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I want to estimate the processor resources required for this project. I first need to know how many data bits I need and what the iteration rates will be for various calculations.
I will be using the White's BM950 coil as the baseline for this design. The coil is tuned for approximately 6600 Hz. Because it is tuned it does not need to be driven with a sign wave. It can be driven with rectangular current pulses at 6600 Hz.
The input signal will have to be sampled on 90 degree intervals. So the basic interrupt rate will be 4 * 6600 or 26400 Hz. I do not think I will be able to sample at that rate and still have time to do everything else that needs to be done. So I will plan on sampling every 270 degrees. That still gives samples that are 90 degrees apart. Sampling points are: 0, 270, 540 (180), 810 (90), etc. So I read a sample every 3rd interrupt, or at 8800 Hz. But alternating samples are used for different signal channels. For example the even samples are used for the X channel and the odd sample are used for Y. So the sampling rate per channel is 4400 Hz.
Now let's look at the number of bits needed. With a single bit of information the detector could decide whether a signal is present to not. But the signal we are looking at is AC. If we are going to do all the demodulation inside the processor we need a sign bit and at least one magnitude bit. That would be enough to detect an air target at a fixed distance from the coil.
When the target is lying on the ground the coil will see a signal from the ground as well as from the target. In many cases the signal from the ground is stronger than the signal from a coin on the surface. With the 9.5 inch coil I have measured ground signals almost 10 times stronger than the coin signal. For this project I want to be able to handle a ground signal 8 times stronger than a surface coin. So we need a sign bit plus 3 magnitude bits just to detect the presence of a coin on the surface.
But I don't just want to detect the target I also want to ID it by measuring the phase angle of the signal. With only one magnitude bit of target info I would only be able to classify the phase angle as being less than 45 or more than 45 degrees. I would like to divide the range of conductive targets (90 degree range) into at least 8 parts. That is going to require 3 bits of magnitude instead of 1. So we are up to a sign bit plus 5 bits of magnitude.
Roughly speaking the signal strength falls off exponentially with distance from the coil. That's not accurate real close to the coil, but it approximately applies at normal detection distances of .2 to 1 times the coil diameter. For the BM950 the signal is reduced by a factor of 2 for each additional inch from the coil. If we want to detect a coin at 9 inches from the coil we need another 9 bits. That brings us up to a sign bit plus 14.
So we need 15 bits without even considering other noise sources. I would like a couple of extra bits for safety, but I am determined to keep this down to 16 bits. (I think I could do a real nice job with an 18 bit A/D.)
Now how do we get even 15 bits of information from a 10 or 12 bit A/D? There should not be much change in the target signal from one sample to the next. In fact the signal changes that we are interested in will occur over periods on the order of tenths of a second or hundreds of samples. So we can integrate many samples to get more accurate measurements at a lower rate. Keep in mind that we do not care about the absolute accuracy of our measurements we just want to be able to recognize small changes.
Suppose we pick 1/60 of a second as the integration time. One important source of external noise is 60 Hz power sources, especially since I will be testing this indoors with lots of fluorescent lights. Integrating over a 60th of a second should cancel out most of the 60 Hz noise, except the unsymmetrical noise from some dimmer switches. In some countries 1/50 would be better. 1/60 th of 4400 is approximately 73 samples. this would give 6 extra bits of data, in fact it would not fit in 16 bits, so we will have to do a 24 bit accumulation.
So the plan is to take 10 or 12 bit samples at a 4400 Hz sample rate and integrate them in a 24 bit accumulator. Then they are scaled down to 16 bits and other operations are performed at a 60 Hz rate.
These are the iteration rates we have so far:
26400 Hz - drive transmit coil, update software counters for lower rates.
8800 Hz - read A/D, demodulate, 16 bit median filter, 24 bit accumulate.
60 Hz - low pass filter x 2, create ground balanced channel, high pass filter x 3, evaluate phase, discriminate.
I will try to add up some instruction cycle times in the next part, but I have taken a little peek at the numbers and it looks like I am already in trouble at the 26400 and 8800 rates. I don't think the 4 MHz chip is going to do it.
Robert Hoolko
I will be using the White's BM950 coil as the baseline for this design. The coil is tuned for approximately 6600 Hz. Because it is tuned it does not need to be driven with a sign wave. It can be driven with rectangular current pulses at 6600 Hz.
The input signal will have to be sampled on 90 degree intervals. So the basic interrupt rate will be 4 * 6600 or 26400 Hz. I do not think I will be able to sample at that rate and still have time to do everything else that needs to be done. So I will plan on sampling every 270 degrees. That still gives samples that are 90 degrees apart. Sampling points are: 0, 270, 540 (180), 810 (90), etc. So I read a sample every 3rd interrupt, or at 8800 Hz. But alternating samples are used for different signal channels. For example the even samples are used for the X channel and the odd sample are used for Y. So the sampling rate per channel is 4400 Hz.
Now let's look at the number of bits needed. With a single bit of information the detector could decide whether a signal is present to not. But the signal we are looking at is AC. If we are going to do all the demodulation inside the processor we need a sign bit and at least one magnitude bit. That would be enough to detect an air target at a fixed distance from the coil.
When the target is lying on the ground the coil will see a signal from the ground as well as from the target. In many cases the signal from the ground is stronger than the signal from a coin on the surface. With the 9.5 inch coil I have measured ground signals almost 10 times stronger than the coin signal. For this project I want to be able to handle a ground signal 8 times stronger than a surface coin. So we need a sign bit plus 3 magnitude bits just to detect the presence of a coin on the surface.
But I don't just want to detect the target I also want to ID it by measuring the phase angle of the signal. With only one magnitude bit of target info I would only be able to classify the phase angle as being less than 45 or more than 45 degrees. I would like to divide the range of conductive targets (90 degree range) into at least 8 parts. That is going to require 3 bits of magnitude instead of 1. So we are up to a sign bit plus 5 bits of magnitude.
Roughly speaking the signal strength falls off exponentially with distance from the coil. That's not accurate real close to the coil, but it approximately applies at normal detection distances of .2 to 1 times the coil diameter. For the BM950 the signal is reduced by a factor of 2 for each additional inch from the coil. If we want to detect a coin at 9 inches from the coil we need another 9 bits. That brings us up to a sign bit plus 14.
So we need 15 bits without even considering other noise sources. I would like a couple of extra bits for safety, but I am determined to keep this down to 16 bits. (I think I could do a real nice job with an 18 bit A/D.)
Now how do we get even 15 bits of information from a 10 or 12 bit A/D? There should not be much change in the target signal from one sample to the next. In fact the signal changes that we are interested in will occur over periods on the order of tenths of a second or hundreds of samples. So we can integrate many samples to get more accurate measurements at a lower rate. Keep in mind that we do not care about the absolute accuracy of our measurements we just want to be able to recognize small changes.
Suppose we pick 1/60 of a second as the integration time. One important source of external noise is 60 Hz power sources, especially since I will be testing this indoors with lots of fluorescent lights. Integrating over a 60th of a second should cancel out most of the 60 Hz noise, except the unsymmetrical noise from some dimmer switches. In some countries 1/50 would be better. 1/60 th of 4400 is approximately 73 samples. this would give 6 extra bits of data, in fact it would not fit in 16 bits, so we will have to do a 24 bit accumulation.
So the plan is to take 10 or 12 bit samples at a 4400 Hz sample rate and integrate them in a 24 bit accumulator. Then they are scaled down to 16 bits and other operations are performed at a 60 Hz rate.
These are the iteration rates we have so far:
26400 Hz - drive transmit coil, update software counters for lower rates.
8800 Hz - read A/D, demodulate, 16 bit median filter, 24 bit accumulate.
60 Hz - low pass filter x 2, create ground balanced channel, high pass filter x 3, evaluate phase, discriminate.
I will try to add up some instruction cycle times in the next part, but I have taken a little peek at the numbers and it looks like I am already in trouble at the 26400 and 8800 rates. I don't think the 4 MHz chip is going to do it.
Robert Hoolko