Useful definitions
Dynamic mode of operation provides for continuous movement of the search head (coil). The response of the device will occur only when prenesenie head over metal. If the head stop on the object - the response will disappear.
Static mode - the mode in which the response of the device will be continue until the head is above the object. It does not matter moves like it or not.
Pseudo-static mode , dynamic mode is very slow tunable threshold. That is, if the head is to long to hold on something that is going to happen adjustment of the instrument under this object and the reaction will disappear. When you remove the head from the object will start to reverse the adjustment of the same duration. In computerized devices processes of adjustment of the threshold is controlled by the microprocessor, so the speed adjust when zooming in or out from the object may be different. In normal devices with pseudostatic for quick recovery threshold when deleting coil from the object button RETUNE.
VCO-mode - dependence not only the volume but also the tone (frequency) from the magnitude and depth of the object. Sharpens the response of the human ear on the weak changes in the adopted from the object signal.
The threshold (threshold tone) - weak audible tone, audible in the loudspeaker of the device or headphones in the absence of metallic objects near the head. Threshold tone have as devices having a search mode all metals and appliances from static search modes. This tone can be judged on the performance the detector and changing ground mineralization. In most devices the threshold tone is adjustable.
Regency - no sound signal (ignoring) for a particular objects.
Main principles of construction of the detector: advantages and disadvantages
How build metal detectors, there are several. Here are the main three.
BFO (Beat Frequency Oscillator - generator beats, dependent generator) - the addition of the vibrations of two high-frequency generators and the allocation of the difference between their frequencies lying in the audio range. If in the vicinity of the search coil a metallic object, changes the frequency of the signal of one of generators, and with it the height of the audible sound. I.e. we work on the basis the determination of small changes in the inductance of the search coil under the influence iron objects. Characterized by low sensitivity.
TR/IB (Transmit-Receive/Induction - Balance-transmission, reception and induction balance) - in the sensor set two coils, transmitting and receiving. The presence of metal modifies the inductive coupling between them, which affects the received signal.
PI (Pulse Induction - pulse induction) - transmission-driving pulses, the shape and the amplitude of the received signals depend on the presence of metal objects near the coils of the detector.
On the contrary, the pulse detectors have many advantages:
- the most sensitive among all the detectors, which in modern times available;
- insensitive to the influence of the earth;
- have a simple design.
They used magnetic pulses capable of inducing current in all metallic objects within the magnetic field. In between pulses, the receiver receives the response, which is amplified and processed electronic part.
With its undeniable advantages, they have disadvantages:
- for their work need powerful batteries;
- they are extremely sensitive to small iron objects.
Since the mid 90-ies many pulse metal detectors began to be supplied the discriminator.
Metal detectors with a balanced induction detectors have become a standard for universal use. In the search head there are two coils, one of which induces an alternating magnetic field. The other coil is positioned what field in the normal state balanced around it, and its output has no no electrical signal.
In fact, in the receiving coil is the so-called residual signal, is not due to the ideality of the structure. Metal objects that closer to the coils, change the configuration of this field, razbalansirovat system. The result at the output of the receiver coil a signal. This the signal can be strengthened and inform the operator about the discovery.
Modern metal detectors that use this principle are powerful electronics, signal processing and gives the operator a lot of additional information, such as:
- the relative conductivity of the metal;
- depth, etc.
Each principle peculiar to the specific advantages and disadvantages. Therefore, in expensive and complex detectors that can determine not only the presence but also the kind metal, combine all three.
The functioning of the metal detector
When you turn on the metal detector search coil creates an electromagnetic field that extends into the environment:
- earth;
- stone;
- water;
- tree;
- air.
On the surface of metals that fall within the range of the search coil under the influence of magnetic field eddy currents. These eddy currents create your own counter electromagnetic field, resulting in reduced power electromagnetic field generated by the search coil. It recorded in the electronic circuitry of the instrument.
In addition, the secondary field distorts the configuration of the primary field that also is captured by the device.
Electronic diagram of the detector processes the received information and indicates the detection of metal.
Eddy currents are formed on the surface of any metal objects or conductive minerals. The definition of metal in the object based on the measurement of the conductivity of the object.
Frequency ranges of the detectors
In the technical literature devoted to the detectors, is used a lot special terms. Thus, the designation VLF (Very Low Frequency) means very low operating frequency of the detector.
The ability of the detector to discriminate (recognize) the nature of the findings depends on the operating frequency of the device. At high frequencies is enhanced by the phenomenon skin ffect, and the quality of discrimination is much worse.
So first (70s, early 80's), manufacturers of metal detectors used a very low frequency of about 2 kHz. This led to the emergence of specific problems:
- at these frequencies, despite good sensitivity to copper and silver, was reduced sensitivity to gold and Nickel;
- when designing coils had problems with the quality (q).
Modern metal detectors have a wide variation in the operating frequency, it due to the specifics of their application, as well as for engineering and design reasons.
Most often, the frequency range extends from 6 to 20 kHz, but sometimes below. In this frequency range devices well discriminate the target and does not occur serious problems with the design of the coils.
Devices for gold prospecting uses elevated frequency of 15-20 kHz and above. This is due to the fact that at these frequencies is improved sensitivity to very small objects, e.g., to gold nuggets, often with scanty size and light weight.
In recent years, to improve the depth and quality of discrimination have been use a multi-frequency search, which under certain conditions gives advantages. With the advent of cheap microprocessors, this method has been strong impetus in its development. But I want to make two important points about multi-frequency search:
- first, a significant increase in the depth of discrimination compared to single-frequency devices this method does not give;
- secondly, increases range of findings, which are well discriminated but in the case of a complex configuration of the latter may lead to clipping.
- Symbol TR (Transmitter-receiver) means "transmitter-receiver" and also has to do with the metal detectors work on the principle of induction balance. This type of metal detectors appeared in the 30-ies. In these devices there are spaced, orthogonally positioned coils. Devices of this type have operating frequencies from 50 to 100 kHz. Serious disadvantages of these devices:
- big reaction to: mineralized soil;
- immunity to small items.
When you search for large objects, the size of a quart jar and more in terms brackish and slightly littered the ground these detectors very comfortable. Since the mid 70-ies practically went out of use.
Today, a typical representative of the detectors of this type is Gemini-W by Fisher. Some manufacturers of metal detectors used this designation as a complement to the term VLF apparently, in order once again to emphasize that the work is based on the principle of induction balance.
Adjustment of the system of compensation of land
Mineralization of soil - soil conductive salts and minerals, and also of minerals and rocks containing iron. Leads to the violation of the right work devices. Eliminated by introducing additional circuitry and modes.
In the process of working on the ground may change the nature of the soil, and, therefore, and its mineralization. This will require adjustment of the compensation system the influence of the earth. This is done both manually and automatically, if the detector there is this mode of operation. He received the designation Ground Track. Application this mode is not recommended when installing high sensitivity.
The term VCO is used to indicate the sound mode the detector, which, depending on the size and depth object changes not only the volume but also the tone. It sharpens perception of hearing the slightest change in the audio signal.
Modern high-end metal detectors have rich service that allows an experienced operator to conduct a fruitful search with the smallest expense of time. If the search engine additionally has an idea about the working principle of the detector and about its real capabilities, it brings additional dividends.
On the sensitivity of metal detectors
To obtain high sensitivity, the total gain total receiving channel of the detector can be tens of decibels. Reckless the increase in sensitivity leads to deterioration of stable operation of the device, turning, therefore, the detector in a useless search tool.
Sensitivity is the main, but not the main advantage of modern the metal detector.
This understanding comes with experience and it becomes apparent after search engine hits of trashy metal rubbish plot. Here and high sensitivity no joy!
Sometimes the pursuit of a super case in some individuals degenerates into the disease, when a person focuses on each extra centimeter the depth specified in advertisement. Firms - sellers are comfortable playing that the fact! And last, depth of detection, typically provided by air, E. for idealized conditions, but in the land of the said item at the stated the depth will most likely not detect.
Author: S. L. Dubrovsky