“An ultrasonic probe is a sensor that generates ultrasonic waves and receives their echoes. Ultrasonic probes can convert electrical energy into sound energy, and also can convert sound energy into electrical energy. The ultrasonic probe is a key component of the ultrasonic level meter. Its performance directly affects the intensity of the transmitted ultrasonic wave, the intensity of the received echo signal, and the level measurement range. It can be said that the quality of the ultrasonic probe directly determines the ultrasonic level meter. Can work normally and stably.
An ultrasonic probe is a sensor that generates ultrasonic waves and receives their echoes. Ultrasonic probes can convert electrical energy into sound energy, and also can convert sound energy into electrical energy. The ultrasonic probe is a key component of the ultrasonic level meter. Its performance directly affects the intensity of the transmitted ultrasonic wave, the intensity of the received echo signal, and the level measurement range. It can be said that the quality of the ultrasonic probe directly determines the ultrasonic level meter. Can work normally and stably. Ultrasound is generated by high-frequency electrical pulses that excite the ultrasonic probe, and the reflected ultrasonic energy is converted into a voltage signal by the probe. The piezoelectric ceramic in the piezoelectric probe has the piezoelectric effect, and the most common method to realize the electroacoustic energy is to use the piezoelectric probe. In order to make the user of the instrument have a more in-depth understanding of the ultrasonic level meter, this paper intends to discuss the structure and main performance indicators of the ultrasonic level measurement probe as follows.
1. Piezoelectric probe structure
The structure of the piezoelectric probe is shown in Figure 1: it consists of piezoelectric ceramics, matching layers, damping blocks, protective films, shells, and high-frequency cables.
Figure 1 Structure diagram of piezoelectric probe
1. Damping block: It is configured by some damping materials and adheres to the back of the piezoelectric ceramic, mainly to absorb the ultrasonic waves on the back of the piezoelectric ceramic to reduce noise.
2. High-frequency cable: The ultrasonic probe needs to be connected to the ultrasonic drive circuit board through a high-frequency cable. This special cable can shield the interference of various external interference noises to the drive pulse and echo signal of the ultrasonic probe.
3. Protective film: its function is to protect the piezoelectric ceramics and electrodes to prevent wear and damage. The protective film must have good wear resistance, high strength, low sound attenuation, good sound transmission performance and appropriate thickness.
4. Piezoelectric ceramics: Piezoelectric ceramics are the core components of the probe, and its performance is directly related to the quality of the probe. Its role is to transmit and receive ultrasonic waves. The two sides of the ceramic are coated with silver layers as electrodes, the purpose is to make the voltage supplied to the ceramic sheet uniform.
5. Shell: The outer shell of the probe is made of plastic, which mainly has the function of fixing and protecting the entire probe element.
6. Matching layer: The matching layer of the ultrasonic probe can match the acoustic impedance between the probe and the working load, which can effectively broaden the working frequency band of the probe and further improve the ultrasonic resolution and work adaptability.
Second, the main performance indicators of ultrasonic probe
The main performance indicators of ultrasonic probes include: operating frequency, sensitivity, quality factor, frequency response, electrical impedance, directional characteristics, etc. Now the performance indicators are introduced as follows:
1. Working frequency f: The working frequency of the ultrasonic probe is determined by the resonance frequency of the piezoelectric ceramic, that is, the frequency of the ultrasonic wave emitted by the probe. Ultrasonic probes work at this frequency to output the maximum energy and travel the farthest distance.
2. Quality factor Q: The quality factor Q of the ultrasonic probe is comprehensively determined by the quality factor of the circuit part and the quality factor of the mechanical part.
3. Directional characteristics: The sharpness of the main lobe determines the scope of the ultrasonic exploration area, so the directional characteristics directly determine the working range and range of the ultrasonic level gauge. The main lobe angle is the most important indicator of the ultrasonic directional characteristics. The higher the ultrasonic frequency, the smaller the main lobe angle, the more concentrated the sound energy of the ultrasonic probe, and the narrower the sound speed range. as shown in picture 2:
Figure 2 Sum beamwidth of acute angles
4. Frequency response: The frequency response of the ultrasonic probe refers to the frequency characteristics of the echo signal received by a reflective object and the probe. The frequency spectrum of the ultrasonic probe can be measured by a frequency analyzer to obtain parameters such as center frequency and bandwidth.
5. Sensitivity: Sensitivity refers to the ratio of the peak-to-peak value of the echo signal voltage output after the ultrasonic echo is converted by the probe to the peak-to-peak value of the driving pulse voltage applied to the probe. Sensitivity is a measure of the mutual conversion efficiency of ultrasonic probe electric energy and sound energy.
6. Impedance characteristics: Due to the characteristics of the ultrasonic probe, it is required to achieve impedance matching with the driving voltage, which has achieved the most ideal driving energy and echo receiving signals.
In practical applications, the maximum test range, minimum test range (blind zone) and emission angle (directivity open angle) of the probe are different, so it needs to be considered comprehensively, and the structure, frequency, and radiation surface of the probe need to be comprehensively considered. After weighing it with the external size, make a choice to meet the actual needs as much as possible.
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