Study on characteristic analysis and test method of ultrasonic sensor
Introduction
Ultrasonic flowmeter has the advantages of small initial flow, large range ratio and long life, so it is widely used in flow measurement. Ultrasonic sensor (hereinafter referred to as sensor) is one of the core components of ultrasonic flowmeter. Flow metering, an ultrasonic flowmeter needs to use a pair of sensors, and use a set of signal transceiver circuit. In order to ensure the accuracy of the time difference, the conversion efficiency and characteristics of a pair of sensors are required to be consistent as far as possible. Only by ensuring the consistency of parameters such as resonant frequency and anti-resonant frequency, resonant impedance and anti-resonant impedance, it is possible to make the conversion efficiency of a pair of sensors close to each other.
The admittance circle diagram method based on impedance characteristics is one of the effective testing means. This method has the advantages of high precision and simple operation, but it requires manual judgment of the resonant frequency value and manual calculation of other parameters of the sensor, so it is not suitable for online measurement. In view of the sensitivity of sensor resonant frequency and other parameters to changes in structure and ambient temperature, Ghasemi et al. used digital signal instrument TMS320F28355 and programmable signal generator models respectively to build a measuring device for sensor resonant frequency, and calculated its impedance indirectly by measuring voltage and current. Then the resonant frequency is determined. Ne6dSek et al. designed a impedance analyzer based on ARM, whose principle is to obtain the impedance of the sensor by testing the synchronous voltage and current of the automatic balancing bridge, which has the characteristics of fast speed and strong resolution. The frequency resolution is 1Hz, and the frequency measurement range is 50 ~ 5000Hz.
As mentioned above, domestic and foreign scholars' research on ultrasonic sensors is more biased to the sensor single parameter test method, and for flow measurement, it is also necessary to have a test device to evaluate the consistency of a pair of sensor main parameters online, as a basis for judging whether it is suitable for a flowmeter.
1. Sensor characteristics and analysis
1. 1 Analysis of resonant and anti-resonant characteristics
The ultrasonic sensor is connected in series to a test loop consisting of excitation signal source and sampling resistance. When the frequency of excitation signal changes, the voltage at both ends of the sampling resistance is shown in Figure 1. As can be seen from the curve in Figure 1, there are two frequency points fm and fn within the range of frequency variation, and fm < fn. When the frequency of excitation signal is fm, the current is maximum, and when the frequency is fn, the current is minimum.
Figure 1 The curve diagram of current varing with frequency
FIG. 2 shows the variation of the equivalent impedance with the frequency of the excitation signal. Among them, fm is called the minimum impedance frequency, and there will be resonant frequency fr of ultrasonic sensor near it; fn is also known as the maximum impedance frequency, and there will be an inverse resonant frequency fa near it.
Figure 2 The curve diagram of impedance varing with frequency
The electromechanical equivalent circuit of the ultrasonic sensor is shown in Figure 3. Where C. Is the static capacitance of the sensor; L1 is the equivalent inductance of the sensor; C1 is the equivalent capacitance of the sensor; R1 is the equivalent resistance of the sensor. According to literature [8], the minimum impedance frequency fm and maximum impedance frequency fn of the sensor are shown in Formula (1) and (2), respectively.
Figure 3 Electromechanical equivalent circuit of ultrasonic sensor
The phenomenon of series resonance occurs when the signal frequency fs is equal to fm, and fs is the series resonance frequency. The phenomenon of circuit parallel resonance occurs when the signal frequency fp is equal to fn, and fp is the parallel resonance frequency.
When there is no mechanical loss:
fm= fs = fr , fa = fP = fn (3)
When there is mechanical loss:
fm < fs < fr; fa < fP < fn (4 )
1.2 Influence of main parameters on piezoelectric effect of sensor
Piezoelectric ultrasonic sensor has positive piezoelectric effect and inverse piezoelectric effect. Whether it is positive piezoelectric effect or inverse piezoelectric effect, its high conversion efficiency means that the energy loss is small in the conversion process. In the process of flow measurement, the conversion efficiency of positive piezoelectric effect and inverse piezoelectric effect is expected to be as high as possible, that is, the ratio between the amplitude of the signal after conversion and the amplitude of the signal before conversion is larger, which will be more convenient to identify the small signal after conversion and ensure the accuracy of the calculation of the time difference.
(1) Analysis of the influence of resonant frequency on piezoelectric effect. As shown in Figure 3, at the resonant frequency point, the equivalent circuit of the sensor L1, C1, R1 is equivalent to a pure resistance, so the sensor is equivalent to R1 and Co in parallel, and the conversion efficiency of the sensor is the highest. When the resonant frequency point is far away, there will be L1 and C1 in the right branch circuit L1, C1 and R1, which will generate reactive power and reduce active power because of the existence of the heart and. P = U2/R shows that if the resistance value is unchanged and the power is reduced, the R1 voltage drop will be reduced, thus reducing the conversion efficiency of the sensor.
(2) The influence of static capacitance on conversion efficiency. According to the circuit theory, with the increase of the static capacitance of the sensor, the reactive power will also increase, and the total power generated by the excitation signal P=P +P is unchanged, active power and reactive power is the relationship between the negative and negative. According to P = U2/R, when the sensor impedance remains stable, the decrease of active power will lead to the decrease of output voltage, thus reducing the conversion efficiency of the sensor.
1.3 Analysis of acoustic Transceiver characteristics
The state of ultrasonic sensor is dynamic. The vibration characteristics and the response of electric signal are both functions of time. The dynamic characteristics of the sensor determine the conversion efficiency of the sensor. In flow metering, the sensors work in pairs and send/receive signals to each other, while the signal processing circuit is also used symmetrically, which requires the characteristics of the sensors to be consistent. After receiving and sending signals exchange with each other, the characteristics of the sensors are relatively similar, otherwise, threshold deviation and phase deviation will occur, which will result in large metering errors.
2. Test method and design of sensor
2. 1 Test project and overall structure design
In order to ensure the consistency of the sensor, it is necessary to test the resonant frequency and anti-resonant frequency, resonant impedance and anti-resonant impedance, output amplitude, static capacitance, conversion efficiency, etc., to ensure the consistency of the main characteristics of a pair of sensors used for the same flowmeter. Figure is a block diagram of the design device.
The transmission line method is used to test the resonant frequency and anti-resonant frequency, the alternative method is used to test the resonant impedance and anti-resonant impedance, the pulse excitation signal method is used to test the sensor output amplitude, and the AC capacitive reactance method is used to test the static capacitance.
2. 2 Signal conditioning circuit design
2.2. 1 Wave detection circuit
Because the attenuation of the sensor signal is very large and very weak, at the same time, the received electrical signal is a modulated signal containing the measured information, so it is necessary to detect, filter, amplify and other signal conditioning of the electrical signal received again. Full-wave detection circuit 1 as shown in FIG. 6 was used for frequency, impedance and amplitude test, while full-wave detection circuit 2 as shown in FIG. 7 was used for static capacitance test.
Figure 6 Full wave detection circuit 1
Figure 7 Full wave detection circuit 2
The unit is required to produce frequency in the range of 100 kHz to 4.5 MHz. The operational amplifier AD8063 and diode 1N4 1 4 8 shown in Figure 6 are of high speed type.
In FIG. 7, op amp A, diodes D5, D6 and resistors 55 and constitute the half wave rectification part, while op amp B and resistors R57, R58 and R59 together form a signal adder structure.
2. 2. 2 Filter circuit
The purpose of filtering is to filter out the high frequency dry disturbance existing in the signal after detection and extract the low frequency part of the output signal of the sensor to be measured. Figure 8 shows the second-order voltage-controlled voltage source type low-pass filter.
FIG. 8 Second-order voltage-controlled voltage source low-pass filter
According to Kirchhoff's current law, the current equation at point M is Equation (5):
The current equation at point p is shown in Equation (6):
By combining formula (5) and (6), R27 and R26 can be obtained:
The calculation and selection results of related parameters in the circuit are as follows: C37 = lpF, C38 = lnF, R26 = R27 = 1.2 kΩ, R28 = 4.7 kΩ, R45 = 16.3 kΩ
2. 3 Test module design
2. 3. 1 Frequency test module
As shown in Figure 9, the signal generator injects excitation signals with fixed amplitude and variable frequency to the sensor, and the sampling resistor A converts the working current of the transducer into voltage signals. The sensor at the resonant frequency point has the maximum working current and minimum equivalent impedance, while the sensor at the anti-resonant frequency point has the minimum working current and maximum equivalent impedance. Therefore, the resonant frequency and anti-resonant frequency of the sensor can be obtained by testing the output voltage value in Figure.
2.3.2 Impedance test module
As shown in FIG. 10, the alternative method is adopted. Firstly, the sensor is connected in series with the resistor, then the sensor is excited with a fixed amplitude and frequency signal, and the voltage at both ends of the resistor is tested.
FIG. 1 Impedance test principle of 0 sensor
2. 3. 3 Output amplitude test
FIG. 11 is a schematic diagram of testing a single sensor designed to simulate working conditions. It consists of a reflective bottom plate with a certain finish and a roof plate that can be fixed to install the sensor. During the test, the sensor is placed face down on the round hole located in the top plate. The sensor realizes the conversion of electrical signal (sent out by the sensor), mechanical wave (transmitted in the medium and emitted by the bottom plate) and electrical signal (received by the sensor) twice through the conversion of inverse piezoelectric effect and piezoelectric effect.
FIG. 1 1 Schematic diagram of sensor amplitude test
2. 3. 4 Static capacitance test
The piezoelectric ultrasonic sensor without excitation signal is equivalent to capacitance, and the capacitance value measured under this condition is the static capacitance C of the sensor. Low frequency excitation signal is loaded to the sensor, and the static capacitance value of the sensor is converted into a voltage value proportional to it by C/ f/ conversion circuit.
3 Test results and analysis
The standard instrument and the designed test device are used to test and compare the sensor samples.
(1) resonant frequency and anti-resonant frequency: the frequency characteristic tester is used as the standard instrument, the model is SA1005D;
(2) resonant impedance and anti-resonant impedance: resistance box, oscilloscope and function signal generator are used as standard instruments. The models are ZX32D.TDS2012C.DG1022U.
(3) Amplitude test: oscilloscope and water bath are used as calibration instruments, and oscilloscope model is TDS2012C;
(4) Static capacitance: LCR tester and standard capacitor box are used as standard instruments, models are TH2811C and ATCDB12 respectively.
3.1 Resonant frequency (impedance) and anti-resonant frequency (impedance)
Five consecutive test experiments were conducted, and the data were shown in Table 1. The test data of standard instrument and design device are denoted as I and n in the table. As can be seen from Table 1, in the test results between the designed device and the standard instrument, the maximum deviation of the resonant frequency is 3 kHz, which appears in the first, third and fourth lines of data, and the maximum relative error is 0.3 %. The maximum deviation value of the anti-resonant frequency rate is also 3 kHz, which appears in the fifth row of data, and the maximum relative error is 0.29%. The comparison shows that the reading of the design device is stable, the test results are consistent and meet the design requirements.
As can be seen from Table 1, in the test results of the designed device and the standard instrument, the maximum deviation of the resonant impedance is 3 n, which appears in the fifth row of data, and the maximum relative error is 7.3%. The maximum deviation value of the anti-resonant impedance is also 3 kHz, which appears in the third line of data, and the maximum relative error is 0.34 %. The comparison shows that the output value of the design device is stable, the test results are consistent and meet the design requirements.
3. 2 Static capacitor
The static capacitance test range of the design device is 100 ~ 2 400 PF. The standard capacitor box is used as the calibration device, and the test results are tested once every 1000 p F and repeated for 5 times. The test data are shown in Table 2. The data of 5 points including 1〇, 500, 1 500, 2 000 and 2 400 pF were selected. As can be seen from Table 2, in the test results of the design device and the standard instrument, the maximum deviation of the measured capacitance of the standard instrument is 9, 8, 18, 8 and 68 PF for the 5 capacitance values of the capacitor box (100, 500, 1 000, 1 500 and 2 400 PF), respectively. The maximum relative error is 9%, which appears in the first column of data. The maximum deviation values of the measured capacitance of the design device are respectively 5, 5, 10, 0 and 0 PF, and the maximum phase-matching error is 5%, which is also shown in the data in the first column. The comparison shows that the output value of the design device is stable, the test results are consistent, and the design requirements are met.
3. 3 Emission amplitude and echo amplitude
Table 3 shows the test data of transmitting and receiving amplitudes for 5 times. As can be seen from Table 3, due to the necessary adjustments to the output signal, both the amplitude and the consistency of the output signal have been greatly improved in the design device. The sensor output signal is rectified, filtered and amplified by the design device for reading, so the test data is relatively large.
3. 4 Pairing experiment
The experimental scheme is shown in FIG. 11 and 1. Taking 5 sensors as examples, the test conditions are as follows: 1) The samples are placed in a water bath with a set temperature and heated for 10 min to make the temperature of the two reach the same level; 2) Set the water bath to be adjustable in the temperature range of 30 t ~ 90 T, and the test data sampling interval temperature is 10 T; 3) Test the sensor output amplitude of the corresponding temperature.
The test data of sensor output amplitude under the condition of self-collection and self-collection are shown in Table 4. Figure 1 and 2 are the curves of self-collection and self-collection test data.
As can be seen from Table 4 and Figure 1-2, the characteristics of sensors No. 1 and No. 2 are close, while the characteristics of sensors No. 3 and No. 4 are close. The effectiveness of sensor pairing was selected according to the data of one-take one-take test. No. 1 and No. 2, and No. 3 and No. 4 were respectively carried out the matching experiment of receiving and sending each other under working conditions. The test results are shown in Table 5. "1-2" in the table indicates that sensor No. 1 sends signals and sensor No. 2 receives signals. Figure 13 shows the data curve of one-to-one receiving and one-to-one matching.
Figure 1. Curves of self-collected and self-generated data
Figure 13 - Incoming data curve
According to the data analysis in Table 5 and Figure 13, sensor 1 and sensor 2 have similar transceiver characteristics after pairing, sensor 3 and sensor 4 have similar transceiver characteristics after pairing, sensor conversion efficiency with similar characteristics is similar, signal attenuation during signal transmission is similar, signal growth rate and phase displacement in signal processing circuit are similar. In this way, the accuracy of flow measurement can be better guaranteed.
It should be noted that the accuracy of flow measurement mainly depends on the sensor, but also has a great relationship with the base pipe segment and other process conditions. Therefore, the quantified value of the characteristic similarity of two paired sensors needs to be calibrated under the environment of the determined production process and relatively stable parameters, and the calibrated value can be used as the detection parameter in production practice.
4 Closing Remarks
Aiming at the problems of low accuracy and low qualified rate of ultrasonic flowmeter caused by poor consistency of sensor performance in the current production process of ultrasonic flowmeter, a test method for a pair of ultrasonic sensors on the same ultrasonic flowmeter is proposed through theoretical analysis and large amount of experiments. The basis of pair selection of transducer and the design method of key circuit for specific test are given. The experimental data analysis has proved the effectiveness of theoretical analysis and specific design. The device has been applied to production practice, and the production efficiency and yield have been greatly improved.
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