principle analysis of ultrasonic ranging sensor

Principle of piezoelectric ultrasonic generator

Piezoelectric ultrasonic generator actually uses the resonance of piezoelectric crystal to work. The internal structure of the ultrasonic generator is shown . It has two piezoelectric wafers and a resonance plate. When a pulse signal is applied to its two poles, the frequency of which is equal to the natural oscillation frequency of the piezoelectric wafer, the piezoelectric wafer will resonate and drive the resonance plate to vibrate to generate ultrasonic waves. On the contrary, if no voltage is applied between the two electrodes, when the resonance plate receives ultrasonic waves, it will press the piezoelectric chip to vibrate and convert the mechanical energy into electrical signals. Then it becomes an ultrasonic receiver.

 

Principle of ultrasonic transducer for distance

The ultrasonic transmitter emits ultrasonic waves in a certain direction, and starts timing at the same time as the transmitting time. The ultrasonic waves propagate in the air and return immediately when encountering obstacles on the way. The ultrasonic receiver stops timing immediately after receiving the reflected waves. The propagation speed of ultrasonic waves in the air is 340m/s. According to the time t recorded by the timer, the distance (s) between the launch point and the obstacle can be calculated, namely: s=340t/2. This is the so-called time difference ranging method.

 

The principle of ultrasonic ranging sensor is to use the known propagation speed of ultrasonic waves in the air to measure the time when the sound wave encounters obstacles and reflect back after transmission, and calculate the actual distance from the transmitting point to the obstacle based on the time difference between transmission and reception. It can be seen that the principle of ultrasonic ranging is the same as that of radar.

The formula of ranging sensor is expressed as: L=C&TImes; T where L is the measured distance length; C is the propagation speed of ultrasonic waves in the air; T is the time difference of the measured distance propagation (T is half of the value of the time from emission to reception).

Ultrasonic distance measurement sensor is mainly used for distance measurement in reversing reminders, construction sites, industrial sites, etc. Although the current distance measurement range can reach 100 meters, the measurement accuracy can only reach the order of centimeters.

Because of the advantages of easy directional emission, good directivity, easy control of intensity, and no direct contact with the measured object, it is an ideal method for liquid height measurement. It is necessary to achieve millimeter-level measurement accuracy in precise liquid level measurement, but the current domestic ultrasonic ranging special integrated circuits are only centimeter-level measurement accuracy. By analyzing the causes of the ultrasonic ranging error, improving the measurement time difference to the microsecond level, and using the LM92 temperature sensor to compensate the sound wave propagation velocity, the high-precision ultrasonic range finder we designed can achieve millimeter-level measurement accuracy.

Error Analysis of ultrasonic ranging sensor

According to the ultrasonic distance measurement formula L=C&TImes;T, it can be known that the distance measurement error is caused by the ultrasonic propagation velocity error and the measurement distance propagation time error.

time error

When the distance measurement error is required to be less than 1mm, assume that the known ultrasonic velocity C=344m/s (20℃ room temperature), and ignore the propagation error of the sound velocity. The ranging error s△t<(0.001/344) ≈0.000002907s is 2.907ms.

Under the premise that the propagation speed of the ultrasonic wave is accurate, as long as the accuracy of the propagation time difference of the measured distance reaches the microsecond level, it can ensure that the ranging error is less than 1mm. The 89C51 single-chip timer using a 12MHz crystal as the clock reference can easily count to an accuracy of 1μs, so the system adopts the 89C51 timer to ensure that the time error is within the measurement range of 1mm.

Ultrasonic propagation velocity error

The propagation speed of ultrasonic waves ultrasonic transducer sensor is affected by the density of air. The higher the density of air, the faster the propagation speed of ultrasonic waves, and the density of air has a close relationship with temperature, as shown in Table 1.

The relationship between ultrasonic velocity and temperature is known as follows:

In the formula: r — the ratio of the heat capacity of a gas at a constant pressure to a heat capacity at a constant volume, which is 1.40 for air,

R —The universal constant of gas, 8.314kg·mol-1·K-1,

M—gas molecular weight, air is 28.8&TImes;10-3kg·mol-1,

T—Absolute temperature, 273K+T℃.

The approximate formula is: C=C0+0.607&TImes;T℃

where: C0 is the sound wave velocity at zero degrees 332m/s;

T is the actual temperature (℃).

When the ultrasonic ranging accuracy is required to reach 1mm, the ambient temperature of ultrasonic propagation must be taken into consideration. For example, when the temperature is 0°C, the ultrasonic velocity is 332m/s, and at 30°C it is 350m/s, and the ultrasonic velocity change caused by the temperature change is 18m/s. If the ultrasonic is used to measure a distance of 100m at a sound speed of 0°C in an environment of 30°C, the measurement error will reach 5m, and the measurement error of 1m will reach 5mm.

Precautions for use:

1. Since ultrasound is greatly affected by environmental and climatic conditions, it is best to use it when the weather is clear.

2. Ultrasonic rangefinder calculates the distance based on the principle of the time when the instrument emits and receives the reflected wave of the measured object, so please pay attention to avoid other objects in the space of the measured distance when using it, otherwise it will cause multiple reflections. Measurement accuracy.

3. Since the wave angle of the ultrasonic wave is relatively large, please pay attention not to have objects (such as desktops, etc.) around the front end of the instrument during measurement. When PVDF housing ultrasonic transducer is measuring in a fixed position, the front end of the instrument should be protruding from the surface where the object is placed (for example, protruding from a point outside the desktop).

4. Please keep the instrument at right angles to the surface of the object to be measured when measuring, and keep the instrument itself horizontal or vertical as much as possible.

5. When it is using the ultrasonic rangefinder in summer, if it is hand-held measurement, it is best not to hold it in your hand for too long, so as not to cause the instrument to overheat and affect its normal operation.