【Sensor Principles】The working principle of pressure sensors and their applications

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This post was last edited by Wang Tianze on 2016-3-22 at 15:46. The working principle and applications of pressure sensors – Reposted. Pressure sensors are among the most commonly used sensors in industrial applications; they are widely employed in various industrial automation scenarios, covering sectors such as water resources and hydropower, railway transportation, smart buildings, production automation, aerospace, military industry, petrochemicals, oil wells, power generation, ships, machine tools, and pipelines. Below is a brief introduction to the principles and applications of some commonly used sensors. 1. Principle and Applications of Strain Gauge Pressure Sensors. There are a wide variety of mechanical sensors, such as resistive strain gauge pressure sensors, semiconductor strain gauge pressure sensors, piezoresistive pressure sensors, inductive pressure sensors, capacitive pressure sensors, resonant pressure sensors, and capacitive acceleration sensors. However, the most widely used is the piezoresistive pressure sensor, which features an extremely low price, high precision, and good linear characteristics. Next, we will mainly introduce this type of sensor. When learning about piezoresistive force sensors, we first need to get to know the component known as a resistive strain gauge. A resistive strain gauge is a sensitive device that converts strain changes in the component being measured into an electrical signal. It is one of the main components of piezoresistive strain sensors. The most commonly used strain gauges are metal strain gauges and semiconductor strain gauges. Metal resistive strain gauges are further divided into wire-type strain gauges and metal foil-type strain gauges. Typically, strain gauges are firmly bonded to the substrate that experiences mechanical strain using special adhesives. When stress changes occur in the substrate as a result of applied forces, the strain gauges also deform, which causes a change in their resistance value and thus a change in the voltage applied across them. The change in resistance generated by such strain gauges under stress is usually small; generally, these strain gauges are part of a strain bridge, and the signal is amplified by an instrumentation amplifier before being sent to the processing circuitry (usually an A/D converter and CPU) for display or to drive actuators. Internal structure of metal resistive strain gauges: A resistive strain gauge consists of a substrate material, metal strain wires or strain foils, insulating protective layers, and lead wires. Depending on the intended use, the resistance value of strain gauges can be designed by the manufacturer. However, attention must be paid to the range of possible resistance values: if the resistance is too low, the required drive current becomes very high; at the same time, the heat generated by the strain gauge causes its temperature to rise significantly. In different environments, the resistance value of the strain gauge changes greatly, resulting in noticeable drift in the output zero point, and the zero-setting circuit becomes overly complex. Moreover, the resistance is too high, the impedance is too high, and its ability to resist external electromagnetic interference is poor. They are generally in the range of several dozen ohms to several dozen kiloohms. Working principle of resistive strain gauges: The working principle of metal resistive strain gauges is based on the phenomenon in which the strain resistor attached to the substrate material experiences a change in resistance value as a result of mechanical deformation; this is commonly referred to as the resistive strain effect. The resistance value of a metal conductor can be expressed by the following formula: Where ρ is the resistivity of the metal conductor (Ω·cm2/m), S is the cross-sectional area of the conductor (cm2), and L is the length of the conductor (m). Taking the strain resistor made of a metal wire as an example, when the wire is subjected to an external force, both its length and cross-sectional area change. It is easy to see from the formula that the resistance value will also change; if the wire stretches due to an external force, its length increases while its cross-sectional area decreases, resulting in an increase in resistance value. When a wire is compressed under external force, its length decreases while its cross-sectional area increases, resulting in a decrease in its resistance value. By measuring the change in voltage applied across the resistor (usually by measuring the voltage at both ends of the resistor), it is possible to determine the strain in the strained wire. 2. Principle and Applications of Ceramic Pressure Sensors: Corrosion-resistant ceramic pressure sensors do not rely on any liquid medium for pressure transmission; instead, the pressure acts directly on the front surface of the ceramic diaphragm, causing it to deform slightly. Thick-film resistors are printed on the back side of the ceramic diaphragm, forming a Wheatstone bridge (a closed bridge). Due to the piezoresistive effect of these resistors, a voltage signal is generated in the bridge, which is highly linearly proportional to the pressure as well as to the excitation voltage. The standard signal values are set at 2.0 / 3.0 / 3.3 mV/V, depending on the pressure range, and such sensors can be used in conjunction with strain-based sensors. Through laser calibration, the sensor exhibits high temperature stability and time stability; it features built-in temperature compensation for the range of 0–70°C, and can come into direct contact with the vast majority of media. Ceramics are a well-recognized material with high elasticity, corrosion resistance, wear resistance, as well as resistance to impact and vibration. The thermal stability of ceramics, along with its thick-film resistance, enables it to operate within a temperature range of up to -40～135°C, while also offering high precision and stability in measurements. Electrical insulation level > 2kV, strong output signal, and good long-term stability. Ceramic sensors with high performance and low prices will be the future direction for pressure sensors; in Europe and the United States, there is a trend toward fully replacing other types of sensors with them, and in China as well, an increasing number of users are opting for ceramic sensors instead of diffused silicon pressure sensors. 3. Principle and Applications of Diffuse Silicon Pressure Sensors. Working Principle: The pressure of the medium being measured acts directly on the diaphragm of the sensor (made of stainless steel or ceramic), causing the diaphragm to undergo a slight displacement proportional to the pressure of the medium. This results in a change in the sensor’s resistance value; electronic circuits then detect this change and convert it into a standard measurement signal corresponding to that pressure. 4. Principle and Applications of Sapphire Pressure Sensors: Utilizing the strain-resistive working principle, these sensors employ silicon-sapphire as the semiconductor sensing element, offering unparalleled measurement capabilities. Sapphires are composed of single-crystal insulating elements, and they do not exhibit phenomena such as hysteresis, fatigue, or creep ； Sapphire is more durable than silicon, has a higher hardness, and is not prone to deformation ； Sapphire possesses excellent elasticity and insulating properties (up to 1000 OC). Therefore, semiconductor sensors made using silicon-sapphire are not sensitive to temperature changes, and they maintain good performance even under high-temperature conditions ； Sapphires have extremely strong radiation resistance ； Furthermore, silicon-sapphire semiconductor sensors have no p-n drift, which fundamentally simplifies the manufacturing process, improves repeatability, and ensures a high yield. Pressure sensors and transmitters manufactured using silicon-sapphire semiconductor sensing elements can operate properly under the harshest working conditions, offering high reliability, good accuracy, minimal temperature error, and excellent cost-performance. The gauge pressure sensor and transmitter consist of two diaphragms: a titanium alloy measurement diaphragm and a titanium alloy receiving diaphragm. A sapphire thin sheet printed with a heteroepitaxial strain-sensitive bridge circuit is welded to a titanium alloy sensing diaphragm. The pressure to be measured is transmitted to the receiving diaphragm (which is firmly connected to the measuring diaphragm by tie rods). Under pressure, the diaphragm of the titanium alloy changes shape; when this deformation is detected by the silicon-sapphire sensor element, the output of the bridge circuit changes, with the magnitude of this change being proportional to the pressure being measured. The sensor’s circuit ensures power supply for the strain bridge circuit, and converts the imbalance signal from the strain bridge into a standardized electrical signal output (0-5, 4-20mA, or 0-5V). In absolute pressure sensors and transmitters, sapphire thin sheets, bonded to ceramic base glass solder, act as elastic elements that convert the pressure to be measured into strain in the strain gauges, thereby enabling pressure measurement. 5. Principles and Applications of Piezoelectric Pressure Sensors. The main piezoelectric materials used in piezoelectric sensors include quartz, potassium sodium tartrate, and ammonium dihydrogen phosphate. Quartz (silicon dioxide) is a natural crystal, and the piezoelectric effect was discovered in this crystal. The piezoelectric properties persist within a certain temperature range, but they disappear completely once the temperature exceeds that range (this high temperature is known as the \"Curie point\"). Since the electric field changes only slightly with changes in stress (in other words, the piezoelectric coefficient is relatively low), quartz has gradually been replaced by other piezoelectric crystals. Sodium potassium tartrate has high piezoelectric sensitivity and piezoelectric coefficients, but it can only be used in environments with room temperature and low humidity. Dihydrogen ammonium phosphate is a synthetic crystal that can withstand high temperatures and relatively high humidity, which has led to its widespread use. Now the piezoelectric effect is also applied to polycrystals, such as modern piezoelectric ceramics including barium titanate piezoelectric ceramics, PZT, niobate-based piezoelectric ceramics, lead niobate magnesium titanate piezoelectric ceramics, and so on. The piezoelectric effect is the main working principle of piezoelectric sensors. Piezoelectric sensors cannot be used for static measurements, as the charge generated by an external force can only be retained when the circuit has an infinitely high input impedance. The actual situation is not like this; therefore, this means that piezoelectric sensors can only measure dynamic stress. Piezoelectric sensors are mainly used in the measurement of acceleration, pressure, force, etc. A piezoelectric acceleration sensor is a commonly used accelerometer. It boasts excellent features such as a simple structure, small size, light weight, and long service life. Piezoelectric acceleration sensors have been widely used in the measurement of vibration and shock in aircraft, automobiles, ships, bridges, and buildings, and they hold a particularly important role, especially in the fields of aviation and aerospace. Piezoelectric sensors can also be used to measure the combustion pressure inside the engine as well as the vacuum level. It can also be used in **industry, for example to measure changes in chamber pressure at the moment a gun cartridge is fired, as well as the shock wave pressure at the muzzle. It can be used to measure both high pressures and very low pressures.