The basis of the transducer is a bimorph piezoceramic disk attached along the outer circumference to an elastic support. A cone shaped aluminum foil resonator is glued to the center of the disc to improve power conversion efficiency. The "grounded" metal housing protects the converter from electrical noise.

On fig. 2 shows the dependences on the frequency of the module of the input resistance |Zp| and sensitivity uxx of the transducer with load resistance Rн->oo. The input impedance module has two extreme values: at the frequency fmi - the minimum of the input impedance, at the frequency fa (antiresonant frequency) - the maximum of the input impedance. The sensitivity has one maximum at a frequency close to the antiresonance frequency fa.

Ris.2

The interval between the frequencies fmi and fv is on average about 2 kHz. When the load resistance Rn decreases, the frequency at which the sensitivity of the converter is maximum decreases, tending to fmi at Rn -> 0 in the limit.

Due to the non-linearity of the properties of piezoceramics, the resonant frequency fmi somewhat decreases with increasing input voltage. At the same time, the input resistance also increases. Figure 3 shows the amplitude characteristics of the converter in terms of the resonant frequency fmi and the resonant value of the input resistance modulus |Zp|

Ris.3

If the voltage supplied to the converter exceeds 5 V at the resonant frequency, then irreversible changes may occur in it.

The directivity pattern of the transducer is single-lobe, with a width of about 30 ° (at a level of 0,7 from the maximum)

When one of the transducers is an emitter and the other is a receiver, this pair should be selected in such a way that fmi emitter and fmp receiver are close to each other. The individual values ​​of these frequencies are indicated in the passports of the converters.

With a sufficiently careful selection of a pair of emitter-receiver and a high load resistance of the receiver (for example, 100 kOhm and higher), it is possible to significantly increase the sensitivity of the receiving path by eliminating the shunting effect of the capacitance of the Cin converter. To do this, coils are connected in parallel to the converter, the inductance LK of which is calculated by the formula

where Co=0,8 St. Capacitance Cm is measured at a low frequency, eg 1000 Hz. Usually it is 1140 ± 40 pF The value of the inductance LK, depending on the specific values ​​of fa and Co, lies in the range of 15-20 mH.

In an echo sounder, when the same transducer operates alternately in the transmission and reception modes, it is advisable to use a direct-amplifier path with a low-impedance input (no more than 1 kOhm). In this case, the resonant frequencies fmn and fmp will be close to each other.

Schematic diagrams of the receiver and transmitter of the device for remote control of household equipment (receiver, tape recorder, TV, lighting devices) using the MUP-1 converter are shown in fig. 4 and fig. 5 The transmitter can be made on a digital integrated circuit (Fig. 4, a) or on transistors (Fig. 4, b).

Ris.4

In the first version of the transmitter, the master oscillator is the elements D1.1 and D1.2, and the buffer stage is made on the element D1.3. In the transistor version of the transmitter, the generator is assembled according to the multivibrator circuit (transistors V1, V2), and the output stage is based on the transistor V3. The required operating frequency of the transmitter is set with a tuning resistor R1 (Fig. 4, a), or by selecting resistors R2 and R3 (Fig. 4, b) In order to be able to adjust the generator during operation, it is advisable to replace one of these resistors with two connected in series with a constant with a resistance of 3 5 kOhm and a variable resistance of 22 kOhm The transmitter setting consists in setting the generator frequency equal to the frequency fmi, which is indicated in the converter passport

In the absence of a frequency meter, the setting can be made according to the maximum output of the converter. To do this, in front of the transducer-emitter at a distance of 15 ... 20 cm, a transducer-receiver is installed, which will be used in the remote control system, an AC voltmeter is connected to it and the transmitter is adjusted, achieving the maximum signal at the output of the receiving transducer

The receiver circuit is shown in Figure 5. It consists of an amplifier on the A1 chip, a diode detector V1, V2, a single vibrator and a trigger on the D1 chip, as well as a transistor switch (V5) with an executive relay K1 in the load circuit. Elements R7, C5 are used to delay the inclusion of a single vibrator in order to prevent false triggering of the actuator due to impulse noise in the network or fuzzy closure of the contacts of the transmitter power button.

Fig.5 (click to enlarge)

The ultrasonic signal received by the transducer B1 of the receiving device is converted into electrical vibrations, which are amplified by the microcircuit A1. The gain at a frequency of 40 kHz is approximately 5000. The detected signal, having passed the delay circuit C5R7, starts the one-shot (element D1.1). The single vibrator response delay time is 1 s. The signal from the single vibrator switches the trigger D1.2, as a result of which the transistor key V5 opens and the relay K1 is activated.

Schematic diagram of the actuator is shown in fig. 6. When the contacts K1.1 are closed in the receiver (Fig. 5), the trinistors V2, V3 open and connect the Load Rн to the network. The permissible load power is determined by the direct current of the trinistors.

Ris.6

The transmitters of the remote control system are powered by a battery of galvanic cells, and the receiver is powered by a stabilized rectifier that gives an output voltage of 20 V. The power supply of the D1 chip is additionally stabilized by a parametric stabilizer assembled on the elements V3 and R6.

In laboratory conditions, when testing the remote control unit, the mutual orientation of the transducers was not required. However, in a room with high sound absorption (a lot of upholstered furniture, carpets, curtains, etc.), the ultrasonic signal is noticeably attenuated, which may require the orientation of the emitter and receiver. The ultrasonic transducer can also be used in multichannel remote control systems. In this case, the commands should be transmitted in discrete form using pulse code, time or pulse phase modulation systems.

Authors: N. Borodulin, V. Morozov, E. Koptev, Moscow; Publication: N. Bolshakov, rf.atnn.ru

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