DD-Coil Analysis (Sensitivity Profile)

[Aziz]

Hi,

I am sure, that some of you is interested on the DD coil analysis. It is quite unique analysis, that proof of course typical sensitivity profile of a DD coil. I made a true DD coil shape. The pictures below will show the shape of the coils and dimensions.

Simulation model data:
TX-Coil: 100 windings, 100 mm mean radius, exact half circle
RX-Coil: Same as TX-Coil.
TX Current = 1 A, f=15 kHz, I(t)=1A * sin(2*PI*15kHz)
Target Model: As gold ring coil, co-planar oriented, mean diameter 20 mm, 3 mm thick. Gold ring is moved around the coil to calculate the sensitivity. Keep attention, that on some pictures the sensitivity scaling will be changed.
Enjoy results,
Aziz

[Aziz]

Red: Voltage increase on RX coil
Blue: Voltage decrease on RX coil
Magnitude: see decibels

[Aziz]

Top view (z-planes) of sensitivity profiles.

[Aziz]

You are welcome, to discuss the results of analysis.
Regards,
Aziz

[Aziz]

Some comments on the method of analysis:
Target is just modelled as a receive and transmit coil. Eddy current effects for a given frequency f are not taken into account. The model is a simplification of the basic physical electro-dynamics. Just using the Maxwell's equations, Faraday's and Lenz's law. Also the coils are a simplification, to reduce the number of wire segments. The whole calculation takes quite a long time (billions of vector math, integrations, derivations, etc.). Therefore the resolution is kept small (relative big rectangles).
But the result is a very good approximation which conforms with practical results.
I was surprised about the induced voltage change from decrease to increase. So this will cause phase changes on the received signal.

Now I can focus on some novel coil designs. I will design the best coil on the planet!

[mikebg]

Excellent!

Hi Aziz! Despite today is 1st of April 'Fool's Day", I should say like Sean Goddard in 1st of
february: WOW, U GENIUS!
Now I'm posting for analysis a bulgarian configuration of windings "DODd" capable to
opress ground signal and all interferences caused by remote sources. I'm using it to
detect small meteorites.
Winding L1 is TX coil. The two identical coils L2A and L2B are opposite sections of RX
inductance, forming coplanar gradiometer - figure of 8.
Winding L3 produces reference voltage for synchronous demodulation, containing more
ground signal . The principles are described in US patents 3,002,262 and 3,823,365.

Mike.

[Aziz]

Hi Aziz! Despite today is 1st of April 'Fool's Day", I should say like Sean Goddard in 1st of
february: WOW, U GENIUS!
Now I'm posting for analysis a bulgarian configuration of windings "DODd" capable to
opress ground signal and all interferences caused by remote sources. I'm using it to
detect small meteorites.
Winding L1 is TX coil. The two identical coils L2A and L2B are opposite sections of RX
inductance, forming coplanar gradiometer - figure of 8.
Winding L3 produces reference voltage for synchronous demodulation, containing more
ground signal . The principles are described in US patents 3,002,262 and 3,823,365.

Mike.

Hi Mike,

I have not forgotton your proposal coil design. During some simulations on this, I found it is interesting to see more physical details. So I am just improving my software at the moment. But the results on the coil analysis will show the proof, that this coil design is not efficient. The reason is simple: relative small RX coil flux area. Also, the coil arrangement will have two hot spots - but definitely not on the center position.
Aziz

[vistac2000]

Hi AZIZ,
Thanks for your coil analyze,
I hope you can reach very soon to the best shape and configuration of DD
coil for best performance in sensitivity and depth and discrimination,
this is better if you analyze concentric coil too ...
please share your results to using us ... Thanks again
Be success...

[mikebg]

Hi Aziz,
If the DOD sensor is preciselly balanced for each RX coil, it provides better SNR than DD. To model SNR, you should put underneath each sensor a halfspace (ground), having permeability and conductivity. For best SNR, TX coil should lie more remote from this halfspace than RX coils.
Reduced ground signal by DOD allows to increase gain of preamp without risk of saturation. That means more depth.
When swinging, the demodulated signal from DOD seems like derivative from that one of conventional DD sensor. Because of this, no need to differetiate twice demodulated signal. Twice differentiation was used in GARRETT machines; take a look at patent US 4,700,139 Sheet 2, FIG. 7.
Pinpointing with DOD is much more exact than with DD. Therefore coil configuration OOOO ( the great-grandfather of OOO having two TX coils) is used for demining since 1917.

[Aziz]

I am sorry, but I did some mistakes on the coil configuration:
1. TX, RX coil resistors were extremely high (1 Ohm/m). Now has 1.17 Ohms.
2. I forgot, that I was entered the frequency domain behavior on induction effects on the target. So the induced current now is correctly calculated (depended on R and complex resistance ZL = R + jXL, XL=wL, w=2*PI*f). Capacitive effects not taken into account yet.
For this coil configuration I have on TX:
f = 15000 Hz
R = 1.17 Ohm (coil resistance)
XL=262.2 Ohm (coil reactance)
ZL=262.2 Ohm (coil impedance)
So the complex resistance will be:
_ZL = R +jXL = 1.17 + j*262.2
L = 2.78 mH
Q = 225 (coil quality)
TC= 2.39 ms (time constant)
The reactance resistance dominate over the DC resistance and the effects of AC-currents will dominate. So I took 1 A AC current for the TX coil. 1 A is the Imax and is defined by: i(t)=Imax*sin(wt+phase).
The result has now different sensitivity profiles. Ok, we can continue analysing more coil configurations.
Aziz

PS: Red and Blue zones are alternating due to AC current. Keep in mind. The picture show the static moment.