Development of digital ultrasonic distance meter
Ultrasonic waves propagate at different speeds in gases, liquids and solids. They have good directivity, concentrated energy, small attenuation during transmission, and strong reflection capabilities. Ultrasonic waves can propagate directionally at a certain speed and form reflections when encountering obstacles. Using this characteristic, the actual distance can be calculated by measuring the time it takes for the ultrasonic waves to return, thus achieving non-contact measurement of object distance. Ultrasonic ranging is fast, convenient, and not affected by factors such as light. It is widely used in hydrological level measurement, construction site measurement, on-site location monitoring, vehicle reversing obstacle detection, mobile machine entry detection and positioning, and other fields. The digital ultrasonic distance meter designed in this article counts the clock pulses of a specific frequency input to the counter during the ultrasonic round-trip time, and then displays the corresponding measurement distance.
Ultrasonic distance meter circuit composition and working principle
The ultrasonic distance meter is composed of an ultrasonic generating circuit, an ultrasonic receiving amplifier circuit, and a counting and display circuit.
Ultrasonic generating circuit
The dual timer EN556 (U2b) forms a monostable trigger. R6 and C6 form a differential circuit, and its function is: when button S2 is pressed, the low level becomes a positive and negative peak pulse, and a negative peak pulse is obtained through VD1, triggering the flip of the monostable trigger. The high level of the monostable flip output lasts for about 1ms, that is, tw≈1.1R5C5≈1ms. EN556 (U2n) forms a multivibrator with an oscillation frequency f1=1/T1≈1/{0.7[(R1+R2)+2(R3+R4)]C3≈40kHz. The oscillator oscillation is controlled by the monostable flip-flop output level. When the monostable trigger outputs a high level, the multivibrator oscillates, and pin 5 of the EN556 outputs about 40 rectangular pulses with a frequency of 40kHz and a duty cycle of about 50%. Considering that the multivibrator is unstable at the start-up stage, it is designed to output a larger number of m pulses. If the number of output pulses is too few, the emission intensity will be small and the measurement distance will be short. Humidity sensor probe, , stainless steel electric heating tube PT100 sensor, , cast aluminum heater, heating coil fluid solenoid valve. However, the number of pulses is too many and the emission duration is long. When the distance to the measured object is close, the pulse train has not been completely transmitted, which causes The echo generated by the pulse transmitted first will reach the receiving end, affecting the ranging results and causing the ranging blind zone to increase. U1a~U1e of 74HC04 (U1) form an ultrasonic pulse drive circuit, which can increase the amplitude of the pulse voltage that drives the ultrasonic sending sensor, effectively perform electrical/acoustic conversion, enhance the ability to transmit ultrasonic waves, and increase the measurement distance. One path of the 40kHz pulse train is inverted through U1a, and then inverted through the inverter connected in parallel between U1b and U1e; the other path is inverted through the inverter connected in parallel between U1c and U1d.
