What I would like in a detector

[Anonymous]

I would like a hybrid machine that has characteristics of both PI and VLF machines. It would use a pulse waveform and an IB coil. The diagram below shows a transmit coil voltage waveform and at least 5 points at which I would like to sample the receive signal.
The sample at point A is for the purpose of subtracting off any background signal such as the earth's magnetic field.
Samples B and C are taken during the on time. These contain information about the reactive part of the signal. These can be filtered and used to generate a target ID (tone ID) and for discriminating out ferrous targets. These also contain a lot of ground signal so they would not be usable on the deepest targets.
Samples D and E are taken during the off time. They do not have as much ground signal and would provide more sensitivity than B and C. They also do not have the reactive signal so they cannot provide a good ID, but they can at least give the decay time.
I would like a choice of on-times. A short on-time would give short time constant targets like small gold an advantage.
I would like all the timing to be microprocessor controlled. The user should be able to choose from a selection of pre set timings that are optimized for different conditions and also have a couple of pots for quickly adjusting some parameters like sensitivity and discrimination/delay. I also want a trigger to switch between motion and no-motion modes. The deluxe version would have a digital display of X Vs R and decay time, but I think a basic version could be made without a display.
The signal processing could be done in analog, but because of the number of samples I have asked for and because I want some of them filtered and because of all the calculations I would like performed on those samples this would be a lot of hardware. I would like to have the samples captured by a 16 bit A/D and all the filtering, integration, and other calculations done in software.
The pulse repetition rate should be high enough (in the neighborhood of 1 kHz) to get a lot of samples from each target at a moderate sweep speed.
Robert

 

[Anonymous]

Robert,
The first person to make this type of detector was Eric foster some twenty years ago. I have recently made one. I am still testing it at this time and making various modifications to it.
A very good friend of mine in Eastern Europe has just finished his design. Unlike my design, he uses a microprocessor for most of his detectors functions. He was the one who named the method "Pulse Induction Balance" or "PIB".
I have not seen the need to use any fancy timing circuits when making a PI. This is especially true when you are making a motion detector. I tested a number of very accurate digital timing circuits before I realized that such accuracy was simply not required.
My PIB allows the reactive sample which is taken about 15uS after the transmit pulse begins to rapidly retune it. The retuning speed is controlled by the resistive signal which is the signal we normally use with a PI. As the amplitude of the "R" signal increases, the "X" signal retuning slows down. This method works very well for identifying iron.
Both my friends design and my own use a ground adjust control. I will share more of my design with the group later on as the design becomes closer to being final, Dave. * * *

 

[Anonymous]

Hi Robert,
The PPD1 that I made in the early 1980

 

[Anonymous]

Dave
Yes, I remember your circuit for getting X. But do you generate a target ID with it or are you just using it for iron disc.
The way I use my VLF for coin and jewelry is to just disc out iron and use tone ID on the non-ferrous signals. The detector takes several samples per sweep so I can hear how the ID changes as the coil passes over the target. I am so used to using that information that I would hate to give it up. But I would also like the same machine to be able handle bad ground in All Metal mode.
I realize I could just use two different detectors. But I already carry a gold machine and a general purpose machine, and now I also want a PI. My trunk is full of MD gear already. Soon I will have to tow a trailer when I go out. It would be nice to have one detector with a few coils that could handle many detecting situations. Oh yeah, and a second one for backup.
Robert

 

[Anonymous]

Robert,
The tone ID is one of the circuits that I am working on. It is possible to use the reactive and the resistive signals in combination so as to provide a variable discrimination or of course a tone ID.
To test this, simply apply the reactive signal and the same signal inverted to either side of a potentiometer. Connect a resistor of about half the value of the potentiometer to the wiper.
The other end of the resistor goes to the resistive signal. Take the output from the wiper of the potentiometer. Targets can be made positive, negative, or nulled out depending on the setting of the potentiometer. The same method can of course be used for ground canceling.
There is no reason why one could not add a notch control. I never use anything more than enough discrimination to get rid of iron myself. Still, there are coin hunters who hunt in some pretty trashy places where a notch control would be of great value.
With the reactive and resistive signals separated using the Foster PIB method, it is possible to adapt many of the old VLF/TR and motion VLF circuits for PIB use.

 

[Anonymous]

Eric
I don't know how you were removing the ground signal from the reactive sample (B), so I will just discuss a simple method.
You can track the samples until you start to see an R and then hold that last sample to subtract it off of all subsequent samples.
In the diagrams below I use a blue curve to represent the target's contribution to the X signal. The red curve is the ground's contribution. What the detector actually measures is the sum of the red and blue curves which I don't show anywhere. The magenta curve is the ground signal after subtracting off the held sample.
In the left graph I show a ground signal that is twice as strong as the target signal. After subtracting the held signal the magenta signal is much smaller than the blue signal. So any calculations you make using the combined blue and magenta signals will give results that depend mostly on the target. You will get a decent ID.
In the right graph I show a case where the ground signal is 100 times as strong as the target signal. I left the red curve out of the graph this time because it would be way off the screen. After subtracting the held signal, the magenta signal is still much larger than the blue signal. So any calculations based on the sum of the blue and magenta curves will give results that depend mostly on the ground. So you are likely to get an ID that says IRON regardless of what the target is.
A target does not have to be very deep to be 100 times weaker than the ground.
A problem with just subtracting off an old sample from the current sample is that it only removes a DC value. The first derivative (slope) and any higher derivatives of the ground signal still get through. A second order (2 pole) filter can remove both the DC and the first derivative.
I would not expect a PIB machine to be quite as good as a VLF at ID's. But it could easily be a lot worse if it does not filter as effectively as a VLF.
The B sample delay plus the sampling time should be less than the shortest target time constants you are interested in.
I am assuming that the effective diameter of an IB coil is about 70% of the size of the outer winding. That is, a 10 inch IB would have about the same range as a 7 inch mono.
Robert