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Dec 27 2017

Types of Pressure Sensor

I Introduction

Devices that can measure pressure and provide remote electrical signals are collectively called pressure sensors. The pressure sensor is an important part of the pressure detection instrument, and its structural types are various. Common types include strain gauge pressure sensor, piezoresistive pressure sensor, capacitive pressure sensor, piezoelectric pressure sensor, and vibration frequency type pressure sensor.

In addition, there are photoelectric, optical fiber, and ultrasonic pressure sensors. The pressure sensor can directly convert the measured pressure into various forms of electrical signals, which is convenient to meet the requirements of centralized detection and control of automated systems, and is therefore widely used in industrial production.

Catalog

I Introduction

II What is A Pressure Sensor?

III Common Terms For Pressure Sensors

IV Parameters

V Types of Pressure Sensors and Their Principles

5.1 Strain Gauge Pressure Sensor

5.2 Piezoresistive Pressure Sensor

5.3 Capacitive Pressure Sensor

5.4 Piezoelectric Pressure Sensor

5.5 Inductive Pressure Sensor

5.6 Hall Pressure Sensor

5.7 Eddy Current Pressure Sensor

5.8 Resonant Pressure Sensor

VI Power Supply And Signal Processing Circuit of Piezoresistive Pressure Sensor

6.1 Power Supply Circuit

6.2 Processing Circuit

VII One Question Related to Pressure Sensor

7.1 Question

7.2 Answer

II What is A Pressure Sensor?

A pressure sensor (Pressure Transducer) is a device that can sense pressure signals and convert them into usable output electrical signals in accordance with certain rules.

While talking about pressure sensors, we must derive the concept of a pressure transmitter.

Usually the sensor consists of two parts, namely the sensitive element and the conversion element. The sensitive element refers to the part of the sensor that can directly feel or respond to the measured; the conversion element refers to the part of the sensor that converts the measured strain felt or responded by the sensitive element into an electrical signal suitable for transmission or measurement.

Since the output signal of the sensor is generally very weak, it needs to be modulated and amplified. With the development of integrated technology, people also install this part of the circuit and the power supply and other circuits inside the sensor. In this way, the sensor can output usable signals that are easy to process and transmit. When the technology is relatively backward in the past, the so-called sensor refers to the above sensitive element, and the transmitter is the conversion element above. The pressure sensor generally refers to a sensitive element that converts a changed pressure signal into a corresponding resistance or capacitance signal, such as a piezoresistive element, a piezoresistive element, and so on. 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. It can generally directly output a standard voltage signal or current signal that is linearly related to the pressure for the instrument, PLC, acquisition card, etc. The device collects directly.

Pressure Sensors

Figure1. Pressure Sensors

III Common Terms For Pressure Sensors

Pressure is one of the important parameters in industrial production. In order to ensure the normal operation of production, the pressure must be monitored and controlled. The following are commonly used terms when selecting a pressure sensor:

(1) Standard Pressure

The pressure expressed in terms of atmospheric pressure is greater than atmospheric pressure is called positive pressure; less than atmospheric pressure is called negative pressure.

(2) Absolute Pressure

The pressure is expressed in absolute vacuum.

(3) Relative Pressure

The magnitude of pressure for the comparison object (standard pressure).

(4) Atmospheric Pressure

Refers to atmospheric pressure. The standard atmospheric pressure (1atm) is equivalent to the pressure of a 760mm mercury column.

(5) Vacuum

Refers to a state of pressure below atmospheric pressure. 1Torr = 1/760 air pressure (atm).

(6) Detection Pressure Range

Refers to the adaptable pressure range of the sensor.

(7) Can withstand Pressure

When the test pressure is restored, the performance can withstand the pressure without degradation.

(8) Round-trip Accuracy

At a certain temperature (23 ° C), when the pressure is increased or decreased, the pressure fluctuation value of the operating point obtained by dividing the output reversed pressure value by the full scale value of the detected pressure is obtained.

(9) Accuracy

At a certain temperature (23 °C), when zero pressure and rated pressure are added, the value obtained by removing the value deviating from the specified value of the output current (4mA, 20mA) with the full scale value. The unit is expressed in% FS.

(10) Linear

The analog output changes linearly with the detected pressure, but it deviates from the ideal straight line. Expressing this deviation as a percentage of the full-scale value is called linearity.

(11) Hysteresis (linear)

Use zero voltage and rated voltage to draw an ideal straight line between the output current (or voltage) value, find the difference between the current (or voltage) value and the ideal current (or voltage) value as an error, and then find the pressure rise and fall Time error value. The value obtained by dividing the full scale current (or voltage) value by the maximum value of the absolute value of the above difference is the hysteresis. The unit is expressed in% FS.

(12) Hysteresis

The value obtained by dividing the difference between the output ON point pressure and the OFF point pressure by the full scale value of the pressure is hysteresis.

(13) Non-corrosive Gas

Refers to substances (nitrogen, carbon dioxide, etc.) and inert gases (argon, neon, etc.) contained in the air.

Pressure Sensor

Figure2. Pressure Sensor

IV Parameters

There are many types of pressure sensors, and their performances are also quite different. How to choose a more suitable sensor for economical and reasonable use?

1Rated Pressure Range

The rated pressure range is the pressure range that meets the value specified in the standard. That is, between the highest and lowest temperatures, the sensor output pressure range meets the specified operating characteristics. In actual application, the pressure measured by the sensor is within this range.

2Maximum Pressure Range

The maximum pressure range refers to the maximum pressure that the sensor can withstand for a long time without causing permanent changes in the output characteristics. Especially for semiconductor pressure sensors, in order to improve linearity and temperature characteristics, the rated pressure range is generally greatly reduced. Therefore, even if it is continuously used above the rated pressure, it will not be damaged. Generally, the maximum pressure is 2-3 times the highest value of the rated pressure.

3Damage Pressure

Damage pressure refers to the maximum pressure that can be applied to the sensor without damaging the sensor element or the sensor housing.

4Linearity

Linearity refers to the maximum deviation of the linear relationship between the sensor output and pressure within the working pressure range.

5Pressure Hysteresis

It is the difference between the sensor output when the minimum working pressure and the maximum working pressure approach a certain pressure at room temperature and working pressure range.

6Temperature Range

The temperature range of the pressure sensor is divided into a compensation temperature range and an operating temperature range. The compensated temperature range is due to the temperature compensation applied, and the accuracy enters the temperature range within the rated range. The operating temperature range is the temperature range to ensure that the pressure sensor can work normally.

Tiny Pressure Sensor

Figure3. Tiny Pressure Sensor

V Types of Pressure Sensors and Their Principles

5.1 Strain Gauge Pressure Sensor

5.1.1 Definition and Composition of Strain Gauge Pressure Sensor

Strain-type pressure sensor is a sensor that indirectly measures pressure by measuring the strain of various elastic elements. According to different materials, the strain element can be divided into two categories: metal and semiconductor. The working principle of the strain element is based on the "strain effect" of the conductor and the semiconductor, that is, when the conductor and the semiconductor material are mechanically deformed, the resistance value will change.

5.1.2 Working Principle of Strain Gauge Pressure Sensor

When the metal wire is subjected to external force, its length and cross-sectional area will change, and its resistance value will change. If the metal wire is elongated by external force, its length will increase, while the cross-sectional area will decrease, the resistance value will increase. Big. When the metal wire is compressed by external force, the length decreases and the cross section increases, and the resistance value decreases. As long as the change in the voltage across the resistor is measured, the strain of the strained wire can be obtained.

Strain Gauge Pressure Transducer

Figure4. Strain Gauge Pressure Sensor

5.2 Piezoresistive Pressure Sensor

5.2.1 Definition of Piezoresistive Pressure Sensor

The piezoresistive pressure sensor refers to a sensor made using the piezoresistive effect of single crystal silicon material and integrated circuit technology. After the single crystal silicon material is subjected to the force, the resistivity changes, and the electrical signal output proportional to the force change can be obtained through the measurement circuit. It is also called diffused silicon piezoresistive pressure sensor. It is different from the adhesive strain gauge which needs to indirectly sense the external force through the elastic sensitive element, but directly sense the measured pressure through the silicon diaphragm.

5.2.2 Principle of Piezoresistive Pressure Sensor-Piezoresistive Effect

Piezoresistive pressure sensors are mainly based on Piezoresistive effect. The piezoresistive effect is used to describe the resistance change of a material under mechanical stress. Unlike the piezoelectric effect, the piezoresistive effect only produces impedance changes, and does not generate charges.

Most metal materials and semiconductor materials have been found to have piezoresistive effects. Among them, the piezoresistive effect in semiconductor materials is much greater than that of metals. Since silicon is the main raw material of integrated circuits today, the application of piezoresistive elements made of silicon becomes very meaningful. The resistance change of silicon comes not only from the stress-related geometric deformation, but also from the stress-related resistance of the material itself, which makes its degree factor hundreds of times greater than that of the metal.

The resistance change of N-type silicon is mainly due to the redistribution of carriers between the conduction band valleys of different mobility caused by the displacement of its three conduction band valley pairs, which in turn causes the mobility of electrons to change in different flow directions. The second is due to the change in Effective Mass from the change in the shape of the conduction band valley. In P-type silicon, this phenomenon becomes more complicated, and it also leads to equivalent mass changes and hole conversion.

The piezoresistive pressure sensor is generally connected to the Wheatstone Bridge through the lead wire. Normally, the sensitive core has no external pressure, and the bridge is in a balanced state (called zero position). When the sensor pressure is changed, the chip resistance changes, and the bridge will lose its balance. If you add a constant current or voltage power supply 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 by the bridge. The bridge detects the change in resistance value, after amplification, and then through the conversion of voltage and current, it is transformed into the corresponding current signal, which is compensated by the nonlinear correction loop, that is, the input voltage is linearly corresponding to the relationship 4 ~ 20mA standard output signal.

In order to reduce the influence of temperature changes on the core resistance value and improve the measurement accuracy, pressure sensors adopt temperature compensation measures to maintain a high level of technical indicators such as zero drift, sensitivity, linearity, stability and so on.

Piezoresistive Pressure Sensor

Figure5. Piezoresistive Pressure Sensor

5.3 Capacitive Pressure Sensor

5.3.1 Definition and Principle of Capacitive Pressure Sensor

Capacitive pressure sensor is a pressure sensor that uses capacitance as a sensitive element to convert the measured pressure into a change in capacitance value. This type of pressure sensor generally uses a round metal film or a metal-plated film as an electrode of the capacitor. When the film is deformed by the pressure, the capacitance formed between the film and the fixed electrode changes. Electrical signals of a certain relationship.

5.3.2 Classification of Capacitive Pressure Sensors

Capacitive pressure sensors are capacitive sensors with variable pole distances, which can be divided into single capacitive pressure sensors and differential capacitive pressure sensors.

(1) Single Capacitive Pressure Sensor

It consists of a circular film and a fixed electrode. The membrane deforms under the influence of pressure, thereby changing the capacity of the capacitor. Its sensitivity is roughly proportional to the membrane area and pressure and inversely proportional to the membrane tension and the distance from the membrane to the fixed electrode.

Another type of fixed electrode takes a concave spherical shape, and the diaphragm is a tension plane fixed around the periphery. The diaphragm can be made by plating a metal layer of plastic. This type is suitable for measuring low voltage and has a high overload capacity. A single-capacitance pressure sensor that measures high pressure can also be made with a moving-pole diaphragm with a piston. This type can reduce the direct compression area of ​​the diaphragm, so as to use a thinner diaphragm to improve sensitivity. It is also packaged with various compensation and protection parts and amplifier circuits in order to improve anti-interference ability. Such sensors are suitable for measuring dynamic high voltages and telemetry of aircraft. The single-capacitance pressure sensor also has microphone type (that is, microphone type) and stethoscope type.

(2) Differential Capacitive Pressure Sensor

The pressure-bearing diaphragm electrode of the differential capacitive pressure sensor is located between two fixed electrodes to form two capacitors. Under the effect of pressure, the capacity of one capacitor increases and the other decreases accordingly. 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. During overload, the diaphragm is protected by the concave surface and will not break.

The differential capacitive pressure sensor has higher sensitivity and better linearity than the single capacitive type, but it is more difficult to process (especially difficult to ensure symmetry), and can not achieve the isolation of the measured gas or liquid, so it is not suitable for working in corrosive or impurity fluids.

Capacitive Pressure Sensor

Figure.6 Capacitive Pressure Sensor

5.4 Piezoelectric Pressure Sensor

Piezoelectric pressure sensors are mainly based on the piezoelectric effect (Piezoelectric effect), which uses electrical components and other machinery to convert the pressure to be measured into electricity, and then performs related measurement work. Precision instruments such as many pressure transmitters and pressure sensors.

Piezoelectric effect can be divided into: positive piezoelectric effect and reverse piezoelectric effect.

(1) Positive Piezoelectric Effect

When the crystal is subjected to an external force in a fixed direction, an electrical polarization phenomenon occurs inside, and a charge opposite to the sign is generated on some two surfaces; when the external force is removed, the crystal returns to the uncharged state; When it changes, the polarity of the charge changes accordingly; the amount of charge generated by the crystal force is proportional to the magnitude of the external force. Most piezoelectric sensors are made using the positive piezoelectric effect.

(2) Inverse Piezoelectric Effect

The inverse piezoelectric effect refers to the phenomenon of applying an alternating electric field to the crystal to cause mechanical deformation of the crystal, also known as the electrostrictive effect. Transducers made with the inverse piezoelectric effect can be used in electroacoustic and ultrasonic engineering. There are five basic forms of piezoelectric deformation: thickness deformation, length deformation, volume deformation, thickness shear, and plane shear. Piezoelectric crystals are anisotropic, and not all crystals can produce piezoelectric effects in these five states. For example, quartz crystal has no piezoelectric effect of volume deformation, but has a good piezoelectric effect of thickness deformation and length deformation.

Piezoelectric Pressure Sensor

Figure7. Piezoelectric Pressure Sensor

5.5 Inductive Pressure Sensor

Electromagnetic pressure sensor is the general name of a variety of sensors using electromagnetic principles, mainly including inductive pressure sensor, Hall pressure sensor, eddy current pressure sensor and so on.

The working principle of the inductive pressure sensor is due to the different magnetic materials and permeability. When the pressure acts on the diaphragm, the size of the air gap changes. The change of the air gap affects the change of the inductance of the coil. The corresponding signal output, so as to achieve the purpose of measuring pressure. This type of pressure sensor can be divided into two types according to the change of magnetic circuit: variable reluctance and variable permeability.

(1) The main components of the variable reluctance pressure sensor are the iron core and the diaphragm. The air gap between them forms a magnetic circuit. 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 iron core coil, the current will change with the change of the air gap, thereby measuring the pressure.

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

Inductive Pressure Sensor

Figure8. Inductive Pressure Sensor

5.6 Hall Pressure Sensor

Hall pressure sensors are made 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 passes, the charge carriers in the conductor are biased to one side by the Lorentz force, and then a voltage (Hall voltage) is generated. The electric field force caused by the voltage will balance the Lorentz force. Through the polarity of the Hall voltage, it can be confirmed that the current inside the conductor is caused by the movement of negatively charged particles (free electrons).

Applying a magnetic field perpendicular to the direction of the current on the conductor will cause the electrons in the wire to be gathered by the Lorentz force, thereby generating an electric field in the direction of electron concentration. This electric field will make the subsequent electrons balanced by electricity The Lorentz force caused by the magnetic field allows subsequent electrons to pass smoothly without drifting. This is called the Hall Effect. The built-in voltage generated is called the Hall Voltage.

Hall Effect Sensor

Figure9. Hall Effect Sensor

5.7 Eddy Current Pressure Sensor

The pressure sensor is based on the Eddy Current Effect. The eddy current effect is caused by the intersection of a moving magnetic field and a metal conductor, or by the perpendicular intersection of a moving metal conductor and a magnetic field. In short, it is caused by the electromagnetic induction effect. This action produces a current circulating in the conductor.

The eddy current characteristics make the eddy current detection have zero frequency response and other characteristics, so the eddy current pressure sensor can be used for static force detection.

Eddy Current Probes

Figure10. Eddy Current Probes

5.8 Resonant Pressure Sensor

The resonant pressure sensor is a pressure sensor that uses a resonant element to convert the measured pressure into a frequency signal. The important applications of resonant sensors are vibrating wire pressure sensor, vibrating cylinder pressure sensor, diaphragm pressure sensor and quartz crystal resonant pressure sensor.

When the measured parameter changes, the natural vibration frequency of the vibrating element changes accordingly. Through the corresponding measurement circuit, an electrical signal that has a certain relationship with the measured parameter can be obtained.

Resonant Pressure Sensor

Figure11. Resonant Pressure Sensor

VI Power Supply And Signal Processing Circuit of Piezoresistive Pressure Sensor

6.1 Power Supply Circuit

The piezoresistive sensor can be powered by a constant voltage source or a constant current source. However, compared with the constant current source, there is a problem that the influence of the ambient temperature cannot be eliminated.

Power Supply Circuit

Figure12. Power Supply Circuit

Assuming that the initial resistances of the four diffusion resistors are all equal and R, when there is stress, the resistances of the two resistors increase by an increment of △R, and the other two resistors decrease by a decrease of △R. Due to the influence of temperature, each small resistance value has a variation of △RT. Therefore, the output of the bridge:

At constant pressure:Formula

It can be seen that the output voltage V is related to temperature and is non-linear, so when power is supplied by a constant voltage source, the influence of temperature cannot be eliminated. In a constant current V = IR.

This shows that the output voltage V is independent of temperature, which eliminates the influence of temperature on the output signal of the sensor. Therefore, the constant current source power supply circuit as shown in the figure below can be used, which USES double power supply to avoid common mode interference. The stability of current I 0= 215/R depends on the stability of reference voltage source 1403 and resistance R.

Circuit

Figure13. Circuit

6.2 Processing Circuit

The full-scale output signal of the piezoresistive sensor ranges from 70 to 350mV, and its output impedance is very high. This requires that the amplifier circuit must have a higher input impedance and do not absorb current from the sensor output to avoid destroying the sensor's working state. The amplifier circuit introduced here is shown below.

Processing Circuit

Figure14. Processing Circuit

This circuit has a high input impedance and a high common-mode rejection ratio and open-loop gain; the offset current, voltage, noise, and drift are small. In the figure, A 1, A 2 form the first stage of in-phase parallel differential amplifier, the amplified output of this stage is V 0′= V 01 -V 02 = [ 1+ (R 1+ R 2)/W ]V i, A 1,A 2. The input terminal does not absorb current, and the circuit structure is symmetrical. Drift and offset cancel each other out, which has the ability to suppress common mode signal interference; A 3 constitutes the second stage of differential amplification to increase the amplification factor. To effectively suppress common-mode signal interference, the circuit must have R 3= R 4= R , R 5= R 6=R f and the total output of the amplifier is

Formula

Adjust the potentiometer W to change the gain of the amplifier, let R 1= R 2, then V 0= - (R f/R) [1+ 2R /W ]V i.

7.1 Question

Which sensor measures the pressure relative to perfect vacuum?

a) Absolute pressure sensor

b) Gauge pressure sensor

c) Vacuum pressure sensor

d) Differential pressure sensor

7.2 Answer

Answer: a

Explanation: Pressure sensor can be classified in terms of pressure ranges they measure, temperature ranges of operation, and most importantly the type of pressure they measure. Absolute pressure sensor is a sensor that measures the pressure relative to perfect vacuum.

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