So far, we’ve looked at how the metal detector works (Issue 78) and how to validate the test pieces (Issue 79). Now we’re going to look at each of the tests, so that we understand why they are performed.
A 3 piece test, where the non-ferrous, ferrous and stainless steel test pieces are tested through the machine is the most basic of tests. So, I’m not going to explain this test, but I would like to raise one important point that sometimes does get forgotten…
The 3 piece test must be carried out in normal product flow. Normal product flow, means the test packs must go through the detector at, at least the same speed as the normal product would go through it. And, it must also go through at the same spacing too. Sometimes you will find that when the line is not filled with product (which may be case when the operator is trying to do a test), the packing machine may slow down to allow for the reduction in product throughput. If the operator does the test at this speed, it is not reflective of normal product flow, therefore you have not proven that the machine would work at normal product flow. You also need to consider how you are going to get the test packs onto the line, in normal product flow, if the line is sealed off with guarding.
Leading, middle and trailing edge test…
What we’re going to cover is the leading, middle and trailing edge test and why we do it – because although it’s essentially the same as the 3 piece test (as in, you can use the 3 test pieces), there is a reason why we go to the effort of putting the test pieces in these particular places on the product (as in leading, middle and trailing edges). And, more importantly, why sometimes it’s just relevant, or even possible if your product is square or round!
So, the reason we do the leading, middle and trailing edge test is to prove that the reject mechanism is set up and working as it should. To understand this, we need to look at how the metal detector rejects the product. The reject signal is normally triggered in two ways:
- The signal from the metal – causes the metal detector to reject,
- the signal from the metal, plus the signal from the product passing through a sensor which is positioned either before or after the search head – causes the metal detector to reject. (Both things must happen or the rejection won’t happen.)
If the metal detector triggers the rejection sequence using the first option, and long packs of product are being produced, there is a risk that the rejection system may not be set up correctly to ensure that the pack in question is rejected – because the reject arm for example, may trigger too early or too late and therefore either miss the product entirely, or hit the back end or front end, meaning it doesn’t reject fully into the reject bin.
This is because the time and distance from the signal from a metal at the front of a long pack and the time and distance from the signal from a metal at the end of a long pack may differ greatly. This means, that the reject system needs to be set up so the timing works for both cases. This is where a leading and trailing edge test would prove that the timing for metal in the back end and the front end of the product, works – and so, would be rejected.
For metal detection systems that use the second option to trigger the rejection sequence and where packs are smaller than the aperture of the search head, then the leading and trailing edge test does not need to be carried out to prove that the reject mechanism is set up correctly, because the sensor will make sure that the rejection happens at the right time. However, the test should be carried out (perhaps as a fail safe test) to show that the sensor is actually working.
To do the test, or not to do the test – that is the question!
So, this means that the test is not always applicable – if the length of the product is smaller than the length of the search head, as it is moving through the detector. Also, doing a leading and trailing edge test does not add any value, if the product does not have a leading and trailing edge side – in the case of square or round products.
To prove that the test is not applicable, you need to measure the length of the search head. Then, go through all your pack sizes and record the length of each one (in the orientation it would be going through the search head) on your metal detector validation report.
Where the length of the product is less than the length of the search head, the test is not needed.