Using RF waves in balanced coil metal detectors.

Hi. I have all this painted timber I need to check for nails before putting through a machine. I was thinking of making a simple BFO detector but not sure if they will be good enough to pick up a 1.5mm pin in a 500mm by 200mm coil. Looked online it seems that balanced coil detectors seem to be the way but no real details. Sigh.

I think balanced coil detectors have 3 concentric coils through which the material being tested moves. The center coil emits radio waves which are picked up by the outer coils which are "in balance".

Whien a piece of metal passes through one othe outer coils, this upsets the balance which sets off an alarm. Does anyone have any ideas how that works exactly?

I am thinking that if all 3 coils were the same and each tuned to the same frequency with their own capacitor, then when metal went through one of the outer couls, the signal generated in that coil would increase (if the metal was ferromagnetic).

Then the outer coils could be connected to a detectors (using crystal diodes say) and their signal fed into a comparitor which would cause a signal when this imbalance occured.

How does that sound?
Would the signals be strog enough to trigger a comparitor?
What RF frequency would be best?

Thanks.

Here is a good one, with explanation how to build the coil.
As you see from the schematic operating frequency is very low in this case.

https://www.hobby-hour.com/electronics/s/groundtrack-gr1290.php

I have a metal detector (Radio Shack). From what I understand it works on the principle you describe.

I have not taken it apart. I believe the coils are a few inches across. To detect a piece of metal, it must be directly below the coils. The buzzing is louder according to the mass of the metal. The frequency is always the same.

After turning the unit on, it will buzz as I lower the sensor head down to the soil. It must be calibrated before use. I calibrate it by pressing a button while holding it an inch or two above the ground. Then it is quiet most of the time as I move it over the soil.

To detect a small nail, I'm pretty sure you will need a small coil head. As a result each pass will cover less of a swath. Hence you will need to make more passes. Or make a long narrow coil. Or an array of multiple coils.

My detector is inexpensive. It will detect the size of metal objects that would be easily spotted in the wood planks. It is not sensitive enough to spot a 1/2 inch tack, which is still a large enough size to cause trouble to a sawblade.

The better units are more sensitive. No doubt they provide better information about what you've detected. Such as a direct audio output of the BFO signal. The frequency may vary in that case. This may allow you to discriminate types of metals.

I have seen a few projects for making a metal detector. I have not built one. It seems to me like a case where the store-bought one does what I would have to go to a lot of effort to accomplish. I have found my metal detector useful for finding nails in the grass, after workmen had been doing projects on the building where I live.

I don't know if greater sensitivity requires that you run less or more current in the coils, or a higher or lower frequency in the coils.

You can use e.g. an AM radio and a suitable oscillator in a metal detector, but if the searched object is located in a wet soil, it will not work. Most metal detectors use lower frequencies, 10-100 kHz, to penetrate wet sand and soil.

Thanks. The question is really about using RF waves to detect metal so I will rephrase it.

If we have a 300 mm air coil oscillating as a tuned circuit at say 100kHz, and an identical coil next to it, the first coil will act as the primary winding and the secondry coil will have an emf across it from the primary coil. How will putting a small piece of metal (1.5mm say) inside this "transformer" affect the amount of energy being picked up by the second coil.

BradtheRad, smaller coils increase the sensetivity but reduce the depth of detection I think.

The trick is "the other coil NEXT to it". The metal detector needs these two coils, induction and pickup, be located close to each other but shifted in the common plane, so the pickup coil output is close to zero. Some capacitive transfer remains but you can use a screen strip to reduce it.
Then, if you put any metal or dielectric object in the "common" field, the balance is broken, and the pickup coil output increases.
Some metal detectors use more than one pickup coil, connected so that no output is obtained. Varying the frequency one could even detect different-conductivity materials, dielectrics, etc.

How will putting a small piece of metal (1.5mm say) inside this "transformer" affect the amount of energy being picked up by the second coil.

Click to expand...

The metal alters the flux field around the coils. In so doing it alters their inductance value. Thus altering the frequency of oscillations, or adding a harmonic, or changing the amplitude, etc.

Whether it is ferrous or non-ferrous, it has eddy currents within it. These currents generate their own fields, creating a secondary influence on what goes on in the coils.

If it is ferrous metal then I think it attracts the field loops (or distorts them, or channels them) in a similar manner as metal in a manufactured coil or transformer.

That's my meager understanding.

People here have been talking about two different principles of operation. One is using a single coil which is part of a self oscillating LC whose frequency is changed when permeable or conductive bodies are brought near it. The other method is using two or more coils which are mutually decoupled from each other. One coil is excited at some frequency and the signal on one or more pickup coils is measured. Normally there is no signal on the pickup coils because they are mutually decoupled, but when a conductive or permeable object is placed near the coils, it will create its own magnetization and effectively cause the coupling between the coils to increase, causing a signal on the pickup coil.

Both methods have issues with accuracy, especially when you ambient humidity and moisture. They need to be carefully tuned for offsets. You might want to look into something using static magnetic fields, like how stud finders work.