Modern measurement technology involves a large number of non-electrical quantities, such as weight, force and moment, acceleration, pressure, flow and other mechanical quantities; temperature, heat and other thermal quantities, as well as chemical composition of gas, liquid composition and concentration. These physical quantities are very difficult and inconvenient to perform direct measurements, and a conversion device is required to convert them into electrical quantities that are easy to measure, transmit, and process. This device is a sensor. There are many types of sensors, and the pressure sensor is one of the most mature technologies. In terms of market sales, it is the first sensor and has an annual growth rate of 20%, which has broad application prospects. The application of pressure sensors with different technical performances to different measurement requirements has become an important topic in the development and application of current pressure sensors.

1 Pressure sensor selection

With the development of modern measurement and automation technology, the amount of pressure sensors is increasing at a rate of 20% per year. At present, there are many kinds of pressure sensors on the market, the specifications and technical performance are different, and the price difference is also very large. The question before the user is what pressure sensing should be used to meet the needs? Which indicators are the most important? What issues should I consider? This involves the selection of sensors. The principle of selection is to buy pressure sensors that meet their application, pressure range, accuracy requirements, temperature range, electrical and mechanical requirements at the most economical price.

After the sensor is mounted on the device, the operation is normal, stable, and accurate. The following are some important aspects that must be considered when selecting a pressure sensor.

1.1 Use

Due to the different structure, the pressure sensor can be divided into absolute pressure, relative pressure to the atmosphere and differential pressure. When the absolute pressure is measured, the sensor itself has a vacuum reference pressure, which is independent of atmospheric pressure and is the pressure relative to the vacuum. The relative pressure to the atmosphere is based on atmospheric pressure, so the sensor elastic membrane side is always in communication with the atmosphere. The atmospheric pressure is related to the height from the ground, the change in the moisture content of the atmosphere in the four seasons, and the changes in the content of various gases at different locations and in the atmosphere. Therefore, the relative pressure measured is related to the above factors. In addition, fluid pressure can be introduced from both sides of the sensor elastic membrane to determine the differential pressure between different locations or fluids. Different configurations of pressure sensors should be used for different purposes.

1.2 Pressure range

The pressure range of the pressure sensor is graded. This is because the elastic membrane of the pressure sensor has a limit to the fluid pressure. This is what is commonly referred to as the withstand voltage limit beyond which the elastic film breaks. In general, each sensor has an overpressure of 20 - 300%. Therefore, the maximum pressure range on the product manual is 30 - 80% of the withstand voltage limit. It is not necessary to use an excessive pressure range.

The selection of the pressure range should mainly consider three factors:

That is, the relationship between the maximum overpressure capability of the sensor, the accuracy and the pressure range, and the relationship between the price of the sensor and the pressure range.

For the maximum overpressure capability of the sensor, the sensor is very different from static pressure and dynamic pressure. The latter tends to have shock pressures and even shock waves. The impact pressure is much higher than the static pressure. If the maximum working pressure range selected is static pressure, the sensor should use a large overpressure capability when subjected to dynamic pressure. Otherwise, the impact pressure easily reaches the ultimate withstand voltage, causing damage to the pressure sensor.

For the relationship between accuracy and pressure range. The thermal zero drift and thermal sensitivity drift coefficient and nonlinear error of the pressure sensor are important indicators that affect the accuracy of the sensor. For the same pressure sensor, the thermal zero drift coefficient decreases as the working pressure increases, while the thermal sensitivity coefficient and nonlinear error increase as the working pressure increases. Therefore, the increase of working pressure is beneficial to reduce the thermal zero drift, which is not conducive to thermal sensitivity drift and nonlinear error. When the thermal zero drift is relatively large, increasing the working pressure range is beneficial to improve the accuracy of the pressure sensor. When the thermal zero drift is relatively small, reducing the working pressure range is beneficial to improve the accuracy. Sensitivity is different for sensors with different pressure ranges. The sensitivity of the low-pressure force range sensor is naturally high.

For the relationship between the price of the sensor and the pressure range, in general, the pressure sensor of 013 - 1 MPa is cheaper, and the pressure sensor of 011 MPa or less or 1 MPa or more is more expensive. A pressure sensor of 10 - 50 kPa is available for measurement of 2 - 3 kPa pressure. In particular, when the user designs and selects the compensation circuit by himself, the accuracy can be further improved. This can greatly reduce the cost. In general, a good quality pressure sensor can achieve 100mV / 10V at full scale output. If only half of the pressure range is used, the corresponding output will be only 50mV / 10V. Therefore, the maximum working range should be as close as possible to the product manual. Indicates the range level of the pressure sensor.

1.3 Precision

The pressure sensor can be used as a pressure metering element or as a sensor element for automatic control. Especially for the previous use, it puts forward a relatively high precision requirement. Since the accuracy of a pressure sensor made of a semiconductor chip is affected by temperature, attention should be paid to the operating temperature range of the sensor.

Static accuracy is the precision that should be achieved at a specific temperature (room temperature 25 °C). Can be divided into four files: 0101 - 011 %FS for ultra-high precision: 011 - 1 %FS for high precision: 1 - 2 % FS for normal accuracy; 2 - 10 % FS for low precision.

The full temperature range accuracy is the accuracy that the pressure sensor should achieve over the entire operating temperature range. The same can be divided into four files:

0101 - 011 %FS; 011 - 1 %FS; 1 - 2 %FS; 2 - 10 %FS. Static accuracy reaches 011 - 1 %FS, maybe the full temperature range accuracy is only 1 - 2 %FS, even only 2 - 10 %FS.

For the user, it is often desirable that the accuracy of the pressure sensor be as high as possible. However, when the pressure sensor reaches high precision, it will inevitably add many additional processes, as well as the calibration process and compensation technology, and the corresponding cost will increase. Of course, the selling price will also increase greatly. Therefore, according to the actual application and requirements of the pressure sensor, reasonable accuracy requirements and corresponding temperature ranges should be proposed.

 1.4 Electrical requirements

Generally, the output of the common pressure sensor is an analog signal. The output voltage of the full-scale full-range range can reach 100 - 150mV, and the output current is 0- 0101mA. The voltage of the long-distance output signal will be attenuated, and the current signal should be output. After the current is amplified by the pressure transmitter, a current signal of 20 mA or less can be output. In this way, the price will increase exponentially.

In addition, digital and frequency signals can only be obtained after A/D and V/F conversion.

Both the constant current source and the constant voltage source are two types of excitation sources used by conventional sensors. The two incentive methods are different and their effects are different.

Constant current source excitation is beneficial to the compensation of thermal sensitivity drift.

Because the temperature coefficient of the bridge arm resistor is positive, and the temperature coefficient of sensitivity is negative. The temperature coefficient of the output signal voltage when the constant current source is excited is the algebraic sum of the two. Constant voltage excitation cannot directly provide sensitivity temperature compensation. However, when excited by a constant voltage source, a thermistor or diode can be connected in series outside the bridge to compensate for thermal sensitivity drift. This sensitivity compensation method does not work when excited with a constant current source. It can be seen that the constant voltage source excitation and the constant current source excitation cannot be interchanged with each other.

The general accuracy is measured by constant current source excitation. When the constant voltage source is excited, the accuracy of the measurement depends on the accuracy of the constant voltage source voltage regulator device.

In addition, the excitation power of the pressure sensor can be divided into a proportional excitation and a fixed excitation. The former is to connect the pressure transmitter bridge directly to the power supply. When the power supply changes, the sensitivity and zero point of the pressure sensor change. The latter has a reference voltage inside, and the pressure sensor bridge is energized by the reference voltage. The reference voltage is constant regardless of the supply voltage. As long as the supply voltage varies within a specified voltage range, the reference voltage does not change. Thus the output of the sensor remains unchanged and is not affected by the supply voltage.

Pressure sensors can be battery powered, but more commonly DC regulated power supplies are used. The noise is small when the battery is powered, but with the use of the battery, the supply voltage is gradually reduced, especially when the sensor is positively excited, the sensitivity is gradually reduced. This will result in inaccurate readings. Therefore, compensation should be used (for example, pressure sensor and A/D converter share one battery), or low-power, low-current pressure sensor, long-life battery, or power supply when measuring pressure. After measurement, The battery is turned off to save energy. After replacing the new battery, the pressure sensor needs to be recalibrated. This is because there are certain differences in the electromotive force and internal resistance of batteries of different grades. A change in the bridge excitation voltage of the pressure sensor causes a change in sensitivity.

1.5 mode of operation

The way of working is also an important issue to consider. For example, the sensor is used for the measurement of gas pressure and the measurement of liquid pressure is different. The gas is a compressible fluid, which stores a certain amount of compression energy when it is replenished, and releases it with kinetic energy when decompressing, and applies a shock wave to the elastic film of the sensor. The pressure sensor is required to have a large overload capacity. The liquid is an incompressible fluid. When the pressure sensor is installed, tightening the screw and having no compressible space can raise the liquid pressure beyond the pressure limit of the elastic film, causing the elastic film to rupture. Since this situation occurs frequently, the pressure sensor is also required to have a large overpressure capability.

When the working environment of the pressure sensor is bad, for example, there are large vibrations, impacts, and large electromagnetic interference, which imposes stricter requirements on the sensor. Not only the over-pressure capability is strong, but also the mechanical seal is required to be reliable, anti-loose, and the sensor is installed correctly. The sensor's own leads, leads and external leads should be electromagnetically shielded and the shield grounded well.

In addition, consideration should be given to the compatibility of the pressure sensor with the fluid medium being measured. For example, the elastic membrane structure of the sensor should be separated from the corrosive medium. At this time, a stainless steel bellows sensor is used, and the silicone oil is used as a pressure transmitting medium in the sensor. When the sensor detects the pressure of flammable and explosive media, use a small excitation current to prevent sparks and sparks from breaking when the elastic film breaks, and increase the pressure resistance of the pressure sensor casing.

1.6 Temperature requirements

A pressure sensor fabricated using a semiconductor chip is characterized by a large temperature, and there is not only a thermal zero drift but also a thermal sensitivity drift. Temperature significantly affects the accuracy of the pressure sensor. In order to eliminate the effects of temperature, various temperature compensation techniques are required. The wider the temperature range, the greater the difficulty of the compensation technique, and the greater the calibration workload, the lower the accuracy of the full temperature range that can be guaranteed. To this end, reasonable requirements should be made based on the actual temperature range and accuracy requirements applied by the pressure sensor.

Generally, the temperature range of the pressure sensor is divided into four categories:

Common commercial grade, range - 10 - 60 °C; industrial grade, range -25 - 80 °C; military grade, range - 55 - 125 °C; special grade, range - 60 - 350 °C.

Pressure sensors are available in commercial grades for indoor applications and industrial grades for outdoor applications. Measures can also be taken to thermally isolate the sensor from the environment or to heat or cool it, using a commercial grade below -10 °C or above 60 °C. The temperature range and mechanical temperature characteristics of the sensor should also be considered in selecting the temperature range.

1.7 Requirements for pressure seals

Commonly used pressure seals are rubber mats (or O-rings), epoxy resins, Teflon gaskets, tapered bore fittings, pipe thread fittings, and welding. The sealing material used determines the operating temperature range of the pressure sensor.

2 Application

Broadly speaking, all kinds of sensors that use various physical effects of piezoelectric materials can be called pressure sensors. They have certain applications in industrial, military and civilian applications.

Piezoelectric pressure sensors can be used to measure dynamic pressures in the range of 104 - 106Pa, Hz to tens of Hz (or even hundreds of Hz), pressure measurement in cylinders, tubing, intake and exhaust pipes of internal combustion engines, guns The fields of rolling, biomedical and aerospace are widely used.

Their elastic elements are collected by a diaphragm, a bellows, etc., and converted into concentrated forces, which are then transmitted to the piezoelectric elements. In order to ensure static characteristics and stability, quartz crystal parallel combination is often used.

The pressure sensor should pay attention to the following points in the application: (1) Ensure that the elastic diaphragm has good surface contact with the rear force transmitting member to reduce the hysteresis and linearity error and improve the static and dynamic characteristics; (2) Sensor The base and the casing must have sufficient rigidity to ensure that the measured pressure can be transmitted to the piezoelectric element as much as possible: (3) The selection of the piezoelectric elastic element should take into account the frequency coverage: bending (014 - 100 kHz), compression (40 kHz - 15MHz), shear (100kHz - 125MHz); (4) The components involved in the force transmission should adopt high-sonic material and thin fan structure to facilitate the fast and lossless transmission of the elastic wave of the elastic element and improve the dynamic characteristics: (5) It is necessary to consider the compensation of environmental disturbances such as acceleration and temperature.

3 Conclusion

In recent years, China's pressure sensor technology is booming and its application field is expanding rapidly. Due to the wide range of technologies involved in pressure sensing technology, it has penetrated into various professional fields. Therefore, the discussion of new theories of pressure sensors, the application of new technologies and methods, new materials and new processes will become the development trend.