metals, such as 304 and 316 in stainless steel, pose the greatest challenge to metal detectors. They have poor electrical conductivity and are difficult to detect, but they are often used in food and packaging machinery. However, as long as the detectors have good performance, they can still be reliably identified.