BASICS AND THEORY
The following paper is both informative and helpful for metal detector users with an interest in technology. This article offers an insight into the basic theory and electronics of metal detectors. Whilst a technical paper, this is not a formal “scientiﬁc paper” and the language used is deliberately more “reader friendly.” Additionally, some terms are used loosely. For example; the terms “magnetic soils” or “mineralised soils” indicates soil that contains materials with signiﬁcant magnetic permeability (or susceptibility). Minelab spends a higher percentage of annual revenue in research and development than any of its competitors. We, as the engineering team, appreciate that our company supports this approach allowing the freedom to dream of what might be and act upon that vision. The result of this effort is demonstrated in the break-through technologies that Minelab has incorporated its worldclass detectors. A basic approach to creating a superior metal detector includes: 1. Products that offer the most useful features and best possible performance 2. Products that are highly reliable
3. Products that exceed expectations every time they are used. To achieve these goals we must know advanced detector theory intimately. A sound working knowledge of electronics, mathematics, and mechanical engineering are essential as is familiarity with government regulations. We also pride ourselves on our practical knowledge of hands-on detecting in the ﬁeld.
This paper will give you a basic overview of the subject and some insight into the way we at Minelab approach the challenges of creating the world’s ﬁnest metal detectors.
Obviously, the majority of our IP is conﬁdential and is thus absent from this paper.
1. Basic operation.
Written by Bruce Candy. 1
Metal detectors work on the principle of transmitting a magnetic ﬁeld and analyzing a return signal from the target and environment. The transmitted magnetic ﬁeld varies in time, usually at rates of fairly high-pitched audio signals. The magnetic transmitter is in the form of a transmit coil with a varying electric current ﬂowing through it produced by transmit electronics. The receiver is in the form of a receive coil connected to receive and signal processing electronics. The transmit coil and receive coil are sometimes the same coil. The coils are within a coil housing which is usually simply called “the coil,” and all the electronics are within the electronics housing attached to the coil via an electric cable and commonly called the “control box”.
This changing transmitted magnetic ﬁeld causes electric currents to ﬂow in metal targets. These electric currents are called eddy currents, which in turn generate a weak magnetic ﬁeld, but their generated magnetic ﬁeld is different from the transmitted magnetic ﬁeld in shape and strength. It is the altered shape of this regenerated magnetic ﬁeld that metal detectors use to detect metal targets. (The different “shape” may be in the form of a time delay.)
The regenerated magnetic ﬁeld from the eddy currents causes an alternating voltage signal at the receive coil. This is ampliﬁed by the electronics because relatively deeply buried targets produce signals in the receive coil which can be millions of times weaker than the signal in the transmit coil, and thus need to be ampliﬁed to a reasonable level for the electronics to be able to process. In summary: 1.
Transmit signal from the electronics causes transmit electrical current in transmit coil. Electrical current in the transmit coil causes a transmitted magnetic ﬁeld. Transmitted magnetic ﬁeld causes electrical currents to ﬂow in metal targets (called eddy currents.) Eddy currents generate a magnetic ﬁeld. This ﬁeld is altered compared to the transmitted ﬁeld. Receive coil detects the magnetic ﬁeld generated by eddy currents as a very small voltage. Signal from receive coil is ampliﬁed by receive...