How to overcome orientation effect in food metal detectors

Many variables will determine the theoretical sensitivity of a metal detector. Among them the aperture size (the smaller the aperture, the smaller the piece of metal that can be detected), the type of metal (ferrous, non-ferrous or stainless steel), product effect, and the orientation of the contaminant as it passes through the detector. Environmental conditions, such as airborne electrical interference - static, earth loops - vibration and temperature fluctuation may also affect performance.

Orientation effect is a result of asymmetrical metal contaminant shards being more easily detected if they pass through the metal inspection system in one direction rather than another. A typical scenario occurs when equipment is calibrated to detect a stainless steel sphere that's 2mm in diameter. While it may identify and reject this contaminant, the machine may fail to detect a stainless steel wire that is smaller in diameter but up to 24mm long, depending on the orientation of the wire as it travels through the detector.

Typically, it is easier to detect stainless steel and non-ferrous wires when they pass through the aperture space sideways or upright, rather than in alignment with the conveyor. The reason for this is down to the magnetic permeability of the metal, which for stainless steel is much lower than other metals.

Phil explains: "Most test pieces use a spherical object, so the signal they give off is the same in any orientation. Yet for a sliver of metal, wire or pins, the product needs to be tested in different orientation to ensure maximum effectiveness. It's also critical to remember that locating a metal contaminant in one orientation doesn't automatically mean another won't slip through at a different orientation." So what are the options?

Some suppliers may recommend positioning several metal detectors at various angles along the conveyor, improving the chances of picking up and rejecting a contaminated product. Additional investment and the costs incurred for maintaining multiple machines are the downside to this option. "You would also have to ensure that spherical sensitivity is not compromised and the benefits of having several machines on a line are not offset," notes Phil. Reducing the aperture size is widely regarded as the simplest and most effective way to increase metal detector sensitivity. Phil puts this into context: "Because sensitivity is measured at the geometric centre of the aperture, the ratio of the aperture to the size of the product is an essential consideration. Maximum sensitivity occurs when the belt and food item is closest to the edge of the metal detector portal, so it makes sense that the smaller the aperture, the more failsafe your system is."

Factoring in frequency

Frequency settings must also be factored in. While dropping the frequency can enhance your ability to find ferrous metals, it can limit performance when it comes to non-ferrous metals, because the lower end of the frequency is more responsive to magnetic effects of the contamination. By the same token, the reverse happens when taking the frequency higher - you start to limit your ferrous detection capability but enhance non-ferrous detection.

To help combat product and orientation effect, Fortress developed and launched the new Interceptor range in late 2015. Using multi-frequency technology, the higher frequency signal can improve detection levels for stainless steel by as much as 100 per cent, as well as performing better on lower density metals like aluminum. The high and low frequencies emitted by the Interceptor machine also help to remove the product effect signals caused by wet and conductive products, emphasises Phil. He says: "The size of contaminant that's detectable depends on the product being inspected, its size and temperature, as well as the machine's aperture size and coil structure."

During the regular testing of metal detectors, manufacturers are advised to insert test pieces in various locations and orientations within products, for example in the front, centre and back, and run consecutive tests. Phil says: "This provides extra assurance that your metal detectors are performing as they should and picking up contaminants regardless of metal type, size or orientation."

For more information about food metal detectors and the orientation effect go to www.fortresstechnology.co.uk.