In recent years, with the in-depth study of vibration experiments, vibration measurement technology has been developed and applied in aviation, aerospace, electronics, ships, and other fields, showing a good development trend, which has become a hot topic for scholars to study.
Vibration measurement technology is to use modern detection means to detect the vibration of mechanical structures, measure the vibration frequency of the fluid, and then determine the flow rate and flow rate of the fluid in the pipeline. The vortex flowmeter is a flow measurement instrument based on the vibration measurement principle, which is mainly used to measure the vortex frequency generated by the vortex flow in the pipeline and works according to the principle of the proportional relationship between the fluid vibration frequency and the flow rate.
In the application process of the vortex flowmeter, it is necessary to use vibration detection technology to convert the vibration quantity into the corresponding charge quantity, and convert the charge quantity into the voltage quantity. The charge amplifier plays a key role in this, which is also the focus of this passage. In order to improve the measurement accuracy of the vortex flowmeter, in this article we conduct in-depth analysis and research on the vibration measurement and detection technology and uses a low-noise charge amplifier design, The function and structure of related circuits are discussed in detail.
Introduction to vibration measurement
Vibration measurement technology is one of the modern commonly used flow measurement methods. It detects the vibration of mechanical structures according to the basic principle of vibration measurement and converts the vibration movement into electrical or other physical signals that are easy to observe, analyze and process. Finally, through the calculation and analysis of the signal frequency, the vibration characteristics of mechanical vibration structures are obtained, so as to realize the measurement of mechanical equipment vibration.
Vibration measurement can divide the vibration signals of instruments and equipment into two categories according to the type of structural vibration: deterministic vibration and random vibration. The vibration caused by the fluid in the vortex flowmeter studied in this article is deterministic vibration. Vibration sensors are used to collect the vibration signals and measure the vibration frequency of the fluid so that the flow rate and velocity of the fluid can be calculated. The vortex flowmeter based on the principle of vibration measurement technology has been widely used to measure the flow of gas, liquid, and steam.
At the same time, the vortex flowmeter is a characteristic parameter for detecting fluid vibration for vibration measurement. There are three methods that can be used in the specific measurement process: mechanical method, optical method, and electrical method. The vibration measurement technology in the vortex flowmeter studied in this passage uses the electrical method. The mechanical vibration signal is obtained by using a vibration measurement sensor, and it is converted into an electrical signal, which is modulated and amplified by a charge amplifier, then determine the flow velocity and flow rate of the fluid in the pipeline.
Composition of vibration measurement system
It can be seen from the above that the electrical measurement method is used in the research of vibration measurement systems in this article, which is a common method in vibration measurement. Compared with optical measurement and mechanical measurement, this method has the advantages of a wide frequency range, wide dynamic range, and high measurement sensitivity. Moreover, the electrical measurement method can be applied to different vibration measurement sensors, and the signal is also easy to be recorded, processed, and transmitted. Therefore, the vibration measurement system in this article is mainly composed of a vibration sensor, charge amplifier, signal analyzer, etc.
Fig. 1 Structure of vibration measurement system
Vibration measurement sensor
The vibration measurement sensor is an important device to obtain vibration information and is the core part of the vibration measurement system. There are many kinds of vibration measurement sensors. In specific application measurements, different sensors should be selected according to different measurement methods and purposes to ensure the measurement effect. The modern vibration measurement sensor has completely changed the traditional independent mechanical measurement device and has become an important part of the entire vibration measurement system, which is used to detect displacement, velocity, acceleration, frequency, and phase, and is also closely related to the charge amplifier and other related electronic circuits.
At the same time, there are differences in the electromechanical transformation principle of the vibration measurement sensor. The form of the output electricity is different. Generally, the vibration signal of the mechanical quantity will be converted into the change of electrical parameters such as resistance and inductance. Moreover, a special measurement route should be set to convert the output electricity of the sensor into the form of electrical signals accepted by the subsequent display, recording, and analyzer according to different electromechanical change principles.
The charge amplifier can convert the weak charge signal from the sensor to the amplified voltage signal, and at the same time, it can convert the high impedance output of the sensor to the low impedance output, and successfully drive the subsequent circuit. At the same time, in the vibration measurement, in view of the differences in the characteristics of the vibration sensor, the measurement parameters involve displacement, velocity, and acceleration, and the peak value, vibration frequency, period, phase difference, and other relevant parameters of the measured vibration momentum should also be included. Therefore, in order to obtain the measurement parameters in the best way, a charge amplifier needs to be set between the vibration sensor and the signal analyzer to realize the displacement Amplification of signals of different charge quantities, such as velocity and acceleration, and converting them into voltage or current signals.
Vibration signal analyzer
The vibration signal analyzer can display the vibration measurement parameters "acceleration, speed, displacement value", and has powerful on-site data acquisition and analysis functions. It can also accurately diagnose the excessive vibration (such as imbalance, misalignment, mechanical looseness, bearing fault, or gearbox fault), point out the location of the fault and the extent of damage, so as to comprehensively grasp the operation status and development trend of the machine and equipment.
Working principle of vortex flowmeter
The vortex flowmeter studied in this article is based on "Karman vortex street "This is a new type of flow measurement instrument developed from the principle, which uses the fluid vibration principle to measure the flow. A vortex street flowmeter is a device that installs one or more nonstreamlined vortex generators in the vertical flow direction of the fluid. When the flow rate of the fluid reaches a specific ratio, it will release and separate two strings of regular vortices with opposite rotation directions on both sides of the barrier, and the fluid vibration within a certain Reynolds number range The frequency is related to the flow rate. The average flow rate and flow rate of the fluid can be calculated by measuring the vortex separation frequency using vibration detection technology. The specific working principle is shown in Figure 2.
Fig. 2 Working principle of vortex flowmeter
At present, the vortex flowmeter has become one of the main flow measurement instruments. Because of its good measurement reliability and wide measurement range, it is widely used in the petroleum, and chemical industry, power generation, and other fields. It has a good value in flow measurement, detection, and control of liquid, gas, and steam. However, because the vortex flowmeter uses vibration measurement technology to realize fluid measurement, it is vulnerable to external interference, which affects its measurement accuracy; At the same time, due to the weak signal transmitted by the vortex sensor, the small flow measurement is limited under the influence of noise. In order to ensure the measurement accuracy of the vortex flowmeter, it is necessary to make full use of vibration detection technology and reduce the noise of the charge amplification circuit to improve its application performance.
Application of Vibration Detection Technology in Vortex Flowmeter
Fluid vibration detection of vortex flowmeter
The vortex flowmeter uses the above vibration measurement technology to achieve fluid detection. For the vibration detection of a vortex flowmeter, it only needs to detect the characteristic parameters of fluid vibration, that is, the vortex frequency generated by the fluid in the specific application pipeline to achieve vibration measurement and obtain vibration measurement signal. At present, the vibration detection methods used in vortex street flowmeters can be piezoelectric and capacitive. The piezoelectric type is used to detect the vibration through the electric pulse generated by the piezoelectric crystal as the detection element due to the pressure pulsation caused by the alternating vortex, while the capacitive type is used to detect the vibration through the capacitance change difference caused by the pressure difference generated by the vortex. The piezoelectric element is not easy to be affected by the fluid density Due to the influence of viscosity and temperature, it has good stability, so it can be widely used in vortex flowmeters.
The weak electrical signal output by the piezoelectric sensor in the vibration measurement system of the vortex flowmeter in this passage, like the above vibration measurement system, needs to exchange the charge signal through the impedance of the preamplifier with high input impedance before it can transmit the sensor signal to the display instrument or realize remote transmission by means of the amplification and detection circuit. This process is completed according to the relevant principles and technologies of vibration detection.
Vortex flowmeter sensor
The vortex flowmeter sensor adopts a piezoelectric sensor, which has high measurement efficiency and can realize fluid measurement without directly contacting the measuring medium. Usually, the stress detection method of the piezoelectric element is used to measure the vibration frequency. The specific operation process is to convert the alternating pressure generated by the vortex into the charge signal of the piezoelectric sensor. The change frequency of the charge signal is the same as the vortex shedding frequency, The alternating charge after being processed by the electronic circuit is converted into the vortex frequency. Since the vortex frequency is positively correlated with the fluid flow, the fluid flow is also obtained.
The piezoelectric crystal element is used to detect the vortex separation frequency, and the alternating separation of vortices is realized on both sides of the rear of the cylinder, so as to promote the generation of pressure pulsation. The probe installed in the main standby wake detects the alternating force, and the piezoelectric crystal element located inside the probe generates a variable charge under the pressure of the alternating force. The alternating charge signal is output after being processed by the detection amplifier or digital signal processing, Or it can be converted into 4~20mA DC standard signal output proportional to the flow.
Amplification circuit of vortex flowmeter
The voltage signal output by the sensor in the vortex flowmeter needs to be amplified and processed by the amplification circuit. The key to the measurement circuit using the vibration measurement technology is the setting of the preamplifier, which can not only change the output signal of the sensor from high impedance to low impedance but also amplify the weak electrical signal of the sensor. The preamplifier circuit can be a voltage amplifier or a charge amplifier with electric fusion feedback. In this article, the charge amplifier is used as the conversion device of the vortex flowmeter. After amplification, filtering and shaping, it becomes a pulse signal whose frequency is proportional to the flow rate and then counts the flow signal to improve the anti-interference ability of the vortex flowmeter.