Photo courtesy Garrett Electronics
A less common form of metal detector is based on pulse induction (PI). Unlike VLF, PI systems may use a single coil as both transmitter and receiver, or they may have two or even three coils working together. This technology sends powerful, short bursts (pulses) of current through a coil of wire. Each pulse generates a brief magnetic field. When the pulse ends, the magnetic field reverses polarity and collapses very suddenly, resulting in a sharp electrical spike. This spike lasts a few microseconds (millionths of a second) and causes another current to run through the coil. This current is called the reflected pulse and is extremely short, lasting only about 30 microseconds. Another pulse is then sent and the process repeats. A typical PI-based metal detector sends about 100 pulses per second, but the number can vary greatly based on the manufacturer and model, ranging from a couple of dozen pulses per second to over a thousand.
If the metal detector is over a metal object, the pulse creates an opposite magnetic field in the object. When the pulse's magnetic field collapses, causing the reflected pulse, the magnetic field of the object makes it take longer for the reflected pulse to completely disappear. This process works something like echoes: If you yell in a room with only a few hard surfaces, you probably hear only a very brief echo, or you may not hear one at all; but if you yell in a room with a lot of hard surfaces, the echo lasts longer. In a PI metal detector, the magnetic fields from target objects add their "echo" to the reflected pulse, making it last a fraction longer than it would without them.
A sampling circuit in the metal detector is set to monitor the length of the reflected pulse. By comparing it to the expected length, the circuit can determine if another magnetic field has caused the reflected pulse to take longer to decay. If the decay of the reflected pulse takes more than a few microseconds longer than normal, there is probably a metal object interfering with it.
The animation above demonstrates PI technology.
The sampling circuit sends the tiny, weak signals that it monitors to a device call an integrator. The integrator reads the signals from the sampling circuit, amplifying and converting them to direct current (DC). The direct current's voltage is connected to an audio circuit, where it is changed into a tone that the metal detector uses to indicate that a target object has been found.
PI-based detectors are not very good at discrimination because the reflected pulse length of various metals are not easily separated. However, they are useful in many situations in which VLF-based metal detectors would have difficulty, such as in areas that have highly conductive material in the soil or general environment. A good example of such a situation is salt-water exploration. Also, PI-based systems can often detect metal much deeper in the ground than other systems.