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Hi Dave,
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Anonymous
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Eric, thank you for your post.
Here are several lines of reasoning which lead me to the conclusion that the transmit pulse in a conventional PI is a "waste of energy from the target's point of view".
REASONING FROM SIGNAL POLARITY
I think we all agree that in the case of a low-conductivity target, the flyback pulse is what kicks the target. The kick from the transmit risetime occurred too far back in time to have any effect.
The voltage induced in the target by the transmit risetime is of a polarity opposite that induced in the target by the flyback pulse. Therefore, whatever happens in the target during transmit risetime is opposed to what happens during flyback.
If the transmit were more important for high-conductivity (or iron) targets, then the polarity of those signals during the receiver period would be opposite that of low conductivity targets. The thing would be a natural discriminator.
Our actual experience is that the polarity of high-conductivity target signals is the same as that of low-conductivity targets, and that PI's are not natural discriminators.
This means not only that it's the flyback that kicks the target, even for high-conductivity targets; it also means that whatever happens in the target during the transmit risetime actually reduces sensitivity. The transmit risetime is a necessary evil, not a principle of operation.
REASONING FROM TIME-VOLT PRODUCT
The current induced in a high-conductivity target is, to a first approximation, proportional to the voltage induced in the target by the field (i.e., the rate of change of transmit current) and the duration that the voltage is applied. In other words it corresponds to the time integral of induced voltage.
The field strength (i.e., current) established during the transmit period equals the field strength (current) which gets reduced to zero during the flyback. Another way of saying the same thing is that the current at the end of the transmit on-time equals the current at the beginning of the flyback, since those two things happen at the same instant. The inductance is the same in either case; therefore the volt-time product is the same.
When the receiver is turned on, the volt-time product of the flyback is something that happened a lot more recently than the volt-time product which happened during the transmit period. The difference between the two things is what the receiver will see, and the effect from flyback will always be greater than the effect from the transmit risetime.
THE EVIDENCE OF ACTUAL PI TRANSMIT RISETIME CUSTOMARY PRACTICE
In the interest of efficient conversion of battery watts to field strength, it would seem to make good sense to operate the transmitter coil at a moderately high Q, in other words, constant current slope.
What actually happens when you do this, is that the sensitivity to high-conductivity targets is really lousy. If you want to preserve good sensitivity to high-conductivity targets, you have to stretch the transmitter timing out so far that in typical PI practice (as I gather from reading patents etc.) the transmit coil actually approaches resistive current limiting. From a power efficiency point of view, this is terrible, but it's what ya gotta do to see those big deep targets.
In VLF detectors, having good transmitter Q is considered important. In conventional PI practice, there is much less concern for Q. The reason is that engineers have discovered that raising the Q of the transmitter coil does not offer the kind of improvement in sensitivity versus power consumption that one would expect based on analogy to VLF practice.
Apart from any theoretical reasoning behind it, that's what the actual conventional practice is, as I understand it. The rule of thumb you offer in your post is "5 times the target time-constant".
Now to the theoretical reason behind it. If it were the transmit risetime that energized the target so that it could be detected, one target time-constant would be the point of diminishing returns. You'd want to get that sucker kicked and then look at it as soon as possible. But anyone who tries this discovers that it doesn't work very well at all.
The reason it takes several time-constants, is because you need to give the target time to forget what happened during the transmit risetime. This happens best if the rate of change of current in the transmitter tapers off and approaches resistance current limiting, allowing the eddy current induced by the transmit current rise to decay back down to relative insignificance.
Modern coin-beach type PI's tend to use shorter transmit on-times and faster rep rates, in order to get good sensitivity to jewelry and acceptable sensitivity to the common coins. What happens on a US silver dollar doesn't matter much to a coin-beach machine, since there are very few silver dollars found on beaches.
The second paragraph of your post provides a clear and detailed description of these phenomena and how they relate to customary industry practice.
--------------
In the third paragraph of your post, you point out that due to skin effect, it takes time for a signal to penetrate to the bulk interior of a large conductive target. Although I agree with this, it does not lead me to the conclusion that transmit pulses have to be of long duration so that the target will be completely energized.
The actual field strength of the transmitter does not induce eddy currents in the target, nor does the actual eddy current in the target induce a voltage in the receiver. It is only the rate of change of the transmitter field, and the rate of change of the current flowing in the target, that matter. No change in current/field, no induction, regardless of how strong the field or the current might be.
When a high-conductivity target is energized to its center by a long-duration transmit pulse, the currents flowing in the target are already diminishing because the transmit field itself has a rate of change which is diminishing. That rate of change was never very high anyway, since the applied voltage to the coil was just battery voltage. If you were able to look at the target signal component without it being disturbed by flyback, the target signal would be weak because the current has stablized and its rate of change is low.
Now, hit it with flyback. Sudden high rate of change of field, a solid chunk of volt-time product that happened only a few microseconds ago. Substantial current is now flowing in the surface of the target. At the end of flyback, this substantial current flowing in the surface of the target redistributes itself quickly into thicker layers of the target, reducing the resistance in the current path and lengthening the decay time-constant. Even though the target is a high-conductivity one, it starts out looking like a physically large low-conductivity target, enabling it to induce a large voltage into the receiver coil. This is the physics behind the rule-of-thumb observation that in PI, it's surface area that counts, not mass. It has as much to do with how PI's are built, as it does with the properties of targets.
---------------
The fourth paragraph of your post says that "the flyback pulse in a conventional PI... is not something that is transmitted to the object." I would disagree. The flyback pulse is what collapses the field, producing the high rate of change of field needed to induce eddy currents in the target close enough in time to the receive period that the receiver will have something substantial to look at. Without flyback, a conventional PI would be nearly useless from the standpoint of sensitivity. (Not that there is any way to avoid flyback in a conventional PI.)
---------------
The UTEM system sounds like it may be something a bit similar to the PI system I'm proposing.
--------------
I expect people to argue over whether or not this should be called "pulse induction". For sure, it is neither conventional PI nor Fisher PI. But the basic idea of hitting the target with a pulse and then taking a look at what happened, in a time sequence, that's pulse and it's induction. The underlying thinking (if not the circuit topology) is from the PI camp, and is quite different from that of VLF/MF practice where signals are treated as continuous.
Figuring that this system needed a name, I decided to call it "continuous current pulse induction" to distinguish it from the prior art discontinuous current PI systems. Maybe someone else will suggest a better name for it.
--Dave J.
Here are several lines of reasoning which lead me to the conclusion that the transmit pulse in a conventional PI is a "waste of energy from the target's point of view".
REASONING FROM SIGNAL POLARITY
I think we all agree that in the case of a low-conductivity target, the flyback pulse is what kicks the target. The kick from the transmit risetime occurred too far back in time to have any effect.
The voltage induced in the target by the transmit risetime is of a polarity opposite that induced in the target by the flyback pulse. Therefore, whatever happens in the target during transmit risetime is opposed to what happens during flyback.
If the transmit were more important for high-conductivity (or iron) targets, then the polarity of those signals during the receiver period would be opposite that of low conductivity targets. The thing would be a natural discriminator.
Our actual experience is that the polarity of high-conductivity target signals is the same as that of low-conductivity targets, and that PI's are not natural discriminators.
This means not only that it's the flyback that kicks the target, even for high-conductivity targets; it also means that whatever happens in the target during the transmit risetime actually reduces sensitivity. The transmit risetime is a necessary evil, not a principle of operation.
REASONING FROM TIME-VOLT PRODUCT
The current induced in a high-conductivity target is, to a first approximation, proportional to the voltage induced in the target by the field (i.e., the rate of change of transmit current) and the duration that the voltage is applied. In other words it corresponds to the time integral of induced voltage.
The field strength (i.e., current) established during the transmit period equals the field strength (current) which gets reduced to zero during the flyback. Another way of saying the same thing is that the current at the end of the transmit on-time equals the current at the beginning of the flyback, since those two things happen at the same instant. The inductance is the same in either case; therefore the volt-time product is the same.
When the receiver is turned on, the volt-time product of the flyback is something that happened a lot more recently than the volt-time product which happened during the transmit period. The difference between the two things is what the receiver will see, and the effect from flyback will always be greater than the effect from the transmit risetime.
THE EVIDENCE OF ACTUAL PI TRANSMIT RISETIME CUSTOMARY PRACTICE
In the interest of efficient conversion of battery watts to field strength, it would seem to make good sense to operate the transmitter coil at a moderately high Q, in other words, constant current slope.
What actually happens when you do this, is that the sensitivity to high-conductivity targets is really lousy. If you want to preserve good sensitivity to high-conductivity targets, you have to stretch the transmitter timing out so far that in typical PI practice (as I gather from reading patents etc.) the transmit coil actually approaches resistive current limiting. From a power efficiency point of view, this is terrible, but it's what ya gotta do to see those big deep targets.
In VLF detectors, having good transmitter Q is considered important. In conventional PI practice, there is much less concern for Q. The reason is that engineers have discovered that raising the Q of the transmitter coil does not offer the kind of improvement in sensitivity versus power consumption that one would expect based on analogy to VLF practice.
Apart from any theoretical reasoning behind it, that's what the actual conventional practice is, as I understand it. The rule of thumb you offer in your post is "5 times the target time-constant".
Now to the theoretical reason behind it. If it were the transmit risetime that energized the target so that it could be detected, one target time-constant would be the point of diminishing returns. You'd want to get that sucker kicked and then look at it as soon as possible. But anyone who tries this discovers that it doesn't work very well at all.
The reason it takes several time-constants, is because you need to give the target time to forget what happened during the transmit risetime. This happens best if the rate of change of current in the transmitter tapers off and approaches resistance current limiting, allowing the eddy current induced by the transmit current rise to decay back down to relative insignificance.
Modern coin-beach type PI's tend to use shorter transmit on-times and faster rep rates, in order to get good sensitivity to jewelry and acceptable sensitivity to the common coins. What happens on a US silver dollar doesn't matter much to a coin-beach machine, since there are very few silver dollars found on beaches.
The second paragraph of your post provides a clear and detailed description of these phenomena and how they relate to customary industry practice.
--------------
In the third paragraph of your post, you point out that due to skin effect, it takes time for a signal to penetrate to the bulk interior of a large conductive target. Although I agree with this, it does not lead me to the conclusion that transmit pulses have to be of long duration so that the target will be completely energized.
The actual field strength of the transmitter does not induce eddy currents in the target, nor does the actual eddy current in the target induce a voltage in the receiver. It is only the rate of change of the transmitter field, and the rate of change of the current flowing in the target, that matter. No change in current/field, no induction, regardless of how strong the field or the current might be.
When a high-conductivity target is energized to its center by a long-duration transmit pulse, the currents flowing in the target are already diminishing because the transmit field itself has a rate of change which is diminishing. That rate of change was never very high anyway, since the applied voltage to the coil was just battery voltage. If you were able to look at the target signal component without it being disturbed by flyback, the target signal would be weak because the current has stablized and its rate of change is low.
Now, hit it with flyback. Sudden high rate of change of field, a solid chunk of volt-time product that happened only a few microseconds ago. Substantial current is now flowing in the surface of the target. At the end of flyback, this substantial current flowing in the surface of the target redistributes itself quickly into thicker layers of the target, reducing the resistance in the current path and lengthening the decay time-constant. Even though the target is a high-conductivity one, it starts out looking like a physically large low-conductivity target, enabling it to induce a large voltage into the receiver coil. This is the physics behind the rule-of-thumb observation that in PI, it's surface area that counts, not mass. It has as much to do with how PI's are built, as it does with the properties of targets.
---------------
The fourth paragraph of your post says that "the flyback pulse in a conventional PI... is not something that is transmitted to the object." I would disagree. The flyback pulse is what collapses the field, producing the high rate of change of field needed to induce eddy currents in the target close enough in time to the receive period that the receiver will have something substantial to look at. Without flyback, a conventional PI would be nearly useless from the standpoint of sensitivity. (Not that there is any way to avoid flyback in a conventional PI.)
---------------
The UTEM system sounds like it may be something a bit similar to the PI system I'm proposing.
--------------
I expect people to argue over whether or not this should be called "pulse induction". For sure, it is neither conventional PI nor Fisher PI. But the basic idea of hitting the target with a pulse and then taking a look at what happened, in a time sequence, that's pulse and it's induction. The underlying thinking (if not the circuit topology) is from the PI camp, and is quite different from that of VLF/MF practice where signals are treated as continuous.
Figuring that this system needed a name, I decided to call it "continuous current pulse induction" to distinguish it from the prior art discontinuous current PI systems. Maybe someone else will suggest a better name for it.
--Dave J.
A
Anonymous
Guest
Hi Dave,
Thanks for your detailed reply. One point that needs clarifying is, what do we mean by flyback pulse. It could be that we are thinking of different things here. To me, the flyback pulse is the large voltage pulse that occurs across the coil when the TX current is rapidly reduced to zero. Bill Lahr, in his patent 5,414,411 refers to a
Thanks for your detailed reply. One point that needs clarifying is, what do we mean by flyback pulse. It could be that we are thinking of different things here. To me, the flyback pulse is the large voltage pulse that occurs across the coil when the TX current is rapidly reduced to zero. Bill Lahr, in his patent 5,414,411 refers to a
A
Anonymous
Guest
Eric, we're in agreement on our labelling of "transmit pulse" and "flyback pulse" in a conventional PI unit.
I am not saying that the transmit pulse is "redundant". From the standpoint of producing a target signal in the receiver, I'm saying that the transmit pulse is worse than redundant. That's why we put its risetime as far away from the receiver turn-on time as is practical. However, in order to have the (necessary) flyback pulse, something has to establish the field, so the transmit pulse is necessary for the operation of the machine even though it is worse than useless for the target itself.
I figured that someday, in conventional PI they'd dump the flyback into a high voltage capacitor, and then use a switching DC-DC convertor to put the energy back into the power supply. If Minelab's current waveform is a sawtooth, that means a big chunk of the energy that's going into the coil is making it all the way to the capacitor. It surprises me that Minelab would just dissipate the energy as heat. Here's my theory: they were intending to switch the power back down to low voltage, but ran into a few snags with the switcher at the last minute, so they said "Hey, we need to start shipping this thing, so the heck with the switcher, just dump the energy as heat, and we'll stick the switcher on the next model."
Here's another theory. When a PI unit having a sawtooth current waveform is lowered to ground having a high magnetic susceptibility, the transmitter inductance increases, reducing the transmitter current. In principle, the flyback (if at fixed voltage, as when controlled by an avalanche semiconductor) will still take the same length of time, because the inductance increase applies to the flyback, too. But, there are two flaws in the ointment: 1. The sawtooth isn't really a sawtooth, which breaks the time symmetry which would otherwise exist between transmit and flyback; and, 2. The increased inductance affects the decay time of unwanted residual stuff, which can matter a lot if you're pushing the receiver turn-on as close to the flyback as possible. It may be that Minelab has discovered that constant-current rather than constant-voltage flyback loading reduces unwanted responses to ground magnetic susceptibility. Furthermore, a constant-current arrangement such as you describe can easily be modulated to allow real-time adjustment of flyback time to correct for operating conditions such as ground mineral properties and temperature.
Anyone who knows the facts surrounding these mysteries care to comment in this forum? (I'm NOT suggesting that someone who holds a position of confidence should divulge trade secrets which don't belong to them.)
I'm not familiar with the engineering details of the SD's, but it doesn't surprise me that they'd be using a sawtooth current waveform. Although that reduces sensitivity to large objects when you look it on a pulse-by-pulse basis, it allows you to cram more pulses into unit time, which buys back some of what was otherwise lost. Anyway, large gold nuggets are not electrically like a silver dollar-- they have convoluted surfaces which allow them to respond as large surface area low conductivity targets.
The key to understanding target response in a PI is not to look at the transmitter current waveform, but to look at its first derivative with respect to time. The first derivative is what the target is responding to. And in a conventional PI, the flyback wins the first derivative contest hands down. Furthermore, since the receiver is looking at the target immediately after the flyback pulse, it gets nice fresh information from the target response to the flyback, which sure beats the auld lang syne decaying remnants of the information from the transmit pulse.
Dave Emery tells me that this whole business of the importance of flyback got thoroughly debated sometime in the past. I missed out on the Great Debate that time around, so it's fun to throw the whole thing back on the front burner and restir the pot.
Merry Christmas! (Hey, Christmas isn't over yet!The "12th day of Christmas" is January 5.)
--Dave J.
I am not saying that the transmit pulse is "redundant". From the standpoint of producing a target signal in the receiver, I'm saying that the transmit pulse is worse than redundant. That's why we put its risetime as far away from the receiver turn-on time as is practical. However, in order to have the (necessary) flyback pulse, something has to establish the field, so the transmit pulse is necessary for the operation of the machine even though it is worse than useless for the target itself.
I figured that someday, in conventional PI they'd dump the flyback into a high voltage capacitor, and then use a switching DC-DC convertor to put the energy back into the power supply. If Minelab's current waveform is a sawtooth, that means a big chunk of the energy that's going into the coil is making it all the way to the capacitor. It surprises me that Minelab would just dissipate the energy as heat. Here's my theory: they were intending to switch the power back down to low voltage, but ran into a few snags with the switcher at the last minute, so they said "Hey, we need to start shipping this thing, so the heck with the switcher, just dump the energy as heat, and we'll stick the switcher on the next model."
Here's another theory. When a PI unit having a sawtooth current waveform is lowered to ground having a high magnetic susceptibility, the transmitter inductance increases, reducing the transmitter current. In principle, the flyback (if at fixed voltage, as when controlled by an avalanche semiconductor) will still take the same length of time, because the inductance increase applies to the flyback, too. But, there are two flaws in the ointment: 1. The sawtooth isn't really a sawtooth, which breaks the time symmetry which would otherwise exist between transmit and flyback; and, 2. The increased inductance affects the decay time of unwanted residual stuff, which can matter a lot if you're pushing the receiver turn-on as close to the flyback as possible. It may be that Minelab has discovered that constant-current rather than constant-voltage flyback loading reduces unwanted responses to ground magnetic susceptibility. Furthermore, a constant-current arrangement such as you describe can easily be modulated to allow real-time adjustment of flyback time to correct for operating conditions such as ground mineral properties and temperature.
Anyone who knows the facts surrounding these mysteries care to comment in this forum? (I'm NOT suggesting that someone who holds a position of confidence should divulge trade secrets which don't belong to them.)
I'm not familiar with the engineering details of the SD's, but it doesn't surprise me that they'd be using a sawtooth current waveform. Although that reduces sensitivity to large objects when you look it on a pulse-by-pulse basis, it allows you to cram more pulses into unit time, which buys back some of what was otherwise lost. Anyway, large gold nuggets are not electrically like a silver dollar-- they have convoluted surfaces which allow them to respond as large surface area low conductivity targets.
The key to understanding target response in a PI is not to look at the transmitter current waveform, but to look at its first derivative with respect to time. The first derivative is what the target is responding to. And in a conventional PI, the flyback wins the first derivative contest hands down. Furthermore, since the receiver is looking at the target immediately after the flyback pulse, it gets nice fresh information from the target response to the flyback, which sure beats the auld lang syne decaying remnants of the information from the transmit pulse.
Dave Emery tells me that this whole business of the importance of flyback got thoroughly debated sometime in the past. I missed out on the Great Debate that time around, so it's fun to throw the whole thing back on the front burner and restir the pot.
Merry Christmas! (Hey, Christmas isn't over yet!The "12th day of Christmas" is January 5.)
--Dave J.
A
Anonymous
Guest
Hi Dave and Eric
I am am no electronic man,but after reading your posts,would I be right in saying that to get a coil to detect deep large nuggets that you would need to keep the inductance,resistance low,and the Q as high as possible,but that is tied to the other figures.
Regards Frank Wallis
I am am no electronic man,but after reading your posts,would I be right in saying that to get a coil to detect deep large nuggets that you would need to keep the inductance,resistance low,and the Q as high as possible,but that is tied to the other figures.
Regards Frank Wallis
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Anonymous
Guest
Frank, the Q and the resistance relate primarily to power consumption and circuit topology, not depth or target size except indirectly. Inductance is primarily a matter of how the designer wants to scale the circuit in relation to voltage and current; damping time is also related to inductance. If the objective is to detect large deep nuggets, independently of other performance and design issues, then the things that will help the most are having a large searchcoil; having relatively slow timing on everything, all the way from the transmit pulse to the quasi-DC differentiators (if any); and having a ground balance system which is effective in the kind of soil where the nugget is buried.
In an actual commercial product, all that stuff gets compromised to some extent in order to have good response to smaller and shallower nuggets, obtain reasonable battery life, have a machine which is not too mechanically cumbersome, keep manufacturing cost within bounds, etc.
To my knowledge, the only PI detectors designed specifically for serious gold prospecting in the consumer market, are the Minelab SD's. Geophex (geophex.com) is aimed more at the professional market, and might be good on the big deep nuggets.
If Eric has time to post a response, it may be quite different from mine. He's been in the thick of PI for decades: I dabbled in it for several years a while back. As my (Fisher's) patent shows, my way of thinking about PI is somewhat different from that of the rest of the industry, so it's possible that between myself and Eric you could get two worthwhile yet totally contradictory opinions on a question such as yours.
--Dave J.
In an actual commercial product, all that stuff gets compromised to some extent in order to have good response to smaller and shallower nuggets, obtain reasonable battery life, have a machine which is not too mechanically cumbersome, keep manufacturing cost within bounds, etc.
To my knowledge, the only PI detectors designed specifically for serious gold prospecting in the consumer market, are the Minelab SD's. Geophex (geophex.com) is aimed more at the professional market, and might be good on the big deep nuggets.
If Eric has time to post a response, it may be quite different from mine. He's been in the thick of PI for decades: I dabbled in it for several years a while back. As my (Fisher's) patent shows, my way of thinking about PI is somewhat different from that of the rest of the industry, so it's possible that between myself and Eric you could get two worthwhile yet totally contradictory opinions on a question such as yours.
--Dave J.
A
Anonymous
Guest
Hey, instead of wondering how to convert this high
voltage flyback pulse to low voltage (can be done with a big capacitor), leave it high voltage, store it on a somewhat smaller capacitor, and then switch say 200 volts onto the coil for the transmit pulse. This will cause the current to rise a whole lot faster on the coil, while the voltage on the cap drops, eventually reaching say
12 volts or 6 volts from the battery which is diode or'd with the cap (have to put some energy into this system to keep it up, and build it up to start with). Timed right (cap and coil) you would reach steady current in the coil faster and you could use a higher inductance (more turns) coil.
And the transmit coil is the step up DC-DC converter. Saves battery too. I have actually been working on this, realizing the need for high voltage on the transmit coil and a bunch of current through it.
Patent Pending.
JC
voltage flyback pulse to low voltage (can be done with a big capacitor), leave it high voltage, store it on a somewhat smaller capacitor, and then switch say 200 volts onto the coil for the transmit pulse. This will cause the current to rise a whole lot faster on the coil, while the voltage on the cap drops, eventually reaching say
12 volts or 6 volts from the battery which is diode or'd with the cap (have to put some energy into this system to keep it up, and build it up to start with). Timed right (cap and coil) you would reach steady current in the coil faster and you could use a higher inductance (more turns) coil.
And the transmit coil is the step up DC-DC converter. Saves battery too. I have actually been working on this, realizing the need for high voltage on the transmit coil and a bunch of current through it.
Patent Pending.
JC
A
Anonymous
Guest
JC, I like your idea.
I wouldn't view higher inductance as a major objective, because higher inductance will stretch out the flyback pulse and prolong damping, reducing sensitivity to the tiny stuff. However, in those situations where sensitivity to tiny stuff is not an important goal, the higher inductance would offer the opportunity to reduce power consumption esp. where the objective is to stretch pulsewidth to improve performance on high-conductivity targets.
The system you propose can have power efficiency in the same league as a VLF. Not only are you recovering flyback energy, the low voltage which is applied during most of the transmitter-on period provides a slow current ramp rate (esp. with increased inductance) eliminating the incentive to run the thing into resistive current limiting. High Q is rewarded.
The role of the "small capacitor" is not obvious to me. Since you'd need a transistor switch to connect it to the coil, it seems to me you could just switch the big storage capacitor directly to the coil for an on-time roughly comparable to the flyback time, and dispense with the additional capacitor.
The current waveform is a trapezoid with sharply slanting rising and trailing edges, and a fairly level top. This waveform puts the transmit risetime nearly as far from the flyback pulse as it could possibly get, improving response to high-conductivity objects and to iron.
Unlike my CCPI scheme, yours does not require an induction balance loop, nor does it require a reactive ground balancing system. CCPI is probably a bit more efficient in terms of converting battery power to signal (your system throws away the target information which is generated during the early part of the transmit pulse), but the difference would not be great enough to prefer CCPI over your scheme for that reason alone.
--Dave J.
I wouldn't view higher inductance as a major objective, because higher inductance will stretch out the flyback pulse and prolong damping, reducing sensitivity to the tiny stuff. However, in those situations where sensitivity to tiny stuff is not an important goal, the higher inductance would offer the opportunity to reduce power consumption esp. where the objective is to stretch pulsewidth to improve performance on high-conductivity targets.
The system you propose can have power efficiency in the same league as a VLF. Not only are you recovering flyback energy, the low voltage which is applied during most of the transmitter-on period provides a slow current ramp rate (esp. with increased inductance) eliminating the incentive to run the thing into resistive current limiting. High Q is rewarded.
The role of the "small capacitor" is not obvious to me. Since you'd need a transistor switch to connect it to the coil, it seems to me you could just switch the big storage capacitor directly to the coil for an on-time roughly comparable to the flyback time, and dispense with the additional capacitor.
The current waveform is a trapezoid with sharply slanting rising and trailing edges, and a fairly level top. This waveform puts the transmit risetime nearly as far from the flyback pulse as it could possibly get, improving response to high-conductivity objects and to iron.
Unlike my CCPI scheme, yours does not require an induction balance loop, nor does it require a reactive ground balancing system. CCPI is probably a bit more efficient in terms of converting battery power to signal (your system throws away the target information which is generated during the early part of the transmit pulse), but the difference would not be great enough to prefer CCPI over your scheme for that reason alone.
--Dave J.
A
Anonymous
Guest
The flat-topped trapezoidal waveform lends itself nicely to use with an induction balance loop, where the receiver issues become pretty much the same as with bipolar CCPI. If the transmitter on-time equals the sum of the flyback pulse and the dead time, the current waveform is a square wave with somewhat less than rectangular edges, and as far as the receiver is concerned it's nearly indistinguishable from CCPI.
In my prior post, I should have said that the "top" of the current trapezoid has a slight slope upward with time, assuming a high-Q coil.
If you can find some way to make the top of that trapezoid flat, then it won't induce reactive components into the receiver coil, and the receiver circuit would not require reactive ground balancing.
--Dave J.
In my prior post, I should have said that the "top" of the current trapezoid has a slight slope upward with time, assuming a high-Q coil.
If you can find some way to make the top of that trapezoid flat, then it won't induce reactive components into the receiver coil, and the receiver circuit would not require reactive ground balancing.
--Dave J.
A
Anonymous
Guest
I just meant that for the transmit coil (tx and rx coils) that increasing the number of turns increases the magnetic field for the same current through the coil(Amp-turns). The problem with just increasing the number of turns is the inductance goes up and then the current rise is even longer requiring longer tx times wasting more power and not getting as many pulses/second.
The bit about the smaller cap had to do with the fact that a large cap will integrate the flyback into a low voltage, and therefore use the a smaller value which will allow a higher voltage to be integrated first of all and that will drop the voltage down to the battery level toward the end of tx period.
Somehow more energy must be put into the system from the battery or it quits working.
You are correct that for one coil for tx and rx that the increase in inductance will still present problems in detecting small objects.
Most of my thoughts tend to have to do with a 52" octagon monocoil designed for large targets at depth (So I can stand the inductance). Also on this coil, unclamped, the flyback voltage is about 3000 volts. So far the Mosfet has been able clamp it to 400 volts without blowing up (IRF data sheets tell you the energy this thing can take in joules), so if I add a storage capacitor to catch it and integrate it I have a good amount of energy even at 200 volts. I thought later this might cause cofussion since some little PIs might only generate 200 volts peak. In fact I need to take some of this clamp load off the mosfet or the next step up will kill it (which I have proven). So pulling this energy into a cap helps that.
If you just want the energy back and don't care about the high voltage bit, than a large capacitor is the answer.
If you wish to be able to shorten the tx pulse to a few time constants instead of five to reach steady state current, save power, get more pulses per second, increase magnetic field strength, and get more depth, and the expensive of more complex electronics (not that bad actually) then maybe this has some merit. But it has to be done right.
JC
The bit about the smaller cap had to do with the fact that a large cap will integrate the flyback into a low voltage, and therefore use the a smaller value which will allow a higher voltage to be integrated first of all and that will drop the voltage down to the battery level toward the end of tx period.
Somehow more energy must be put into the system from the battery or it quits working.
You are correct that for one coil for tx and rx that the increase in inductance will still present problems in detecting small objects.
Most of my thoughts tend to have to do with a 52" octagon monocoil designed for large targets at depth (So I can stand the inductance). Also on this coil, unclamped, the flyback voltage is about 3000 volts. So far the Mosfet has been able clamp it to 400 volts without blowing up (IRF data sheets tell you the energy this thing can take in joules), so if I add a storage capacitor to catch it and integrate it I have a good amount of energy even at 200 volts. I thought later this might cause cofussion since some little PIs might only generate 200 volts peak. In fact I need to take some of this clamp load off the mosfet or the next step up will kill it (which I have proven). So pulling this energy into a cap helps that.
If you just want the energy back and don't care about the high voltage bit, than a large capacitor is the answer.
If you wish to be able to shorten the tx pulse to a few time constants instead of five to reach steady state current, save power, get more pulses per second, increase magnetic field strength, and get more depth, and the expensive of more complex electronics (not that bad actually) then maybe this has some merit. But it has to be done right.
JC
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Anonymous
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Hi Dave,
The expression for the return signal in a PI is the product of three terms. The first of these is a function of the object (which is usually a sphere for theoretical studies). The second is a property of the detecting system and the third is a geometrical factor expressing the fall off of the signal with distance of the object from the coil. The object is described by its radius, conductivity and time constant. The coil by its number of turns, radius and current at the time of switch-off. The geometric factor relates to the coil radius and the range at which the object is placed. From the expression, the voltage induced by the object in the receiving coil can be calculated from all times from toff onwards. Note that toff must be fast compared to the object TC. For toff faster than 1/5 of the object TC, the eddy currents generated are virtually the same as if the field had been removed instantaneously. Note that it is the current, and hence the field, being reduced to zero that is THE CAUSE. The derivatives of this are effects, not causes. The major effect is the generation of a high voltage spike across the ends of the coil, and a separate formula can be used to calculate this. The next effect is the derivative emf induced in the object which drives the eddy current, whose further derivative gives the signal induced in the receive coil. This whole subject is rigorously dealt with in a paper written in 1956 by F.A.Johnson of the Signals Research and Development Establishment, and later revised by Dr John Alldred who worked with me at GeoElectronics Ltd in the 1970
The expression for the return signal in a PI is the product of three terms. The first of these is a function of the object (which is usually a sphere for theoretical studies). The second is a property of the detecting system and the third is a geometrical factor expressing the fall off of the signal with distance of the object from the coil. The object is described by its radius, conductivity and time constant. The coil by its number of turns, radius and current at the time of switch-off. The geometric factor relates to the coil radius and the range at which the object is placed. From the expression, the voltage induced by the object in the receiving coil can be calculated from all times from toff onwards. Note that toff must be fast compared to the object TC. For toff faster than 1/5 of the object TC, the eddy currents generated are virtually the same as if the field had been removed instantaneously. Note that it is the current, and hence the field, being reduced to zero that is THE CAUSE. The derivatives of this are effects, not causes. The major effect is the generation of a high voltage spike across the ends of the coil, and a separate formula can be used to calculate this. The next effect is the derivative emf induced in the object which drives the eddy current, whose further derivative gives the signal induced in the receive coil. This whole subject is rigorously dealt with in a paper written in 1956 by F.A.Johnson of the Signals Research and Development Establishment, and later revised by Dr John Alldred who worked with me at GeoElectronics Ltd in the 1970
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Anonymous
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Dave said:
>I think we all agree that in the case of a low-conductivity target, the flyback pulse is what kicks the target.
>
Dave
It seems to me that your terminology is confusing when you talk about a voltage pulse on the transmit coil kicking the target.
The signal that you want to detect comes from the decaying magnetic field in the target. The magnetic field in the target is affected by the magnetic field of the transmit coil. The magnetic field of the transmit coil is a function of the coil current. And the coil current is a function of the transmit voltage. So it is true that the transmit voltage affects the signal from the target but it is not as direct as you make it sound.
At the end of the transmit on time there is a magnetic field around the coil. If the on time was long enough for the target to reach steady state and the coil current was fairly constant near the end of the on time then the eddy currents in the target will be approximately zero and the magnetic field through the target will be equal to the nearby field from the coil.
When you turn off the transmit drive transistor, the magnetic field around the coil starts to collapse. The collapsing magnetic field keeps current flowing through the transmit coil. If the transmit coil circuit has a high impedance at this time the collapsing magnetic field will cause a high voltage pulse (the flyback pulse). The duration of the pulse depends on how long it takes to dissipate all the energy in the magnetic field. The higher the impedance the faster the energy is dissipated (power = R*I^2).
At the same time, the magnetic field through the target is decaying and causing current to flow in the target (eddy current). The duration of this current depends on the impedance of the circuit in the target. The higher the conductivity the longer the current will last. But the flyback pulse does not cause the eddy current in the target. The collapsing magnetic field through the target causes the eddy current, and the collapsing magnetic field through the coil causes the pulse from the coil.
Robert
>I think we all agree that in the case of a low-conductivity target, the flyback pulse is what kicks the target.
>
Dave
It seems to me that your terminology is confusing when you talk about a voltage pulse on the transmit coil kicking the target.
The signal that you want to detect comes from the decaying magnetic field in the target. The magnetic field in the target is affected by the magnetic field of the transmit coil. The magnetic field of the transmit coil is a function of the coil current. And the coil current is a function of the transmit voltage. So it is true that the transmit voltage affects the signal from the target but it is not as direct as you make it sound.
At the end of the transmit on time there is a magnetic field around the coil. If the on time was long enough for the target to reach steady state and the coil current was fairly constant near the end of the on time then the eddy currents in the target will be approximately zero and the magnetic field through the target will be equal to the nearby field from the coil.
When you turn off the transmit drive transistor, the magnetic field around the coil starts to collapse. The collapsing magnetic field keeps current flowing through the transmit coil. If the transmit coil circuit has a high impedance at this time the collapsing magnetic field will cause a high voltage pulse (the flyback pulse). The duration of the pulse depends on how long it takes to dissipate all the energy in the magnetic field. The higher the impedance the faster the energy is dissipated (power = R*I^2).
At the same time, the magnetic field through the target is decaying and causing current to flow in the target (eddy current). The duration of this current depends on the impedance of the circuit in the target. The higher the conductivity the longer the current will last. But the flyback pulse does not cause the eddy current in the target. The collapsing magnetic field through the target causes the eddy current, and the collapsing magnetic field through the coil causes the pulse from the coil.
Robert
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Anonymous
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You're right, Robert, that phrase about the "flyback pulse kicking the target" was rather sloppy use of the English language.
The voltage that appears across the transmit coil during flyback is what drives the current back to zero. If the voltage were somehow maintained, the current would go through zero and reverse itself, a process which actually happens in resonant circuits.
The rapid change in the magnetic field which occurs during the flyback pulse (not literally the voltage seen on the coil) is what generates an e.m.f. in the space surrounding the coil. If there is a conductive object in this space, the e.m.f. will cause current to flow in the object.
(Incidentally, if anyone reading this doubts that the changing magnetic produces a free space e.m.f., they can measure that e.m.f. for themselves by simply making a small loop of wire and connecting it to an oscilloscope, then bringing it near the transmitter coil.)
When the current in the transmitter coil stops changing (whether or not that current is zero is irrelevant), there is no more free space e.m.f. being produced by the processes taking place in the coil. The voltage drop across the resistance of the target drives the current flowing in the target toward zero in a quasi-negative-exponential fashion. If the target is a ring of nonferrous metal, the current decay is a true negative exponential which follows the rules for an RL time-constant, R and L being the resistance of the current loop and L being the inductance of the current loop.
As the current flowing in the target decays, the field produced by that current also decays. The roles of transmitter coil and target are now reversed. The target is a little one-turn transmitter coil which is producing an e.m.f. in the surrounding space because the current is changing, and the thing with all that wire in it we previously called "the transmitter coil" is now a target with voltage being induced in it.
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In the case of a target which has a time-constant shorter than the duration of the flyback, whatever happened prior to the flyback doesn't matter to the target or to the receiver system. If energy was expended for any reason other than to make a current available which can be pushed back to zero during flyback, that energy was wasted.
In the case of a high-conductivity target, the situation is not so simple. The rising field strength during the transmit on-time (commonly called "the transmit pulse" because it delivers energy to a coil which we call "the transmit coil", not because any useful signal is being transmitted) produces a free space e.m.f. which induces current to flow in the target in a direction opposite that of the current that will be induced during the flyback pulse. This being a linear process, by superposition the currents tend to cancel each other during the time the receiver is turned on.
In order to minimize the amount of current which is induced in the target during the transmit on-time which will still remain when the receiver is turned on, it is customary to lengthen the transmit on-time and allow the coil to go into resistance current limiting. This puts most of the "target action" tens or hundreds of microseconds before the receiver is turned, allowing it to decay sufficiently that it will not substantially cancel the current induced in the target during flyback. It also means that the coil is being used as an electrical heating element rather than an energy storage device.
When it comes to our explanations of how PI works, I think we're all describing the same processes, but using words a bit differently to do it. Does "the target care about flyback?" If by flyback we mean the voltage across the transmitter coil, independently of the implications of that voltage to coil current, and independently of the connection between coil current and field, then no, the target doesn't care one whit about flyback. If, on the other hand, we take the question to mean, "is the target signal that the receiver sees, primarily the result of process that take place during flyback?" then the answer is yes. And, to the extent that high electrical conductivity of the target leads to dependence on processes that take place during the transmit on-time, those processes are detrimental to target detection.
If there's something we're not agreeing on, I'm not sure at this point what it is, except differences in wording. In particular, it looks like we're in agreement that ideally the current in the target would be zero at the beginning of flyback.
--Dave J.
The voltage that appears across the transmit coil during flyback is what drives the current back to zero. If the voltage were somehow maintained, the current would go through zero and reverse itself, a process which actually happens in resonant circuits.
The rapid change in the magnetic field which occurs during the flyback pulse (not literally the voltage seen on the coil) is what generates an e.m.f. in the space surrounding the coil. If there is a conductive object in this space, the e.m.f. will cause current to flow in the object.
(Incidentally, if anyone reading this doubts that the changing magnetic produces a free space e.m.f., they can measure that e.m.f. for themselves by simply making a small loop of wire and connecting it to an oscilloscope, then bringing it near the transmitter coil.)
When the current in the transmitter coil stops changing (whether or not that current is zero is irrelevant), there is no more free space e.m.f. being produced by the processes taking place in the coil. The voltage drop across the resistance of the target drives the current flowing in the target toward zero in a quasi-negative-exponential fashion. If the target is a ring of nonferrous metal, the current decay is a true negative exponential which follows the rules for an RL time-constant, R and L being the resistance of the current loop and L being the inductance of the current loop.
As the current flowing in the target decays, the field produced by that current also decays. The roles of transmitter coil and target are now reversed. The target is a little one-turn transmitter coil which is producing an e.m.f. in the surrounding space because the current is changing, and the thing with all that wire in it we previously called "the transmitter coil" is now a target with voltage being induced in it.
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In the case of a target which has a time-constant shorter than the duration of the flyback, whatever happened prior to the flyback doesn't matter to the target or to the receiver system. If energy was expended for any reason other than to make a current available which can be pushed back to zero during flyback, that energy was wasted.
In the case of a high-conductivity target, the situation is not so simple. The rising field strength during the transmit on-time (commonly called "the transmit pulse" because it delivers energy to a coil which we call "the transmit coil", not because any useful signal is being transmitted) produces a free space e.m.f. which induces current to flow in the target in a direction opposite that of the current that will be induced during the flyback pulse. This being a linear process, by superposition the currents tend to cancel each other during the time the receiver is turned on.
In order to minimize the amount of current which is induced in the target during the transmit on-time which will still remain when the receiver is turned on, it is customary to lengthen the transmit on-time and allow the coil to go into resistance current limiting. This puts most of the "target action" tens or hundreds of microseconds before the receiver is turned, allowing it to decay sufficiently that it will not substantially cancel the current induced in the target during flyback. It also means that the coil is being used as an electrical heating element rather than an energy storage device.
When it comes to our explanations of how PI works, I think we're all describing the same processes, but using words a bit differently to do it. Does "the target care about flyback?" If by flyback we mean the voltage across the transmitter coil, independently of the implications of that voltage to coil current, and independently of the connection between coil current and field, then no, the target doesn't care one whit about flyback. If, on the other hand, we take the question to mean, "is the target signal that the receiver sees, primarily the result of process that take place during flyback?" then the answer is yes. And, to the extent that high electrical conductivity of the target leads to dependence on processes that take place during the transmit on-time, those processes are detrimental to target detection.
If there's something we're not agreeing on, I'm not sure at this point what it is, except differences in wording. In particular, it looks like we're in agreement that ideally the current in the target would be zero at the beginning of flyback.
--Dave J.
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Anonymous
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please see my reply to Robert Hoolko above.
I think that you and I and Robert are all giving the same description of how conventional PI works; but, coming from different backgrounds, we are using different words to say it.
Our discussion over the last several days has probably produced at least 5 or 6 accurate but different-sounding descriptions of how PI works.
It's easy to get confused about inductors and magnetic fields-- so much of what they do is contrary to common sense. Once I had a patent rejected by the Examiner because I claimed a circuit in which the current flow was negative when the voltage was positive. Sounds like a perpetual motion machine, what eh? He finally relented when I Xeroxed a page out of the International Rectifier Handbook showing the same waveform, but even then I suspect he thought he was being bamboozled.
--Dave J.
I think that you and I and Robert are all giving the same description of how conventional PI works; but, coming from different backgrounds, we are using different words to say it.
Our discussion over the last several days has probably produced at least 5 or 6 accurate but different-sounding descriptions of how PI works.
It's easy to get confused about inductors and magnetic fields-- so much of what they do is contrary to common sense. Once I had a patent rejected by the Examiner because I claimed a circuit in which the current flow was negative when the voltage was positive. Sounds like a perpetual motion machine, what eh? He finally relented when I Xeroxed a page out of the International Rectifier Handbook showing the same waveform, but even then I suspect he thought he was being bamboozled.
--Dave J.
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Anonymous
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Dave
The energy that is released from target during the sampling period was actually stored there during the coil on time. It is easiest to describe this for the case where the coil rise time is much less than the target time constant. This is faster than in Thomas' experiment. So consider a PI circuit in which a high voltage is used to raise the coil current in less than one tenth of the target time constant, and then the current is held constant for several time constants, and then the coil is turned off.
The magnetic field builds up quickly around the coil, but the field does not immediately penetrate the target because the rising field creates eddy currents in the target that oppose the coil's field. The resistance of the target causes the eddy current to decay over time allowing the coil's field to gradually penetrate the target. During the first target time constant the magnetic field inside the target will reach 63% of its maximum possible value. During the second it will add another 23% bringing up to 86% of max. During the third it will add 9% for a total of 95%. After the fourth it will be at 98% of max. The expression is (1 - e^(-t/TC).
When the coil is turned off, the field around the coil will collapse, and the field inside the target will collapse at the rate of e^(-t/TC). It is important that the coil field collapse quickly to get it out of the way so the target signal can be measured. But what is being detected from the target is the decay of the magnetic field that was stored there during the first couple of time constants of the on time. If the on time were too short, the magnetic field inside the target would not have reached its full strength and there would be less to detect after the coil is turned off.
If the coil current rises more slowly the math is more difficult but the principle is the same, the coil field has to be applied long enough for the field inside the target to build up to reasonable value. After 4 or 5 time constants there is no more significant change, so any on time past that is a waste as far as that particular target is concerned.
Robert
The energy that is released from target during the sampling period was actually stored there during the coil on time. It is easiest to describe this for the case where the coil rise time is much less than the target time constant. This is faster than in Thomas' experiment. So consider a PI circuit in which a high voltage is used to raise the coil current in less than one tenth of the target time constant, and then the current is held constant for several time constants, and then the coil is turned off.
The magnetic field builds up quickly around the coil, but the field does not immediately penetrate the target because the rising field creates eddy currents in the target that oppose the coil's field. The resistance of the target causes the eddy current to decay over time allowing the coil's field to gradually penetrate the target. During the first target time constant the magnetic field inside the target will reach 63% of its maximum possible value. During the second it will add another 23% bringing up to 86% of max. During the third it will add 9% for a total of 95%. After the fourth it will be at 98% of max. The expression is (1 - e^(-t/TC).
When the coil is turned off, the field around the coil will collapse, and the field inside the target will collapse at the rate of e^(-t/TC). It is important that the coil field collapse quickly to get it out of the way so the target signal can be measured. But what is being detected from the target is the decay of the magnetic field that was stored there during the first couple of time constants of the on time. If the on time were too short, the magnetic field inside the target would not have reached its full strength and there would be less to detect after the coil is turned off.
If the coil current rises more slowly the math is more difficult but the principle is the same, the coil field has to be applied long enough for the field inside the target to build up to reasonable value. After 4 or 5 time constants there is no more significant change, so any on time past that is a waste as far as that particular target is concerned.
Robert
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Anonymous
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Robert:
Let us assume for the sake of argument that the transmitter has been on, and at a constant current, forever. That's the ultimate "transmit pulse" (a misnomer, but we use it anyway). What is happening in the target? Nothing, and nothing is what's been happening ever since we can remember. The (nonferrous) target doesn't know or care what the actual field strength is, so we may as well not care either.
Then we suddenly produce a step function in the transmit current. "Flyback" is one way to do that. But the target doesn't know or care how we do it, nor does the target know or care what the current was at the beginning of "flyback", or at the end of "flyback". The only thing that affects the target is the magnitude of the change. (We're assuming a fast enough change that it looks instantaneous to the target). There could be 100 amperes constant DC transmit current superimposed on the whole process, keeping the searchcoil glowing red: doesn't matter. It could happen in earth field, and usually does-- doesn't matter.
This step change in field induces a unit pulse of e.m.f. (voltage) in the space in which the target is located, which causes current to flow in the target. One instant no current, next instant, finite current. It is the slope of the decay of this current (not its actual value at any instant) which is detected by the receiver coil.
In real life, there are no perfect step functions or unit pulses, but those concepts can be useful approximations, and are here. Used properly, they are good reality checks.
The process by which the target induces a voltage in the receiver coil is the same as the process by which the changing field of the transmitter coil induces a voltage in the target. In the case of the receiver, however, we prefer for minimum current to flow, because we have a preamp sitting there which will do a better job of changing voltage to current than the coil itself can, and we can do the integration downstream of the demodulators.
The concept of "the field building up inside the target" during the transmit pulse can be misleading, as can the concept of "the field collapsing" during the flyback pulse. These two concepts are at best convenient mental pictures, not statements of physics. Because they so easily lead to erroneous conclusions, I never use the "field collapse" picture, and the "field buildup" picture I use only when thinking about magnetic hysteresis.
Robert, it is evident to me from your post of 3 January that if you and I were to draw voltage and current waveforms of a PI system (including the target), we'd probably come up with the same waveforms, no matter what words we used to describe those waveforms. I'm under the impression you've built quite a few PI machines yourself, so I don't doubt your grasp of the essentials.
If we abandon the words associated with mental pictures ("transmit", "flyback", "field collapse", etc.), and stick with statements like "if this is the current waveform, this will be the voltage waveform", then we'll be talking about things about which we agree on the physics, and which we have actually seen on oscilloscopes. But, alas, I doubt that we have that much self-discipline.
In future posts I hope to expound on the concept of current being proportional to volt-time product, and the concept of superposition of currents as a way of breaking up events in coils and targets into easily describable functions which can then be summed to get an accurate result.
-Dave J.
Let us assume for the sake of argument that the transmitter has been on, and at a constant current, forever. That's the ultimate "transmit pulse" (a misnomer, but we use it anyway). What is happening in the target? Nothing, and nothing is what's been happening ever since we can remember. The (nonferrous) target doesn't know or care what the actual field strength is, so we may as well not care either.
Then we suddenly produce a step function in the transmit current. "Flyback" is one way to do that. But the target doesn't know or care how we do it, nor does the target know or care what the current was at the beginning of "flyback", or at the end of "flyback". The only thing that affects the target is the magnitude of the change. (We're assuming a fast enough change that it looks instantaneous to the target). There could be 100 amperes constant DC transmit current superimposed on the whole process, keeping the searchcoil glowing red: doesn't matter. It could happen in earth field, and usually does-- doesn't matter.
This step change in field induces a unit pulse of e.m.f. (voltage) in the space in which the target is located, which causes current to flow in the target. One instant no current, next instant, finite current. It is the slope of the decay of this current (not its actual value at any instant) which is detected by the receiver coil.
In real life, there are no perfect step functions or unit pulses, but those concepts can be useful approximations, and are here. Used properly, they are good reality checks.
The process by which the target induces a voltage in the receiver coil is the same as the process by which the changing field of the transmitter coil induces a voltage in the target. In the case of the receiver, however, we prefer for minimum current to flow, because we have a preamp sitting there which will do a better job of changing voltage to current than the coil itself can, and we can do the integration downstream of the demodulators.
The concept of "the field building up inside the target" during the transmit pulse can be misleading, as can the concept of "the field collapsing" during the flyback pulse. These two concepts are at best convenient mental pictures, not statements of physics. Because they so easily lead to erroneous conclusions, I never use the "field collapse" picture, and the "field buildup" picture I use only when thinking about magnetic hysteresis.
Robert, it is evident to me from your post of 3 January that if you and I were to draw voltage and current waveforms of a PI system (including the target), we'd probably come up with the same waveforms, no matter what words we used to describe those waveforms. I'm under the impression you've built quite a few PI machines yourself, so I don't doubt your grasp of the essentials.
If we abandon the words associated with mental pictures ("transmit", "flyback", "field collapse", etc.), and stick with statements like "if this is the current waveform, this will be the voltage waveform", then we'll be talking about things about which we agree on the physics, and which we have actually seen on oscilloscopes. But, alas, I doubt that we have that much self-discipline.
In future posts I hope to expound on the concept of current being proportional to volt-time product, and the concept of superposition of currents as a way of breaking up events in coils and targets into easily describable functions which can then be summed to get an accurate result.
-Dave J.
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Dave
I agree that the magnitude of the step determines the strength of the target response. It does not matter if the change is 10 to 8, or 2 to 0, or 1 to -1. They are all the same step size. But in a simple PI design with a single winding coil you want the drive voltage and drive current to be zero while you are trying to measure the target signal. That is what fixes the endpoint of the step at 0. My use of terminology was based on this special case where the final current and final magnetic field is going to be 0. If you want to consider more complex designs where the drive current does not end up at 0 while measuring the target response then terms like collapse and flyback are probably not very useful.
Looking back over my posts I think the point that I really wanted to make is that the total target response is not determined just by the final drive current step. It is determined by the history of the drive current. If the changes in current happen rapidly you can use superposition as you suggested in your last paragraph. Each current change produces a target response. Just add all the target responses together. (If the current changes slowly you have to use convolution.) The consequence of this is that all the current during the past 4 or 5 time constants is important. The farther back in time it was, the less important it is, e^(-t/TC).
By the way, I have not built any PI's. I have never even used one. I don't normally post here because I don't know anything about PI's. But I was sick last week and was not working. Idle hands get into all kinds of trouble.
Robert
I agree that the magnitude of the step determines the strength of the target response. It does not matter if the change is 10 to 8, or 2 to 0, or 1 to -1. They are all the same step size. But in a simple PI design with a single winding coil you want the drive voltage and drive current to be zero while you are trying to measure the target signal. That is what fixes the endpoint of the step at 0. My use of terminology was based on this special case where the final current and final magnetic field is going to be 0. If you want to consider more complex designs where the drive current does not end up at 0 while measuring the target response then terms like collapse and flyback are probably not very useful.
Looking back over my posts I think the point that I really wanted to make is that the total target response is not determined just by the final drive current step. It is determined by the history of the drive current. If the changes in current happen rapidly you can use superposition as you suggested in your last paragraph. Each current change produces a target response. Just add all the target responses together. (If the current changes slowly you have to use convolution.) The consequence of this is that all the current during the past 4 or 5 time constants is important. The farther back in time it was, the less important it is, e^(-t/TC).
By the way, I have not built any PI's. I have never even used one. I don't normally post here because I don't know anything about PI's. But I was sick last week and was not working. Idle hands get into all kinds of trouble.
Robert
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Dave. The voltage that appears across the transmit coil during flyback is what drives the current back to zero. If the voltage were somehow maintained, the current would go through zero and reverse itself, a process which actually happens in resonant circuits.
Eric. The voltage that appears across the transmit coil during flyback is what tries to keep the current going. However, because the coil circuit has a high resistance at this time, the energy is quickly dissipated, resulting in the collapse of the magnetic field.
Dave. The rapid change in the magnetic field which occurs during the flyback pulse (not literally the voltage seen on the coil) is what generates an e.m.f. in the space surrounding the coil. If there is a conductive object in this space, the e.m.f. will cause current to flow in the object.
Eric. It is not the free space emf that causes the current to flow in an object. It is the collapse of the magnetic field in which the object is immersed. This generates an internal emf in the object that drives a current, which in turn tries to maintain the internal field. This current is then dissipated in the internal resistance.
Dave. Incidentally, if anyone reading this doubts that the changing magnetic produces a free space e.m.f., they can measure that e.m.f. for themselves by simply making a small loop of wire and connecting it to an oscilloscope, then bringing it near the transmitter coil.
Eric. A small loop of wire brought near the coil and connected to a scope is not measuring the free space emf, but the emf induced in the coil, that appears across the scope input impedance by the collapsing magnetic field.
Eric.
Eric. The voltage that appears across the transmit coil during flyback is what tries to keep the current going. However, because the coil circuit has a high resistance at this time, the energy is quickly dissipated, resulting in the collapse of the magnetic field.
Dave. The rapid change in the magnetic field which occurs during the flyback pulse (not literally the voltage seen on the coil) is what generates an e.m.f. in the space surrounding the coil. If there is a conductive object in this space, the e.m.f. will cause current to flow in the object.
Eric. It is not the free space emf that causes the current to flow in an object. It is the collapse of the magnetic field in which the object is immersed. This generates an internal emf in the object that drives a current, which in turn tries to maintain the internal field. This current is then dissipated in the internal resistance.
Dave. Incidentally, if anyone reading this doubts that the changing magnetic produces a free space e.m.f., they can measure that e.m.f. for themselves by simply making a small loop of wire and connecting it to an oscilloscope, then bringing it near the transmitter coil.
Eric. A small loop of wire brought near the coil and connected to a scope is not measuring the free space emf, but the emf induced in the coil, that appears across the scope input impedance by the collapsing magnetic field.
Eric.
simonbaker
New member
Eric Foster said:Dave. The voltage that appears across the transmit coil during flyback is what drives the current back to zero. If the voltage were somehow maintained, the current would go through zero and reverse itself, a process which actually happens in resonant circuits.
Eric. The voltage that appears across the transmit coil during flyback is what tries to keep the current going. However, because the coil circuit has a high resistance at this time, the energy is quickly dissipated, resulting in the collapse of the magnetic field.
Dave. The rapid change in the magnetic field which occurs during the flyback pulse (not literally the voltage seen on the coil) is what generates an e.m.f. in the space surrounding the coil. If there is a conductive object in this space, the e.m.f. will cause current to flow in the object.
Eric. It is not the free space emf that causes the current to flow in an object. It is the collapse of the magnetic field in which the object is immersed. This generates an internal emf in the object that drives a current, which in turn tries to maintain the internal field. This current is then dissipated in the internal resistance.
Dave. Incidentally, if anyone reading this doubts that the changing magnetic produces a free space e.m.f., they can measure that e.m.f. for themselves by simply making a small loop of wire and connecting it to an oscilloscope, then bringing it near the transmitter coil.
Eric. A small loop of wire brought near the coil and connected to a scope is not measuring the free space emf, but the emf induced in the coil, that appears across the scope input impedance by the collapsing magnetic field.
Eric.
But isn't Dave basically correct using the "free space emf" terminology? When you say the emf induced in the coil, how do you think that happens? Doesn't it boil down to Maxwell's equations affecting the electrons in the wire?
Regards,
-SB
Eric Foster
New member
Hi Simon,
Only the other day I was reading the posts on the Geotech Forum and wondered if the subject would leak over to this one. If you put a 0.1 ohm sensing resistor in the ground end of the coil, you can observe with a scope exactly what the current and hence the magnetic field from the coil is doing. First there is an exponential rise as the current grows in the coil inductance, then there is a flatter portion after which the current ramps down almost linearly toward the zero line. No high voltage spikes show up, neither do they as far as the target object is concerned. Basically we are dealing with Faraday's law of electromagnetic induction where the coil and target are in fact a transformer. The link between target and coil is solely the changing magnetic field. You can put a Faraday screen between the coil and target without affecting the result. It is the collapsing magnetic field that induces an emf in the target, which then drives the eddy current.
Eric.
Only the other day I was reading the posts on the Geotech Forum and wondered if the subject would leak over to this one. If you put a 0.1 ohm sensing resistor in the ground end of the coil, you can observe with a scope exactly what the current and hence the magnetic field from the coil is doing. First there is an exponential rise as the current grows in the coil inductance, then there is a flatter portion after which the current ramps down almost linearly toward the zero line. No high voltage spikes show up, neither do they as far as the target object is concerned. Basically we are dealing with Faraday's law of electromagnetic induction where the coil and target are in fact a transformer. The link between target and coil is solely the changing magnetic field. You can put a Faraday screen between the coil and target without affecting the result. It is the collapsing magnetic field that induces an emf in the target, which then drives the eddy current.
Eric.