In the human social environment, pressure is everywhere, so the pressure sensor naturally becomes the most commonly used sensor in industrial practice. It is widely used in various industrial self-control environments, involving water conservancy and hydropower, railway transportation, intelligent building, production automation, Aerospace, military, petrochemical, oil wells, power, ships, machine tools, pipelines and many other industries.
Pressure Sensor
A pressure sensor is a sensor that converts pressure into an electrical signal output.
While describing the pressure sensor, we must derive the concept of a pressure transmitter.
Usually the sensor consists of two parts, namely the sensitive element and the conversion element. Wherein the sensitive component refers to a portion of the sensor that can directly sense or respond to the measured component; the conversion component refers to a portion of the sensor that converts the measured strain experienced or responded by the sensitive component into an electrical signal suitable for transmission or measurement.
Since the output signal of the sensor is generally weak, it needs to be modulated and amplified. With the development of integration technology, people also installed this part of the circuit and power supply circuit inside the sensor. In this way, the sensor can output an available signal that is easy to process and transmit. While the prior art is relatively backward, the so-called sensor refers to the sensitive component above, and the transmitter is the conversion component above.
A pressure sensor generally refers to a sensitive component that converts a varying pressure signal into a correspondingly varying resistance or capacitance signal, such as a piezoresistive element, a pressure-capacitive element, and the like. The pressure transmitter generally refers to a complete set of circuit units for measuring pressure composed of a pressure sensitive element and a conditioning circuit, and generally can directly output a standard voltage signal or a current signal linearly related to pressure for instruments, PLCs, acquisition cards, etc. The device collects directly.
Classification of pressure sensors
There are many types of pressure sensors, such as resistance strain gauge pressure sensors, semiconductor strain gauge pressure sensors, piezoresistive pressure sensors, inductive pressure sensors, capacitive pressure sensors, and resonant pressure sensors.
At present, the widely used pressure sensors include: diffusion silicon piezoresistive pressure sensor, ceramic piezoresistive pressure sensor, sputtered film pressure sensor, capacitive pressure sensor, and sapphire pressure sensor with high temperature resistance. But the most widely used is a piezoresistive pressure sensor, which has a very low price, high precision and good linearity.
Principles of various pressure sensors Piezoresistive pressure sensors
The sensor adopts an integrated process to integrate the resistor strip on the single crystal silicon film to form a silicon piezoresistive chip, and the periphery of the chip is fixedly packaged in the outer casing to lead the electrode lead. When it comes to piezoresistive sensors, the first thing to talk about is its piezoresistive effect.
The piezoresistive effect is used to describe the change in electrical resistance of a material under mechanical stress. Unlike the piezoelectric effect, the piezoresistive effect only produces impedance changes and does not generate charge.
As shown above, when the pressure changes, the resistors R1, R2, R3, and R4 change, causing a change in the voltage applied across the resistor, which reflects the pressure value.
The piezoresistive sensor is also called a diffusion silicon piezoresistive pressure sensor. The pressure of the measured medium directly acts on the diaphragm of the sensor (stainless steel or ceramic), so that the diaphragm generates a micro-displacement proportional to the pressure of the medium, so that the sensor The resistance value changes, and the change is detected by an electronic circuit, and a standard measurement signal corresponding to this pressure is converted and output.
Ceramic pressure sensor
The corrosion-resistant ceramic pressure sensor has no liquid transfer, the pressure acts directly on the front surface of the ceramic diaphragm, causing a slight deformation of the diaphragm. The thick film resistor is printed on the back of the ceramic diaphragm and connected into a Wheatstone bridge (closed) Bridge), due to the piezoresistive effect of the varistor, the bridge produces a highly linear voltage signal proportional to the pressure, which is proportional to the excitation voltage. The standard signal is calibrated to 2.0 / 3.0 / 3.3 depending on the pressure range. mV/V, etc., compatible with strain gauge sensors. With laser calibration, the sensor has high temperature stability and time stability. The sensor comes with temperature compensation of 0 to 70 ° C and can be in direct contact with most media.
Ceramic is a recognized material that is highly elastic, resistant to corrosion, abrasion, shock and vibration. The thermal stability of ceramics and its thick film resistance allow it to operate over a temperature range of -40 to 135 °C with high precision and high stability. The electrical insulation degree is 2kV, the output signal is strong, and the long-term stability is good.
Piezoelectric pressure sensor
The piezoelectric pressure sensor principle is based on the piezoelectric effect. The piezoelectric effect is that when some dielectrics are deformed by an external force in a certain direction, polarization occurs inside, and positive and negative opposite charges appear on the two opposite surfaces thereof. When the external force is removed, it will return to the uncharged state. This phenomenon is called the positive piezoelectric effect. When the direction of the force changes, the polarity of the charge also changes. On the contrary, when an electric field is applied in the polarization direction of the dielectric, these dielectrics are also deformed, and the deformation of the dielectric disappears after the electric field is removed. This phenomenon is called an inverse piezoelectric effect.
Piezoelectric pressure sensors are available in a wide variety of types and models, and can be classified into diaphragm type and piston type in the form of elastic sensing elements and force receiving mechanisms. The diaphragm type is mainly composed of a body, a diaphragm and a piezoelectric element. The piezoelectric element is supported on the body, and the pressure is transmitted from the diaphragm to the piezoelectric element, and the piezoelectric element outputs an electrical signal in a certain relationship with the measured pressure. This type of sensor is characterized by small size, good dynamic characteristics, and high temperature resistance. Modern measurement technology is increasingly demanding the performance of sensors.
For example, using a pressure sensor to measure the dynamometer of an internal combustion engine, water cooling is not allowed in the measurement, and the sensor is required to withstand high temperatures and small volume. Piezoelectric materials are best suited for the development of such pressure sensors. Quartz is a very good piezoelectric material, and the piezoelectric effect is found on it. A more effective method is to select a quartz crystal cutting method suitable for high temperature conditions. For example, an XYδ (+20° to +30°) cut quartz crystal can withstand a high temperature of 350 °C. The LiNbO3 single crystal has a Curie point of up to 1210 ° C, making it an ideal piezoelectric material for high temperature sensors.
Resistance strain gauge pressure sensor
When the strain gauge is attached to a specific surface of the elastic member, when physical quantities such as force, torque, speed, acceleration, and flow rate act on the elastic member, the stress and strain of the member are changed, thereby causing a change in resistance of the strain gauge. The change in resistance is output as an electrical signal after the circuit is processed. This is the working principle of the resistance strain gauge sensor.
The most widely used resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. The metal resistance strain gauge has two kinds of filament strain gauges and metal foil strain gauges. Usually, the strain gauge is tightly bonded to the mechanical strain matrix by a special adhesive. When the stress changes due to the force of the substrate, the strain gauges are also deformed together, so that the resistance of the strain gauge is changed, thereby The voltage applied to the resistor changes. The strain gauges typically have a small change in resistance when stressed. Typically, such strain gauges form a strain bridge and are amplified by a subsequent instrumentation amplifier and transmitted to the processing circuitry (usually A/D conversion). And CPU) display or actuator.
The semiconductor strain gauge is a sensitive component made by utilizing the piezoresistive effect of the semiconductor single crystal silicon. The semiconductor strain gauge needs to be attached to the test piece to measure the strain of the test piece or adhere to the elastic sensitive element to indirectly sense the external force to be measured. The elastic components of different configurations can be used to measure the mechanical quantities such as stress, strain, pressure, torque and acceleration of various objects. Compared with the resistance strain gauge, the semiconductor strain gauge has the advantages of high sensitivity coefficient (about 50 to 100 times higher), small mechanical hysteresis, small volume, and low power consumption.
Inductive pressure sensor
Inductive pressure sensors are meters that measure the pressure by varying the inductance of the inductor.
Commonly available are air gap type and differential transformer type. The working principle of the air gap type is that the measured pressure acts on the diaphragm to cause displacement, causing the magnetic resistance of the differential inductance coil to change. At this time, the diaphragm increases from the air gap of the core, and the other side decreases. The amount is reduced while the other side is increased, thereby forming an inductance differential change, and the bridge composed of the inductor outputs an AC voltage corresponding to the measured pressure. It has the advantages of small volume, simple structure and the like, and is suitable for use in an environment with vibration or impact.
The working principle of the differential transformer type is that the side pressure acts on the spring tube _L to generate a displacement proportional to the pressure, and at the same time drives the iron core connected at the end of the spring tube to move, so that the two symmetrical and opposite of the differential transformer The series-connected secondary winding is out of balance and outputs a voltage proportional to the measured pressure. It can also output a standard current signal in combination with the electric unit combination meter to form an automatic control system.
Capacitive pressure sensor
A capacitive pressure sensor is a pressure sensor that utilizes a capacitive sensing element to convert a measured pressure into a quantity of electrical output that is related to it. It is characterized by low input force and dwarf energy, high dynamic response, small natural effects, and good environmental adaptability.
It generally uses a circular metal film or a metallized film as an electrode of a capacitor. When the film is deformed by pressure, the capacitance formed between the film and the fixed electrode changes, and the output can be related to the voltage through the measuring circuit. electric signal. Capacitive pressure sensors are pole-to-change capacitive sensors that can be divided into single-capacitor pressure sensors and differential capacitive pressure sensors.
Single capacitive pressure sensor
It consists of a circular film and a fixed electrode. The film is deformed under pressure to change the capacity of the capacitor, and its sensitivity is roughly proportional to the area and pressure of the film and inversely proportional to the tension of the film and the distance from the film to the fixed electrode. Another type of fixed electrode adopts a concave spherical shape, and the diaphragm is a peripherally fixed tensioning plane, and the diaphragm can be made by a plastic metal plating layer. This type is suitable for measuring low pressure and has a high overload capacity. It is also possible to use a single-capacitance pressure sensor with a piston moving pole diaphragm to measure high voltage. This type reduces the direct compression area of ​​the diaphragm to increase sensitivity with a thinner diaphragm. It is also packaged with various compensation and protection sections and amplifier circuits to improve immunity to interference. This sensor is suitable for measuring dynamic high pressures and telemetry of aircraft. Single-capacitor pressure sensors are also available in microphone (ie microphone) and stethoscope models.
Differential capacitive pressure sensor
Its pressurized diaphragm electrode is located between two fixed electrodes to form two capacitors. Under the action of pressure, the capacity of one capacitor increases and the other decreases accordingly, and the measurement result is output by the differential circuit. Its fixed electrode is made by plating a metal layer on the concave curved glass surface. When overloaded, the diaphragm is protected by a concave surface without breaking. Differential capacitive pressure sensors have higher sensitivity and better linearity than single-capacitance type, but they are difficult to process (especially difficult to ensure symmetry), and can not achieve isolation of the gas or liquid to be measured, so it is not suitable for working in corrosion. Sexual or impurity fluid.
Resonant pressure sensor
A pressure sensor that converts the measured pressure into a frequency signal using a resonant element. It is an important application aspect of resonant sensors, mainly vibrating wire pressure sensors, vibrating cylinder pressure sensors, diaphragm pressure sensors and quartz crystal resonant pressure sensors.
When the measured parameter changes, the natural vibration frequency of the vibrating element changes accordingly. Through the corresponding measuring circuit, an electrical signal having a certain relationship with the measured parameter can be obtained. Its advantages are small size, light weight, compact structure, high resolution, high precision and easy data transmission, processing and storage.
Membrane pressure sensor
The sputtered film pressure sensor also works with the resistance strain effect, which is the same principle as the traditional strain gauge pressure sensor. The main difference between the two is in the production process. The sputtered film pressure sensor extends the principle of measuring the pressure of the metal elastic diaphragm, and uses the ion beam sputtering and etching process to directly form the strain bridge on the metal pressure measuring film. Since there are no moving parts, it is anti-vibration and anti-vibration. It has a strong impact and can be used in harsh environments.
Silicon-sapphire is used as a semiconductor sensor with unparalleled metrology.
The gauge pressure sensor and transmitter consist of a double diaphragm: a titanium alloy measuring diaphragm and a titanium alloy receiving diaphragm. A sapphire sheet printed with a heteroepitaxial strain sensitive bridge circuit is soldered to the titanium alloy measuring diaphragm. The pressure to be measured is transmitted to the receiving diaphragm (the receiving diaphragm and the measuring diaphragm are firmly connected by a tie rod). Under the action of pressure, the titanium alloy receiving diaphragm is deformed. After the deformation is sensed by the silicon-sapphire sensing element, the bridge output changes, and the magnitude of the change is proportional to the measured pressure.
The sensor's circuitry ensures power to the strained bridge circuit and converts the strain bridge's unbalanced signal into a uniform electrical signal output (0-5, 4-20mA or 0-5V). In the absolute pressure sensor and transmitter, the sapphire sheet is connected with the ceramic base glass solder to act as an elastic element to convert the measured pressure into strain gauge deformation for pressure measurement.
The sapphire is composed of a single crystal insulator element, which does not cause hysteresis, fatigue and creep; sapphire is stronger than silicon, has higher hardness and is not afraid of deformation; sapphire has very good elasticity and insulation properties (within 1000 OC), so use Silicon-sapphire-made semiconductor sensitive components, which are insensitive to temperature changes, have excellent operating characteristics even under high temperature conditions; sapphire has excellent radiation resistance; in addition, silicon-sapphire semiconductor sensitive components have no pn drift. Therefore, the manufacturing process is fundamentally simplified, the repeatability is improved, and high yield is ensured.
Pressure sensors and transmitters made of silicon-sapphire semiconductor sensitive components operate under the toughest operating conditions with high reliability, high accuracy, minimal temperature error and cost-effectiveness.
Pressure sensors are the most widely used in industrial applications. They also face problems in selection optimization, installation detection, troubleshooting, zero drift, etc. We will come together for you.
everyone enjoys luck , https://www.eeluck.com