IB Metal Detector Project, Part 32

[Robert Hoolko]

In Part 30 I forgot to mention that you can get negative signals from the coil when the target is in certain regions of the coil's field.

With a concentric coil the signal is positive when the target is centered under the coil. But if the target is close enough to the coil the signal may go negative at one or more points as the target is moved from the center to the edge of the coil. If the target is far enough away from the coil the signal will always be positive.

Differentiating coils will give a negative signal half the time. These coils have a figure 8 winding and have a signal null along the centerline of the coil. On one side of the null the signal will be positive, and on the other side of the null it will be negative. In some coils the null runs from front to back. In this case as you sweep left and right all targets cross the null and produce both + and - signals. In the Bigfoot coil the null runs side to side so targets under the front half of the coil give a + signal, and targets under the back half give a -.

I don't have any DD coils but I would expect them to have negative signal areas to the right and left of the center positive area.

Robert Hoolko

[Carl]

Re: IB Metal Detector Project, Part 32




Is this the basic configuration of a bigfoot coil? Seems like this would give better ground balance as the ground signal will cancel. Also, DD coils should be all-positive as the TX signal is all-positive (there is no reverse TX signal that can create a reverse RX signal).

[Robert Hoolko]

Re: IB Metal Detector Project, Part 32




I think it is the receive winding that is figure 8. It does give good ground signal rejection as long as the coil is parallel to the ground. You have to tip the front down when ground balancing otherwise it might try to balance to a negative signal. Unfortunately, a cold rock (phase angle less than ground) under the back half will give a positive signal.

The DD could possibly give a negative signal. Consider the red magnetic lines from the transmit winding. They go down through target A but up through target B. If A gives a + signal then B gives a -. Whether this actually happens in practice would depend on the geometry of the coil. In any case I would expect the - signal to be weaker than the + signal.

Robert

[Carl]

Re: IB Metal Detector Project, Part 32

OK, I agree on the DD, hadn't considered the backside of the flux lines.