cti4sw
Bronze Member
- Joined
- Jul 2, 2012
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- Minelab Equinox 600, Garrett AT Pro, Pro Pointer
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- Relic Hunting
Actually, anyone here with a VLF MD might want to read this, as they all function from the same basic principle.
There have been a few AT Pro threads this past week or so. I know this is technically the wrong forum for this, but the group that collectively posts in the Today's Finds forum is the audience I am targeting, not the guys in the MD forum, who already know this stuff.
So, you have your basic metal detector
:
There are three metal detecting technologies out there, and they all have their pros and cons. The most popular (and cheapest by comparison) is the VLF method, which sends and receives analog electromagnetic signals, and analyzes the target(s) based on the return signal's phase. "Phase" in electronics refers to the harmony of the two signals being compared. If the return signal is completely in phase with the transmitted signal, then they are completely aligned in frequency and magnitude. See the first signal in the image below.
When a target is "seen" by the detector, it absorbs the transmitted signal and then reflects a signal back that is based on the physical properties of the metal, including (but not limited to) density, polarity, size, shape, signal strength, surface area...etc. Anyways, the signal received by the detector is rarely ever in phase with the transmitted signal. This is known electronically as being out of phase. You can see examples of phasing in the image below:
The detector's circuitry then compares the return signal to the transmitted signal and a stored database (the size and complexity of this database depends on the quality of the detector) of known target signatures before presenting the output conclusion to the user. Phases are measured in degrees; if anyone remembers their trigonometry or physics (or electronics) classes, sine waves (which represent signal waves) start at 0 degrees, crest at 90 degrees, cross at 180 degrees, trough at 270 degrees, and return to complete the period (a period = 1 wavelength) at 360/0 degrees.
If the received signal starts at the crest of the transmitted signal, it's said that the received signal is 90 degrees out of phase.
If the received signal starts at the cross of the transmitted signal, then the received signal is 180 degrees out of phase.
And so on. Signals can be any degree out of phase, from 1 to 359; although once past 270, negative values are often used, for instance instead of saying 300 degrees OOP it may be said that it's -60 degrees OOP because the return signal is received more than halfway through the previous period and before the start of the next period. The internal software of the detector will use the trig functions sin, cos, tan, sec, and csc to determine the phase signature as well to provide a detailed conclusion for the target metal.
Does this mean that your detector is "smart" and "knows" which metal is in the ground? Absolutely not. Remember the factors that help/impede with ID: density, polarity, size, shape, signal strength, surface area...etc.
Density can cause completely different metals to return similar signal phases. Purified, processed, and tempered metals are far more dense than handworked metals. Alloys also have mixed densities. Corrosion and concretion can affect densities, because corrosion erodes mass while concretion adds to it (although most oxidation concretion is far less dense than the oxidizing metal).
Polarity refers to the metal's "reaction" to electromagnetic radiation. All metal has polar properties whether it's actually "magnetic" or not. Metal targets in the ground will cause eddy currents in the transmitted signal, which disrupts the transmitted signal and reflects an OOP signal back to the receiver coil.
Size, shape, & surface area contribute (or interfere) in a similar manner to density. The larger (and flatter) the object, the higher the signal may be because the detector will receive more of the return phase signal. If an object is large and flat but is angled in the ground, much of the return phase signal will be deflected away from the receiver, giving the appearance that the target is smaller than its actual size. Can slaw, for instance, does not have a uniform, symmetrical shape. There may be tears, shards, points, crumples, bends, holes, and so on that all cause the signal to deflect, reflect, and bounce... but since the can is made of purified processed aluminum it has a uniform density and the nature of flattened cans is that they lay and get buried flat, resulting in a high toned varying signal or a mid-toned solid signal. Afterthought: One trick you can use against this is with your pinpointer. Foil and can slaw tend to be larger than most "keeper" targets and have more varying signals than coinage or tools. Use your AT Pro's built-in pinpointer to determine the size of the item by holding the pinpoint button down and sweeping across the target area. Larger items will buzz longer than smaller items.
Signal strength can also be affected by these other factors in itself. Any return signal will be much weaker than the signal it reflects; think of bouncing a tennis ball: when you drop the ball, the bounce back height is significantly less than the height from which it is dropped. if you drop a flat rock into a puddle, the main splash can go in any direction (although SOME will ALWAYS go back UP). Lighter rocks have smaller splashes than heavier rocks. This is how the previous factors also metaphorically affect signal strength.
Considering these contributing factors to the received signal, the detector takes the phase signal and compares it to the pre-programmed criteria in its database. Each pre-programmed criteria has a given output procedure for the user to read; one criteria may tell the detector to activate the LCD segment that the user sees as pointing to "nickel" and the 10 LCD segments that the user sees forming the number "53". Another criteria from the same analyzed signal may send the output to the LCD that points to "iron" on the display label and the 7 segments the user sees forming the number "21". The AT Pro's depth indicator is determined solely by signal strength, and why it sometimes seems like larger objects read shallow when they are in fact deeper and tiny objects read deep despite being just below the surface.
Sensitivity is actually a reference to the strength of the signal transmission, which adds more power to the transformer used to propagate (send) the signal through the transmitting coils. High sensitivity will deplete your battery much faster.
Ground balancing allows the detector to establish a baseline reading for the soil in which it will be detecting so that it can ignore any inherent mineralization instead of "detecting" it. Ground balancing will therefore have an effect on the way the receiver and control box analyze signal phases, because the mineralization will otherwise automatically return an OOP signal even when no target is detected. Once the baseline is established by the ground balance, a compensation factor will likely be applied by the software to the OOP receiver signal prior to analysis.
Hope this provides an inside look from an electronics engineer's point of view. I've always said that the more I understand how something works, the better I can use it. Hope this also applies to you!
There have been a few AT Pro threads this past week or so. I know this is technically the wrong forum for this, but the group that collectively posts in the Today's Finds forum is the audience I am targeting, not the guys in the MD forum, who already know this stuff.
So, you have your basic metal detector
: There are three metal detecting technologies out there, and they all have their pros and cons. The most popular (and cheapest by comparison) is the VLF method, which sends and receives analog electromagnetic signals, and analyzes the target(s) based on the return signal's phase. "Phase" in electronics refers to the harmony of the two signals being compared. If the return signal is completely in phase with the transmitted signal, then they are completely aligned in frequency and magnitude. See the first signal in the image below.
When a target is "seen" by the detector, it absorbs the transmitted signal and then reflects a signal back that is based on the physical properties of the metal, including (but not limited to) density, polarity, size, shape, signal strength, surface area...etc. Anyways, the signal received by the detector is rarely ever in phase with the transmitted signal. This is known electronically as being out of phase. You can see examples of phasing in the image below:
The detector's circuitry then compares the return signal to the transmitted signal and a stored database (the size and complexity of this database depends on the quality of the detector) of known target signatures before presenting the output conclusion to the user. Phases are measured in degrees; if anyone remembers their trigonometry or physics (or electronics) classes, sine waves (which represent signal waves) start at 0 degrees, crest at 90 degrees, cross at 180 degrees, trough at 270 degrees, and return to complete the period (a period = 1 wavelength) at 360/0 degrees.
If the received signal starts at the crest of the transmitted signal, it's said that the received signal is 90 degrees out of phase.
If the received signal starts at the cross of the transmitted signal, then the received signal is 180 degrees out of phase.
And so on. Signals can be any degree out of phase, from 1 to 359; although once past 270, negative values are often used, for instance instead of saying 300 degrees OOP it may be said that it's -60 degrees OOP because the return signal is received more than halfway through the previous period and before the start of the next period. The internal software of the detector will use the trig functions sin, cos, tan, sec, and csc to determine the phase signature as well to provide a detailed conclusion for the target metal.
Does this mean that your detector is "smart" and "knows" which metal is in the ground? Absolutely not. Remember the factors that help/impede with ID: density, polarity, size, shape, signal strength, surface area...etc.
Density can cause completely different metals to return similar signal phases. Purified, processed, and tempered metals are far more dense than handworked metals. Alloys also have mixed densities. Corrosion and concretion can affect densities, because corrosion erodes mass while concretion adds to it (although most oxidation concretion is far less dense than the oxidizing metal).
Polarity refers to the metal's "reaction" to electromagnetic radiation. All metal has polar properties whether it's actually "magnetic" or not. Metal targets in the ground will cause eddy currents in the transmitted signal, which disrupts the transmitted signal and reflects an OOP signal back to the receiver coil.
Size, shape, & surface area contribute (or interfere) in a similar manner to density. The larger (and flatter) the object, the higher the signal may be because the detector will receive more of the return phase signal. If an object is large and flat but is angled in the ground, much of the return phase signal will be deflected away from the receiver, giving the appearance that the target is smaller than its actual size. Can slaw, for instance, does not have a uniform, symmetrical shape. There may be tears, shards, points, crumples, bends, holes, and so on that all cause the signal to deflect, reflect, and bounce... but since the can is made of purified processed aluminum it has a uniform density and the nature of flattened cans is that they lay and get buried flat, resulting in a high toned varying signal or a mid-toned solid signal. Afterthought: One trick you can use against this is with your pinpointer. Foil and can slaw tend to be larger than most "keeper" targets and have more varying signals than coinage or tools. Use your AT Pro's built-in pinpointer to determine the size of the item by holding the pinpoint button down and sweeping across the target area. Larger items will buzz longer than smaller items.
Signal strength can also be affected by these other factors in itself. Any return signal will be much weaker than the signal it reflects; think of bouncing a tennis ball: when you drop the ball, the bounce back height is significantly less than the height from which it is dropped. if you drop a flat rock into a puddle, the main splash can go in any direction (although SOME will ALWAYS go back UP). Lighter rocks have smaller splashes than heavier rocks. This is how the previous factors also metaphorically affect signal strength.
Considering these contributing factors to the received signal, the detector takes the phase signal and compares it to the pre-programmed criteria in its database. Each pre-programmed criteria has a given output procedure for the user to read; one criteria may tell the detector to activate the LCD segment that the user sees as pointing to "nickel" and the 10 LCD segments that the user sees forming the number "53". Another criteria from the same analyzed signal may send the output to the LCD that points to "iron" on the display label and the 7 segments the user sees forming the number "21". The AT Pro's depth indicator is determined solely by signal strength, and why it sometimes seems like larger objects read shallow when they are in fact deeper and tiny objects read deep despite being just below the surface.
Sensitivity is actually a reference to the strength of the signal transmission, which adds more power to the transformer used to propagate (send) the signal through the transmitting coils. High sensitivity will deplete your battery much faster.
Ground balancing allows the detector to establish a baseline reading for the soil in which it will be detecting so that it can ignore any inherent mineralization instead of "detecting" it. Ground balancing will therefore have an effect on the way the receiver and control box analyze signal phases, because the mineralization will otherwise automatically return an OOP signal even when no target is detected. Once the baseline is established by the ground balance, a compensation factor will likely be applied by the software to the OOP receiver signal prior to analysis.
Hope this provides an inside look from an electronics engineer's point of view. I've always said that the more I understand how something works, the better I can use it. Hope this also applies to you!
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