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METAL TUBE DETECTOR

METAL TUBE DETECTOR

The device allows to find underground cables and metal pipelines of all kinds, located at a depth of 1.5 - 2 m , to detect (at the same depths) individual metal objects (pipe sections, manhole covers, sheets of at least 250 X 250 mm ), and also to determine the location of the detected objects in the plan with an error of up to 20 30 cm .

Structurally, the searcher consists of two main nodes of an induction installation with mutually orthogonal (spaced along the axis) exciting and receiving frame antennas and an electronic unit, which includes a generator and a receiver. Frame antennas are placed in a rigid cylinder with a diameter 260 mm and length 700 mm made of fiberglass. They have an electrostatic screen in the form of a copper layer deposited on the outer surface of the plastic frame. The electronic unit and the cylinder are mounted on a rigid bracket, equipped with shoulder straps for carrying. Adjustment organs (switches, potentiometers, as well as connectors, a visual indicator and telephone jacks) are placed on the top panels of the generator and receiver.

In overseas pipe-and-wire detectors , the installation with vertical exciting and horizontal receiving frame antennas has become prevalent. When moving such an installation along a profile perpendicular to the cable or pipeline route, the change in curve of the secondary induced signal turns out to be asymmetrical and has two extreme values ​​(maximum and minimum, and with phase-sensitive display, two maximums) of different magnitudes away from the cable route (pipeline). At the same time, to determine the route, it is necessary to pass the profile twice in mutually opposite directions and take measurements on the ground (determine the middle distance between two marks). This device is free from this drawback: due to the location of the frame antennas at an angle of 45 0 and 135 0 to the horizon, it allows you to get maximum readings directly above the track of the object, as well as to divide objects that are parallel to each other at small distances in the horizontal plane (about 1 2 m ).

Schematic diagram

The electronic unit consists of a generator loaded on the exciting frame antenna, and an amplifier with a receiving frame antenna. The latter also includes a compensation device that serves to finely (fine-tune) compensate the primary induced signal (that is, the signal induced directly from the excitation antenna to the receiving one). The input of the compensation device is connected directly to the generator. Both units are powered by batteries located in the generator unit. The generator unit (Fig. 1) consists of a master oscillator (T 1 ), a matching stage (T2), two amplifier stages (T3, T4, T5) and a terminal power amplifier (T6, T7). The master oscillator tuned to a frequency of 12 kHz is assembled according to a three-point scheme with capacitive coupling. The coil L 1 of the oscillating circuit is wound on a core of the type TCHK-55P and has an inductance of about 25 mg . The number of turns of the winding and retraction are selected during adjustment.

To reduce the influence of the parameters of the transistor T 1 on the generation frequency, a partial connection of the circuit to the collector circuit has been applied. The supply voltage of the master oscillator is stabilized by the Zener diode D 1 . The emitter follower assembled on the transistor T 2 eliminates the effect of load changes on the master oscillator. The thermistor R 5 maintains the necessary stability of the output voltage in the operating temperature range. Transformer Tr 1 - matching. The pre-terminal and output stages of the amplifier are made on T4-T7 transistors using a push-pull transformer circuit. Exciting loop antenna is connected through a capacitor C 9 to transistors T6, T7. Diodes D2-D5 serve to prevent thermal breakdown of transistors T 6 , T7 with an increase in their temperature; to protect these transistors in case of a short circuit of the amplifier output, the Pr1 fuse with a rated current of 0.15 a serves. From the output of the generator, the voltage is applied to the input of the compensation device through a voltage divider and a transformer Tp 2 . To facilitate electrical compensation, a polarity switch Tp 2 can be entered into the device. The exciting loop antenna BP is matched to the output impedance of the generator by means of partial switching. To increase the radiated power and reduce the nonlinear distortion of the excitation signal, the BP antenna is tuned to resonance at the operating frequency. To protect the generator from improper inclusion of batteries, a diode D 6 is installed. Mars batteries (373) are used in the power supply battery to ensure that the searcher will operate continuously for approximately 50 hours. During the work with breaks service life of elements increases. The reception of the searcher (see the diagram in Fig. 2) consists of the compensation device of the amplifier and the block of indicators.

The main attenuation of the primary signal, induced in the receiving frame antenna PR , is due to its location orthogonal and symmetrical with respect to the exciting frame antenna BP. A compensation device connected with the PR antenna to the amplifier input serves to bring the residual primary signal to a level comparable to the secondary induced signal that appears when the seeker approaches the desired metallic objects. Adjusting the amplitude and phase of the compensating voltage is carried out by variable resistors R 1 and R 4, respectively. The amplifier contains six stages: the input on the transistor T 1 , aperiodic (T2) resonant (T3), limiter (T4), two aperiodic (T5 and T6). The connection between the transistors T 2 , T3 and T5, T6 is direct. In the block of indicators, two types of indications are used: visual (by microammeter) and acoustic (by sound in phones). The visual indication channel has a detector (diode D 1 ), to which a 100 micrometer microammeter is connected, the acoustic indication channel consists of an amplifier (transistor T7), a detector (diode D2) and a multivibrator (transistors T8, T9). The base voltage T8 is applied, the detected diode D 2 . Its constant component is the bias voltage on the base of T8, with a change in which the frequency generated by the multivibrator changes, and consequently, the pitch of the sound in the phones.

The sensitivity of the amplifier can be adjusted roughly with switch P 1 and a smoothly variable resistor R 37, and the sensitivity of the acoustic indication channel is variable with resistor R 26. The cascade on the transistor T 4 is the minimum signal of the resulting signal (secondary and uncompensated primary). Such a limiter allows the use of an uncompensated primary signal, the amplitude of which is comparable to or greater than the amplitude of the secondary signal. As a result, irregular interference (eg, pulsed) will be effectively suppressed. In addition, if there is a limiter, the requirements for the compensation depth of the primary signal can be relaxed. Finally, the limiter allows to increase the sensitivity of the amplifier to small changes in the secondary signal and to facilitate the adjustment of the compensation device.

Design

The device consists of a housing in which the excitation and reception framework antennas are located at an angle of 45 0 to the horizon; the bracket that houses the case with antennas, as well as blocks of the amplifier and the generator; generator block with power supply, amplifier unit with compensation device. The case, in which the excitation and receiving frame antennas are fixed, is made of fiberglass with a thickness of 4 mm in the form of a cylinder with an internal diameter 260 mm and length 700 mm . To increase the rigidity inside the body, two rings are also made of fiberglass, arranged perpendicular to the generator of the cylinder. The body is made by gluing fiberglass with epoxy resin. Instead of fiberglass, other layers of mass (for example, vinyl plastic) can be used.

Inside the cylinder along its axis, the exciting and receiving frame antennas are inserted at both ends and fixed them with brackets. Antennas to brackets and bevels to the body are attached with screws; after setting the device attachment points, pour epoxy resin. Antenna spacing along cylinder axes (the distance between their centers) should be approximately 450 mm . Cylinders of the exciting and receiving antennas of cylindrical shape are machined from PCB and impregnated with bakelite lacquer. The average diameter of the winding antenna 225 mm . The annular groove for stacking the antenna coils has dimensions (in cross section) of 18 X 18 mm. A layer of copper, which serves as a static screen, is deposited on its inner surface by electrochemical deposition. Receiving antenna contains 800 turns of PEL wire 0.1, exciting - 400 turns (tap from the 10th turn) wires of the PEV-2 0.6. From the outside, the frame turns are shielded with copper or brass tape. To reduce the losses made by the screens, in 1-2 places of the screen should be left narrow slits (slits). The bracket on which the device nodes are located is made of thick plywood. Its back wall has two protrusions for fastening the belts covering the back of the operator. On this wall are brackets where the case with antennas is laid, pulled by straps. The front wall of the bracket has a hinge on the left and a latch on the right, which allows the operator to put on the bracket and then close the front wall. The carrying straps are attached to the bracket so that the load on the shoulders of the operator from the blocks located on the front wall of the bracket and the case with antennas located on the back wall is evenly distributed. On the side walls of the bracket are fixed cables from the excitation antenna to the receiving antenna. The generator of the device is assembled in a metal case. In the lower compartment of this building is placed a battery of 15 elements of the type 373 ("Mars"). To allow access to the battery compartment, its bottom cover is made removable. It has a rubber seal. The top cover (also removable and sealed) serves as the front panel of the generator. It is attached to the getinaksovaya board, which is mainly mounted parts of the generator. On the top cover of the generator are placed: switch Bk 1 ; a socket part for connecting the generator with the exciting loop antenna and the amplifier unit; fuse holder Pr1. The amplifier is assembled in a separate metal case and mounted on two getinaksovy circuit boards arranged in parallel. Boards are attached on racks to the top (front) panel. Between them there is an electrostatic screen, made of foil getinaksa. On the board, located closer to the front panel, are the details of the first four amplifier stages, and on the second board - the remaining stages. Boards can be swiveled at the hinges, allowing access to all parts. On the front panel of the amplifier are placed: microammeter; connector base; amplifier controls; headphone jacks. The winding data of the transformers and the choke of the generator and amplifier are tabulated. To protect the installation from moisture between the front panel of the amplifier and its body placed rubber gaskets.

Customization.

The device is set up in the following order: first, frame antennas, then a compensator, and finally, a generator and an amplifier. The tuning of the frame antennas is reduced to the adjustment of their relative position to obtain the minimum primary signal, measured in dozens of microvolts. Found the optimal position of the antennas are rigidly fixed by pouring fasteners with epoxy resin. To obtain the required depth of suppression of the primary signal (about 100,000 times), you can use small adjustment frames or metal plates located near the receiving loop antenna. After setting, they are also poured with epoxy resin. Adjustment of the compensator is reduced to a further reduction of the primary signal at the amplifier input using sequential adjustments of variable resistors R 1 and R 4. The setting of the generator and the amplifier does not have any specific features and therefore is not described here. When properly configured, the generator of the device should have the following parameters: operating frequency 12 kHz ± 60 Hz; instability in the temperature range –20 0 - +50 0 С - ± 120 Hz; Q-factor of the exciting frame - about 30; active power allocated in the exciting frame - not less than 0.8 W; output voltage instability in the temperature range -20 0 - +50 0 С - not worse than ± 5%. The amplifier parameters should be as follows: tuning frequency 12 kHz ± 120 Hz; the relative instability of the tuning frequency in the temperature range -20 0 - +50 0 С is about ± 2%, and the gain factor is about 2 times.