Principle of piezoelectric ultrasonic generator
The piezoelectric ultrasonic generator actually uses the resonance of the piezoelectric crystal to work. The internal structure of the ultrasonic generator is shown in Figure 1. It has two piezoelectric wafers and a resonance plate. When a pulse signal is applied to its poles and its frequency is equal to the natural oscillation frequency of the piezoelectric wafer, the piezoelectric wafer will resonate and drive the resonance plate to vibrate, which generates ultrasonic waves. Conversely, if no voltage is applied between the two electrodes, when the resonance plate receives ultrasonic waves, it will press the piezoelectric wafer to vibrate and convert mechanical energy into electrical signals. At this time, it becomes an ultrasonic receiver.
Principle of Ultrasonic Ranging
The ultrasonic transmitter emits ultrasonic waves in a certain direction, and starts timing at the same time as the transmission time. The ultrasonic waves propagate in the air. When an obstacle is encountered on the way, it returns immediately. When the ultrasonic receiver receives the reflected wave, it immediately stops timing. The propagation speed of ultrasonic waves in the air is 340m / s. According to the time t recorded by the timer, the distance (s) from the launch point to the obstacle can be calculated, namely: s = 340t / 2. This is the so-called time difference ranging method.
The principle of ultrasonic distance measurement is to use the propagation speed of ultrasonic waves in the air to be known. Measure the time when the sound wave meets the obstacle and reflects back after transmission, and calculate the actual distance from the transmission point to the obstacle according to the time difference between transmission and reception. This shows that the principle of ultrasonic ranging is the same as that of radar.
The formula for ranging is expressed as: L = C & TImes; T
In the formula, L is the measured distance length; C is the propagation speed of the ultrasonic wave in the air; T is the time difference of the measured distance propagation (T is half the value of the time from transmission to reception).
Ultrasonic ranging is mainly used for distance measurement of reversing reminders, construction sites, industrial sites, etc. Although the current ranging range can reach 100 meters, the measurement accuracy can often only reach the order of centimeters.
Due to the advantages of easy directional emission of ultrasonic waves, good directivity, easy control of strength, and no direct contact with the measured object, it is an ideal method for liquid height measurement. In precision liquid level measurement, it is necessary to achieve measurement accuracy of millimeter level, but at present, domestic integrated circuits for ultrasonic distance measurement only have measurement accuracy of centimeter level. By analyzing the causes of ultrasonic ranging errors, increasing the measurement time difference to the microsecond level, and using the LM92 temperature sensor to compensate for the sound wave propagation speed, our designed high-precision ultrasonic rangefinder can achieve millimeter-level measurement accuracy.
Analysis of ultrasonic ranging error
According to the ultrasonic ranging formula L = C & TImes; T, it can be known that the ranging error is caused by the propagation speed error of the ultrasonic wave and the time error of the measured distance propagation.
Time error
When the ranging error is required to be less than 1mm, assuming that the known ultrasonic velocity C = 344m / s (20 ℃ room temperature), the propagation error of the sound velocity is ignored. The ranging error s △ t <(0.001 / 344) ≈0.000002907s or 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, the error of the ranging distance can be guaranteed to be less than 1mm. The 89C51 microcontroller timer using the 12MHz crystal as the clock reference can easily count to an accuracy of 1μs, so the system uses the 89C51 timer to ensure that the time error is within the measurement range of 1mm.
Ultrasonic speed error
The propagation speed of ultrasonic waves 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 speed and temperature is known as follows:
In the formula: r —the ratio of the constant pressure heat capacity of the gas to the constant volume heat capacity, which is 1.40 for air,
R — gas universal constant, 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 ℃
In the formula: The sound wave velocity of C0 is 332m / s at zero degrees;
T is the actual temperature (° C).
For the ultrasonic ranging accuracy requirement of 1mm, the ambient temperature for ultrasonic propagation must be taken into account. For example, when the temperature is 0 ℃, the ultrasonic velocity is 332m / s, and when it is 30 ℃, it is 350m / s. The ultrasonic velocity change caused by the temperature change is 18m / s. If an ultrasonic wave measures the distance of 100m at a sound speed of 0 ° C under a 30 ° C environment, the measurement error will reach 5m, and the error of 1m measurement will reach 5mm.
Precautions for use:
1. Because ultrasonic waves are greatly affected by environmental and climatic conditions, it is best to use it when the weather is sunny.
2. The 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 be careful not to have other objects in the space of the measured distance when using it, otherwise it will cause multiple reflections and affect Measurement accuracy.
3. Since the wave angle of the ultrasonic wave is relatively large, please be careful not to have objects (such as a tabletop) around the front end of the instrument when measuring. When measuring in a fixed position, the front of the instrument should protrude from the surface where the object is placed (such as protruding a little outside the table).
4. When measuring, please keep the instrument and the surface of the measured distance at a right angle, and keep the instrument as horizontal or vertical as possible.
5. When using an ultrasonic rangefinder in the summer, if it is a handheld 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 normal work.
4000 Puffs Disposable ecig have a completely enclosed design, reducing the need for charging and replacing cartridges. The no-charge design also reduces the occurrence of faults. It is understood that with rechargeable e-cigarettes, each cartridge needs to be charged at least once and the battery efficiency is extremely low, while the design of disposable ecig can solve this problem very well.
4000 Puff E-Cigarette,4000 Puff E-Cigarette For Sale,Best 4000 Puff E-Cigarette,Best 4000 Puff E-Cigarette For Sale
Shenzhen E-wisdom Network Technology Co., Ltd. , https://www.globale-wisdom.com