Beyond this basic similarity, there are a wide range of different variations used: in the number of coils (one, two or three); the "shape" (spatial extent) of the primary magnetic field; the frequency of the transmitter; the waveform transmitted (sinusoidal or pulsed); the dominant target property responded to (magnetic permeability or electrical conductivity); whether the head coils(s) have a magnetic core or are air-cored; and how the electronics separate the (very weak) received voltage out from the (potentially much larger) voltages present in the search coils even in the absence of any metal target. Although all these factors can affect the sensitivity to any one particular target, the last factor is probably the most important, as it determines the stability or "zero-drift" of the instrument:-- if the zero-point is unstable, high sensitivity will never be achieved, however much the other factors are optimised.
If your browser supports animated GIFs, you should see below
a "movie" of the Pulse Induction process locating a steel bar
(This is in slow-motion: things actually happen 5000 times faster!)
The primary (or transmitted) magnetic field will vary with time exactly in step with the figure 1a current waveform, and propagates (rapidly -- at the speed of light) down to and through the target. When the pulse is switched off, and if the target is a conductor, eddy currents are induced to flow in the target.
These eddy currents always flow in such a direction as to try to re-create the magnetic field that has just disappeared, and, initially at least, they actually succeed in this; but once the primary field has all gone, there is no source of energy to maintain these currents, so they decay gently away -- nevertheless persisting for about a hundred microseconds; see figure 1c.
. Figure 1: Coil Waveforms
The eddy currents generate a secondary magnetic field which propagates in all directions, including back towards the search head, where it induces a (small) voltage in the coil; this voltage also decays away at the same rate (see figure 1d), and has the same sign (polarity) as the back-emf spike.
The received voltage from a target at the limit of the detection range may only be a few microvolts: one ten-millionth of the back-emf spike! It would be quite out of the question for the electronics to notice such a tiny change actually during the back-emf spike, and that is not the way it's done.
The signal is "sampled" by an electronic switch which ignores the signal during the transmit pulse and immediately after (during the back-emf), and only "looks at" the signal after a short delay which ensures that the switch-off transient is over (see figure 1e). In this way, the transmitted and received signals are separated from each other.
If the target had been purely magnetic, but non-conductive, it would have become magnetised by the transmit pulse, and then de-magnetise just as promptly at switch-off; by the time of the delayed sample pulse, nothing would be happening down at the target, and therefore nothing would be happening up at the search coil.
If the target is both conductive and magnetic (eg a ferrous metal), the eddy currents would be produced exactly as in the purely conductive case; the effect of the target's magnetic permeability is to enhance the magnitude of the effect (and also to modify the "time-constant" of the decay of the eddy currents).
If there is no target at all . . . . . . . . nothing happens!
Actually, there will always be a certain inescapable amount of electrical "noise" in the receiver coil and circuitry, and three techniques are used to filter this out to produce a final signal (in the absence of a target) which is very close to zero and absolutely rock-steady.
The decay time-constant (persistence) of the eddy-currents, and hence received signal, depends (predominantly) on the target's electrical conductivity and size. Targets such as low-conductivity alloys or thin foils have a very short decay time; and the choice of a short or long delay between switch-off and sample can be arranged to either detect or ignore such targets. The ionic conductivity of sea- or brackish water is so low, and its decay time so short, that such signals have always decayed away before the sample is taken; so the P.I. technique is not affected by moisture.
Some more recent rebar locators and cover meters have used methods which are electromagnetic in nature (rather than purely magnetic), but the coil configuration dictated by the detection technique is invariably far short of optimum for practical bar location:- either the field is too widespread and diffuse, which makes resolution of closely-spaced bars impossible (see figure 3b); or else (if more compact) the field is non-directional, and cannot allow distinction between horizontal and vertical bars (figure 3c).
Since a Pulse-Induction coil can (in principle) be any shape required, the "shape" and extent of the field can be optimised for both bar-resolution and bar-orientation, with total zero-point stability (figure 3d).