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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Ultrasonic fire alarm. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Safety and security The proposed signaling device is intended for systems of distributed control of extended fire hazardous objects, for example, fuel lines, electric cables, gas pipelines, tanks with combustible substances, as well as various units. It reacts to the heat of the flame. The sensitive element is an ultrasonic waveguide made in the form of a flexible wire made of heat-resistant metal, equivalent to a plurality of temperature sensors distributed along its length. Cord-like design allows you to lay such an element along the surface of the controlled object, following its shape. Fire alarm devices are known, the sensitive element of which is made in the form of an extended heat-resistant tube filled with low-melting material, and through it acoustic communication of the emitter and ultrasound receiver located at opposite ends of the tube is carried out [1, 2]. The material inside it melts when heated by a flame, as a result of which the acoustic connection between the emitter and the receiver changes, which serves as the basis for the formation of an alarm signal. The disadvantage of such devices is the complex design of the sensing element, which should prevent leakage of the melt from the tube. In addition, the response temperature is always equal to the melting temperature of the material filling the sensing element. It can be regulated only by changing its chemical composition. In practice, for different conditions, it is necessary to have a stock of sensitive elements made of different materials, which is not always acceptable. The signaling device described in [3] operates on a similar principle, but the sensitive element (ultrasonic waveguide) in it is made not from a tube, but from a solid heat-resistant wire. Its advantage is the simplicity of the design of the sensitive element. The signal at the input of the receiving device changes as a result of complex interference phenomena occurring in the ultrasonic waveguide when the wave propagation velocity changes as a result of its heating. The response temperature can be adjusted by changing the threshold of the comparison node at the output of the receiver. The disadvantage is that in order to obtain the desired sensitivity, it is often necessary to adjust the frequency of the generated ultrasonic vibrations. The fact is that without it, the received signal during a fire can both decrease and increase, and the comparison unit in the device [3] reacts only to its decrease. The signaling devices discussed above have one more common drawback. They should have two piezoacoustic transducers - transmitting and receiving, installed at different ends of the sensitive element. This complicates the design of the signaling device as a whole, and in some cases makes it difficult to install it at the facility. The proposed fire detector is free from the above disadvantages. Main Specifications
The structure of the signaling device is shown in fig. 1. It includes an ultrasonic frequency signal generator G, power amplifiers UM1 and UM2, a piezo-acoustic transducer PP with an ultrasonic waveguide (sensing element) attached to it, a resistive equivalent of a piezoelectric transducer EPP, sensors for the current consumed by amplifiers UM1 and UM2 DT1 and DT2 , differential amplifier DU, integrating circuit AND, threshold devices PU1 and PU2, ignition indication unit IND. Nodes UM 1 and UM2, DT1 and DT2 are identical in pairs. The ultrasonic frequency signal from the output of the generator G is fed to the inputs of the amplifiers UM1 and UM2. An ultrasonic piezo transducer PP is connected to the output of UM1, and its equivalent is connected to the output of UM2 The PP excites longitudinal ultrasonic oscillations in the waveguide-sensitive element, which propagate to its end, are reflected and returned to the transducer. As a result, a standing acoustic wave is established in the waveguide. This mode corresponds to a certain input acoustic impedance of the waveguide, which serves as a load for the transducer. It determines the power taken by the transducer from PA1 and the current consumed by this amplifier from the power source. In the absence of ignition, all these parameters remain unchanged. However, when a section of the waveguide is heated by a flame, the velocity of propagation of ultrasound through it changes. Accordingly, the pattern of standing waves and the input acoustic impedance of the waveguide change. The result of this is the deviation of the current consumed by UM1 from the steady value. The resistance of the resistive equivalent of the EPP emitter connected to the output of AM2 is chosen so that in the absence of ignition, the values of the current consumed by AM1 and AM2 are equal. At the same time, the voltage values coming from the current sensors DT1 and DT2 to the inputs of the differential amplifier DU that calculates their difference do not differ.
The output signal of the remote control, having passed through the integrating circuit AND, further attenuating its ultrasonic component, is fed to the inputs of the threshold devices PU1 and PU2. One of them is configured so that it reacts to an increase in voltage relative to the stationary value, and the second - to its decrease. When any threshold device is triggered, the IND indication unit generates sound and light alarm signals. After the fire is eliminated and the sensitive element cools down, the signaling device is ready for operation again. Other destabilizing factors (for example, a change in the supply voltage) do not violate the mutual equality of the current consumed by UM1 and UM2, therefore, an alarm signal is not generated when they are affected.
The diagram of the fire alarm is shown in fig. 2 The generator G is assembled on the comparator DA1. Its output signal is a sequence of rectangular pulses with a duty cycle of about two. Capacitor C2 and resistors R4, R7 are frequency-setting, tuning resistor R7 provides the ability to change the pulse frequency. Their amplitude is reduced to the desired value by a resistive voltage divider R9R10. Capacitor C1 and resistor R1 form a filter that reduces the penetration into the power supply circuit of the signaling device of impulse noise that occurs during the operation of the generator. The UM1 amplifier is assembled on transistors VT3, VT4 and VT1, and UM2 is assembled on VT5 VT6 and VT2. The voltage gain of each of them is given by the resistance ratios of the resistors R13 to R1 1 and R14 to R12, respectively. Resistors R15, R17 are the load first stages of the corresponding amplifiers. Resistors R13, R14, R16 R18, R20-R23 stabilize the DC amplifier mode. Diodes VD1-VD4 set the bias voltage of transistors VT3-VT6. A piezoacoustic transducer BQ1 (PP) is connected to the UM1 output. Resistors R24 and R25 form the equivalent of such a converter (ECP) Sensors DT1 and DT2 are resistors R19 and R26 connected in series in the power amplifier circuits. A remote control is assembled on the DA3 OS. Resistors R27-R29, R33 set its gain. Resistors R30: R34 and capacitor C9 ensure the normal operation of the op-amp with a unipolar supply. Capacitor CU reduces the amplitude of the ultrasonic frequency voltage between the inputs of the control The integrating circuit AND is formed by a resistor R37 and a capacitor C13. PU1 and PU2 are assembled respectively on comparators DA4 and DA5. Resistive voltage dividers R31R35 and R32R36 set the thresholds for their operation. Capacitors C11 and C12 - filtering The fire indication unit consists of an electromagnetic sound emitter HA1 with a built-in generator, a filter capacitor C14 and a blinking LED HL2. The integral stabilizer DA2 and filter capacitors C3, C4 form a +15 V voltage source. The HL1 LED with resistor R8 is the unit for indicating the device is turned on. Details of the signaling device are mounted on a breadboard. They are interconnected by thin insulated wires. The sensitive element is a piece of copper wire with a diameter of 2 mm and a length of 1,5 m, soldered at one end to the working surface of the BQ1 piezoacoustic transducer. Instead of the K554SAZ comparator, you can use K554SAZB, K521SAZ, 521 SAZ or their imported analogue LM311 with different indices. OA K140UD6 can be replaced by 140UD6A, 140UD6B, 140UD601A 140UD601B KR140UD6 KR140UD608 and other general purpose OA. Import analogues of the integrated stabilizer KR142EN8V - 7815 with various prefixes and indices. KT503G transistors can be replaced by transistors of the same series or others with similar parameters. Transistors KT814G, KT815G can be replaced with the same ones with other letter indices or series KT816 and KT817, respectively. Diodes KD522B are replaced by other low-power pulsed silicon diodes, for example, from the KD503, KD521 series. The AL307VM LED can be any other, and L-816BID can be a blinking LED, for example, L-796BID The signaling device uses imported oxide capacitors, but domestic ones are also suitable, for example, K50-35. Ceramic capacitors - K10-17a, K10-176 and other similar ones. Fixed resistors - C2-33 with a possible replacement for C2-23, MLT, OMYAT Trimmer resistors - SP4-3, instead of them you can use SPZ-16a, SPZ-37, SPZ-ZEA and other similar ones. Electromagnetic sound emitter HCM1212X can be replaced by HCM1612X. The BQ1 piezoacoustic transducer is a foreign-made frameless transducer (probably type VSB35EW-0701 B), another one with a resonant frequency of 80 kHz can be used instead. Switch SA1 can be of any type, for example MT-1. The establishment of a properly assembled signaling device begins with setting the frequency of the generator G, equal to the frequency of the series resonance of the piezoacoustic transducer BQ7, with a tuning resistor R1. The amplitude of the output signal of this generator should be about 1 V, which, if necessary, is achieved by selecting resistor R9. By selecting resistors R13 and R14, the operating modes of the amplifiers (PA1 and PA2, respectively) are set for direct current such that the maximum signals at their outputs have the least distortion. Equal gains UM1 and UM2 at the operating frequency are achieved by selecting resistors R11 and R12. The signaling device is balanced with a tuning resistor R25 - the minimum possible constant voltage is achieved between the terminals of the capacitor SYU (inputs of remote control) with the sensing element evenly heated to room temperature. After balancing, the constant voltage at the output of the op-amp DA3 should be approximately 7,5 V - half the supply voltage of the DA3 chip. If now we heat small areas of the sensing element, for example, with a flame of an alcohol lamp or a candle, the output voltage of the op-amp should decrease or increase (depending on the place and degree of heating) by at least 1 V relative to the initial value. R4 and R5, while the HL31 LED should start flashing, and the emitter HA32 should emit an intermittent sound. You should make sure that when the sensitive element cools down, the signaling device returns to its original state, in which the HL2 LED and the sound emitter are turned off and the voltage at the output of the op-amp DA1 has taken its previous value. When installing a signaling device on an object, it is necessary to take measures to exclude the influence of the vibration of the object and the acoustic noise it creates on the sensitive element. To do this, it is mounted, for example, on vibration-isolating supports. An object of a large area or volume is controlled by bending the sensitive element around it. Literature
Author: O.Ilyin
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