Measures to Prevent False Rejects in Metal Detectors: External Noise Part 2

In the previous technical note*, we explained false rejects in metal detectors caused by electrical noises such as radiation noise, conduction noise, and static noise. In this paper, we will focus on false rejects and countermeasures caused by two types of noises. One is the noise that occurs when metal parts of production equipment move within the magnetic field generated by the metal detector, and the other is the noise caused by electromagnetic induction occurring in peripheral equipment such as roller conveyors and packaging machines.

*Please refer to "Measures to Prevent False Rejects in Metal Detectors: External Noise Part 1".

From this experiment, we can see that the metal parts near the detection head could potentially affect a metal detector's magnetic field when they vibrate. Now, let's take a look at some examples that frequently occur on the actual production line.

1. Regularly tighten the bolts ensuring there is no looseness.

2. If that does not solve the problem, change the metal product guide to a resin one.
Yet, be cautious as the resin can degrade and may break off. We provide a Guide Bank that does not come into direct contact with the inspected item for non-packaged products.

The same phenomenon might occur if the bolt of the stainless steel crossing plate which smoothly transfers from the upstream conveyor to the conveyor of the metal detector loosens up. As the camera stand attached to the chassis may shake if the fixing stay does not have adequate strength, it is recommended to place it in a different location.

Glue or bolt the frame to prevent it from vibrating.

The copper wire is coiled up, but the indicator hardly fluctuates in a state where both ends are separated.

【Released state (enlarged view)】

When both ends of the wire are made to touch, creating a loop shape, the indicator oscillates significantly for a while.

【In contact (enlarged view)】

By wrapping one side with vinyl tape it becomes insulated, and even when both ends touch the swing of the indicator is minimal.

【Insulated state where vinyl tape was wrapped (enlarged view)】

Here, we introduce some examples of electromagnetic induction which occur on the actual line.
Electromagnetic waves are still produced via electromagnetic induction even when the power supply of surrounding equipment is OFF.

After cleaning, check that the insulating bushing has not come off, and if it has come off, reattach the bushing.

(Note) There are free roller conveyors that don't include an insulating bushing.

Please refer to the reference material "Insulation Measures" at the end of this paper.

Conclusion

In this paper, we explained false rejects caused by the noise generated when metals move around metal detectors. The principle rule of metal detectors is not to place any metallic parts or stands nearby; however, if placement is unavoidable, visually or manually check for any looseness or rough points. The insulation measures for the upstream and downstream equipment involve things like fitting resin sheets or bushings at the appropriate positions. Please implement the measures while considering whether it does not hinder the opening and closing of the cover, and whether the insulator does not fall off. When a series of false rejects occur suddenly, it can be disturbing, but please try to identify the cause using this paper as well as the previous technical note as references.

Reference Material

【Distance to avoid installing metal objects】

Excerpt from page 5 of M6-h Series Metal Detector Installation Guide (84W279066-02) issued by Anritsu Corporation.

Caution: If the size of the moving metal is large, false rejection may occur even if the metal object is outside the following range.

(7) To avoid the effect of magnetic fields, do not allow any of the metallic objects listed in (1) to (4) near the metal detector within the shaded parts in the drawing below.

[1] Fixed metallic objects;
[2] Rotating metallic objects such as rollers and bearings;
[3] Moving metallic objects such as cams and links;
[4] Electromagnetic parts such as solenoids and clutches that might generate magnetic fields.

See the table below for X and Y.

Note: The mentioned values are typical and vary with metal type and dimensions.