Managing EMI in dairy metal detection

Noise is a common occupational hazard in dairy processing factories. From vibrating panels to mechanical rotors, stators, fans, conveyors, pumps, compressors, palletizers and fork lifts. Additionally, some less audible disturbances can impair the performance of highly sensitive metal detection equipment. The most overlooked being EMI/RFI noise generated by ground loops and electric motor drives. 

There are several causes and effects of these disturbances, with measures which can be implemented to reduce noise interference.

Many factors determine the performance of a metal detector. The main factors are the aperture size, product effect and operating frequency.  However, environmental conditions, such as airborne electrical interference – static, radio or earth loops – and vibration, for example moving metal, may also affect performance.

Unique features such as Noise Immunity Structure and AutoPhase can suppress some of this interference noise, which may otherwise require reducing the sensitivity levels manually.

The main sources of electromagnetic interference and radio frequency interference include AC motor drives – for instance variable frequency drives and servo motors - two-way radios, including walkie talkies, electric loops, electrical contacts and static discharge.

Identifying EMI/RFI noise

The most widespread challenge is quite a common issue in food processing plants, particularly on end-to-end lines incorporating robots, bagging, flow wrapping and conveyors. The effects of electromagnetic interference (EMI) can negatively impact the performance of metal detectors resulting in false detections, false rejections, and consequently increased food safety risks.

Packaging machines, for instance paper and plastic film rollers and conveyor belts, can create a certain amount of electrostatic interference. However, for this to cause an issue with the metal detector it must be very close to the coils and would typically only occur when the slider beds and belt material on the metal detector conveyor are rubbing. This could cause a build-up that eventually discharges to the metal detector case close to the coils.

EMI/RFI noise occurs when different electrically powered machines and peripheral equipment operating in close proximity to each other are not shielded or filtered correctly and it could come from an AC motor drive on the conveyor. 

The weak points often observed are from are cables that power the AC motor drive. If not shielded correctly, this can radiate EMI as AC drives work on the principle of switching the line voltage. This switching causes a fluctuating current draw on the line resulting in broadcasting RFI/EMI noise back on the AC motor drive input line.  A filter can attenuate the draw to eliminate the noise. However, even when this filtering is applied, the output cable to the motor can still radiate noise. This is when a Variable Frequency Drive (VFD) shielded cable should be used, ensuring that noise doesn’t broadcast from the cable.

The closer the source of the EMI gets to the metal detector coils, the greater the strength of the signal broadcast. Worst case scenario is when the aperture opening is in line with EMI generator as there’s a clear line of sight. If the EMI is to the side, the body of the metal detector helps to shield the internal coils from the broadcast noise. 

Radio waves

The susceptibility of a metal detector to electromagnetic interference is very dependent on its sensitivity and operating frequency. If one metal detector is transmitting a at a frequency very close to another, they risk cross talking with each other if positioned close together. 

To prevent this happening, the recommendation is that metal detectors should be spaced at least four meters apart, or staggered so they aren’t directly aligned.  As part of a site audit, it is also good practice to note the operating frequencies of metal detectors in close proximity so that a different frequency can be selected for the new equipment. 

Long and medium wave transmitters – such as walkie talkies – rarely cause problems. Providing they are operating at three watts or less, and not used in very close proximity to the metal detector coil receiver and digital communication devices, for example smart phones, are never a problem for metal detectors because they operate at a much higher frequency, well out of the band that is optimal for metal detection.

Static troubleshooting

Static electricity build up is more likely to occur on gravity and vertical metal detection applications if the pipework has not been earthed correctly. It can also occur when slider and belt material are incorrectly selected causing static to build in the metal detector aperture.

Locating a metal detector on a mezzanine floor can create potential issues. Notably more mechanical noise infractions, particularly from chutes, hoppers and conveyors. 

To ensure the most reliable performance and avoid vibration, all support structures and reject devices should ideally be of welded construction. Additionally, bolted connections should be avoided across the frame where they may form very distinct loops. 

Finding the source of the problem quickly and accurately is critical, as ongoing interference on automated processing lines can cause service disruptions. While it is impossible to completely eliminate noise interference in food production environments, taking these precautions and seeking expert guidance, can help to significantly reduce the EMI feedback and ensure metal detection performance is not compromised. 

Fortress Technology Ltd. is a privately-owned Toronto based company. Fortress Technology is the only metal detection manufacturer that, since its inception in 1996, custom manufactures metal detectors to suit its customers’ needs, application and specification while ensuring optimal performance, stated the company.

Any opinions presented in this expert column are the thoughts of the author alone. They do not necessarily reflect the views of Dairy Foods or its parent company, BNP Media.