Detector for food

[DiggerBarns]

I would like to build a metal detector that I would call an apeture detector. A detector that is made of wood/plastic, with no metal parts, and having a tunnel or apeture that is say 10" by 4" through which I would pass items such as food, or wood, or other non metalic items. The idea is to be able to detect any pieces of stray metal that contaminate the items. I do appreciate that there will some limitation to finding very small pieces of metal especially Stainless Steel in a home made detector like this. I intend to build some of the PI projects that are on the Geotech site. What I am asking is. What sort of pulse frequency/Sample delay/Sample width for this type of application. Bearing in mind that no soil or sand is involved. Would very much apreciate any help with this.
Thanks Digger.

 

[Prospector Al]

Hi Digger,
I don't have any schematics for you, but I know that detectors for food are common for scanning cereal boxes, turkey loaves, etc.

They usually have a rectangular window, just large enough to pass the product. These detectors usually work at a high frequency--higher than 1 MHz. The transmitter coil is a high-Q circuit, which causes a
very high current to circulate in the coil, while supplying a reasonable current from the source.

The high frequency enables the detection of metal contamination of a very small size. Products that are conductive will cause a "product
effect", however and this must be dealt with. A synchronous demodulator is generally used, with a band-pass filter after the preamplifier. The demodulator gate is placed so that the quadrature
voltage is nulled out. ( The voltage crosses zero in the middle of the gating pulse. ) The gating pulse can be shifted so that the product voltage--an in-phase voltage--is cancelled by the quadrature voltage.

Apart from the special coil and the high operating frequency, this detector is very similar to the early sine-wave detectors.

The receiver coil is usually split in two, with the transmitter coil
between them. The coils are phased so that in the absence of a target
the induced voltages cancel...

Good Luck with you project,

Prospector Al

 

[DiggerBarns]

Thank you for your advice Al

 

[Eric Foster]

Hi Digger,

I have been involved with PI detectors for the food industry on many occasions. A lot depend on what you want to find, and a lot of tweaking is necessary to get good sensitivity to small items. Eventually the high frequency balanced coil systems win out, once you get down to the equivalent of 2mm diameter and less. Even bigger if it is stainless steel. An aperture detector of the size you mention, can be made quite sensitive and you should be able to get down to 5uS delay, or a bit less, with no real problems. What's needed is a low capacitance coil, and the electronics as close as you can get, to minimise the length of cable.

Another, often necessary, trick is to have a second identical coil spaced a few inches back from the first, but still a part of the aperture tunnel. This acts a second RX coil that is connected to the other input of the preamp, so that the signals subtract. Gets rid of all the electrical noise in an industrial environment. You can then wind up the gain to get good sensitivity. The receiver output usually goes to a comparator so that when a preset level is exceeded, the comparator flips and operates a relay circuit, that can operate flashing lights, sound horns, stop the conveyor, or shut down the whole plant.

Good screening of the coils is necessary at high sensitivities to prevent "product effect", or capacitance to whatever is on the conveyor. I have seen damp timber give false signals on improperly screened coils.

Eric.

 

[DiggerBarns]

Thank You Eric