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Jul 17 2019

What is Pressure Sensor? Types and Applications of Pressure Sensor


Catalog


1. Introduction to pressure sensor

2. Types of pressure sensors and measuring principles

2.1 piezoelectric pressure sensor

2.2 piezoresistive pressure sensor

2.3 capacitive pressure sensor

2.4 electromagnetic pressure sensor

(1) inductive pressure sensor

(2) Hall pressure sensor

(3) eddy current pressure sensor

2.5 vibrating wire pressure sensor

3. Common applications of pressure sensors

 

3.1 Pressure sensor application in the water treatment industry

3.2 Pressure sensor application in the petrochemical industry

4. How to choose the right pressure sensor

 

4.1 What is the pressure value for sensor measurement?

4.2 What kind of accuracy does the sensor need to achieve?

4.3 What is the temperature resistance of the sensor?

4.4 What kind of output is selected?


1 Introduction to pressure sensor


A pressure sensor is a device that can sense a pressure signal and convert the pressure signal into a usable output electrical signal according to a certain rule.

 

Pressure sensors are usually composed of pressure sensitive components and signal processing units. According to different test pressure types, pressure sensors can be divided into gauge pressure sensors, differential pressure sensors and absolute pressure sensors.

 

Pressure sensor is 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 well, electric power, ship, machine tool , pipelines and many other industries,

 

2 Types of pressure sensors and measuring principles 

 

2.1 piezoelectric pressure sensor


piezoelectric pressure sensor

 

It is a precision measuring instrument that based on the piezoelectric effect, using electrical components and other machinery to convert the pressure to be measured into electricity, and then perform measuring work. Piezoelectric sensors can only be used in dynamic measurements. The main piezoelectric materials are: dihydrogen phosphate, sodium potassium tartrate and quartz. With the development of technology, the piezoelectric effect has also been applied to polycrystals. For example, piezoelectric ceramics, barium magnesium titanate piezoelectric ceramics, tantalate-based piezoelectric ceramics, and barium titanate piezoelectric ceramics are included.

 

Sensors that operate on the piezoelectric effect are electromechanical conversion and self-generating sensors. Its sensitive components are made of piezoelectric materials. When the piezoelectric material is subjected to an external force, a charge is formed on the surface. After the charge is amplified by the charge amplifier, the measuring circuit, and the impedance is transformed, it is converted into a power output proportional to the external force. It is used to measure forces and non-electrical quantities that can be converted into forces, such as acceleration and pressure.

 

The advantages are: light weight, reliable operation, simple structure, high signal to noise ratio, high sensitivity and wide signal bandwidth.

 

The disadvantage is that some voltage materials are not wet, so it is necessary to take a series of moisture-proof measures; and the output current response is relatively poor, it is necessary to use a charge amplifier or high input impedance circuit to make up for this shortcoming.

 

2.2 piezoresistive pressure sensor


piezoresistive pressure sensor


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. Most metallic materials and semiconductor materials have been found to have a piezoresistive effect. Since silicon is the main material of today's integrated circuits, the application of piezoresistive components made of silicon has become very meaningful. The change in resistance is not only due to stress-dependent geometric deformation, but also from the stress-dependent electrical resistance of the material itself, which makes the degree factor greater than hundreds of times the metal.

 

The piezoresistive pressure sensor is typically connected to the Wheatstone bridge via a lead. Usually, the sensitive core has no external pressure, and the bridge is in equilibrium (called zero position). When the sensor is pressed, the resistance of the chip changes and the bridge will lose its balance. If a constant current or voltage supply is applied to the bridge, the bridge will output a voltage signal corresponding to the pressure, so that the resistance change of the sensor is converted into a pressure signal output through the bridge. The bridge detects the change of the resistance value. After amplification, it is converted into a corresponding current signal through the conversion of the voltage and current. The current signal is compensated by the nonlinear correction loop, which produces a 4 to 20 mA standard output signal which has a linear relationship with the input voltage.

 

In order to reduce the influence of temperature change on the core resistance value and improve the measurement accuracy, the pressure sensor adopts temperature compensation measures to maintain its high level of zero drift, sensitivity, linearity and stability.

 

2.3 capacitive pressure sensor


capacitive pressure sensor


The capacitor is used as a sensitive component to convert the measured pressure into a pressure sensor whose capacitance value changes. Such a pressure sensor 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 voltage and voltage can be measured by the measuring circuit. A certain relationship with the electrical signal. Capacitive pressure sensors are pole-to-change capacitive sensors that can be divided into single-capacitor pressure sensors and differential capacitive pressure sensors.

 

The single-capacitance pressure sensor 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 plane that 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-capacitance pressure sensors are also available in microphone and stethoscope models.

 

The pressurized diaphragm electrode of the differential capacitive pressure sensor is located between the 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 a 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-capacitor type, but they are difficult to process (especially difficult to ensure symmetry), and can not achieve isolation of the gas or liquid to be tested, so it is not suitable for working in corrosive or impurity fluid.

 

2.4 electromagnetic pressure sensor

 

The sensors using the electromagnetic principle are collectively referred to as electromagnetic pressure sensors, and mainly include an inductive pressure sensor, a Hall pressure sensor, an eddy current pressure sensor, and the like.

 

(1) inductive pressure sensor


inductive pressure sensor


The working principle of the inductive pressure sensor is due to the difference in magnetic material and magnetic permeability. When the pressure acts on the diaphragm, the size of the air gap changes. Then the change of the air gap affects the change of the inductance of the coil. The processing circuit can transform the change of the inductance. Corresponding signal output, so as to achieve the purpose of measuring pressure. The pressure sensor can be divided into two types according to the magnetic circuit change: variable magnetic resistance and variable magnetic permeability. Inductive pressure sensors have the advantages of high sensitivity and large measurement range; the disadvantage is that they cannot be applied to high-frequency dynamic environments.

 

The main components of the variable reluctance pressure sensor are the iron core and the diaphragm. They form a magnetic circuit with the air gap between them. When there is pressure, the size of the air gap changes, that is, the magnetic resistance changes. If a certain voltage is applied to the core coil, the current will change as the air gap changes, thereby measuring the pressure.

 

In the case where the magnetic flux density is high, the magnetic permeability of the ferromagnetic material is unstable, and in this case, the variable permeability magnetic pressure sensor can be used for measurement. The variable permeability pressure sensor replaces the core with a movable magnetic element, and the change in pressure causes the movement of the magnetic element, so that the magnetic permeability changes, thereby obtaining a pressure value.

 

(2) Hall pressure sensor


Hall pressure sensor


Hall pressure sensors are based on the Hall effect of certain semiconductor materials. The Hall effect refers to the phenomenon that when a solid conductor is placed in a magnetic field and a current is passed, the charge carriers in the conductor are biased to one side by the Lorentz force, which in turn generates a voltage (Hall voltage). The electric field force caused by the voltage will balance the Lorentz force. By the polarity of the Hall voltage, it can be confirmed that the current inside the conductor is caused by the movement of the negatively charged particles (free electrons).

 

Applying a magnetic field perpendicular to the direction of the current on the conductor causes the electrons in the wire to be concentrated by the Lorentz force, thereby generating an electric field in the direction in which the electrons are concentrated, which will cause the later electrons to be balanced by the action of electricity. The Lorentz force caused by the magnetic field makes the subsequent electrons pass smoothly without offset, which is called the Hall effect. The built-in voltage generated is called the Hall voltage.

 

When the magnetic field is an alternating magnetic field, the Hall electromotive force is also an alternating electromotive force of the same frequency, and the time for establishing the Hall electromotive force is extremely short, so the response frequency is high. The materials of commonly used Hall elements are mostly semiconductors, including N-type silicon (Si), indium antimonide (InSb), indium arsenide (InAs), germanium (Ge), gallium arsenide (GaAs), and multilayer semiconductor structural materials.

 

(3) eddy current pressure sensor


eddy current pressure sensor


Based on the eddy current effect, a moving magnetic field intersects the metal conductor or is formed by a perpendicular intersection of the moving metal conductor and the magnetic field. In short, it is caused by electromagnetic induction. This action produces a current that circulates within the conductor. The eddy current characteristics make the eddy current detection have characteristics such as zero frequency response, so the eddy current pressure sensor can be used for static force detection.

 

2.5 vibrating wire pressure sensor

 

The vibrating wire pressure sensor is a frequency sensitive sensor. This frequency measurement has a high accuracy because time and frequency are physical quantity parameters that can be accurately measured, and the frequency signal can ignore the influence of other factors such as the resistance, inductance and capacitance of the cable during transmission. At the same time, the vibrating wire pressure sensor also has advantages such as strong anti-interference ability, small zero drift, good temperature characteristics, simple structure, high resolution, stable performance, easy data transmission, processing and storage, easy digitization. Therefore, vibrating the strings Pressure sensors can also be used as one of the directions in the development of sensing technology.

 

The sensitive component of the vibrating wire pressure sensor is a tensioned steel string, and the natural frequency of the sensitive component is related to the magnitude of the tension. The length of the string is fixed, and the amount of vibration frequency change of the string can be used to measure the magnitude of the tension, that is, the input is the force signal, and the output is the frequency signal. The vibrating wire pressure sensor is composed of two upper and lower parts, and the lower part is mainly a combination of sensitive components. The upper member is an aluminum casing, which contains an electronic module and a terminal block and are placed in two small chambers so that the sealing of the electronic module chamber is not affected when wiring.

 

The vibrating wire pressure sensor can be selected from the current output type and the frequency output type. The vibrating wire pressure sensor is in operation, and the vibrating wire keeps vibrating at its resonant frequency. When the measured pressure changes, the frequency changes. This frequency signal is converted into a 4~20m current signal by the converter.

 

 

3 Common applications of pressure sensors

 

3.1 Pressure sensor application in the water treatment industry

 

The use of pressure sensors in water and wasted water treatment processes provides important control and monitoring tools for system protection and quality assurance.

 

Among them, the pressure sensor MSP300 series has set a new performance-price ratio model for high-volume commercial and industrial applications due to its low cost. The sensor is suitable for gas and liquid pressure measurement, even including sewage, steam, mildly corrosive liquids and gases.

 

The pressure sensor MSP300 converts the pressure (generally the pressure of the liquid or gas) into an electrical signal output, which in turn can be used to measure the level of the static fluid and thus can be used to measure the liquid level. The sensitive components of the pressure sensor mainly comprise a silicon cup sensitive component, a silicone oil, an isolating diaphragm and an air guiding tube. The measured medium pressure P is transmitted to one side of the silicon cup component through the isolating diaphragm and the silicone oil, and the atmospheric reference pressure po passes through the air guiding tube. Acting on the other side of the silicon cup component, the silicon cup component is a cup-shaped single crystal silicon wafer that is thinly processed at the bottom. The bottom film of the cup produces elastic deformation with minimal displacement under the pressure P and Po. The single crystal silicon is an ideal elastic body, and its deformation has a strict proportional relationship with the pressure, and the recovery performance is excellent.

 

On the silicon diaphragm, the four bridge resistors formed by the semiconductor diffusion process are arranged in a square shape. When the silicon diaphragm is deformed by pressure, the two resistors on the diagonal are stressed, and the other two resistors are subjected to Tensile stress. Due to the piezoresistive effect of diffused silicon, the relative resistance of the two resistors increases, and the resistance of the other two resistors decreases. If a voltage is applied to the A-A terminal, there is a voltage signal output proportional to the P-Po differential voltage between the C-Ds.

 

3.2 Pressure sensor application in the petrochemical industry

 

The pressure sensor is one of the most used measuring devices in the automatic control of the petrochemical industry. In large chemical projects, almost all pressure sensor applications are included: differential pressure, absolute pressure, gauge pressure, high pressure, differential pressure, high temperature, low temperature, and remote flange pressure sensors of various materials and special processing. .

 

It can be seen that the demand for pressure sensors in the petrochemical industry is mainly concentrated on reliability, stability and high precision. Among them, reliability and many additional requirements, such as range ratio, bus type, etc., depend on the structural design of the transmitter, the level of machining technology and structural materials. The stability and high precision of the pressure transmitter are mainly guaranteed by the stability and measurement accuracy of the pressure sensor. Corresponding to the measurement accuracy of the pressure transmitter is the measurement accuracy and response speed of the pressure sensor. Corresponding to the stability of the pressure transmitter is the temperature characteristics and static pressure characteristics of the pressure sensor and long-term stability.

 

The demand for pressure sensors in the petrochemical industry is reflected in measurement accuracy, rapid response, temperature characteristics, static pressure characteristics, and long-term stability. The micro pressure sensor is a new type of pressure sensor manufactured by semiconductor materials and MEMS technology. It has the advantages of high precision, high sensitivity, good dynamic characteristics, small volume, corrosion resistance and low cost. The material of pure single crystal silicon is fatigued, and the micro pressure sensor made of this material has good long-term stability. At the same time, the micro pressure sensor is easy to integrate with the micro temperature sensor, which increases the temperature compensation accuracy and greatly improves the temperature characteristics and measurement accuracy of the sensor. If two micro-pressure sensors are integrated, static pressure compensation can be implemented to improve the static pressure characteristics of the pressure sensor. Today, micro-pressure sensors have many of the advantages that traditional pressure sensors do not have, and are well suited to the needs of pressure sensors in the petrochemical industry.

 

Examples of pressure sensors in the petrochemical industry: high-precision, high-stability, high-reliability pressure transmitters U7100. U7100 ultra-stable series of pressure sensors set a new performance-price ratio model for demanding engine and motor vehicle applications, which is suitable for liquids and gas pressure measurement, even including medium such as sewage, steam and corrosive liquids. The U7100's pressure chamber is machined from 316L stainless steel. It has no O-rings, and is not directly in contact with the measuring medium. A variety of leak-free pressure ports are available. Automotive-grade pressure transmitters feature sealed pressure ports and electrical connectors with a range of 0 to 15 to 150 psi (10 Bar) for excellent durability. The sensor complies with the latest heavy industry CE standards, including surge protection, and 16Vdc forward and reverse over-voltage protection. Stability and accuracy are guaranteed in the petrochemical industry.

 

4. How to choose the right pressure sensor

 

4.1 What is the pressure value for sensor measurement?


The first thing you need to know is the maximum pressure required in your system. Then the maximum pressure sensor pressure range required should be 1.5 times the maximum pressure required by the system. These additional pressure ranges are due to many systems, particularly water pressure and process control, with pressure spikes or continuous pulses. These spikes may reach five to ten times the “maximum” pressure and may cause damage to the sensor. Continuous high-voltage pulses that approach or exceed the sensor's maximum rated pressure also reduce sensor life. Therefore, simply increasing the rated pressure of the sensor is not a perfect solution, because it will sacrifice the resolution of the sensor. Buffers can also be used to attenuate spikes, but this is only a compromise because it reduces the sensor's response speed.

 

4.2 What kind of accuracy does the sensor need to achieve?

 

Accuracy is a commonly used term in the industry to describe sensor output errors. It comes from nonlinearity, hysteresis, non-reproducibility, temperature, zero balance, correction and humidity effects. Usually we specify the accuracy as the combined effect of nonlinearity, hysteresis and non-reproducibility. For many sensors, the “accuracy” is lower than the nominal value due to factors such as temperature and zero balance. The cost of a sensor with higher accuracy will be higher, so does the corresponding system really need such high precision? Systems using high-precision sensors and low-resolution instruments are an inefficient solution.

 

4.3 What is the temperature resistance of the sensor?

 

Pressure sensors, like all physical equipment systems, can cause errors or even be unusable in extreme temperature environments. Generally each sensor will have two temperature ranges, the working range and the compensation range. The compensation range is included in the scope of work.

 

The working range means that within this range, the sensor can be exposed to the medium without damage after it is energized. However, this does not mean that the performance can reach the nominal specification (temperature coefficient) when it is outside the compensation range.

 

The compensation range is generally a narrower range within the working range. Within this range, the sensor ensures that the nominal specifications are met. The change in temperature affects the sensor in two ways, one causing zero drift and the other affecting the output of the entire range. The sensor specification should list these errors in the following form: ±x% full scale / °C, ±x% reading / °C, ±x% full scale over temperature compensation range, or ±x% over temperature compensation range reading. Failure to do so will cause uncertainty in your use. So is the change in sensor output due to pressure changes or temperature changes? Temperature effects will be the most complex part of understanding how to use the sensor.

 

4.4 What kind of output is selected?

 

A typical sensor has a millivolt output, or a voltage amplification, or milliamp, or frequency output. The type of output selected depends on the distance between the selected sensor and the system control or display component, noise, and other electrical disturbances, as well as whether amplification is required, where the amplifier is best placed. For many original equipment manufacturers, their control components and sensors are very short, so millivolt output is generally sufficient and less expensive.

 

If sensor output amplification is required, it is easier to use another sensor with a built-in amplifier. In long-distance cables, or in areas with large electrical noise, mA output or frequency output is required. In environments with strong RF interference and electromagnetic interference, it is also necessary to consider adding additional shielding or filtering equipment outside the milliamp and frequency output.

 

 


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