Let's made a PC-base metal detector with usb interface !!!

[Aziz]

Hi all,

another issue on the coil current at frequency f2 (regarding last picture):
This would induce a much higher target response of course (direct proportional to frequency), so the magnitude can be lowered to avoid the rx amplifier overload. It could be better to have almost same target response for f1 and f2 at the end.

But the interesting question is:
Is there any useful information, which can be derived from the response f1 and f2 to make a good ground balancing and iron mineralisation rejection?

I don't know the answer yet. Let's investigate this.

Aziz

[Aziz]

Hi all,

I should remember you about the inductive reactance XL:
XL = w*L, w=2*PI*f,
where f = frequency, L = coil inductance

The impedance (=frequency dependent resistance) of the coil is
ZL = RL + jwL (complex notation)
where RL = coils DC resistance

So, if we have a simple coil (coil only) and have a voltage source (like an voltage amplifier), the current through the coil will be frequency dependent. The higher the frequency, the lower the current through the coil (impedance ZL gets higher). If we want two frequencies to be passed optimally through the coil, we have to make the XL almost zero on these frequencies. So the higher frequency part needs a higher voltage from the source but we usually haven't the higher voltage source. We will make the higher voltage through an another coil and capacitor (like a step-up converter).

BTW, we could take a current source and pass it to the plain coil. But then, a higher voltage is again necessary to implement the current source.

Ok, there is a very simple solution possible:
A parallel resonant tank (chocke with parallel capacitor) is connected in series with the voltage source. The search coil gets now very simple having only one coil and a series resonant capacitor (series LC circuit). The choke resonant tank can be placed near the voltage source.

Below is the frequency response of an ideal solution (the current through the search coil is shown this time, which really counts).

Aziz

[Aziz]

New Idea on Dual Frequency VLF

Hi all,

no problem . There are many ways to get it working.

I will try to make the coil coupling zero. So one of the coils will be wound on a seperate ferrite core and the other one for magnetic field emissions (search head). It should give a better response this time.

The trick is to reduce the reactance on both frequencies (ideally making zero) so the frequency independent part of the impedance (coil resistance) gets only active.

If this will work, than I have a new very nice powerful and very simple detector. A simple power amplifier will push more current to the coil and a simple rx amplifier will do the rest.

Ok, let's make some more coils (bloody hell, I need a big ferrite core).

Aziz

[simonbaker]

Hi all,

I am sorry, but the dual frequency VLF detector doesn't work as hoped. Due to the complex coil architecture, it gives different responses, which can not be modelled easy. Even the transmitter works quite well with two different frequencies (6.28 kHz and 23.5 kHz), the response does not make easy ground balancing and hot rock rejection possible.
---------
It seems, the krauts are not interested to arrest me anymore. I should get my equipment back to work on my projects further.

Cheers,
Aziz

But very interesting idea. Maybe you'll get another thought about it.

Regards,

-SB

[WM6]

Due to the complex coil architecture, it gives different responses, which can not be modelled easy.

Sadly, cannot be some way simplified?

[Aziz]

Dual Frequency VLF failed

Hi all,

I am sorry, but the dual frequency VLF detector doesn't work as hoped. Due to the complex coil architecture, it gives different responses, which can not be modelled easy. Even the transmitter works quite well with two different frequencies (6.28 kHz and 23.5 kHz), the response does not make easy ground balancing and hot rock rejection possible.
---------
It seems, the krauts are not interested to arrest me anymore. I should get my equipment back to work on my projects further.

Cheers,
Aziz

[Aziz]

Dual Frequency VLF Detector

Hi all,

I have been working on a new type of VLF detector to kill the boring time. I am wondering, whether the new ground balance system can be transferred to this type of detector. As I have put all my equipment away, it will be a battery-less detector (passive coil again). I have only magnet wire, capacitors and resistors at home. This should be enough to start with a laptop/sound-card solution.

Below is the frequency response of the dual frequency transmitter coil. As you can see, it amplifies on two different resonant frequencies, which will be processed. I don't know yet, whether it will give a positive result.

The transmitter coil consists of two (less) coupled coils with a combined series and parallel resonant tank. It is very sensitive to coupling of the coils, which will hopefully make detecting reactive response and resistive response easy. The receive coil is seperate (induction balanced configuration).

I'll keep you informed, whether it is working or not.
Cheers,
Aziz

[Aziz]

Part 3

Here is the missing diameter variation (see below):
The sqrt(2)/2 diameter (half of the area of the TX coil)

Tinkerer,

it depends on your demands. If you want a deep going coil, look at the tables. This can be seen directly.
To get the mono coil performance, just increase the RX winding turns (but higher EMI picking). Doubling it, will double the target response voltage. But will make 4 times higher RX coil inductivity.
Note, that on every coil configuration: TX with coupled bucking coil = 300 µH, RX = 300 µH.

Aziz

[Tinkerer]

Aziz, thanks for the excellent work.

Let's see if I understand it right.

With the RX-BU coils at 75% diameter at a distance of 2r, we get about 1.23 times the amplitude of the target response, than at 50% diameter.

However, we need more turns on the BU coil to obtain balance. More turns on the BU reduces the target response received by the RX coil.

Where is the sweet spot?

Tinkerer

[Aziz]

Concentric Co-Planar Coil Analysis Part 2

This is part 2 of the analysis. Now RX and bucking coil will have the same diameter but the diameter itself is varied.
Look at the tables below. You can see, at which range the diameter gives good depth performance. Smaller diameter Bucking/RX coils tend to give a stronger near detection field (good for pin-pointing) but suffers in the far detection range. Bigger diameter tend to be like a bigger mono coil.

To the name convention of the tables:
CoilComp-CC2-0.33.txt means: RX/Bucking diameter = 0.33*10" = 3.3"
CoilComp-CC2-0.5.txt means: RX/Bucking diameter = 0.5*10" = 5"
CoilComp-CC2-0.75.txt means: RX/Bucking diameter = 0.75*10" = 7.5"
and so on...

Who can see the optimum range of diameter? Now, it should be possible to make a better concentric co-planar IB coil.

Cheers,
Aziz

[Aziz]

Concentric Co-Planar Coil Analysis Part 1

Hi all,

this is the first part of the coil analysis, which excludes inefficient coil designs. I have made three coils and can exclude two of them.
See below the induced target response voltages:
Left: TX=10" diameter, Bucking= 0.67*10" = 6.7", RX = 0.5*10" = 5", RX coil is smaller
Middle: TX=10" diameter, Bucking= 6.7", RX = 0.8*10" = 8", RX coil is between TX and bucking coil
Right: TX=10" diameter, Bucking= 6.7", RX=6.7", RX coil is 4 mm above the bucking coil (same diameter, a typical standard configuration)

TX coil is placed at z=0 mm, Bucking at z=-2 mm, RX at z=+2 mm.

As you can see directly, the left and middle design is inefficient. It does not make sense to follow similar configurations with different diameters. If the RX coil is between bucking and TX, the induction balancing gets quite difficult (not practical design). The standard coil configuration (right) is more efficient and practical, as the bucking and RX coil can be wound in the same winding core.

Cheers,
Aziz

[Aziz]

Updated Coil Software

Hi all,

I have extended the functionality of my coil software. It will save a lot of work now and makes two excel calculation sheets obsolete.

- Inductance normalization of single or multiple coupled coils can be done easily
- Output of coil coupling coefficients and inductances (single, mutual, coupled) (good for spice simulations)

I will start the analysis as soon as possible.
Cheers,
Aziz

[Tinkerer]

Hi Tinkerer,

regarding the concentric co-planar IB coil:
We can make an own comparison between different parameters:
- well, mainly differing the diameter of the bucking coil (RX and bucking same diameter)
- putting the receive coil either more inner of the configuration or
putting it between TX and bucking coil. Everything is possible.

The optimum performance can directly be seen then. As a result of this, one of my mentioned coil findings will be revealed . And I expect from the coil makers, that this idea will be a public domain then and they shouldn't try patenting it. Otherwise, I'll come and blast them off into the space.

I will add some functionality to my coil software to avoid more work to me. Now it's time to do this as I have to make some things by hand (excel calculations, manual input of parameters, inductivity normalizations, etc. ) . We can start the new coil analysis next week.

Cheers,
Aziz

Hi Aziz,

The concentric, coplanar coil is very complex. I have come to understand a good part of the complex interactions between the 3 coils, but there are still other parts of the interaction that I can not quantify. So I would love to see all of the interactions quantified.
For example, I have noticed differences with the positioning of the Bucking coil, above, below, outside, inside or spiral wrapped around the RX coil.
This difference is extreme with spiral wound coils, where the BU coil pushes the field so much, that the reach of the coil becomes very asymmetric, something like 2:5, top and bottom. The size of the RX damping resistor, has an influence on the Bucking coil coupling(k) to the RX coil. In receive mode, the BU coil signal is of opposite polarity from the RX coil. It is therefore interesting to have a low k factor at this time. The turns ratio TX:BU gets very important there.
At a certain distance from the coil, the RX fields of the TX and RX coil interaction changes. The pin-pointing with the concentric coplanar IB coils is good, but I have not measured the variation in the target response for off center targets. I have noticed strange behavior at a distance of 2r and more and lateral distance from the center, out to way beyond the diameter of the coil.

Off course, all the factors play an important role in the GB and IB balance.

These are just some of the complexities. There are to be defined and explored.

Tinkerer

[Aziz]

Concentric Co-planar IB Coil

Hi Tinkerer,

regarding the concentric co-planar IB coil:
We can make an own comparison between different parameters:
- well, mainly differing the diameter of the bucking coil (RX and bucking same diameter)
- putting the receive coil either more inner of the configuration or
putting it between TX and bucking coil. Everything is possible.

The optimum performance can directly be seen then. As a result of this, one of my mentioned coil findings will be revealed . And I expect from the coil makers, that this idea will be a public domain then and they shouldn't try patenting it. Otherwise, I'll come and blast them off into the space.

I will add some functionality to my coil software to avoid more work to me. Now it's time to do this as I have to make some things by hand (excel calculations, manual input of parameters, inductivity normalizations, etc. ) . We can start the new coil analysis next week.

Cheers,

Aziz

[Aziz]

Anti-Interference Coil Thoughts

Hi all,

consider the following:

Interference cancelling coil has the potential for up to 30 dB external EMI noise reduction (this is the most limitting factor for a detector). This range could be used to amplify the signal for potential performance losses. 30 dB is a gain of 10^(30/20)= 31.62. So this would not contribute much amplifier noise.

My novel AI coil would outperform the mono loop coil . It will be a super deep and super quiet detector . Better I don't show you the performance comparision graph . You won't believe it. This could give a real breakthrough in detector performance.

Will follow it further.

Aziz