Pressure Sensor Working Principle

The pressure sensor is most commonly used in industrial practice a sensor, which is widely used in various industrial automation environment, involving water utilities, rail transportation, intelligent buildings, production automation, aerospace, military, petrochemical, oil, electric power, shipbuilding, machine tools , pipes and many other industries, the following brief introduction to some of the commonly used sensor principle and its application.

1. Strain gauge pressure sensor Principles and Applications
A wide variety of mechanical sensors, such as resistance strain gauge pressure sensor, semiconductor strain gauge pressure sensors, piezoresistive pressure sensor, inductive pressure sensor, a capacitive pressure sensor, resonant pressure sensor and capacitive accelerometer sensor. However, the most widely used is a piezoresistive pressure sensor, it has a very low price and high accuracy and good linearity characteristics. Here we introduce the type of sensor.

In understanding the piezoresistive force sensor, we are the first to recognize this element of resistance strain gauge. Resistance strain gauge is a sensitive device on the DUT strain change is converted into an electrical signal. It is the main components of piezoresistive strain sensors. Resistance strain gauge is the most widely used metal resistance strain gauges and semiconductor strain gauges two. Metal resistance strain gauge there are two kinds of the filamentous strain gauge and metal foil strain gauge. Usually strain gages through special binder closely adhered to the the generated mechanical strain on the substrate, the resistance strain gauge with deformation stress change occurs when the substrate is subject to change, so that the resistance of the strain gage, thereby the voltage across the resistor changes. The change in resistance of the strain gauge force is usually small, general strain gauge composed of strain bridge, and follow-up instrumentation amplifier to amplify, and then transmitted to the processing circuit (usually A / D conversion and the CPU) or actuator.

The internal structure of the metal resistance strain gauge
It is part of the matrix materials, metal wire strain or strain foil, insulating protection sheet and terminations. In accordance with the type of use, the resistance of the resistance strain gauge can be designed by the designer, but the range of the resistance should be noted that: the resistance is too small, the required drive current is too large, while the strain gages of excessive heat causing the temperature of itself different environments, too much strain gauge resistance change, output zero drift, nulling circuit too complex. The resistance is too large, the impedance is too high, poor ability of anti-electromagnetic interference from outside. Generally are tens of ohms to several kilohms.

The working principle of the resistance strain gauge
The working principle of the metal resistance strain gauge adsorption strain resistance in the base material with mechanical deformation resistance change phenomenon, commonly known as resistance strain effect. The metal conductor resistance value can be represented by the formula:
R = ρ * L / S

Wherein: ρ - metallic conductor resistivity (Ω · cm2 / m)
S - cross-sectional area of ​​the conductor (cm2)
L - the length of the conductor (m)

Strain resistance of the wire, for example, when the metal wire by external force, its length and cross-sectional area will be changed, can be easily seen from the above formula, the resistance value is changed, if the metal wire by external force and elongation, and its length increases, while the cross-sectional area is reduced, the resistance value will increase. Compressed by external force when the wire length is reduced and increased cross-section, the resistance value is reduced. As long as the change in resistance is measured (usually to measure the voltage across the resistor), get the the strain wire strain situation

2. Ceramic pressure sensor principle and application
The corrosion-resistant ceramic pressure sensor is not the transmission fluid pressure acts directly on the front surface of the ceramic diaphragm, the diaphragm to produce slight deformation of the thick-film resistor printed on the back surface of the ceramic diaphragm plate, connected into a Wheatstone bridge (closed bridge), so that the bridge due to the piezoresistive effect of the pressure-sensitive resistor to generate a highly linear and proportional to the pressure, is also proportional to the excitation voltage of the voltage signal, the standard signal according to the type of calibration of the pressure range of 2.0 / 3.0 / 3.3 mV / V, and the strain gauge sensor compatible. By laser calibration, the sensor has a very high temperature stability and time stability, the sensor comes with temperature compensation from 0 to 70 ° C, and the vast majority of media and direct contact.

Ceramic is a recognized high flexibility, corrosion resistance, abrasion resistance, shock and vibration material. The thermal stability characteristics of the ceramic and thick film resistor can make up to -40 ° C ~ 135 ° C operating temperature range, and having a measurement accuracy, high stability. Electrical Insulation extent> 2kV, the output signal is strong, long-term stability. High characteristics, low-cost ceramic sensor will be the development direction of the pressure sensor, comprehensive alternative to other types of sensors in Europe and the United States, in China, more and more users use ceramic sensor instead of diffused silicon pressure sensor.

3. Diffusion silicon pressure sensor principle and application
Working principle
Measured medium pressure acts directly on the sensor diaphragm (stainless steel or ceramic), the diaphragm will produce medium pressure proportional to the micro-displacement sensor resistance value changes, and electronic circuits detect this change and The converted output is a standard measurement signal corresponding to this pressure.

4. Sapphire pressure sensor Principles and Applications
The use of strain resistance working principle, the use of silicon - sapphire as semiconductor sensitive components, with unparalleled metrological characteristics.

Sapphire by single-crystal insulator element, hysteresis, fatigue, and creep phenomenon does not occur; sapphire sturdy than silicon, and higher hardness, are not afraid of deformation; sapphire has a very good flexibility and insulation properties (1000 oC), therefore, the use of silicon - sapphire manufacturing a semiconductor sensitive element is not sensitive to changes in temperature, even under high temperature conditions, also has a good working characteristics; sapphire strong anti-radiation characteristics; Furthermore, the sensitive element of the silicon - sapphire semiconductor pn drift, Therefore, to radically simplify the manufacturing process, improve the repeatability ensure a high yield.

The sensitive component manufacturing silicon - sapphire semiconductor pressure sensor and transmitter, normal work in the most adverse operating conditions, and high reliability, good accuracy, the temperature error is very small, cost-effective.

By double diaphragm, the the table pressure sensor and transmitter components: titanium diaphragm measuring diaphragm and titanium reception. Printed with a sapphire sheet, the heteroepitaxial strain sensitive bridge circuit is welded to the titanium alloy on the measuring diaphragm. The measured pressure is transferred to the receiving diaphragm (receiving solid connection between the diaphragm and the measuring diaphragm rod). Under pressure, titanium receiver diaphragm is deformed, the deformation of the sensitive element of the silicon - sapphire perception, the bridge output will change, the magnitude of change is proportional to the measured pressure.

The sensor circuit is able to guarantee a strain bridge circuit power supply and strain bridge imbalance signal into a unified signal output (of 0-5,4-20 mA or 0-5V). In absolute pressure sensor and transmitter, sapphire sheet, ceramic base glass solder connection, play the role of the elastic element, the measured pressure is converted to a strain gauge deformation, so as to achieve the purpose of pressure measurement.

5. Piezoelectric pressure sensor Principles and Applications
The piezoelectric sensor in the main use of the piezoelectric material comprises quartz, sodium potassium tartrate, and ammonium dihydrogen phosphate. Wherein the quartz (silica) is a natural crystal, piezoelectric effect is found in this crystal piezoelectric properties, within a certain temperature range, has always existed, but after the temperature exceeds this range, the piezoelectric nature of the incomplete disappeared (the high temperature is called the Curie point). Varied slightly with the stress changes in the electric field (that is to say the piezoelectric coefficient is relatively low), quartz gradually other piezoelectric crystals instead. Sodium potassium tartrate having a large piezoelectric sensitivity and piezoelectric coefficients, but it can be applied only in a relatively low ambient temperature and humidity environment. Ammonium dihydrogen phosphate is a synthetic crystal, able to withstand the high temperatures and relatively high humidity, it has been a wide range of applications.

The piezoelectric effect is also applied in the polycrystalline body, such as a piezoelectric ceramic including barium titanate piezoelectric ceramic PZT-based piezoelectric ceramic Niobate, magnesium niobate-lead piezoelectric ceramic, etc..

The piezoelectric effect is the operating principle of the piezoelectric sensors, piezoelectric sensors can not be used for static measurements, because the charge after the external force has infinite input impedance only preserved only in the loop. The actual situation is not the case, so this decision piezoelectric sensor can only measure the dynamic stress.

The piezoelectric sensors used in the measurement of acceleration, pressure and force. The piezoelectric acceleration sensor is an accelerometer. It has a simple structure, small size, light weight, long life and other excellent features. Piezoelectric acceleration sensor vibration and shock measurements in aircraft, automobiles, ships, bridges, and buildings have been widely used, especially in the fields of aviation and aerospace more of its special status. The piezoelectric sensor can also be used to measure the internal engine combustion pressure measurement and the measurement of the degree of vacuum. Also be used for military industry, for example, use it to measure the change of the moment of firing guns and bullets in the chamber in chamber pressure and muzzle shock wave pressure. It can be used to measure the pressure, can also be used to measure a slight pressure.

Piezoelectric sensors are widely used in biomedical measurements, such as the the ventricular catheter decline sound is made by the piezoelectric sensor measuring dynamic pressure is so common, the piezoelectric sensor applications is very wide.

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The Difference in Performance of Touch Screen

Capacitive touch screen with high resolution, light transmission rate is also good, can satisfy the various demands in public places is the touch screen. However, the capacitive touch screen human body as a capacitor electrode use when the the conductor near large enough capacitance coupling between the laminated ITO face, the current flow away will cause capacitive touch screen malfunction; another gloved or handheld insulating object touching when no response, this is because the increase of the insulating medium.
      
Infrared touch screen is off to rely on the determination of the infrared touch position is determined, the a transparent bezel material is chosen with the touch screen contents (simply do not use any shutter). Therefore, the choice of good light transmission properties baffle, and to be anti-reflective treatment, you can get a good visual effect. But by the infrared emission tube size restrictions, it is not possible to transmit high-density infrared touch screen resolution is not high. In addition, because the infrared touch screen relies on infrared sensors to work, changes in ambient light, such as sunlight or indoor light, etc. will affect its accuracy.
Resistive touch screen work environment completely isolated from the outside world, it is not afraid of dust, water, gas and oil, can be used to touch any object, more suitable for use in the field of industrial control. The disadvantage is that the outer layer of the composite film plastic, too hard or use sharps touch may scratch the touch screen.
Surface acoustic wave technology is very stable, and surface acoustic wave touch screen controller by measuring the decay time on the timeline to calculate the touch location, so its accuracy is very high. The surface acoustic wave touch panel having a third axis (z-axis) is pressure axis - can be obtained by calculating the attenuation amount of the reception signal attenuation at the strength of the user touches the screen size can be divided into up to 2 5 6 efforts. The greater the force, and also the wider the reception signal on the waveform attenuation notch deeper, in all of the touch screen, only the surface acoustic wave touch screen having the perception of the performance of the touch pressure.
Applications
Based on the analysis of the structure, principle and performance characteristics of the touch screen, the type of touchscreen applicable Occasion follows:
Wire resistive touch screen: not afraid of dirt, oil and photoelectric interference afraid of scratching their main weakness. Applies to fixed users in public places, such as industrial control field, office, home and so on.
Five-wire resistive touch screen: excellent sensitivity and light transmission, long service life, not afraid of dust, grease and optical interference, applies to all public places, especially for applications that require precision industrial control field.
Capacitive sensing touch screen: capacitance with temperature, humidity, or ground conditions different changes, so the stability is poor, tend to produce drift. Afraid of electromagnetic interference, drift, not readily available in the industry control and interference. Can be used to require less sophisticated public information inquiries; need for frequent calibration, positioning.
Surface acoustic wave touch screen: plain glass, translucent best, long life, good scratch resistance, suitable for various public places unknown user. But the fear of a long period of accumulation of dirt and oil dip, so use the environment clean place better. Otherwise, the need for regular cleaning service.
Development trends
Touchpad: touchpad with a pressure-sensitive capacitive touch technology, the screen area. It consists of three parts: the bottom center sensor is used to monitor the touch panel is touched, and then processes the information, the intermediate layer provides the interaction with the graphics, text, etc., the outermost layer is the touch surface by high strength the plastic material. Fingers tap the outer surface within a thousandth of a second can this information to the sensor and the registration processing. In addition to PC-compatible, it also has high brightness, image clarity, ease of interaction, which is applied to the pointing information query system (such as electronic bulletin boards), has received very good results.
Touch-screen technology to facilitate the operation of the computer used is a very promising interactive input. Countries in the world this generally give attention to, and put a lot of manpower and material resources for research and development continue to emerge, the new touch screen.
Touch pen: using the touch pen to operate the touch screen similar to the whiteboard, in addition to the display interface, windows, icons, touch pen also has a signature, the function of the mark. Touch pen than earlier only light pen to select the menu function is greatly enhanced.
In short, the development of touch screen showing the trend of specialization, multimedia, three-dimensional and large-screen and so on. With the development of the information society, people need access to a variety of public information. Public information transmission system with touch screen manufacturer for interactive window through the use of advanced computer technology, the use of text, images, music, narration, animation, video and other forms, intuitive, to introduce a variety of information to the people, to people with to great convenience. It can be foreseen that with the rapid development of touch screen technology, touch screen applications will be more and more widely, the performance will be getting better and better.
Infrared sensor touch screen: a lower resolution, but is not current, voltage and electrostatic interference, suitable for harsh environmental conditions; suitable for infrared and light interference of various public places, offices and the requirements are not very sophisticated industrial control scene.