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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Metal detector disk sensor. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The disk sensor is traditional for metal detectors on beats, for metal detectors based on the "transmit-receive" principle with a coplanar arrangement of coils, it is also convenient for induction, pulse metal detectors and for a metal detector based on the principle of an electronic frequency meter. The sensor is fixed on the rod (see fig. 35).
Rice. 35. Appearance of the disk sensor on the rod The rod can be metal or fiberglass. Telescopic extendable bars are convenient. For amateur purposes, you can use 2-3 knees from a telescopic fiberglass fishing rod of suitable thickness. A non-metallic rod is preferable because it does not have large masses of metal near the sensor. The sensor is in the form of a hollow disk made of plastic. Coils, fasteners and other elements are placed inside. Below we consider the practical design of such a single-coil sensor, proposed by the author (Fig. 36).
The sensor consists of top and bottom covers 1 and 2, which are polyethylene covers from a set of plastic dishes. The covers are connected to each other by welding. Welding of polyethylene covers is carried out along the shoulder on the edge of each cover, using a soldering iron with a special nozzle. The nozzle on the soldering iron tip for such welding is a "V"-shaped tray in cross section. The inner surface of the nozzle-tray is heated by the contacting edges of the covers until melted in one place, after which, smoothly moving the soldering iron with the nozzle-tray, bypass the entire length of the contact circle lid edges. Before performing such a clean welding, it is recommended to practice on prototypes. It will be necessary to select the desired temperature of the nozzle-tray (soldering iron power 40 W), the speed of movement of the tray during welding. It is recommended to fix the soldering iron with a tray attachment fixedly, for example, in a vice, and hold the parts to be welded in your hands. In order for molten polyethylene not to accumulate in the nozzle-tray and to obtain a more accurate seam, the movement of the nozzle-tray along the circumference must be combined with its reciprocating oscillations with an amplitude of 2 ... 5 mm. The described operation of welding polyethylene parts is laborious, but the resultant sealed, lightweight and durable sensor housing justifies the cost. In addition to welding, additional fastening of covers 1 and 2 is carried out using a central bar made of vinyl plastic. It is attached to the bottom cover with self-tapping screws. After assembling the sensor, the corners of the sensor suspension are also screwed to the central bar. The main "highlight" of the sensor is an unusual combination of foam and epoxy resin for any other designs. The filler disk for 3 sensors is made of foam plastic. Styrofoam can be used almost any. Widely available packaging and insulation foam (loose, easily crumbled by hand into individual balls) is even more preferable than solid, as it has larger and even through pores, which are subsequently filled with epoxy resin. Disk 3 occupies a large part of the sensor's volume and, with a small mass, gives it the necessary rigidity. The disk is cut using a hot metal thread (from the coil of an electric heater) connected to a low-voltage power source. If there is a foam sheet of the required thickness, then only a jigsaw or a safety razor blade will be needed from the necessary tools. In disk 3, grooves of the required shape and size are cut out for the central bar, for cable 8, for capacitors 6 and for other elements 7 of the electrical circuit of the sensor. Winding 4 is placed along the outer edge of disk 3 and filled with epoxy resin 5. The winding is wound with wire of the required brand and thickness on a mandrel with a diameter exceeding the diameter of disk 3 by approximately 5 mm. Carefully removed from the mandrel, the winding is fixed in four places with adhesive tape to give it the shape of a thin ring. Then this ring is wrapped with adhesive tape (it is better to use paper) with a width of 5 - 10 mm, with maximum tension (Fig. 37). Tape wrapping must be carried out in such a way that adjacent turns of adhesive tape have sufficient overlap. This overlap gives the sensor winding the required rigidity.
Figure 37 Wrapping the spool with adhesive tape In a similar way, a layer of aluminum foil is applied over the layer of adhesive tape, which serves as a screen for the sensor winding. To do this, the foil is cut into strips about 10 mm wide. To prevent the formation of a short-circuited turn, which reduces the quality factor of the circuit, the foil winding should not occupy the entire surface of the sensor winding ring - a small section 1–2 cm long is left free from the foil To prevent the foil from unwinding, its last turns are fixed with adhesive tape. The tap from the screen is carried out with a single-core tinned wire without insulation. The wire is fixed with a knot or with adhesive tape at the beginning of winding the screen foil and then also wound with tension over the screen to its other end. The remaining end of the wire is fixed with adhesive tape and its free end 5 ... 10 cm long is left for connection. The described technology for manufacturing shielded coils of metal detector sensors is available for repetition in amateur conditions. It does not require such time-consuming procedures as impregnation of the windings (with epoxy resin, paraffin, etc.), and the mechanical rigidity of the winding is high, especially when poured with epoxy resin from the outside. The sensor is assembled in the following sequence. A central bar is installed on the bottom cover 2 of the sensor from its inner side, a foam disk 3 and a winding 4 are installed, a cable 1 is threaded through the holes in the top cover 8 and the central bar and its end is cut. Then the remaining elements of the electrical circuit of the sensor are installed - capacitors, resistors, etc. - and they are unsoldered with the leads of the cable and the winding of the sensor coil. In this case, it is advisable to install the compensation thermistor of an induction metal detector in close proximity to the winding. For convenience, radioelements can be mounted on a small breadboard printed circuit board. After that, the sensor is installed on a horizontal surface and the foam disk 3 is pressed with a load so that it does not float in the epoxy resin. Then the sensor is filled with resin or epoxy adhesive with a hardener. The curing ability of the resin or adhesive must be checked in advance to avoid damaging the sensor! It is recommended to pour the sensor up to the edges of the bottom cover 2. It is not recommended to work with epoxy resin at temperatures below +15 °C, since curing may take several days, and above +25 °C due to the rapid course of the curing reaction with the release of a large amount of heat, which may deform the sensor. After the resin has hardened, the cable 8 is placed in the groove cut out for it in the disk 3, its exit into the hole of the top cover 1 is sealed with silicone sealant and the top cover is put in place, fixing it with self-tapping screws on the central bar along with the corners of the suspension. In conclusion, the upper and lower covers of the sensor housing are welded in the manner described earlier. Author: Shchedrin A.I.
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