EMI sensor ---removal of primary field & Tx power

[Mizha Mekka]

Hi everyone!

First I want to know that generally how much power is required for an frequency domain EMI sensor's transmitter to produce enough eddy current in a soil having a resitivity of 30 Ohm/m, also.

Right now I customised a EMI receiver that consists of an sigma-delta ADC, a coil and a microcontroller, it could pick up signals in the background enviroment...

However, when I input the data into PC and use matlab to analyse it, I found some difficulties in extracting the InPhase and Quadrature components, because basically the data I obtained is coming from the superimposition of both primary field and secondary field... I need to find a way to removal this signal without doing something physically, since the core of coil is fixed and would be pretty hard to get a bucking coil inside... Is there a way that I could retrieve induced current solely generated by the secondary field in software (particularly, Matlab), would I/Q demodualtion be a good idea? and How should I do it?

[mikebg]

Hi Mizha,
The removal of primary field and calculation of target's impulse response (time domain) or target's frequency response (spectral domain) is described in patent US 4,506,225.

However there is more simple signal processing method. Instead frequency response, is calculated "color coefficients" of target.
Follows description of discrimination in Bulgarian metal detector PD3200:

"It is quite difficult to achieve good discrimination with pulse induction metal detectors, especially if they use monocoils. Discrimination is based on the time to decay for target signal, and this time depends on the chemical nature of the object, its size and shape, too."

"PD3200 calculates “color coefficients” of the signal received, and they show approximately the metal type. “Color” objects normally have short response, but if they are big size, the response gets longer and similar to the response of iron and steel. That concerns to bigger objects, and is unwelcome fact for discrimination abilities of PI-detectors with mono coils."
"Situation for iron objects is more stable – iron objects are always shown as “black” targets, exceptions are very small objects, they may give short response and show themselves as “color” objects."

"Other disadvantage is difference between small and big gold objects. If detector/coil is tuned for small gold hunting (gold rings), which means low inductance of the coil and short delay for sampling, it doesn’t work well on gold nuggets. Big gold objects produce long response and that fact makes them similar to iron objects."

"Some PI-detectors use preliminary sampling in pulse “on” time to achieve better form of discrimination. This is not available for monocoils, which are simplest to make up. That is because PD3200 does not use this technique."

"VLF detectors have similar “problems” with size, orientation and form of the target. Most of them show big iron object as “very color” object. The instrument can be tuned to solve this problem, but that affects to errors in recognizing of small objects."

"In other words, no form of discrimination is perfect. Some principles are better than others, but all can be fooled, and that concern both VLF and PI detectors."

[Mizha Mekka]

Thanks to the link, mikebg Now its time to convert math to matlab code

[Aziz]

GEM sensors

Hi Mizha Mekka,

have a look at these sensors (GEM-2, GEM-3, .. ) . Might give you some stimulating ideas.

http://www.geophex.com/GEM%203/GEM-3%20home.htm

Have a look to the publications too:
http://www.geophex.com/Publications/publications.htm

Aziz