METAL DETECTOR
Introduction:
Metal detectors are protection equipment. They protect processing equipment, products people consume, companies’ brand image, and limit their liability due to contaminated product.
Metal detectors are both effective and inexpensive. Processing and packaging equipment is often expensive and susceptible to damage from other equipment such as crushers, extruders, shredders, cutters, grinders, choppers, mixers, etc., can be costly to repair, not to mention the down time.
Although 75-80% of metal detectors are used in food related industries, many nonfood industries (rubber, plastic, aggregate, mining, wood, textile, glass, product and environmental security) use them as well. Metal detector usage can be broken into three main categories.
à Packaging – where higher sensitivity (the smallest piece of metal) is desired,
à Bulk Processing – where gravity drop or pipeline metal detectors are used to
protect equipment (e.g. blenders, mixers, cutters, choppers, etc.),
à Industrial – where industries have lower sensitivity requirements (mining,
aggregate, gravel, plastic, lumber, etc.).
Anatomy of a Metal Detector
A typical metal detector is light-weight and consists of just a few parts:
- Stabilizer (optional) - used to keep the unit steady as you sweep it back and forth
- Control box - contains the circuitry, controls, speaker, batteries and the microprocessor
- Shaft - connects the control box and the coil; often adjustable so you can set it at a comfortable level for your height
- Search coil - the part that actually senses the metal; also known as the "search head," "loop" or "antenna"
Operating a metal detector is simple. Once you turn the unit on, you move slowly over the area you wish to search. In most cases, you sweep the coil (search head) back and forth over the ground in front of you. When you pass it over a target object, an audible signal occurs. More advanced metal detectors provide displays that pinpoint the type of metal it has detected and how deep in the ground the target object is located.
Metal detectors use one of three technologies:
- Very low frequency (VLF)
- Pulse induction (PI)
- Beat-frequency oscillation (BFO)
VLF Technology
Very low frequency (VLF), also known as induction balance, is probably the most popular detector technology in use today. In a VLF metal detector, there are two distinct coils:
- Transmitter coil - This is the outer coil loop. Within it is a coil of wire. Electricity is sent along this wire, first in one direction and then in the other, thousands of times each second. The number of times that the current's direction switches each second establishes the frequency of the unit.
- Receiver coil - This inner coil loop contains another coil of wire. This wire acts as an antenna to pick up and amplify frequencies coming from target objects in the ground.
The current moving through the transmitter coil creates an electromagnetic field, which is like what happens in an electric motor. The polarity of the magnetic field is perpendicular to the coil of wire. Each time the current changes direction, the polarity of the magnetic field changes. This means that if the coil of wire is parallel to the ground, the magnetic field is constantly pushing down into the ground and then pulling back out of it.
As the magnetic field pulses back and forth into the ground, it interacts with any conductive objects it encounters, causing them to generate weak magnetic fields of their own. The polarity of the object's magnetic field is directly opposite the transmitter coil's magnetic field. If the transmitter coil's field is pulsing downward, the object's field is pulsing upward.
The animation above demonstrates VLF technology:
The receiver coil is completely shielded from the magnetic field generated by the transmitter coil. However, it is not shielded from magnetic fields coming from objects in the ground. Therefore, when the receiver coil passes over an object giving off a magnetic field, a small electric current travels through the coil. This current oscillates at the same frequency as the object's magnetic field. The coil amplifies the frequency and sends it to the control box of the metal detector, where sensors analyze the signal.
The metal detector can determine approximately how deep the object is buried based on the strength of the magnetic field it generates. The closer to the surface an object is, the stronger the magnetic field picked up by the receiver coil and the stronger the electric current generated. The farther below the surface, the weaker the field. Beyond a certain depth, the object's field is so weak at the surface that it is undetectable by the receiver coil.
PI Technology
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 than a few microseconds longer than normal, there is probably a metal object interfering with it.
The figure 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.
BFO Technology
The most basic way to detect metal uses a technology called beat-frequency oscillator (BFO). In a BFO system, there are two coils of wire. One large coil is in the search head, and a smaller coil is located inside the control box. Each coil is connected to an oscillator that generates thousands of pulses of current per second. The frequency of these pulses is slightly offset between the two coils.
As the pulses travel through each coil, the coil generates radio waves. A tiny receiver within the control box picks up the radio waves and creates an audible series of tones (beats) based on the difference between the frequencies.
If the coil in the search head passes over a metal object, the magnetic field caused by the current flowing through the coil creates a magnetic field around the object. The object's magnetic field interferes with the frequency of the radio waves generated by the search-head coil. As the frequency deviates from the frequency of the coil in the control box, the audible beats change in duration and tone.
The animation above demonstrates BFO technology.
The simplicity of BFO-based systems allows them to be manufactured and sold for a very low cost. But these detectors do not provide the level of control and accuracy provided by VLF or PI systems.
APPLICATIONS OF METAL DETECTOR:
- Airport security - screen people before allowing access to the boarding area and the plane (see How Airport Security Works)
- Building security - screen people entering a particular building, such as a school, office or prison
- Event security - screen people entering a sporting event, concert or other large gathering of people
- Item recovery - help someone search for a lost item, such as a piece of jewelry
- Archaeological exploration - find metallic items of historical significance
- Geological research - detect the metallic composition of soil or rock formations
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