| NEW CABLE TRANSDUCER FOR SEISMOMAGNETOMETRIC DETECTION MEANS.
ZVEZHINSKY Stanislav Sigismundovich, Candidate of Technical Sciences
NEW CABLE TRANSDUCER FOR SEISMOMAGNETOMETRIC DETECTION MEANS As is known, a seismic magnetometric detection device (SO), combining two passive physical detection principles — seismic and magnetometric, is the most effective for border protection, extended unfenced boundaries of objects [1]. Compared to a magnetometric one, it solves the problem of a “magnetically clean” intruder, compared to a seismic one, it has significantly greater noise immunity, and is less susceptible to the effects of climatic and geological conditions. Compared to an active detection device based on the leaky wave line effect, it has several times lower energy consumption and cost. The seismic magnetometric CO “MILES”, first developed in the USA in the mid-70s [2], is being modernized and used to protect military and especially important facilities abroad. In the State Unitary Enterprise “Dedal” (Dubna) in the late 90s, the combined seismic magnetometric CO “Grezy-12-2” was developed [3]. However, it has its drawbacks, including those caused by the not entirely satisfactory technical characteristics of the specialized KTPEDEP cable, designed to form the sensitive element of the CO. Therefore, the development of an improved combined cable converter (CTC) of seismic and magnetometric signals is relevant. Such a converter, while maintaining cost parameters, should have:
The development of the converter has significant technical difficulties, the main one being the provision of acceptable and reproducible seismic sensitivity based on the triboeffect. Numerous works in this area have failed to develop either a scientific or even a phenomenological theory of the construction (optimization) of a distributed triboconverter. In known sources of information, including patents [4, 5], there are numerous contradictions on this issue. Therefore, when creating such a cable, one has to go “by touch” in many ways, carrying out trial iterations on the manufacture of numerous prototypes. Triboelectric cable transducers are used in the air, attached to the fence, registering its vibrations under the action of the intruder (in vibration CO), as well as in the ground, where they register seismic vibrations caused by the intruder's walking or the passage of transport. The triboelectric cable located in the ground experiences useful deformations 2 … 3 orders of magnitude smaller than on the fence, therefore the useful signals are extremely small — in unfavorable natural conditions (for example, frozen ground) their value is (in terms of the input charge) ~ 0.1 pC and less. In addition, as experiments show, signal formation in the ground differs significantly from that on the fence, therefore the development of seismic CP options «in the air» is not entirely adequate. In a cable converter, the contacting and rubbing (during deformation) surfaces are a metal screen and a polymer insulator separating the screen from the internal (central) conductor, which are in a state of thermodynamic equilibrium. The type of their contact can be most adequately defined as sliding, multi-point, reversible. The development of an effective triboconverter encounters a number of technical contradictions, including the coefficient of friction and freedom of movement of the contacting surfaces metal — insulator, the thickness of the shell and the outer diameter of the cable, the presence of an additional screen, etc. It is a well-known and little-disputed fact that the triboeffect, i.e. the appearance of free electrons in a metal or dielectric — polymer during their friction (sliding, contact), is caused mainly by contact electrification [6 — 8]. It is believed that the electrons injected into the polymer are captured by deep local states (traps) lying near the Fermi level of the metal (~4 eV) [9]. Such traps are caused by defects in the polymer structure, for example, impurities, and oxidation of the polymer surface significantly increases the tribocharge. The higher the difference in the work functions or the contact potential difference (CPD) of the materials, the more useful the signal is — the charge Q is proportionally larger [8]. There is also a direct correlation between the value of Q and the density of surface traps of the dielectric, the contact area S [6 — 8]. Other observed dependencies are due to many interrelated factors (including those related to the chemical and atomic structure of the polymer, for example, acceptor-donor properties) and are in the field of theoretical models. With increasing temperature, the CPD increases, but practically no dependence of tribosignals on temperature was found. Specific conductivity of the surface does not play a role in triboelectrification, but accelerates the dissipation (degradation) of the transferred charge. The tribocharge depends on the condition (contamination) of the surface: in metal these are oxides, sulfates, on an insulator – various oil films, dirt, dust; the higher the contamination of the surface, the lower the tribocharge. The presence of water and humidity sharply reduce the signal, including due to its shunting” (for water r ~ 105…106 Ohm•m). In a converter, where the contacting surfaces are a metal screen (aluminum, tinned, copper) and a polymer (polyethylene, fluoroplastic, polyvinyl chloride, lavsan), an increase in S causes an increase in its diameter. Micromechanical properties of the contacting surfaces (for example, the pressure force) affect the transferred charge, some minimum pressure is required [9], which may not be provided in a real cable. The creation of a tribo cable is an “inverse case” with respect to the more frequently encountered task of creating a communication cable with minimal tribo noise [10]. It is interesting to note that in both cases [4, 10], the so-called semiconducting polyethylene (SPE) (r ~ 1015 Ohm•m) with the addition of graphite chips is sometimes used, due to which it acquires anomalous electrical properties (r ~ 105…107 Ohm•m). The mechanism for reducing tribo noise (for example, by applying graphite chips) consists in the implementation of a semiconducting electrostatic screen that “diverts” free charges. The mechanism for increasing tribo noise with the help of SPE is not entirely clear. However, the use of SPE is limited by a significant increase in the linear capacitance of the Сп cable, which negatively affects the intrinsic noise of the CO signal amplifier. Indeed, the classic “linear capacitance” (per 1 m) of a coaxial cable is equal to [11]: Сп » 24•e/log (D/d), pF/m, where e is the relative permittivity (for PE ~2.3), D is the screen diameter, d is the diameter of the inner conductor. The presence of PE between the cable plates leads to an effective increase in d, and therefore, in the linear capacitance. The “triboelectric series” is well-known and widely used in technology [12] — a sequential list of materials that electrify each other when rubbed, and the further the contacting materials are in this series, the greater the tribocharge. In Fig. 1, this series is presented in the application to components used in the cable industry, the arrows show the direction of increasing work function or the ability to acquire free electrons, charging negatively. The minimum triboeffect is characteristic of a pair of materials standing next to each other in the triboelectric series (for example, aluminum — paper).
As can be seen from Fig. 1, fluoroplastic has the best triboelectric properties with respect to metals, but it is relatively expensive and rigid. Polyethylene (PE) is cheap and widely used, but, for example, uncontrolled mixing of its various grades (for example, types 153-01K and 107-02K) minimizes the charge that occurs when it comes into contact with copper. The advantages of copper or aluminum for making a triboscreen are discussed in the literature contradictorily due to their different microstructure (mesh or foil), the effect of oxidation; in general, if there is a difference, it is insignificant [12]. Aluminum foil is oxidized almost immediately during manufacture, while its work function increases (from 4.3 eV to 5.4 eV) and, in general, the CRP does too. Annealed copper used to make screens does not change its physical properties for 5…10 years if the cable is not damaged. Studies of the magnitude of the triboeffect in copper cables depending on the date of manufacture (for example, PVChS, RK, KMM) did not reveal any stable correlation [7]. Lavsan film, which is used in the TPPep cable, has good triboelectric properties. Compared to PE, lavsan has an order of magnitude higher density of surface traps (~ 1015 eV-1m-2). Nylon with a low work function (4.1 eV) and Mylar are also promising materials for cables. The maximum density of the surface charge formed on the surface of the contacting surfaces is limited by a value sufficient for electrical corona breakdown: Epr » 30 kV/cm. In this case, the surface charge density is limited by the value spr = e0Epr = 2.7•10-9 C/m2. The heat-stabilized high-pressure PE used for the sheath is relatively hard and inflexible, especially at subzero temperatures, so the use of thermoplastic elastomer (TPE) seems promising, into which reagents can be added that repel rodents. If the cable sheath is damaged and it “sucks in moisture, the screen (copper, aluminum) oxidizes and the triboeffect decreases by 3…5 times. Studies of the triboeffect in copper screens have revealed a proportional dependence of its value on the saturation of the winding up to a certain value, after which saturation occurs” and the effect practically does not increase. Apparently, there is an optimum associated not only with the contact area, but also with ensuring minimal friction and intermittent” contact. A longitudinal winding of the screen (at an angle of ~450 to the cable axis) compared to an overlap with the same saturation increases the triboeffect by 1.5…1.7 times, however, the linear capacity of the cable also increases, but less noticeably (by ~1.2 times). The development of an effective cable triboconverter is essentially heuristic and is based on solving the above-described alternative technological and compositional private problems, implementing combinations in the form of prototypes, checking their characteristics, analyzing shortcomings, and developing ways to improve. For example, as practice shows, a successful combination of contacting surfaces is aluminum — lavsan (TPPep cable), copper — low-pressure PE (PVChS cable), copper — Kapton with Mylar film [13]. In order to determine the transfer function WKP of the cable converter, equal to the ratio of the output charge to the tabulated mechanical effect, a special test bench can be used, shown in Fig. 2. Its feature is the integral (over 2 m of the length of the tested CP section) mechanical effect — bending, which in its amplitude (fractions of mm) approximately corresponds to the bending of the cable in the ground (~ 0.01 … 0.1 mm). Using a low-noise amplifier, useful signals are recorded, and WKP of different CPs is compared. The amplifier's own noise, referred to the input, is ~ 10-14 C (peak) in the low-frequency range of ~ 0.1…1 Hz, which allows us to evaluate the useful signal with an amplitude of at least 3•10-14 pC.
A 2 m long section of cable is placed in a groove between two pine boards, which are then pulled together along their entire length with a rubber band. The mass P of the load, which is applied to a 70 cm long lever (Fig. 2), is 0.1 … 1 kg; varying the load allows us to estimate the linearity of the transfer characteristic of the CP. For better repeatability, in order to exclude the influence of knocks, the load is installed on the stand and useful signals from the «removal of the load» are recorded. The initial deflection of the structure does not exceed fractions of a mm, the repeatability (reproducibility) of the experiments is at least 80%. Upon contact, electrons from the metal always pass into the polymer, which is charged negatively, and the charge is «bound». When the metal and polymer are separated, the released charge (in the form of current) flows through the amplifier circuit, initially always in one direction. This is confirmed by the same polarity of useful signals recorded on the stand, under the same mechanical influences. Table 1 summarizes the identified technological and design parameters required for the implementation of a highly sensitive cable seismic transducer.
Table 1. Required technological parameters of a triboelectric CP with high sensitivity
The development of a new improved tribotransducer is based on the implementation of the above-described alternatives (in technology and design) in the form of prototypes, testing their characteristics (in comparison with the KTPEDEP), analyzing shortcomings, and developing ways to improve. The manufacturer of the special cable samples is ZAO RusInterna (Moscow), which has experience in developing special cables (KTV-mf, KTVU) designed to record fence vibrations using the triboeffect. As a result of the preliminary work, according to the recommendations in the table, about 30 experimental prototypes were manufactured, tested, and analyzed, from which a prototype of the new seismomagnetometric KP was selected. Its design in section is schematically shown in Fig. 3.
It is a coaxial cable with three screens (2 working, 1 electrostatic) made of aluminum-lavsan tape. The outer diameter of the cable is ~ 17 mm, the insulation thickness is 1.5 mm. The triboelectric signal is taken from two internal screens. In the center of the converter there are 20 twisted insulated conductors (with a copper diameter of 0.5 mm), necessary for the implementation of the magnetometric converter. The internal working screen surrounds the conductors, the splined shell surrounds the screen. Lavsan tape is applied to the tube, which is the main medium for generating triboelectricity. Next comes the second screen (working), an insulator layer made of easily crumpled foamed PE, an external (electrostatic) screen and a shell made of thermoplastic elastomer. Tribosignals are taken from the drainage conductors. The technical characteristics of the new combined cable converter sample generally meet the requirements. Its full-scale tests have shown high seismic sensitivity, exceeding the analogue by 2…3 times, with an increase in electromagnetic compatibility by almost an order of magnitude.
Literature
|
