How to Choose & Use Industrial Metal Detectors - ERIEZ - #9

The transmitter coil emits periodic field pulses that are received by the receiver coils, resulting in signals with characteristic amplitudes and decay times. When metal is present, some of the transmitter energy is absorbed by the generation of eddy currents in the metal, and those eddy currents themselves generate fields that are sensed by the receiver coils (along with the-modified-emitted pulse). Thus, the characteristic amplitude and decay time of the received signals are affected by the metal (as shown in Figure 7), and these effects are identified and reported by the electronic filter package.

Transmitted Signal

On-Pulse Off-Pulse

Received Signal

Metal Passing - Eddy Currents in Metal Affect D . Received Signal

. . . . , . | ^{No Metal}

Filtered and

Amplified

Signal

I_I

Figure 7. Signal Decay Affected by Metal in Pulse Induction Detector.

The received signal from the sensor is fed to the electronic filter. In modern metal detectors the filter is always solid state, and is generally digital in operation. Lower cost or special purpose detectors may incorporate analog filters. Each manufacturer develops its own signal processing circuitry (or licenses from other developers) and each emphasizes features and performance areas that it considers most important. Eriez signal processing circuitry is almost 100% in-house developed, and emphasizes sensitivity, stability and durability.

The most basic output device is a voltage measured at the output terminals of the electronic filter. The presence, absence, or sign of this voltage-depending on the design of the particular detector-indicates that metal has been detected. In practice, either solid­state or mechanical relays are generally incorporated into the output side of the detector so that external device(s) can be actuated when metal is found.