FEATURE Food & Beverage
Noise sources that aff ect food metal detector sensitivity
Nathan Tankard, Technical Applications Support at Fortress Technology, explains how to identify and remove noises in food-processing factories that can affect the final product’s quality
same bandwidth as processing equipment, which is a good thing.
Static troubleshooting
Any movement inside the metal detectors may cause vibrations and these can trigger false rejects. Static electricity build-up is more likely to occur on gravity and vertical metal detection applications if the pipework has not been earthed correctly.
Locating a metal detector on a mezzanine M
any factors determine the theoretical sensitivity of a metal detector: type of metal, aperture size, product
orientation and its eff ect as it passes through the detector are just some of them. However, environmental conditions such as airborne electrical interference (static, radio or earth loops), vibration (moving metal) and temperature fl uctuations can also aff ect performance. Unique features like noise immunity
structure and digital fi lters on digital metal detectors can suppress some of these, which may otherwise require reducing the sensitivity levels manually. Main sources of electromagnetic
interference (EMI) and radio frequency interference (RFI) include electric motor drives; for instance, variable frequency drives and servo motors, motor cables not shielded correctly, two-way radios (including walkie talkies), ground loops, electrical contactors and static discharge.
Ground loop feedback
One common issue in food factories is EMI aff ecting the performance of metal detectors, resulting in false detections/rejections – particularly in end-to-end processing lines incorporating robots, bagging, fl ow wrapping and conveyors. Packaging machines such as fl ow wrappers and conveyor belts tend to be
28 May 2022 | Automation
the biggest cause of ground loop issues due to worn-out or loose fi xings and rollers. Ground- loop feedback occurs when any metallic part in close proximity to the detector connects to make a conductive loop, for example an idle roller that has not been correctly insulated on one side of the frame. A loop forms that induces current, which in turn can cause signal noise that disrupts the metal detection signal and can cause processing issues, such as false product rejects.
Radio waves
The susceptibility of a metal detector to interference is very dependent on its sensitivity and detection bandwidth. If one metal detector is transmitting a similar frequency to another in a busy factory environment, there could be crosstalk. To prevent this, Fortress recommends spacing metal detectors at least four metres apart, or staggering the metal detector frequencies so they aren’t directly aligned.
Long- and medium-wave transmitters (such as walkie talkies) shouldn’t be cranked too high or used in very close proximity to the metal detector coil receiver. Keep walkie talkies operating at 3W or less.
Digital communication devices, like smart phones, emit even fewer noise interferences, which depends on how sensitive the coil unit is and the proximity of the device to the metal detector. But mobile devices are rarely on the
fl oor can create potential issues. Notably more mechanical noise infractions, particularly from chutes, hoppers and conveyors. Metal detectors that are phased to wet products are generally even more sensitive to this type of vibration and noise. To ensure the most reliable performance and avoid vibration, all support structures and reject devices should be welded. Fortress also avoids using anti- static belting material, as this too can reduce metal detector performance. Finding the source of the problem quickly and accurately is critical, as ongoing interference on automated processing lines can cause service disruptions. Fortress can deploy a sniff er unit to swiftly track the source of nearby EMI and RFI. Like an antenna, the white disc measures wavelengths and can swiftly locate the source of the competing frequencies. With this information, engineers can shield, supress or alter the path of the emissions. Fortress also off ers the option to upgrade to a high-voltage oscillator. For extremely noisy production settings, including highly- automated plants, this solution makes the Fortress metal detector the dominant noise source. Features like automated single- pass learning and calibration can deliver an accurate system setup within seconds and eliminate human errors. Additionally, built-in noise immunity structure can dramatically reduce the eff ects of external electrical noise, again, resulting in fewer false product rejects.
CONTACT:
Fortress Technology
www.fortresstechnology.com
automationmagazine.co.uk
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