Piezoelectric pressure sensors operate on the principle of the piezoelectric effect. The piezoelectric effect occurs when certain dielectric materials deform under a force in a specific direction, resulting in internal polarization and the appearance of opposite charges on their two opposing surfaces. When the force is removed, the material returns to its uncharged state; this phenomenon is called the direct piezoelectric effect. When the direction of the force changes, the polarity of the charges also changes.
Conversely, when an electric field is applied along the polarization direction of the dielectric material, it deforms; when the electric field is removed, the deformation disappears; this phenomenon is called the inverse piezoelectric effect. Piezoelectric pressure sensors come in many types and models, and can be classified into diaphragm and piston types based on the form of the elastic sensing element and the force-bearing mechanism. Diaphragm type sensors mainly consist of a body, a diaphragm, and a piezoelectric element. The piezoelectric element is supported on the body, and the diaphragm transmits the measured pressure to the piezoelectric element, which then outputs an electrical signal that is proportional to the measured pressure. This type of sensor is characterized by its small size, good dynamic characteristics, and high-temperature resistance. Modern measurement technology places increasingly higher demands on sensor performance.
For example, when using pressure sensors to measure and plot the indicator diagram of an internal combustion engine, water cooling is not allowed during the measurement, and the sensor must be able to withstand high temperatures and be small in size. Piezoelectric materials are best suited for developing such pressure sensors. Quartz is an excellent piezoelectric material, and the piezoelectric effect was discovered in it. A relatively effective method is to select a quartz crystal cutting method suitable for high-temperature conditions; for example, XYδ (+20°～+30°) cut quartz crystals can withstand temperatures up to 350°C. LiNbO3 single crystals have a Curie point as high as 1210°C, making them an ideal piezoelectric material for manufacturing high-temperature sensors.
Diffused silicon type: The pressure of the measured medium acts directly on the diaphragm of the sensor (stainless steel or ceramic), causing a micro-displacement of the diaphragm proportional to the medium pressure. This causes a change in the sensor's resistance value, which is detected by electronic circuitry and converted into a standard measurement signal corresponding to that pressure.
Sapphire type: Utilizing the strain gauge principle, it employs silicon-sapphire as the semiconductor sensing element, possessing unparalleled metrological characteristics.
Sapphire is composed of a single-crystal insulating element, exhibiting no hysteresis, fatigue, or creep. Sapphire is stronger and harder than silicon, and is resistant to deformation. Sapphire has excellent elasticity and insulation properties (up to 1000°C). Therefore, semiconductor sensing elements made using silicon-sapphire are insensitive to temperature changes and maintain excellent operating characteristics even at high temperatures. Sapphire has strong radiation resistance. Furthermore, silicon-sapphire semiconductor sensing elements have no p-n drift, fundamentally simplifying the manufacturing process, improving repeatability, and ensuring high yield.
Pressure sensors and transmitters made with silicon-sapphire semiconductor sensing elements can operate normally under the harshest conditions, exhibiting high reliability, high accuracy, minimal temperature error, and high cost-effectiveness.
Gastric pressure sensors and transmitters consist of a dual-diaphragm design: A titanium alloy measuring diaphragm and a titanium alloy receiving diaphragm. A sapphire wafer printed with a heterogeneous epitaxial strain gauge bridge circuit is soldered onto a titanium alloy measuring diaphragm. The measured pressure is transmitted to a receiving diaphragm (the receiving diaphragm and measuring diaphragm are securely connected by a tie rod). Under pressure, the titanium alloy receiving diaphragm deforms. This deformation is sensed by the silicon-sapphire sensing element, causing a change in the bridge output, the magnitude of which is proportional to the measured pressure.
The sensor circuitry ensures power supply to the strain gauge bridge circuit and converts any imbalance signal from the strain gauge bridge into a uniform electrical signal output (0-5, 4-20mA, or 0-5V). In absolute pressure sensors and transmitters, the sapphire wafer, connected to a ceramic-based glass solder, acts as an elastic element, converting the measured pressure into strain gauge deformation, thereby achieving pressure measurement.