According to the characteristics of the tested object, such as its volume, material and whether it is mobile or not, the ultrasonic sensor adopts different detection methods. There are four common detection methods: Penetration: the transmitter and receiver are located on both sides, and the detected object is detected according to the attenuation (or occlusion) of the ultrasonic wave when passing between them.
Limited distance: the transmitter and receiver are on the same side. When the detected object passes within the limited distance, it is detected according to the reflected ultrasonic wave.
Limited range: the transmitter and receiver are at the center of the limited range, the reflector is at the edge of the limited range, and the attenuation of reflected wave when no object is occluded is used as the reference value. When the detected object passes within the limited range, it is tested according to the attenuation of the reflected wave (the attenuation value is compared with the reference value).
Regressive reflection: the transmitter and receiver are located on the same side, with the detection object (flat object) as the reflecting surface, which is detected according to the attenuation of reflected wave.
Test quality Nothing is reflected in the direct test of the ultrasonic sensor with the table. An audio oscillator circuit can be built to test the performance of an ultrasonic sensor. When C1 is 390O holding F, an audio signal of about 1.9khz can be generated between the legs of the inverter. Attach the ultrasonic sensor (transmit and receive) to the foot. If the sensor is able to produce audio sound, it can be basically determined that the ultrasonic sensor is good.
Note: when C1=3900 times F, it is about 1.9khz; When C1=0.
Liquid level test The basic principle of ultrasonic liquid level measurement is: the ultrasonic pulse signal emitted by the ultrasonic probe is propagated in the gas, and is reflected when it encounters the interface between air and liquid. After receiving the echo signal, the transmission time of ultrasonic round-trip is calculated, and the distance or liquid level height can be converted. There are many non-comparable advantages of ultrasonic measurement: (1) it has no mechanical transmission components and does not touch the tested liquid. It is a non-contact measurement. It is not afraid of electromagnetic interference and strong corrosive liquid such as acid and alkali.
(2) its short response time can conveniently realize real-time measurement without delay.
The working frequency of the ultrasonic sensor adopted by the system is about 40kHz. The ultrasonic pulse emitted by the transmitting sensor is transmitted to the liquid surface and returned to the receiving sensor after being reflected. The time required for the ultrasonic pulse to be transmitted to the receiving sensor is measured. According to the sound velocity in the medium, the distance between the sensor and the liquid surface can be obtained, so as to determine the liquid surface. Considering the influence of ambient temperature on ultrasonic velocity, the velocity is corrected by temperature compensation to improve the measurement accuracy. The formula is: V = 331.5 + 0.607 T
In equation (1), V is the velocity of ultrasonic wave in air; T is the ambient temperature. S is V times t over 2 is V times t1 minus t0 over 2 is less than br over > In formula (2), S is the measured distance; T is the time difference between transmitting ultrasonic pulse and receiving its echo; T1 is the time when ultrasonic echo is received. T0 is the time of ultrasonic pulse emission. The capture function of MCU can be used to conveniently measure the t0 time and t1 time. According to the above formula, the measured distance S can be obtained by software programming. As the MCU of this system adopts the mixed signal processor with SOC characteristics and integrates the temperature sensor, the software can be used to conveniently compensate the temperature of the sensor.