Balance metal detector sensitivity for smaller pieces of metal

Question

As suggested by Andy in these posts proximity sensor technology, range of 150mm, Metal detector "Head sensor" and How can I drive an LC tank circuit to get a nice and clean sine wave?, I built my version with add-ons of graphite sheet shielding inside and with a little different circuit than maijaz99 built.

Andy wrote in one post that the more the current to drive the coils the higher the sensitivity of the detector will be, so I set the current at 7 amperes by adding a transistor and resistor to it.

I tried with Maijaz99's circuit first which was circulating 0.25A in it, but the sensitivity was very limited. Only large pieces of metal were getting detected, and only by a very small shift in phase and amplitude.

My circuit is a basic current control circuit.

• R calculation : 0.7V / 7A = 0.1 Ohm
• Resistor wattage calculation = 0.7V X 7A = 5 watt. (0.7V is needed to turn on the transistor, 0.6 ideally)

The problem is when no LC tank is connected and 255 kHz are triggering the MOSFET, at no-load condition my multimeter shows 7 amperes passing through the circuit very easily. When the LC tank is connected as shown in the schematic attached it takes only 100-150 mA. What is going wrong?

There was only one reason for changing the circuit. In that circuit, I would have needed to add a very high wattage resistor to control the current. In my circuit, a smaller wattage resistance can be used. Also as shown in the schematic, the frequency was calculated at 366 kHz, but I achieved resonance at 255 kHz switching frequency.

With my circuit also, I have achieved resonance both in Tx and Rx coils, but as the current is smaller the same thing is happening. Only large pieces of metal are detected.

How can I circulate more current through the coil so that it can detect objects that are in size 1mm or smaller?

Answer 1

R calculation : 0.7V / 7A = 0.1 Ohm

You are missing the point here. The DC current that flows through the tuned tank circuit in the drain of the MOSFET should be just a few hundred milliamps. The AC current that circulates in the tank will be several amps. Control the gate voltage through a resistor and potentiometer and drive the gate with a small AC voltage. In fact, close-the-loop and get it to self-oscillate.

255 kHz are triggering the MOSFET

Here's another problem; pharmaceutical metal detectors (due to their quite small coil size/aperture) tend to run at about 1 MHz and will have an oscillator coil inductance of about 400 nH when encased within the metal box they fit in.

I ran all my metal detectors from a 24 volt DC power supply and, as previously mentioned, the standing DC current through the MOSFET (IRFZ44 from memory) was about 250 mA.

but the sensitivity was very limited

The main thing here is a tuned receiver circuit. You have not mentioned anything about how you measured the receive signal but, if you expect to say any movement on the receive waveform with sub mm contaminants using an oscilloscope then forget that. The sort of real signal induced in the coil for a small detectable test sample is less than 100 nV RMS. You'll never see that unless you parallel tune the receive coil (an uplift of 30 ish on voltage to the microvolt level) then a gain of 1000 amplifier to put your detectable signal in the millivolt range. Then you can detect small contaminants.

Maybe you have done all of this but, your post doesn't indicate so.

How can I circulate more current through the coil so that it can detect objects that are in size 1mm or smaller?

• a 24 volt power supply
• a bias current of about 250 mA
• an operating frequency of around 1 MHz
• the best ceramic NP0 tuning capacitors you can get
• a tuned receive circuit
• stable coils that are 3 mm diameter to improve Q factor
• a good low noise gain of 1000 AC amplifier.
• a very good synchronous rectification scheme to help detection
• a great auto-balance circuit that is always trying to actively balance of the offset of the receive coils (except it halts as soon as a signal begins to be detected)
• really good software filters to give more signal to noise.
• mechanical stability in coils, coil former and wires to coils.

You appear to be a good way from reaching those targets.

Answer 2

Images taken here

As you can see, the source current is minimal at resonance. The current through your MOSFET isn't equal to the coil current. You do just cover the losess in the LC tank. Ideally the current would be zero if there would be no resistance.

The capacitor can charge maximally to the source voltage 15V. The energy is tweaking between L and C. So we could say the max energy stored in the capacitor is:

$$W=\dfrac{C\cdot v^2}{2}$$

And it converts into inductor energy:

$$W=\dfrac{L\cdot i^2}{2}$$

Without any losses (R=0), the maximum current is:

$$i_{peak}=\sqrt{\dfrac{C\cdot v^2_{peak}}{L}}$$

The limiting resistor doesn't matter too much, here. Rather the voltage and quality of the LC tank.

Answer 3

EDIT : This assembly will help you understand why the initial assembly "cannot" work as it is because of Q2, I think.

As stated by @Marko Buršič, after choosing other components values ...

Here another view of current variables. Not yet "steady state". Only "start".

Here is a another schematic that can help you ... It is a half-bridge (24 V).Under 12 V, the RMS current will be 4 A.

See the voltage across the inductor \$(50uH, @ 250kHz)\$ ... for a current around 8 A rms.

Here an other alternative.

Unless I am wrong : \$L * di/dt = L * 2 * pi * f * Ipeak = +/- 600 V \$ ...

If R3 is higher, voltage and current will be, of course, lower.