Ground Balance Theory

[Ferric Toes]

We have Ground Balance on the Bench for the "builders", but I have this paper which is more of a head case job and would appeal to the "thinkers".

I met Yogadish Das at a soil magnetism conference and workshop in 2008 and gave him a sample of Australian ironstone to experiment with. Unfortunately he took up a different job, unrelated to detectors or soil science and he was unable to research it.

Eric.

http://www.sic.rma.ac.be/~pdruyts/pu...yCopyright.pdf

[mikebg]

CONTENTS
Abstract
I. INTRODUCTION
II. PROBLEM DESCRIPTION
III. CONVENTIONS AND NOTATIONS
IV. DEVELOPMENT OF SOIL EMI RESPONSE MODELS
A. Soil response in the frequency domain
B. Soil response in the time domain
V. APPLICATIONS
A. Sensitivity maps
B. HS response and implementation validation
C. Zero equi-sensitivity surface
D. Volume of influence
VI. CONCLUSIONS AND PERSPECTIVES
APPENDIX A. TX CURRENT IN PRESENCE OF SOIL
APPENDIX B. MQS RECIPROCITY
APPENDIX C. MUTUAL INDUCTION COEFFICIENT
REFERENCES

Aziz, look, there are scientists who initially make analysis in the Frequency domain, then in the Time domain.
Let us hope that experimentalists in our forum will do so. Then they will see that the order of response does not depend on the number of timeconstans.

[Qiaozhi]

Aziz, look, there are scientists who initially make analysis in the Frequency domain, then in the Time domain.
Let us hope that experimentalists in our forum will do so. Then they will see that the order of response does not depend on the number of timeconstans.

I have only skimmed through the paper, but it appears to me that the section entitled "Soil response in the frequency domain" is focussed on the frequency dependence of the soil magnetic susceptibility.

It is also worth pointing out that the paper is mainly concerned with the response of the soil in the absence of a target.

I guess you must have missed the part where it says "Section IV-B extends the models to apply to the time domain which is better suited to analyze pulse induction detectors".

[Ferric Toes]

Another paper by Yoga Das

http://www.gichd.org/fileadmin/pdf/L...soil_paper.pdf

Eric.

[sido]

Interesting reading. The maths lost me totally though.

Can someone please elaborate from the first document posted. See below quote.

Question is, what parameters is being referred to below that the TX driver is what influences the TX Current in air compared to in the soil??
Ideally soil and air would be both the same, but how does the TX driver achieve close to ideal/optimum results?
Is it Frequency/PRR of use? or the amount of TX Power being used? or both?

If it is TX Power, altering for different power levels in experiments would be interesting to see the results for targets buried in soil tests with varying power levels.

Sid

[mikebg]

I have only skimmed through the paper, but it appears to me that the section entitled "Soil response in the frequency domain" is focussed on the frequency dependence of the soil magnetic susceptibility.

It is also worth pointing out that the paper is mainly concerned with the response of the soil in the absence of a target.

I guess you must have missed the part where it says "Section IV-B extends the models to apply to the time domain which is better suited to analyze pulse induction detectors".

George Overton, even your family name is in Frequency domain.
However Moreland means more - combination of Frequency domain and Time domain in his last design "hibrid VLF+PI" metal detector.

Please don't waste time, but read papers because you should explain in the next edition of your textbook how to make ground balance.

[mikebg]

Interesting reading. The maths lost me totally though.

Can someone please elaborate from the first document posted. See below quote.

Question is, what parameters is being referred to below that the TX driver is what influences the TX Current in air compared to in the soil??
Ideally soil and air would be both the same, but how does the TX driver achieve close to ideal/optimum results?
Is it Frequency/PRR of use? or the amount of TX Power being used? or both?

If it is TX Power, altering for different power levels in experiments would be interesting to see the results for targets buried in soil tests with varying power levels. Sid

Sid, The problem arises only when TX coil moves because ground operates as a core for TX coil.
Eric Foster has a solution to avoid movement: he places a very large TX loop on the ground and searches with RX coil inside the loop.

An old solution in Frequency domain is to use P-I-D controller for amplitude stabilisation of voltage across TX coil:

http://www.geotech1.com/forums/showt...772#post152772

[sido]

Thanks for the reply Mikebg.

Lets say we have no variable TX current in soil when a coil is moved side by side (neutral soil, equal characteristics)
We know that TX current induced in variable soil will differ compared to TX current in air and vice versa.

How does the TX driver determine the amount of current induced in air compared to in soil as per that paragraph statement i posted from the first document from Ferric toes.

I think its reffering to different TX power levels can be used with different certain soil conditions which is what then determines the efficiency of the current induced in soil compared to the original transmitted TX Coil current.

Is the PRR selected and the TX coil current selected have a influence in achieving the strongest Magnetic field in certain soils?

Sid

[mikebg]

Sid, don't think what happens in the Earth. Think what sees the synchronous demodulator. It receives a large combination - AIR&GND signal. I will explain its properties in Frequency domain.

The AIR component is direct proportional to frequency. It is always in quadrature to phase of TX current.

http://www.geotech1.com/forums/showt...552#post163552

However the AIR signal is proportional to primary field. Since the GND signal is generated by primary field, if AIR signal is modulated, then GND signal is also modulated.

The GND signal is proportional to frequency squared when the soil has conductivity.
In common case the GND signal has phase difference with AIR signal - see loci 3 and 4 in this post:

http://www.geotech1.com/forums/showt...886#post163886

At ground balance, the synchronous demodulator can not see the amplitude modulation of GND signal. However it sees the modulation of AIR signal.

The AIR signal is modulated at movement of TX coil with small modulation index. However the TGT signal appears as modulation of AIR&GND signal with very small index. I tested with PI the orthogonal (TWO BOX) loop configuration. It has very small modulation index because TX coil appears relatively far from ground:

http://www.geotech1.com/forums/showt...489#post167489

CONCLUSION: For maximal sensitivity, we should use induction balanced search head to suppress AIR&GND signal and P-I-D controller to suppress amplitude modulation of AIR signal.

[Davor]

... or just stabilise Tx power supply to the same effect. PID controllers are generally of 2nd order, and oscillators have some lag in their response, so you come up to the 3rd order with plethora of non-linearities - and you expect stability!?

I did some simulations of a PID Tx regulator to learn the most out of it, and, surprise, it works fine ... provided the tank is driven by 50% duty cycle and at precisely 0° phase. If it was driven off resonance by only a few degrees, it was a noise generator. Most of the free running oscillators are in fact off resonance because of various odd effects, including Miller capacitance etc. and practically none operate at 50% duty cycle, but deeply in C class, or worse. Controlling the conduction angle is perhaps the worst idea of all.

[mikebg]

BETONKOPF

... or just stabilise Tx power supply to the same effect. PID controllers are generally of 2nd order, and oscillators have some lag in their response, so you come up to the 3rd order with plethora of non-linearities - and you expect stability!?

I did some simulations of a PID Tx regulator to learn the most out of it, and, surprise, it works fine ... provided the tank is driven by 50% duty cycle and at precisely 0° phase. If it was driven off resonance by only a few degrees, it was a noise generator. Most of the free running oscillators are in fact off resonance because of various odd effects, including Miller capacitance etc. and practically none operate at 50% duty cycle, but deeply in C class, or worse. Controlling the conduction angle is perhaps the worst idea of all.

Davor, why you hate to copy the best solutions? Do you know more than the world known masters?
GARRETT uses P-I controller in their TXs more than 30 years and MINELAB - more than 15 years.

Please fill a plastic bowl with salt water and measure the modulation index of AIR signal pumping with search head. You'll get a much larger modulation index of AIR signal than does this the target. The SPICE can't show such small index, but the synchronous demodulator senses it.
Then do the same with a P-I controlled TX to realize that you're wrong.

[Qiaozhi]

George Overton, even your family name is in Frequency domain.

This demonstrates the discrepancies that can occur between theory and practice. To be in the frequency domain, my surname would need an extra "e".

Remember the words of Pascal Druyts M.Sc., Dr. Yogadhish Das, Dr. Christophe Craeye and Dr. Marc Acheroy, "Section IV-B extends the models to apply to the time domain which is better suited to analyze pulse induction detectors".

Could all these exteemed people be mistaken?

Pascal Druyts is from the Signal and Image Centre of the Royal Military Academy in Brussels, is conducting research on the modeling of EMI sensors and ground penetrating radars within the framework of mine clearance.
Yogadhish Das is with the Threat Detection Group, Defence R&D Canada – Suffield since 1977, where most of his work has been in Unexploded Ordnance and landmine detection for military countermine as well as humanitarian demining. His current research interest is in understanding the effect of soil on landmine detection devices.
Christophe Craeye is an Associate Professor at the Universit ́ catholique de Louvain. His research interests are finite antenna arrays, multiple antenna systems, numerical methods for fields in periodic media, small antennas and microwave detection.
Marc Acheroy is a Professor in signal processing and automatic control in the polytechnic Faculty of the RMA and the Head of the Communication, Information, Signal and Sensor Department of the RMA. He has a solid expertise in image restoration and compression, in pattern recognition, data fusion and mine action technologies. He is leading several projects, among which are projects for the Defence, the European Commission, the European Space Agency and the Belgian scientific Policy.

Have you found your soldering iron yet?

[Davor]

Davor, why you hate to copy the best solutions? Do you know more than the world known masters?
GARRETT uses P-I controller in their TXs more than 30 years and MINELAB - more than 15 years.

Please fill a plastic bowl with salt water and measure the modulation index of AIR signal pumping with search head. You'll get a much larger modulation index of AIR signal than does this the target. The SPICE can't show such small index, but the synchronous demodulator senses it.
Then do the same with a P-I controlled TX to realize that you're wrong.

OK, let's have it your way, can you tell me what order is the regulation network in this Garrett oscillator, and why it's regulation overshoots?

I thought of a lengthy answer explaining all that's happened between the 1998 cornerstone paper by A. Hajimiri and T. H. Lee, "A general theory of phase noise in electrical oscillators" which led to better oscillators of nowadays, but let's hear your arguments.

[Ferric Toes]

OK, let's have it your way, can you tell me what order is the regulation network in this Garrett oscillator, and why it's regulation overshoots?

I thought of a lengthy answer explaining all that's happened between the 1998 cornerstone paper by A. Hajimiri and T. H. Lee, "A general theory of phase noise in electrical oscillators" which led to better oscillators of nowadays, but let's hear your arguments.

This discussion seems to have shifted from what happens in the time domain (pulsed TX), to what happens in the frequency domain (CW oscillator TX).

Due to the decoupling of TX and RX by the sample delay and with the correct TC's for the coil /TX and damping circuit, ground modulation of TX field has no effect on the detector operation at the sample delays that are suitable for general use.

Eric.

[mikebg]

This discussion seems to have shifted from what happens in the time domain (pulsed TX), to what happens in the frequency domain (CW oscillator TX).

Due to the decoupling of TX and RX by the sample delay and with the correct TC's for the coil /TX and damping circuit, ground modulation of TX field has no effect on the detector operation at the sample delays that are suitable for general use.

Eric.

Eric, the CW oscillator TX is simply a specific case of excitation with periodic TX current. When something happens in the Frequency domain, it shows what is happened in the time domain. The Frequency domain is similar to a tool for seeing through the fog. The Time domain is filled with a fog. Fourier transform shows things that will turn your existing concept of PI technology. The conventional PI is other specific case of wideband technology with low energy efficiency. For high efficiency, we need ASinc "ringing" of TX tank circuit instead damping.

http://www.geotech1.com/forums/showt...177#post118177

1. Frequency domain shows that there is no decoupling of TX and RX by sample delay. There is convolution with AIR signal, despite TX current is zero. The conventional PI is not a radar.

2. Each delay in TD determines a frequency in FD. The conventional PI makes discrete signal processing with short samples. However we know even your designs with continous signal processing - an integrating window instead series of samples.

3. The value of each short sample determines the response at a frequency. This is low noise narrow band process.

4. We can make a PI detector to operate as a narrow band CW detector, however with bad TX efficiency because magnetic field contains useless frequencies. It has even a component with zero frequency (that means DC).

5. Frequency domain shows that flyback voltage is simply the AIR signal received when search head is lifted. This is frequency spectrum that provokes the eddy currents in environment.

If the AIR signal is modulated by ground, the synchronous demodulator sees the modulated carrier wave as an additional ground sugnal because the modulating frequency band is the same (for example 0.2 ... 0.8Hz). However the AIR signal has different frequency properties than the genuine GND signal. The AIR signal is direct proportional to frequency. The GND signal has ferrite component also direct proportional and conductive component proportional to frequency squared.

Oops, I speak in Frequency domain only. In Time domain, the AIR signal is proportional to derivative of TX current, but conductive groung generates GND signal proportional to the second derivative.

Fortunately, since the beginning of this month I am no longer unemployed, but this drastically reduces my free time for hobby. I will try to participate in this forum because I have ready drawings. Unfortunately I have no prepared text for the drawings. Attached is the drawing explaining the method for measuring the ground modulation of TX (modulation of AIR signal). The method is valid for all types of metal detectors. I do not remember if years ago I published in the forum description of the method. I'll search the forum and if I can not find the description, I will write new.