Ultrasonic sensor performance
Piezoelectric ultrasonic sensors use the principle of piezoelectric effect of piezoelectric materials to work. Commonly used sensitive component materials mainly include piezoelectric crystals and piezoelectric ceramics.
According to the difference between the positive and inverse piezoelectric effects, piezoelectric ultrasonic sensors are divided into generators (transmitting probes) and receivers (receiving probes). According to the structure and the wave type used, they can be divided into straight probes, surface wave probes, and Lamb wave probe, variable angle probe, dual crystal probe, focus probe, water immersion probe, water spray probe and special probe, etc.
The piezoelectric ultrasonic generator uses the principle of inverse piezoelectric utility to convert vibration into high-frequency vibration analysis, and then generates ultrasonic waves. When the frequency of the alternating working voltage is equivalent to the resonance frequency of the piezoelectric material, it will cause resonance, and the ultrasonic wave generated at this time is the strongest. The piezoelectric ultrasonic sensor can generate high-frequency ultrasonic waves of tens of kilohertz to tens of megahertz, and its sound intensity level can reach tens of watts per cubic centimeter.
The key structure of a typical piezoelectric ultrasonic sensor is composed of a piezoelectric wafer, an absorption block (damping block), and a protective film. Piezoelectric wafers are mostly disc-shaped, and the ultrasonic frequency is inversely proportional to its thickness. The two sides of the piezoelectric chip are plated with a silver layer as a conductive pole piece, the bottom side is grounded, and the top side is led to the transformer grounding wire. In order to better prevent the direct contact between the sensor and the DUT and damage the piezoelectric chip, a protective film is bonded under the piezoelectric chip. The effect of the absorption block is to reduce the quality of the mechanical equipment of the piezoelectric chip and absorb the kinetic energy of the ultrasonic wave.
The piezoelectric ultrasonic receiver works by using the principle of the positive piezoelectric effect. When ultrasonic waves act on the piezoelectric wafer to cause the wafer to expand and contract, charges of opposite polarity are generated on the two surfaces of the wafer. These charges are converted into voltages and sent to the measurement circuit diagram after amplification, and finally recorded or displayed. The structure of a piezoelectric ultrasonic receiver is basically the same as that of an ultrasonic generator, and sometimes the same sensor is used as both a generator and a receiver.
The main performance of the ultrasonic sensor includes:
(1) Operating frequency: The operating frequency is the resonance frequency of the piezoelectric wafer. When the frequency of the AC voltage applied to its two ends is equal to the resonance frequency of the chip, the output energy is the highest and the sensitivity is the highest.
(2) Working temperature: Because the Curie point of piezoelectric materials is generally relatively high, especially when the ultrasonic probe used for diagnosis uses low power, the working temperature is relatively low, and it can work for a long time without failure. The temperature of medical ultrasound probes is relatively high and requires separate refrigeration equipment.
(3) Sensitivity: mainly depends on the manufacturing wafer itself. The electromechanical coupling coefficient is large and the sensitivity is high; on the contrary, the sensitivity is low.
Working criteria of ultrasonic sensors:
Ultrasonic sensors can be divided into piezoelectric, magnetostrictive, electromagnetic, etc. according to their working principles. The piezoelectric type is the most commonly used.