Food & Industrial Multi-Frequency Metal Detectors

Metal detection technology overview

Metal detectors used in the food processing industry find small particles of ferrous, non-ferrous and stainless steel using coils wound on a non-metallic frame and connected to a high-frequency radio transmitter. When a particle of metal passes through the coils, the high frequency field is disturbed under one coil, changing the voltage by a few microvolts. The output is used to detect metal.

More specifically, three equally spaced coils surround the aperture or opening through which inspected material passes. The central coil connects to an oscillator circuit to produce a magnetic field. The coils on either side of the central coil receive this signal. These are the receiving or input coils.

[Note that multiple transmit coils can be configured to yield a more effective magnetic field and much improved sensitivity so metal spheres up to 20% smaller in diameter can be detected reliably.]

Since the input coils are equally spaced from the oscillator, they receive equal amounts of signal. The coils are wound in such a way that their signals oppose each other so the net signal across them is zero. When a piece of metal enters the magnetic field, it alters the field strength around it. As this metal passes through the aperture, it changes the balance of the receiving coils so that the net signal is no longer zero.

Multiscan metal detection technology operates using the balanced-coil, full-loop detection system. However, it uses more than three coils in its design. Up to three pairs of oscillator coils provide higher levels of sensitivity over traditional methods. Parallel and series arrangements of coils are used. Two receive coils are used to produce the metal signal.

A digital signal processor (DSP) processes this signal. The DSP performs the product compensation, phasing, residual compensation filtering, and produces a reject signal. The detector used in multiscan technology is a high performance, measuring instrument. The quality of the installation has a direct effect upon performance and reliability.

Frequency

Different sizes of the same metal have different magnetic and conductive reactions. Alloys of metals have different reactions too. And the shape, orientation and position of the metal can change the resulting signals in a metal detector.

What is the “best” frequency for any metal detection application? The answer is: as many as you can get all at once. This is the premise behind Multiscan technology. The operator picks a set of up to five frequencies from 50 kHz to 1000 kHz and Multiscan scans through each frequency at a very rapid rate, effectively acting like five metal detectors in one. As a result, the detectors can run a frequency close to ideal for most any type of metal encountered. The result is that the probability of detection goes up exponentially and escapes disappear. Sensitivity is optimized because the optimal frequency is running for each type of metal of concern. Multiscan technology enables users to identify contaminants that are up to 70 percent smaller in volume than previous technologies.

Types of metal that can be found by metal detection technology

It is widely understood that ferrous is the easiest metal to detect due to its magnetic properties. Magnets attract iron. An electromagnetic field reacts most when a ferrous metal is in it, and the lower the frequency the greater the reaction.

Conversely, stainless steel, which contains only a small amount of ferrous metal, has little or no magnetic property. To find stainless steel with a metal detector requires running a high frequency because the high frequency field induces a current in the stainless steel, which creates a new field that interacts with the original field in the metal detector to create a signal.

Here is a chart of common metals found in the food production process.  The higher number of dots indicates the easier to detect.

Conventional metal detectors use a technique to ignore magnetic and conductive product effects called “phasing.” Anything passing through the metal detector with a known ratio of magnetic to conductive signal below a threshold is ignored because it is the product and not a piece of metal.

If looking for a piece of stainless steel hidden in a piece of cheese, for example, you could vary the frequency a bit (this separates the product and metal phase angles) and detect it, but that could mean that you won’t see a different size of stainless steel because this contaminant phase angle lines up exactly with the cheese. So, the problem isn’t fixed; it just moved somewhere else. It is similar to patching a hole in your roof and then drilling a new hole in a different place. Water still comes in. This is a very subtle but important real-world problem because no single-frequency metal detector ever has perfect detection. Adding a second fixed frequency makes things a little better but in the end you still have holes in your roof.

By scanning up to five user-selectable frequencies running at a time, multiscan metal detection technology provides unmatched sensitivity and the highest probability of finding ferrous, non-ferrous, and stainless steel metal contaminants in challenging applications such as dairy, meat, poultry, bread, and other foods with high product effect.

Learn more about the Sentinel Multiscan Metal Detector