No matter what kind of physical quantities used for detection, capacitive sensors only three types of change: ① variable distance type, is to change the capacitor two pole plate spacing to change the capacitance. ② variable area type, that is, change the capacitor pole plate area to change the capacitance. ③ variable dielectric type, it is by changing the dielectric constant of the medium between the plates to change the capacitance.
For capacitive pressure sensors, it is by the size of the pressure to change the distance between the two metal diaphragms to make the capacitance change, so, it is variable distance type.
What do you know about pressure sensorsWorking principle of pressure sensors
The electrical component of the pressure transmitter that feels the pressure is generally a resistance strain gauge, which is a sensitive device that converts the pressure on the measured part into an electrical signal. Resistive strain gauges are the most widely used metal resistive strain gauges and semiconductor strain gauges. There are two types of metal resistive strain gauges: filament strain gauges and foil strain gauges. Usually the strain gages are tightly bonded to the mechanical strain generating substrate by a special adhesive. When the substrate is subjected to stress changes, the resistance strain gages will also deform together, so that the resistance of the strain gages will change, and thus the voltage applied to the resistor will change.
Types of pressure sensors
There are four common types of pressure sensors:
1, strain pressure sensors
Strain pressure sensors mainly through the measurement of elastic element strain to measure the pressure sensor. According to the different materials used in the production, the strain element is divided into metal and semiconductor. When the conductor and semiconductor material mechanical deformation, its resistance value will change.
2, piezoresistive pressure sensor
Piezoresistive pressure sensor is the use of monocrystalline silicon material piezoresistive effect and integrated circuits made of sensors. Monocrystalline silicon material is subjected to force, the resistivity will change, at this time through the measurement circuit, proportional to the force changes in the electrical signal will be output. Different from the paste type strain gauge, mainly through the silicon diaphragm to feel the measured pressure.
3, capacitive pressure sensors
Capacitive pressure sensors mainly use capacitors, the measured pressure into a capacitance value change in the pressure sensor. Capacitor electrode selection of round metal film or metalized film, when the film feels the pressure deformation, when the capacitance changes, the formation of electrical signals.
4, piezoelectric pressure sensor
Piezoelectric pressure sensor using the piezoelectric effect, with electrical components and other mechanical pressure will be converted into electricity, and then the relevant measurement of the instrument.
Pressure sensor how to choose the type?
There are many models of pressure transmitters, when used according to the need to select the appropriate type of pressure transmitter, pressure transmitter selection needs to be referred to the main factors are:
1, what kind of pressure measurement
The first to determine the system to measure the pressure of the maximum value, generally speaking, you need to choose a pressure transmitter with a larger than the maximum value of 1.5 times or so. range of the pressure transmitter than the maximum value is about 1.5 times larger.
2, what kind of pressure media
Viscous liquids, mud will plug the pressure interface, solvents or corrosive substances will not destroy the transmitter and these media in direct contact with the material. These factors will determine whether to choose a direct isolation membrane and direct contact with the media materials.
3, the accuracy of the pressure transmitter
The factors that determine the accuracy of the pressure transmitter are non-linearity, hysteresis, non-repeatability, temperature, zero bias scale, the effect of temperature, etc., the higher the accuracy, the higher the price.
4, the temperature range of the pressure transmitter
Often a transmitter will be calibrated to two temperature range, one of the temperature range is the normal operating temperature, the other is the temperature compensation range, the normal operating temperature range is the transmitter in the working state is not destroyed when the temperature range, in excess of the temperature compensation range may not reach the performance of its application.
5, need to get what kind of output signal
mV, V, mA and frequency output digital output, choose what kind of output depends on a variety of factors, including the distance between the transmitter and the system controller or display, whether there is "noise" or other electronic interference signals, the need for amplifiers. The location of the amplifier, etc. For many OEMs with short distances between the transmitter and the controller, a transmitter with a mA output is the most economical and effective solution. If amplification of the output signal is required, it is best to use a transmitter with built-in amplification. For long distance transmission or the existence of strong electronic interference signal is best to use mA level output or frequency output. If in the RFI or EMI indicators are very high in the environment in addition to note that the choice of mA or frequency output in addition to special protection or filter.
6, choose what kind of excitation voltage
The type of output signal determines how to choose the excitation voltage. Many transmitters have built-in voltage regulators and therefore have a wide range of supply voltages. Some transmitters are quantitative configuration, the need for a stable operating voltage, therefore, the operating voltage to determine whether to use the sensor with a regulator, the choice of transmitter to consider the operating voltage and system cost.
7, whether the need for interchangeable pressure transmitters
To determine whether the required pressure transmitter can be adapted to the use of multiple systems. Generally speaking, this is very important, especially for OEM products. Once the product is in the hands of the customer, the cost of calibration can be significant. If the product has good interchangeability, then even if you change the pressure transmitter used will not affect the effectiveness of the whole system.
8, pressure transmitter overtime work after the stability
Most of the pressure transmitter after excessive work will produce "drift", so it is necessary to understand the stability of the transmitter before purchasing, this kind of advance work can reduce the future use of all kinds of trouble.
9, pressure transmitter package
Pressure transmitter package, it is often easy to ignore is its rack, but this point in the future use will gradually reveal its shortcomings. In the purchase of transmitters must take into account the future working environment of the pressure transmitter, how humidity, how to install the transmitter, there will be no strong impact or vibration.
10, in the pressure transmitter and other electronic equipment used between how to connect
Including the need to use a short distance connection? If the use of long-distance connection, is it necessary to use a connector? These should be considered clearly.
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Types of pressure sensorsThere are four common types of pressure sensors Capacitive Pressure Sensors:
1, strain pressure sensors
Strain pressure sensors are mainly through the measurement of elastic element strain to measure the pressure of the sensor. According to the different materials used in the production of capacitive pressure sensor, the strain element is divided into metal and semiconductor. When the conductor and semiconductor materials are mechanically deformed Capacitive Pressure Sensors, their resistance value will change.
2, piezoresistive pressure sensor
Piezoresistive pressure sensor is the use of monocrystalline silicon material piezoresistive effect and integrated circuits made of sensors. Monocrystalline silicon material is subjected to force, the resistivity will change, at this time through the measurement circuit, proportional to the force changes in the electrical signal will be output. Different from the paste type strain gauge, mainly through the silicon diaphragm to feel the measured pressure.
3, capacitive pressure sensors
Capacitive pressure sensors mainly use capacitors, the measured pressure into a capacitance value change in the pressure sensor. Capacitor electrode selection of round metal film or metalized film, when the film feels the pressure deformation, when the capacitance changes, the formation of electrical signals.
4, piezoelectric pressure sensor
Piezoelectric pressure sensor using the piezoelectric effect, with electrical components and other mechanical pressure will be converted into electricity, and then the relevant measurement of the instrument.
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Capacitive sensor working principleCapacitive sensor working principle is as follows:
Capacitive sensors are also often referred to as capacitance level meter, capacitance level meter capacitance detection element is based on the principle of cylindrical capacitor work, capacitor is composed of two coaxial coaxial cylindrical plate electrode and outer electrode insulated, between the two cylinders are filled with the dielectric constant ε of the electrolyte When the two cylinders are filled with electrolyte with dielectric constant ε, the capacitance between the two cylinders is L, where L is the length of the mutually overlapping part of the two cylinders.
D is the diameter of the outer cylinder electrode. d is the diameter of the inner cylinder electrode; e is the dielectric constant of the intermediate medium. In the actual measurement of D, d, e is basically unchanged, so measured C can know the level of the high and low, which is also a capacitive sensor with easy to use, simple structure and high sensitivity, inexpensive and so on, one of the reasons for the characteristics of the sensor.
Capacitive sensor is a variety of types of capacitors as the sensing element, due to changes in the measured changes will lead to changes in capacitance of the capacitor, through the measurement circuit, the change in capacitance can be converted into an electrical signal output. The size of the electrical signal can be measured to determine the size of the measurement. This is the basic working principle of capacitive sensors.
Introduction of Capacitive Sensors
Capacitive sensors generally use a round metal film or metalized film as an electrode of the capacitor. Capacitive pressure sensor belongs to the pole distance change type capacitive sensor, can be divided into single capacitive pressure sensor and differential capacitive pressure sensor.
Single capacitance sensor
It consists of a circular film with a fixed electrode. The diaphragm can be made of plastic plated metal layer method. This type is suitable for measuring low pressure and has a high overload capacity. Can also be used with a piston moving pole diaphragm made of high pressure measurement of a single capacitive pressure sensor. This type can reduce the direct pressure area of the diaphragm, in order to use a thinner diaphragm to improve sensitivity.
It is also with a variety of compensation and protection section and amplifier circuitry in the whole package together, in order to improve the anti-interference ability. This sensor is suitable for measuring dynamic high pressures and for telemetry of aircraft. Single capacitive pressure sensors are also available in microphone (i.e. microphone) and stethoscope types.
Differential capacitance sensor
It's pressurized diaphragm electrode is located between the two fixed electrodes, forming two capacitors.