A
Anonymous
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In the early 1960
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A
Anonymous
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It sounds like we're talking half sine current pulses in both cases, since the Barringer thing was probably high Q, and conventional roadway loops have a Q down in the dust in the basement at 60 Hz.
Detection principle is the same as in a conventional pulse detector. Current is induced into the target one way on the rising quarter-sine, and is induced into the target the other way on the falling quarter-sine. The last one "wins" because it's the one closest in time to the receiver turn-on.
Again, it is helpful to remember that the process is linear so superposition applies. The first induced current and second induced current can be analyzed as though they existed as independent entities, the first one having plenty of time to decay, and the other one still fresh at receiver turn-on. To analyze what the receiver is going to see, you have to sum the two (conceptually) independent currents.
--Dave J.
Detection principle is the same as in a conventional pulse detector. Current is induced into the target one way on the rising quarter-sine, and is induced into the target the other way on the falling quarter-sine. The last one "wins" because it's the one closest in time to the receiver turn-on.
Again, it is helpful to remember that the process is linear so superposition applies. The first induced current and second induced current can be analyzed as though they existed as independent entities, the first one having plenty of time to decay, and the other one still fresh at receiver turn-on. To analyze what the receiver is going to see, you have to sum the two (conceptually) independent currents.
--Dave J.
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Anonymous
Guest
OK Dave, I understand that, but the reason I posed the question is that there is no flyback in the terms that we have discussed.
Eric.
Eric.
A
Anonymous
Guest
Hi Eric,
I thought about your question and one solution that comes to mind is monitoring the peak voltage output. If one side of the loop is connected to a simple series resistor and the other to the rectified signal, I would think the inductance change of the loop because of an automobile entering the field would lower the voltage across a resistor in series with the coil. This is an over simplistic answer, but I see no reason it shouldn't work given the fundamental parameters you gave to work with.
A more complex answer would be to analyze a phase/amplitude change across the resistor using synchronous demodulation. However, I would think this would be unnessary since a stepped down voltage should display very little change as the result of any line fluctuations. As such, the voltage fluctions as seen across a series resistor should be the result of the inductance change due to the presence of a vehicle in the loop.
Reg
Sorry about the rather mundane answer, but I still think it should work ok.
I thought about your question and one solution that comes to mind is monitoring the peak voltage output. If one side of the loop is connected to a simple series resistor and the other to the rectified signal, I would think the inductance change of the loop because of an automobile entering the field would lower the voltage across a resistor in series with the coil. This is an over simplistic answer, but I see no reason it shouldn't work given the fundamental parameters you gave to work with.
A more complex answer would be to analyze a phase/amplitude change across the resistor using synchronous demodulation. However, I would think this would be unnessary since a stepped down voltage should display very little change as the result of any line fluctuations. As such, the voltage fluctions as seen across a series resistor should be the result of the inductance change due to the presence of a vehicle in the loop.
Reg
Sorry about the rather mundane answer, but I still think it should work ok.
A
Anonymous
Guest
The weekend's over, but I'll throw in my guess.
Current through the coil (and thus through the rectifier) should lag the voltage somewhat, ideally 90 degrees I suppose. So I would expect the rectifier to stay on longer than a half-cycle, ideally 3/4 cycles is the lag is 90 degrees. At this point the voltage on the coil will be at a negative peak, then when the diode shuts off (and hence the current is zero) the coil voltage will snap to zero volts. Perhaps you can look at this slew to see if there is a tail due to a target.
However, we're talking 10-15ms at 50Hz, and it seems to me that the current slew (and hence the magnetic field slew) would be slow enough that eddy effects would pretty much be dead by the time you tried to use the coil as a receiver. Maybe not, maybe iron mass of a car will cause that long of a tail. Or maybe you can configure the rectifier/coil to have a quicker negative current slew.
OK, give us the real answer...
- Carl
Current through the coil (and thus through the rectifier) should lag the voltage somewhat, ideally 90 degrees I suppose. So I would expect the rectifier to stay on longer than a half-cycle, ideally 3/4 cycles is the lag is 90 degrees. At this point the voltage on the coil will be at a negative peak, then when the diode shuts off (and hence the current is zero) the coil voltage will snap to zero volts. Perhaps you can look at this slew to see if there is a tail due to a target.
However, we're talking 10-15ms at 50Hz, and it seems to me that the current slew (and hence the magnetic field slew) would be slow enough that eddy effects would pretty much be dead by the time you tried to use the coil as a receiver. Maybe not, maybe iron mass of a car will cause that long of a tail. Or maybe you can configure the rectifier/coil to have a quicker negative current slew.
OK, give us the real answer...
- Carl
A
Anonymous
Guest
Don't know how the companies finally did it, but the simplest might be to just measure the change in current in the coil with the introduction of big metal objects.
JC
JC
A
Anonymous
Guest
Hi Carl and all,
I obviously did not phrase the question clearly, as the main point seems to have been missed. This all resulted from the debate about the flyback pulse, and whether it was important or not. There are waveforms other than square or rectangular, that can be used for time domain type detectors. Half sine and triangular being two. Neither half sine or triangular have a voltage flyback pulse, and yet they can be used in a metal detector that samples the object decay waveform, just as for a rectangular pulse. I don't believe that the flyback pulse has a function other than it is just a secondary effect of the collapsing field at the coil terminals. Hence I am happy that the half sine works as expected. I just wanted to see how the pro-flyback camp explained things.
Eric.
I obviously did not phrase the question clearly, as the main point seems to have been missed. This all resulted from the debate about the flyback pulse, and whether it was important or not. There are waveforms other than square or rectangular, that can be used for time domain type detectors. Half sine and triangular being two. Neither half sine or triangular have a voltage flyback pulse, and yet they can be used in a metal detector that samples the object decay waveform, just as for a rectangular pulse. I don't believe that the flyback pulse has a function other than it is just a secondary effect of the collapsing field at the coil terminals. Hence I am happy that the half sine works as expected. I just wanted to see how the pro-flyback camp explained things.
Eric.
A
Anonymous
Guest
Eric
I did not understand the question, but here is an answer to some question using Dave Johnson's superposition method.
This is for a half sine coil current waveform. Since the current gradually goes to 0 there is no flyback. I did not solve for the voltage waveform required to produce this current, but it would look approximately like the part of a sine wave from 90
I did not understand the question, but here is an answer to some question using Dave Johnson's superposition method.
This is for a half sine coil current waveform. Since the current gradually goes to 0 there is no flyback. I did not solve for the voltage waveform required to produce this current, but it would look approximately like the part of a sine wave from 90
A
Anonymous
Guest
Hi Robert,
Thanks for the waveforms you are posting. Makes things a lot clearer. The half sine current pulse is used mainly for geophysical prospecting equipment. As I understand it, a capacitor is charged to a 100V or more and discharged via a thyristor into the TX coil. The current does a half sine at the resonant frequency of the L and C then the thyristor automatically shuts off as the current goes through zero. The L and C are chosen to give the on time required, usually a few milliseconds for the object time constants that ore bodies have. The receive coil waveform is the derivative of the half sine current and the object decay shows as a tail, as in your waveform.
Eric.
Thanks for the waveforms you are posting. Makes things a lot clearer. The half sine current pulse is used mainly for geophysical prospecting equipment. As I understand it, a capacitor is charged to a 100V or more and discharged via a thyristor into the TX coil. The current does a half sine at the resonant frequency of the L and C then the thyristor automatically shuts off as the current goes through zero. The L and C are chosen to give the on time required, usually a few milliseconds for the object time constants that ore bodies have. The receive coil waveform is the derivative of the half sine current and the object decay shows as a tail, as in your waveform.
Eric.
A
Anonymous
Guest
Hi Eric,
here is my attempt to pull you to the flyback camp ;-)
Maybe there is still a misunderstanding: I (and also Dave & the other
here is my attempt to pull you to the flyback camp ;-)
Maybe there is still a misunderstanding: I (and also Dave & the other