All of today's high sensitivity metal detectors use the three coil, balanced field approach.

There is a single transmit coil and two receiver coils. These coils are wrapped around the opening, or aperture, of the metal detector. A signal is applied to the transmitter coil to create an RF field for the product to pass through. The receiver coils are wound on each side of the transmitter and are designed to receive equal signals from the transmitter. The receiver coils are wired so that the equal signals cancel out each other and the total output of the receiver coil is zero. This is called a “balanced field” detector.

The closer the field is to being balanced, the better the sensitivity.

As product enters the field, the signal in first one, then the other coil is changed, or the field becomes unbalanced. This signal is picked up on the electronics card and used to trigger an alarm circuit.

Virtually any product passing through the coils will create some type of signal... The strength of the signal will vary depending on moisture, temperature, mass, speed, saline content and shape. This signal is called "product effect".

All of today's detectors have some mechanism to cancel out the signal that comes from the product.

In our detector this canceling is done by means of the "phase" control.

Once the product effect is reduced as much as possible, the remaining signal is determined to be metal.

Analog vs microprocessor vs digital signal processing(DSP) in Metal Detectors

• Analog - ambiguous signal depending on amplitude and phase.
• Microprocessor - Strictly on/off, phase not important. Need signal converter to change analog to digital. Programmable with memory.
• Digital Signal Processor(DSP) - Accepts analog inputs, also programmable. No converters are required.

SUMMARY