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I decided to add a motion filter. I used the design above which has a delay line that stores the 7 most recent data samples. Each sample can be multiplied by a coefficient, then all the products are summed. This is called a finite impulse response filter.
For the first try I used a very simple filter with coefficients 1,0,-1,0,0,0,0. In other words I used the most recent data minus the data from two samples back. This will differentiate any signals that are less than 7.5 Hz. This will tell us how fast the signal is changing. Since target signals usually change quickly and ground signals change slowly this helps to separate the target signal from the ground signal.
The filter is running all the time and producing output at 60 samples per second, but I can only display about one sample per second. So I put in a moving threshold. I compare each sample out of the filter to the threshold. If the sample is below the threshold I discard it. If the sample is above the threshold I save the sample, replacing the previously saved value, and I also set the threshold to the sample value, this causes lower values to be ignored. The result is to save the largest value from one swing of the coil. I let the threshold decay with a time constant of 2 seconds so that it will eventually get low enough for the next swing of the coil. Then once a second I display the saved value and its phase angle.
In an air test it can detect and ID a penny at 3". The ID is pretty good but it does not trigger every time. At 5" it does not trigger at all. It is a little hard to judge how fast I am swinging the penny past the coil, but I think I am getting the best results at between 1 and 2 feet per second.
With the coil on the concrete floor it can still ID the penny at 3" but the ID is a little sloppier. This is with a fairly strong but constant ground signal. I have not quite figured out yet how I am going to test with a changing ground signal on my concrete floor.
Robert Hoolko