Good Metal Bands Are So Hard To Find Nick Novacco, nan5030 Or: Metal Detectors, how they do that?

Everywhere you look Metal Detectors are an ubiquitous part of modern society, found in industrial, professional, and personal applications throughout the world. Examples being military land mine detectors,“walk-though” detectors as found in schools, airports, etc., and the infamous hand-held, beach-combing devices like Uncle Bill brings on every family outing, insisting on his imminent discovery of buried treasure.

Nope, no WMD's here...

Very Low Frequency (VLF) Metal Detectors in this most popular design utilize generated magnetic waves to effectively “charge up” conductive objects within their field, and to measure the difference in frequency this creates. The way in which these devices operate is easy to understand once you apply concepts from class, namely those of electrostatics and circuits.

Electrical schematics for these devices, however, are beyond the scope of this class...

Robo-Anatomy 101 There are two main components of the device: 1: Control Box 2: Antenna There are also the shaft and arm rest, which are less sophisticated components.

Control Box The control box houses the hardware that interprets the frequency of an object within the field, and produces an audible signal when something interesting is detected. Some more advanced models can even show depth, material composition, or even a rough image of the object itself!

Antenna The antenna is where all the action is, however. In VLF devices there are two wire coils, one inside the other at the business end. The primary, or transmitter coil oscillates thousands of time per second by running a current first one direction along the wire, and then immediately back in the other direction. VLFs typically use frequencies of approximately 30kHz, and thus oscillate 30,000 times per second. This oscillation creates a magnetic field perpendicular to the coils in the antenna.

Antenna, cont... The magnetic field “above” the detector is ignored, while the field “below” the detector is what interacts with objects in the ground.

Antenna, cont... Because of the oscillations in the transmitter coil, the magnetic field changes polarity at the same frequency, and so it “pushes” and “pulls” the magnetic field up and down 30 thousand times per second. The secondary, or receiver coil is what “reads” the magnetic interference caused by conductive materials in the ground. The electromagnetic field created by the transmitter coil charges any conductive object beneath it, allowing the object to create a (weak) field of its own. (Charging by Induction) The polarity of the object is directly opposite of the polarity of the device, and this is what the receiver antenna “reads”.

Antenna, cont... Finding anything with the magnetic field produced at the antenna depends a lot upon the physical orientation and shape of the object in the ground. The more surface area an object is presenting toward the coil, the stronger the field the receiver reads. It is easier to find a car hood than a brick of gold of the same mass, despite gold being a better conductor than steel or aluminum. A gold car hood, on the other hand...

BFO detectors Another method of detecting metal is the use of BFO, or Beat-Frequency Oscillator devices. BFOs still have two coils, except the antenna contains one, and the control box the other. These coils are oscillated at high frequency, creating radio waves. The oscillations of the two coils are out of phase, creating the “beat” phenomena. When an object in the ground interferes with the radio waves, the detector is able to pick up on it, and the regular tempo of its beats are altered, altering the operator to the nearby object.

Summary VLF metal detectors use a transmitter coil and a receiver coil. The transmitter coil produces an oscillating magnetic field that in turn induces an electric charge on objects within the field. These objects then produce a weak magnetic field of their own, which the receiver coil detects and the hardware in the control box interprets. The more surface area of the object, and less interference in the soil (or other media) will produce a stronger “hit” by the metal detector.

Summary The bulk of the understanding of metal detectors comes from weeks 10, 11 and 12, where we learned the properties of magnetic fields, the different methods of charging an object, the properties of waves, circuits and resistivity, and waves as they relate to beats.

Insights It was actually quite interesting to learn about the processes that occur to make a metal detector work. The basic apparatus of it was straightforward enough, but finding that there are multiple methods used for different purposes was intriguing, as it made me think of aspects and difficulties I wouldn't have had otherwise. The more niche aspects are what I found most interesting, such as salt water and many types of earth (dirt) will interfere with a detector. Also the information on the different phase detection to distinguish the composition of the objects was interesting, but I felt it was a little too specific to include here. Basically, it is very fascinating to learn how aspects of physics are used in so many different ways for the mundane objects and gadgets we take for granted every day.

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