How do metal detectors work in the food industry?

Posted October 6, 2011 in
Insight Metal Detector For the Food Industry

Metal detectors are used in the food industry to guarantee the safety and robustness of food products.

They are most widely used throughout the industry to detect any trace element of metal that may have got into the food during the production process. The most commonly used form of metal detector works on the principle of the balanced coil system, utilising a 3 coil system with 1 transmitting coil and 2 receivers. Although the first industrial metal detector was patented in the 19th century, it was only produced in the UK in 1948.

Technological progression has meant that metal detectors no longer work by means of transistors or integrated circuits, but now use microprocessors. These give much greater sensitivity, stability and flexibility, as well as widening the range of output signals and information they provide. However, it should be pointed out from the outset, that no machine is completely infallible or absolute. Even the most advanced metal detector used in the food industry can’t detect every single particle of metal passing through them. All instruments used in measurement are to some extent restricted: for metal detectors, this restriction is size of the detectable metal particle. Even the most accurate metal detection machine may miss some minute metallic particles. None the less, metal detectors still play a valuable, important and cost-effective role in the quality control process in industry, particularly the food industry.

What are the most commonly used types of metal detector used in the food industry?

Modern metal detectors by and large fall into two main categories: systems with a general purpose head, and systems with a ferrous-in foil search head. Metal detectors with a general purpose search head are capable of detecting both ferrous and non-ferrous metals, as well as stainless steel, in fresh and frozen products, whether that be wrapped or unwrapped. Systems utilising a ferrous-in foil search head are capable of detecting only ferrous metals within foil-wrapped fresh and frozen products.

How the balanced coil metal detector works?

All general purpose metal detectors function in the same way and share many similar characteristics, though not all detectors are suitable for every application. There are all constructed in such a way that ensures the independent mechanical movement of the search head components and also prevents the ingress of dirt and water. Most typical detectors are encased in a metal box which houses the coil components. This protects them and acts as a shield. The aperture, or tunnel through which the products pass is usually lined with an hygienic, non-metallic material, generally plastic.

The ratio of the aperture to the size of the product is vitally important in ensuring optimum performance. This is because the sensitivity of the detector is measured at the geometric centre of the aperture. As this is the least sensitive part, this is inversely proportional to the size of the aperture, in particular, to the smaller of the two sides. There are 3 coils in the detector system. The transmitter coil generates a field, very much like a radio transmitter. This process ‘illuminates’ the metallic object and makes it identifiable.  The second and third coils are receivers, and are connected together to detect the presence of the ‘illuminated’ particle. They do this by responding to the conductive and magnetic properties of the metallic particle identified

The controls used to identify the particle can be mounted on the search head itself, or remotely, depending on the design and the application of your system. It doesn’t necessarily matter where the controls are mounted as this doesn’t affect the performance of the system. The signal processor is highly sophisticated. When a typical metal particle is "illuminated", the signal value at the receiver coils is one millionth of a volt. This is initially amplified by a high-performance RF amplifier, then modulated down to lower frequency, providing both amplitude and phase information. The signals are digitised and digitally processed, to optimise the sensitivity.

Magnetic Field Systems for Foil-Packed Products

These systems operate on a totally different detection principle to. They work by incorporating a Tunnel, or passage, which is subjected to a strong magnetic field and, as a result, any magnetic Material, such as a metal fragment with a ferrous content, is magnetised as it passes through. Incorporated into the tunnel are a series of coils. When the magnetised particle passes under them, a current is generated which is then amplified by the electronics of the detection system, and this is used to trigger the detection signal output.

Secondary effects, due to the movement of any conductive material in a magnetic field, will also generate signals for non-magnetic metals. However, these are small compared to the effect generated by materials with a magnetic content. Consequently, only the largest pieces of non-ferrous metals and stainless steel can be detected.