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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Metal detector cable sensor. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The disadvantage disk encoder is the complexity of its manufacture with a winding diameter of more than 30 cm. The cable sensor proposed below can be manufactured with a diameter of up to 1,5 m. . The rigidity of the design of such a sensor is lower than that of a disk one, however, the ease of manufacture justifies everything. Unlike the disk probe, the cable probe is negatively buoyant (sinks in water), which is convenient for underwater searches. In addition, the large diameter cable sensor can be dismantled! To manufacture the sensor, you need a shielded telephone cable of the TPP-30 or TPP-50 brand, with 30 or 50 pairs of wires, respectively. From such a cable, a winding consisting of 60 or 100 turns will be obtained. The appearance of the cable non-separable sensor is shown in fig. 38.
The sensor consists of a piece of cable - winding 1, fiberglass braces 2 with vinyl plastic fasteners 3 to winding 1, junction box 4 and cable 5. The design is clear from the figure, only the method of cable desoldering and the device of the junction box require explanation, which we will dwell on in more detail. To simplify the desoldering of the cable, you must first get acquainted with some of the features of its device, which will greatly simplify the matter. The conductors in the domestic CCI cable (as well as in many other types of telephone cables) are twisted in pairs. One of the conductors of each pair is common and has either white or red insulation. The remaining conductors have colors: brown, yellow, green, blue, gray. Pairs are grouped in a cable by tens, each tens is entwined with its own viscose thread. Each ten has five pairs with a white common wire and five with a red one. Thus, in each ten, each twisted pair of wires is unique in the combination of two colors. Before you start desoldering the ends of the cable, you must carefully cut them (preparation). The latter consists in disassembling the cable conductors in tens and in pairs within each tens. Since the twisting step of each pair is quite large (an ignorant person usually does not notice that the conductors are twisted in pairs), for successful disassembly of the conductors, a free section (reserve) with a length of at least 0,5 m is required from each end of the prepared cable segment. The cutting of the end of the cable begins with the removal of insulation, for which an annular incision of the outer plastic coating is made at the required length. It is necessary to cut the plastic very carefully so as not to cut the inner cores of the cable (especially the bare conductor, which is the screen output). After notching, small bends in the place of the notch (on a break) separate the outer insulation section together with the internal aluminum screen. For convenience, the insulation must be removed in segments of 10 ... 15 cm. After removing the outer insulation and screen, but before unwinding the protective film (!), it is necessary to fix the ends of all conductors (with adhesive tape or by twisting). The bare conductor of the screen is rolled up separately into a small coil so that it does not interfere. After that, you can unwind the protective film and disassemble the cable pairs into dozens (each ten is wrapped with its own viscose thread) Each ten is also fixed at the end with adhesive tape or twisting, after which the bunches of dozens can be parted to the sides. While the cable is still "fresh" after cutting the ends, it is advisable to disassemble each ten in pairs. To do this, without releasing the end of one of the dozens from fixation, by squeezing it longitudinally and gently shaking and turning, they achieve "scattering" of the bundle of wires into twisted pairs. As mentioned above, for this it is necessary to have a length of the cut part of the cable of at least 0,5 m. The conductors in each pair are fastened at the end with a small piece of adhesive tape (for each pair). There is no need for additional marking, since within each ten each pair is unique in terms of the combination of insulation colors. After that, it is already easy to "call out", i.e. identify the tens at each end of the cable, for example by assigning them numbers (1, 2, 3 for a 30-pair cable, or 1-5 for a 50-pair cable). After such preliminary preparation, you can begin to desolder the conductors. It is advisable to desolder in several stages - dozens of pairs. In this case, from each tens of twisted pairs of the cable, an insulated section of the future sensor winding with 20 turns of wire will be obtained. In the future, individual sections are connected in series-accordingly to form a winding with 60 or 100 turns. The desoldering is carried out according to the scheme below.
Before soldering the conductors, the ends of the cable, where the outer protective plastic insulation ends, are as close as possible to each other. In this case, a circle of the required size is formed from the cable. The cable and its ends are fixed in this position. In accordance with the proposed wiring diagram, one of the dozens of conductors is selected from one end of the cable, one from the other end. For example, the brown wire of a white-brown pair on one end and the white wire of a similar pair on the other. The conductors are cut to a length of 5 cm, stripped of insulation, tinned and soldered together. The soldering point is isolated with adhesive tape or with a thin heat-shrinkable tube. Thus, the pairs of each tens are desoldered, and then the tens are soldered together. If you follow the proposed scheme, you should end up with a winding with the beginning output in the form of a wire with white insulation, and with the end output in the form of a wire with gray insulation. After desoldering the winding, the place where the ends of the cable are soldered is placed in a plastic junction box measuring 40x40x80 (mm). The internal volume of such a box allows, with a small seal, to place the soldered ends of a 30- or 50-pair cable. The internal volume of the box is filled with epoxy resin or epoxy glue. Filling guarantees reliable additional insulation of the cable wires, prevents them from breaking off, and provides a strong mechanical connection of the cable ends. In the junction box, the sensor winding leads are connected to the end of a flexible cable passed through a hole in the box. The box also contains the capacitors of the oscillatory circuit of the sensor and its other elements, if any are provided for by the circuit. Shown in fig. 38 appearance corresponds to the non-separable design of the cable sensor with an outer diameter of 40 cm. The cable sensor can easily be made in much larger diameters. The main limitations are the ability of the operator to manipulate such a sensor during operation, as well as the transportability of the sensor. The first limitation corresponds to a sensor diameter of no more than 1,5 m and a weight of up to 5 kg. The second limitation encourages the design of the cable sensor to be collapsible. On fig. 39 shows the design of a collapsible cable sensor.
By its principle, the proposed design resembles the design of a folding umbrella. The sensor consists of a cable ring with a junction box, 12 fiberglass bracing tubes, 2 central disks and auxiliary fastening elements. From the side of the cable, the bracing tubes are fastened with aluminum adapter bushings fixed to the cable with brackets and screws. Transition bushings are freely inserted inside the tubes - braces. Transition bushings of a similar purpose are also located on the central disks (6 on each). However, they are fixed with the possibility of rotation within a small range, so that the bracing tubes can move during the assembly of the sensor in the same way as the spokes of an umbrella move when it is opened. The circumference of the sensor and the length of the bracing tubes are selected so that in the assembled state the sensor winding cable is under strong tension (at least 100 N). In this case, it is necessary to take appropriate measures to prevent rupture and damage to the sensor winding, carefully mechanically connect the ends of the winding cable into a ring in the junction box, round off the sharp edges of the adapter sleeves at the points of contact with the sensor winding cable, etc. In the disassembled state, the sensor consists of separate elements: a cable ring with adapter sleeves fixed on it, 12 bracing tubes (it is convenient to store them in a separate case), 2 central disks, as well as elements with which the sensor is fixed in the working condition - a tightening screw with a nut, a central thrust sleeve and a handle for carrying the sensor or a mounting assembly for docking with a rod. A certain trick lies in the method of folding the cable - the sensor winding - in a disassembled state. The cable, although flexible, is not to the same extent as a clothesline. It absolutely does not allow bends with a small radius of curvature and longitudinal torsion. Any use of force when folding the cable ring can break the inner shield foil and cause other damage! The cable ring is folded into three turns. The sequence of operations when folding is shown in fig. 40.
Rice. Fig. 40. Sequence of folding the sensor cable ring Author: Shchedrin A.I.
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