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I am going to begin outlining an IB metal detector project. The purpose of this project is not to build a working detector that can be used in the field but to build a test bed that can be used to evaluate metal detector algorithms.
The design is going to be constrained by the fact that I have an XLT, and I want to be able to compare my results to what I get from the XLT. I also want to use my XLT coils. So this will be a single frequency detector operating at approximately 6600 Hz. I want to implement both all metal and discriminate modes.
The hardware design will be a PIC microcontroller board with these additional circuits: a driver for the transmit winding, a preamp for the received signal, a few switches for changing modes, a driver for earphones, and a serial interface to a PC.
The output of the preamp will be sampled by an A/D. The signal will be demodulated by sampling at appropriate times to produce two channels of data (X and Y) 90 degrees apart. X and Y contain the phase information that is needed for discrimination. X and Y will be combined to produce a ground balanced signal G. All channels will be low pass filtered to reduce noise.
For all metal mode the G signal will be sent through a high pass filter to provide a self adjusting threshold. Then the signal will be used to create an audio tone for the earphones.
For discriminate mode the X, Y, and G signals will be filtered with identical high pass or band pass filters. The filtered G signal will be used to determine when to look at X and Y. The phase angle will be calculated from X and Y. The phase angle will be evaluated as being in an accepted or rejected range. Accepted and rejected signals cause a beep from the discriminator to begin or end. If a beep is not terminated by a rejected signal it will end automatically after a predetermined amount of time.
Most of the work is in the filtering. Before I start buying hardware I want to convince myself that the amount of processing to be done is within the capabilities of the PIC controllers. So I will start estimating the memory requirement and instruction cycle requirements of the various filters in Part 2 of this project.
Robert Hoolko
The design is going to be constrained by the fact that I have an XLT, and I want to be able to compare my results to what I get from the XLT. I also want to use my XLT coils. So this will be a single frequency detector operating at approximately 6600 Hz. I want to implement both all metal and discriminate modes.
The hardware design will be a PIC microcontroller board with these additional circuits: a driver for the transmit winding, a preamp for the received signal, a few switches for changing modes, a driver for earphones, and a serial interface to a PC.
The output of the preamp will be sampled by an A/D. The signal will be demodulated by sampling at appropriate times to produce two channels of data (X and Y) 90 degrees apart. X and Y contain the phase information that is needed for discrimination. X and Y will be combined to produce a ground balanced signal G. All channels will be low pass filtered to reduce noise.
For all metal mode the G signal will be sent through a high pass filter to provide a self adjusting threshold. Then the signal will be used to create an audio tone for the earphones.
For discriminate mode the X, Y, and G signals will be filtered with identical high pass or band pass filters. The filtered G signal will be used to determine when to look at X and Y. The phase angle will be calculated from X and Y. The phase angle will be evaluated as being in an accepted or rejected range. Accepted and rejected signals cause a beep from the discriminator to begin or end. If a beep is not terminated by a rejected signal it will end automatically after a predetermined amount of time.
Most of the work is in the filtering. Before I start buying hardware I want to convince myself that the amount of processing to be done is within the capabilities of the PIC controllers. So I will start estimating the memory requirement and instruction cycle requirements of the various filters in Part 2 of this project.
Robert Hoolko
I put ALL the aprts of your article into one huge DOC file which I posted on the forum a year or so ago.
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