Phase Shift and Target Identification in Metal Detectors

Target Identification in Metal Detectors

Phase Shift and Target Identification (TID) are terms you come across early in your exploration of metal detecting technology.

The basics of electricity tell us that running a current through a wire creates an electromagnetic field around that wire. If you move another wire through that field it induces current into that second wire. That effect is called electromagnetic force, or “emf.” Now it doesn’t matter if the wire moves through a stationary field or an alternating field moves through a stationary wire. The generation of emf works the same in both cases.

A detector runs a radio frequency current, anywhere from about 5-khz to 20-khz through its coil. This creates a moving field that expands and collapses many times each second. This is a moving field. A coin in the ground is like a tiny 1/2-inch piece of wire. The moving field from the coil induces a very small emf in the coin, called an eddy current.

Now, here’s the trick. The physics of that current in the coin are such that it pushes back, in the opposite direction, against the current in the detector coil. This push-back, or echo, slows the current in the coil, but has no effect on the voltage. The result is that current in the detector coil becomes out of step with the voltage. This is called phase shift in electrical jargon. It means the current waveform lags behind the voltage waveform.

A highly conductive target, such as a silver dollar will create a large phase shift in the detector coil. A zinc penny will cause a smaller shift, and a rusty nail even less.

The push-back from the target also explains why you get mixed signals from asymmetrical targets. A long nail scanned in one direction will produce a different phase shift than if scanned at 90-degrees from the original coil swing.

These changes in the current/voltage phase shift are exceedingly small, so it takes a highly refined signal processing circuit to detect the differences.

For less expensive detectors, the phase shift scale is broken up into segments or categories, generally called notches. These notches can be turned on or off to either select or eliminate phase shift categories. If you want to look just for coins, you can turn off , or gray out, the notches for iron, foil, and pull-tabs.

In a more expensive detector, the phase shift scale is more exquisitely defined, into many more specific readings called Visual Identification (VID) numbers. The engineering for this finer performance is what makes the detector more costly. The payoff, however, is that you have much more specific Target Identification in Metal Detectors. This allows you to distinguish between a zinc penny and a copper penny, or between a silver dime and a clad dime.

The VID scale numbers from any one manufacturer are generally different from other machines. Whichever detector you use, you soon become familiar with the numbers corresponding to specific coins or common junk items, such as pull-tabs. This speeds up your choices on which targets to dig.

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