The ultrasonic level meter travels in the air at a speed of 340 m/s. Therefore, if the propagation time of the ultrasonic wave in the air can be measured, the distance traveled can be calculated. Ultrasonic measurement of the liquid level is to measure the distance of sound wave transmission by measuring the time gap of ultrasonic wave propagation. Specific method: an ultrasonic wave is attached to the liquid surface by an ultrasonic probe installed on the top of the container to be tested, and the sound wave is reflected by the liquid surface and then received by the probe. The controller measures the propagation time t, and according to the sound velocity v, the spatial distance L from the liquid surface to the probe can be obtained. Under the premise that the probe has reached the bottom of the container, that is, the installation height H, the liquid level height h in the container can be obtained. The calculation formula is:
H=H-L where L=v•t/2
In addition, since the air temperature has a certain influence on the sound speed, the ambient air temperature T should also be measured to correct the sound speed. The formula is:
The ultrasonic level meter consists of an ultrasonic probe, a transmission control and reception processing circuit, a controller and a display. The transceiver transceiver and transducer probe are composed of a piezoelectric transducer, a transceiver and a temperature compensation circuit; the transceiver includes a transmission control and reception processing circuit, mainly by ultrasonic emission pulse width control, echo signal detection, comparison, differentiation, and shaping The signal processing circuit is composed of 74LS164 and LED, which completes the display of system parameters and liquid level. The controller is an application system composed of 80C552 single-chip microcomputer, which mainly completes the scanning processing of the keys, the emission control of the ultrasonic waves, and the receiving. Calculation of echo time, and conversion of liquid level. This paper focuses on the hardware and software measures taken to improve the measurement accuracy and reduce the measurement of blind spots on ultrasonic echo detection.
When the ultrasonic wave is emitted, the transmitted wave or the received echo can be input from the In terminal through the receiving loop, and a sharp pulse is formed at the R end of the D flip-flop by detecting, amplifying, comparing, and differentiating. The pulse signal is shaped by a D flip-flop to obtain a narrow pulse of the next jump, which is sent to the INT terminal of the 80C552. Through the interrupt system of 20C552, each interrupt time is recorded. According to the interval between two adjacent interrupts, the time from the transmission to the reception of the echo, that is, the time when the ultrasonic wave travels back and forth, is known, so that the distance of propagation can be calculated. .
When the ultrasonic wave is emitted and there is a strong residual vibration, the echo cannot be detected at the same time. Therefore, it is impossible to detect normally within a certain distance from the emitting surface of the probe to form a detection dead zone. Generally, the range of blind spots of ultrasonic instruments at home and abroad is greater than 0.30m. Through the specific analysis of the measurement accuracy and the cause of blind zone formation, it is concluded that the size of the blind zone is mainly related to the strength of the transmitted wave and the method of echo detection. Under normal circumstances, when the distance is the same, the transmitted wave is strong, and the received echo signal is strong, but the residual vibration is also strong, and the blind zone is large. On the contrary, it is small. When the transmitted wave intensity is the same, the closer the detection distance is, the stronger the received echo signal is, and the larger the blind zone is. Secondly, when the ultrasonic probe emits a beam of ultrasonic waves to the liquid surface, the wave is reflected multiple times between the liquid surface and the probe, so that the receiving circuit generates a plurality of echoes, and a series of interrupt signals are formed at 80C552. The emission pulse width has a great influence on the measurement accuracy. For this reason, the instrument takes the following measures in design: 1 Automatically control the strength of the transmitted wave to reduce the detection dead zone. The specific method is to test the emission and the distance, so that the weak intensity of the transmitted wave changes with the change of the detection distance. 2 Implement some special processing methods in the software design. In the time when the echo is recorded, the county records four echo times continuously for each measurement. When calculating the echo time, the last two time difference is used as the propagation time of the ultrasonic wave. . This treatment avoids the effect of the transmitted pulse width on the measurement, which makes the measurement more accurate and also reduces the dead zone to a minimum of 0.05 m.
Through the above processing, the maximum measurement error is 0.006m for the required measurement range of 0.20m~8m, and the repeatability is good. Because the ultrasonic liquid level meter designed in this paper has the simple hardware structure, reliable operation and the above characteristics, therefore, it It can be used not only in liquid level measurement in chemical, electric power, petroleum and other industries, but also in other ranging systems.