Tweet
Сергей Конструктор Альтаир-2Д
"The fundamentals of the AFS amplitude-frequency sampler method."
Introduction.
The Altair-2D selective metal detector uses a different principle of operation from the methods used in fore ign and domestic - including homemade Russian, selective metal detectors. Therefore, I will describe in more detail both the manufacturing procedure of the Altair-2D, and the basics of the method used in it, as well as the operation of its main circuit nodes.
Most modern selective metal detectors for searching for metal objects in the ground are mainly based on the inductive balance (IB) method. This method makes it possible to implement with high efficiency all the basic necessary requirements for the search parameters of a modern metal detector – high sensitivity (range of operation on metal objects), elimination of the influence of soil on the operability of the device and selectivity by type of metals. All other methods (pulse PI, Q meter, frequency BFO, Fm, OR), individually, are not able to ensure simultaneous compliance with all the above requirements.
PI and Q do not have effective metal species selection, and frequency methods lose their effectiveness in soils with increased mineralization. The only drawback of the IB method from the point of view of self-manufacturing of metal detectors is the difficulty of manufacturing a high–quality and stable sensor at home. Unlike other methods, it is a composite structure of several inductive coils, requiring their high precision balancing and thermal stability. However, it is possible to implement a selective metal detector not only on the basis of the IB method. The symbiosis of two separate methods, Q and Fm, also makes it possible to obtain a device with sufficient sensitivity, the ability to eliminate the influence of soil and selectivity by type of metals.
This uses a simple sensor consisting of a single inductor coil. It is this Q&Fm method (or otherwise AFS - Amplitude-Frequency Sampler) that is used in the Altair-2D metal detector.The essence of the Q-method is that when a metal object appears near the inductor of an LC generator, its quality factor changes and, accordingly, the amplitude on the circuit. These changes in the amplitude of the oscillator are further detected by the amplitude detector, filtered, amplified, and sampled by the metal detector circuit.
Moreover, the closer the generator mode is set to the threshold of generation failure, the more sensitive the metal detector is. The features of the Q-method include the lack of selectivity by type of metals (for any metal, the amplitude of the oscillator decreases) and the reduced influence of soil on the operability of the method.The essence of the Fm method is that when a metal object appears near the inductor of an LC generator, its frequency also changes. These frequency variations of the oscillator can then be detected by a frequency detector, filtered, amplified, and sampled by a metal detector circuit. The features of the Fm method include the presence of selectivity by type of metals (mainly ferrous / non-ferrous) and a complete loss of performance on soils with increased mineralization. Both of these methods, Q and Fm, make it possible, with appropriate circuitry, to obtain high sensitivity comparable to that of most popular front-end IB metal detectors. That is, within 20-40 cm per single coin in the ground, depending on the diameter of the sensor and the specific model.However, the simple independent application of two separate methods Q and Fm (Fig.1.) as a selective metal detector for searching for metal objects in soils with increased mineralization is completely ineffective.
This is due to the increased influence of the soil on the Fm method. The signal from the soil with increased mineralization at the output of the Fm channel will be much higher than the signals from small or deep-lying metal objects. Therefore, weak signals from these objects will be completely masked by the signal from the ground (Fig. 2.).
In order for the Q plus Fm method to work effectively in soils with increased mineralization, both of these methods must be interrelated and help each other to eliminate their disadvantages. This is exactly what happens in Altair-2D: the Q-method significantly reduces the influence of soil in the Fm channel, and also allows you to regulate the nature of its selectivity by any types of metals. And the Fm method, in turn, eliminates the signal from the ground in the Q channel. The result is a selective (Q+Fm) device, comparable in search parameters (sensitivity, adjustable detuning from the ground and adjustable discrimination) with comercial class IB devices.
Current comparison of Altair-2D with popular models of comercial front-end IB metal detectors
A detailed description of the technical and operational characteristics of the Altair-2D, video tests, operating instructions, etc. are available for public. As follows from the description and ground tests, the technical and operational characteristics of the Altair-2D practically do not differ from IB devices of a similar class. However, there are some differences due to the peculiarities of the methods used.
If we consider Altair-2D specifically, rather than the potential capabilities of the Q+Fm method in general then we can highlight its advantages and disadvantages in comparison with popular IB metal detectors.:
Advantages:
1. Energy efficiency.
The current consumption of the Altair-2D is about 6 milliamps, powered by two batteries with a total voltage of 18 volts. Two conventional "salt" batteries of the Crown type are enough for 40-50 hours of continuous operation. Alkaline batteries (Duracell, Energizer, etc.) increase the continuous operation time by another 2-3 times.
2. Stable operation in a wide temperature range from -10 to +40 °C.
3. An easy-to-manufacture and configure inductive sensor. Reduced requirements for coil winding and mechanical strength.
4. Easy configuration of the electronic circuit (one tuning resistor and one selectable capacitor in the sensor and one tuning resistor in the processing unit).
5. Independence of adjustment adjustments from the ground balance and discrimination.
6. In the "All metals" and "Mixferrum" modes, the metal detector detects any metal object in the soil with high increased mineralization without loss of detection range. At the same time, in the "Mixferrum" mode, it is possible to evaluate the type of metal of the detected object.
7. Increased boosted sensitivity to fine natural gold - nuggets and gold dust.
Disadvantages:
1. The adjustment of detuning from soil with increased mineralization is too "sharp" alias detuning is carried out in a very narrow range of the regulator scale and requires precise skilled installation.
2. In practice, the inaccuracy of detuning from soil with increased mineralization is manifested in the appearance of false workings on the soil. It can be eliminated by installing an additional regulator for "smooth" detuning from the ground. - Manual detuning from the soil is carried out only from mineralized (Fe-) soils. There is no manual detuning from salty soils (in particular, some chernozems black soils). To introduce it, it will be necessary to supplement the Altair-2D with one op amp, two resistors and one switch which was supposed to introduce the "Fe-ground / Salty ground" switch mode).
3. Another disadvantage of the Altair-2D AFS is a high operating frequency 32 kHz and need to place the Q-generator directly in the sensor.
4. The influence of salty soil moisture and false erratic signals in salty sea water.
Comment