System and method for checking working characteristics of PVDF target embedded stress sensor

Abstract

The invention relates to a system and a method for checking working characteristics of a PVDF (polyvinylidene fluoride) target embedded stress sensor. Comprising a pressure-making oil cylinder arranged on a working platform of an excitation load applying device, the upper end of the pressure-making oil cylinder is provided with a calibration piston rod communicated with the interior of the oil cylinder, and the upper surface of a mounting platform of the calibration piston rod is rigidly connected with a PVDF stress sensor to be calibrated. The sensitive surface of the PVDF stress sensor to be calibrated is rigidly connected with a force transmission mass block of which the cross section is the same as that of the sensitive surface of the PVDF stress sensor; an acceleration sensor is installed on the lower surface of the installation platform of the piston rod for calibration, and a standard pressure sensor installation hole communicated with the interior of the pressure-making oil cylinder is formed in the side face of the pressure-making oil cylinder. A sensor is installed on the upper end face of a piston rod of a pressure-making oil cylinder, a load is applied to the sensor by knocking a force transmission mass block on the sensor, and equivalent stress generated in the stress sensor is calculated by monitoring the pressure amplitude generated in the oil cylinder through a standard pressure sensor installed on the side face of the oil cylinder. Therefore, the working characteristic detection of the PVDF stress sensor is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of sensor detection, and in particular relates to a system and method for calibrating the working characteristics of a PVDF target embedded stress sensor. Background Art

[0002] Testing the dynamic change of internal stress of concrete targets under implosion, penetration or external explosion loads is of great significance for the vulnerability assessment of targets. Using embedded PVDF stress sensors with good dynamic characteristics to test the internal dynamic stress of targets is a common method for testing the internal stress of concrete targets. Accurately obtaining the working characteristic parameters of PVDF stress sensors is of great significance for ensuring accurate measurement of stress.

[0003] The static characteristics of PVDF stress sensors are generally poor. Due to the existence of charge leakage, static calibration is not applicable to PVDF sensors. At present, the calibration of the working characteristic parameters of PVDF sensors is mostly quasi-static calibration. Quasi-static calibration is a common calibration method for stress sensors. When performing quasi-static calibration, it is usually necessary to install the sensitive surface of the sensor to be calibrated inside the pressure-building cylinder to ensure that a quasi-static pressure load can be applied to the sensor to be calibrated. This requires that the pressure-building cylinder needs to have a mounting interface that matches the sensor to be tested. PVDF stress sensors are usually fixed to the mounting bracket by bonding and do not have fastening structures such as mounting threads. In addition, due to the lack of a unified standard, the size and shape of PVDF stress sensors are not uniform. Most common PVDF stress sensors are not standard circular due to the existence of wire connectors. This makes it difficult to directly install PVDF stress sensors in the pressure-building cylinder. It is necessary to design and process the installation tooling according to the size and structure of the PVDF stress sensor, or even the entire pressure-building cylinder assembly. This brings great inconvenience to the detection and calibration of PVDF stress sensors, resulting in a waste of manpower and material resources, and greatly increases the time and difficulty required for sensor calibration. Summary of the invention

[0004] The object of the present invention is to provide a system and method for calibrating the working characteristics of a PVDF target embedded stress sensor.

[0005] The technical solution to achieve the purpose of the present invention is: a PVDF target embedded stress sensor working characteristic calibration system, including a pressure-building oil cylinder arranged on a working platform of an excitation load applying device, a calibration piston rod connected to the inside of the oil cylinder is arranged at the upper end of the pressure-building oil cylinder, a PVDF stress sensor to be calibrated is rigidly connected to the upper surface of the mounting platform of the calibration piston rod, a force transmission mass block with the same cross section as the sensitive surface of the PVDF stress sensor to be calibrated is rigidly connected to the sensitive surface of the PVDF stress sensor to be calibrated, and the force transmission mass block is a column; an acceleration sensor is installed on the lower surface of the mounting platform of the calibration piston rod, a pressure-building medium is filled inside the pressure-building oil cylinder, and a standard pressure sensor mounting hole connected to the inside of the pressure-building oil cylinder is provided on the side of the pressure-building oil cylinder for mounting a standard pressure sensor;

[0006] The excitation load applying device applies nearly half-sine force loads of different amplitudes to the upper surface of the force-transmitting mass block.

[0007] Furthermore, the area of ​​the mounting platform of the calibration piston rod is larger than the area of ​​the sensitive surface of the PVDF stress sensor to be calibrated.

[0008] Furthermore, the PVDF stress sensor to be calibrated is mounted on the mounting platform at the upper end of the calibration piston rod by gluing.

[0009] Furthermore, the operating frequency band of the standard pressure sensor is larger than the effective frequency band of the pressure signal in the pressure-generating oil cylinder.

[0010] Furthermore, multiple standard pressure sensors are set up at the same time, and the average of the test results of the multiple standard pressure sensors is used as the final measurement result.

[0011] Furthermore, the excitation load applying device is a pendulum-type hydraulic generating device. During calibration, the installed pressure-generating cylinder is placed on the working platform of the pendulum-type hydraulic generating device; and it is ensured that the landing point of the hammer head of the pendulum-type hydraulic generating device is on the central axis of the calibration piston rod; the pendulum-type hydraulic generating device controls the hammer head installed on the connecting rod to fall at different heights, thereby applying nearly half-sine force loads of different amplitudes to the upper surface of the force-transmitting mass block.

[0012] A method for calibrating the working characteristics of a PVDF target embedded stress sensor using the above system comprises the following steps:

[0013] Step (1): The connecting rod is rotated to a specified angle by a pendulum-type hydraulic generating device and the hammer is released; the hammer head will swing freely under the action of gravity and eventually hit the force transmission mass block, generating a nearly semi-sinusoidal stress load on the sensitive surface of the PVDF sensor to be calibrated. At the same time, the downward pressure is transmitted to the calibration piston rod through the PVDF sensor to be calibrated. The calibration piston rod compresses the pressure-generating medium inside the pressure-generating cylinder, generating a nearly semi-sinusoidal pressure signal in the pressure-generating cylinder;

[0014] Step (2): The acceleration sensor monitors the acceleration a(t) of the calibration piston rod, reads the pressure monitoring pressure value p(t) of the standard pressure sensor, and solves the equivalent stress curve of the PVDF stress sensor to be calibrated and the output curve of the PVDF stress sensor to be calibrated according to the dynamic model of the calibration system;

[0015] Step (3): adjusting the angle of the connecting rod and the hammer head to adjust the load generated in the PVDF stress sensor to be calibrated; obtaining the output curve of the PVDF stress sensor to be calibrated under stress load excitation of different amplitudes through multiple tests using the method of step (2);

[0016] Step (4): Relying on the least squares method, the equivalent stress and output value of the PVDF stress sensor to be calibrated are fitted to obtain the working characteristic curve of the PVDF stress sensor to be calibrated, and the working characteristic parameter index of the PVDF stress sensor to be calibrated is calculated to realize the calibration of the working characteristics of the PVDF stress sensor.

[0017] Furthermore, the kinetic model in step (2) is:

[0018] S2σ(t)-S1p(t)-f=(m1+m2+m3)(a(t)-g)

[0019] Where, S2 is the sensitive surface area of ​​the PVDF stress sensor, S1 is the cross-sectional area of ​​the calibration piston rod, σ(t) is the stress load on the PVDF stress sensor, p(t) is the internal pressure value of the pressure cylinder detected by the standard pressure sensor, f is the resistance encountered by the calibration piston rod when it moves, m1 is the mass of the calibration piston rod, m2 is the mass of the PVDF stress sensor, m3 is the mass of the acceleration sensor, a(t) is the test result of the acceleration sensor, and g is the acceleration of gravity;

[0020] Ignoring the influence of the resistance f under ideal lubrication conditions, the relationship between the stress load σ(t) of the PVDF stress sensor (4) to be calibrated, the pressure value p(t) obtained by monitoring the standard pressure sensor (2), and the acceleration a(t) obtained by monitoring the acceleration sensor (6) is:

[0021]

[0022] The equivalent stress curve of the PVDF stress sensor (4) and the output curve of the PVDF stress sensor are calculated by the above formula.

[0023] Compared with the prior art, the present invention has the following significant advantages:

[0024] The PVDF stress sensor is installed on the precision piston rod, and a standard pressure sensor is used to monitor the pressure generated inside the pressure-generating cylinder when the PVDF stress sensor is subjected to force. An acceleration sensor is installed on the calibration piston rod to realize piston rod acceleration monitoring, so as to further calculate the equivalent stress on the PVDF stress sensor, thereby realizing a comparative calibration of the stress sensor. In this calibration method, the installation of the PVDF stress sensor is flexible and convenient, and is closer to the installation method of the PVDF sensor in actual use, avoiding the need to redesign the pressure-generating cylinder with a corresponding installation interface for PVDF stress sensors of different specifications during calibration, making the calibration of the PVDF stress sensor more convenient. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the pressure-generating cylinder and its components of the PVDF target embedded stress sensor calibration system.

[0026] Figure 2 Schematic diagram of the installation of the pressure-generating cylinder on the pendulum-type hydraulic generating device.

[0027] Figure 3 Equivalent stress curve of PVDF stress sensor.

[0028] Figure 4 Output of PVDF strain sensor under equivalent stress.

[0029] Figure 5 Working characteristic curve of PVDF stress sensor.

[0030] Description of reference numerals:

[0031] 1-pressure cylinder, 2-standard pressure sensor, 3-calibration piston rod, 4-PVDF stress sensor to be calibrated, 5-force transmission mass block, 6-acceleration sensor, 7-hammer, 8-connecting rod, 9-pendulum hydraulic generating device. DETAILED DESCRIPTION

[0032] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0033] The present invention proposes a method for calibrating the working characteristics of a PVDF target embedded stress sensor. The method designs a calibration piston rod with a sensor mounting platform for the PVDF stress sensor to replace the piston rod of the pressure-generating cylinder in the original quasi-static calibration equipment. During calibration, the PVDF stress sensor is pasted on the mounting platform at the upper end of the calibration piston rod, and an acceleration sensor is installed on the piston rod to monitor the acceleration of the piston rod. A force transmission mass block is designed on the PVDF stress sensor, and the force transmission mass block is a cylinder with a cross section identical to the sensitive surface of the PVDF stress sensor, which is used to ensure that the PVDF stress sensor is subjected to uniform force during the calibration process. During calibration, a downward force is generated by hammering the upper end surface of the force transmission mass block through a pendulum or a drop hammer hydraulic generating device, thereby generating an approximately semi-sine stress load in the PVDF stress sensor. After the calibration piston rod is subjected to force, the pressure-generating medium inside the cylinder is compressed, and a corresponding pressure signal is generated inside the pressure-generating cylinder. A standard pressure sensor is installed on the side of the pressure-generating cylinder to monitor the pressure signal inside the pressure-generating cylinder. An acceleration sensor is installed on the calibration piston rod to monitor the acceleration change when the calibration piston rod moves. The stress load on the PVDF stress sensor can be calculated by monitoring the pressure curve in the oil cylinder. By comparing the output of the PVDF stress sensor and the stress load of the PVDF stress sensor calculated according to the calculation, the working characteristic parameters of the PVDF stress sensor can be obtained, thereby realizing the comparative calibration of the PVDF stress sensor.

[0034] The present invention proposes a method for calibrating the working characteristic parameters of a stress sensor embedded in a PVDF target. The system composition of the detection method is as follows: Figure 1 As shown, it includes a pressure-generating cylinder 1, a standard pressure sensor 2, a calibration piston rod 3, a PVDF stress sensor 4, a force-transmitting mass block 5 and an acceleration sensor 6;

[0035] The pressure-generating cylinder 1 has a pressure-generating medium inside its cavity, and a calibration piston rod 3 is installed on the pressure-generating cylinder 1. The upper end of the calibration piston rod 3 has a larger mounting plane for mounting the PVDF stress sensor 4, and the area of ​​the mounting plane is larger than the sensitive surface area of ​​the calibrated PVDF stress sensor. The PVDF stress sensor is mounted on the mounting plane at the upper end of the calibration piston rod 3 by gluing. The sensitive surface of the sensor faces upward during installation. The sensitive surface of the PVDF stress sensor remains coaxial with the piston rod during installation. An acceleration sensor mounting hole is provided under the mounting platform of the calibration piston rod 3 for mounting an acceleration sensor 6 to monitor the acceleration change a(t) of the calibration piston rod 3 during the calibration process. The calibration piston rod 3 is connected to the inside of the cylinder. When the calibration piston rod 3 is forced to move downward, it compresses the pressure-generating medium in the pressure-generating cylinder 1, thereby generating a corresponding pressure signal in the pressure-generating cylinder 1. The side of the pressure-building cylinder 1 has a standard pressure sensor mounting hole connected to the inside of the pressure-building cylinder 1, which is used to install a standard pressure sensor 2 to monitor the pressure p(t) inside the pressure-building cylinder 1 to obtain the equivalent stress σ(t) exerted on the PVDF stress sensor 4 during calibration.

[0036] Assume that there is a rigid connection between the calibration piston rod 3 and the PVDF stress sensor 4, and between the PVDF stress sensor and the force transmission mass block 5. The calibration piston rod 3 and the force transmission mass block 5 are rigid bodies. The calibration piston rod 3 and the PVDF stress sensor 4 are regarded as a whole, and their dynamic model can be expressed by formula (1).

[0037] S2σ(t)-S1p(t)-f=(m1+m2+m3)(a(t)-g) (1)

[0038] Where S2 is the sensitive surface area of ​​the PVDF stress sensor, S1 is the cross-sectional area of ​​the piston rod, σ(t) is the stress load on the PVDF stress sensor, p(t) is the internal pressure of the pressure cylinder 1 detected by the standard pressure sensor 2, f is the friction and other resistances encountered by the piston rod when it moves, m1 is the mass of the piston rod, m2 is the mass of the PVDF stress sensor, m3 is the mass of the acceleration sensor, a(t) is the test result of the acceleration sensor, and g is the acceleration of gravity.

[0039] The standard pressure sensor 2 is installed on the side of the pressure cylinder 1 to monitor the pressure changes in the pressure cylinder so as to indirectly measure the force applied to the piston rod. The standard pressure sensor 2 needs to be able to accurately measure the pressure generated in the pressure cylinder 1, and the working frequency band of the standard pressure sensor 2 needs to be greater than the effective frequency band of the pressure signal in the pressure cylinder. In order to improve the pressure monitoring accuracy, multiple standard pressure sensors can be set at the same time, and the average of the test results of multiple standard pressure sensors is used as the final measurement result. This improves the monitoring accuracy of the pressure in the pressure cylinder 1.

[0040] like Figure 2 As shown, a pendulum-type hydraulic generator 9 is used as an excitation load application device. During calibration, the installed pressure-generating cylinder is placed on the working platform of the pendulum-type hydraulic generator 9. Ensure that the landing point of the hammer head of the pendulum-type hydraulic generator 9 is on the central axis of the piston rod. The pendulum-type hydraulic generator 9 controls the hammer head 7 installed on the connecting rod 8 to fall at different heights, thereby applying an approximately half-sine force load of different amplitudes to the upper surface of the force transmission mass block 5. The force transmission mass block 5 converts the force load it receives into a uniform stress load σ(t) and applies it to the sensitive surface of the PVDF stress sensor 4. Under good lubrication conditions, the friction force in formula (1) is very small, and the influence of the friction force f can be ignored during calculation. At this time, according to formula (1), the relationship between the stress load σ(t) on the PVDF stress sensor 4, the pressure value p(t) obtained by monitoring the standard pressure sensor 2, and the acceleration a(t) obtained by monitoring the acceleration sensor 6 is:

[0041]

[0042] When the working characteristic parameters of the PVDF sensor are calibrated by this method, the control connecting rod 8 is rotated to a specified angle by the pendulum hydraulic generating device 9 and the hammer is released. The hammer head 7 will swing freely under the action of gravity and will eventually hit the force transmission mass block 5, thereby generating an approximately semi-sinusoidal stress load on the sensitive surface of the PVDF sensor 4. At the same time, the downward pressure is transmitted to the calibration piston rod 3 through the PVDF sensor 4. The piston rod compresses the pressure medium inside the pressure cylinder 1, thereby generating an approximately semi-sinusoidal pressure signal in the pressure cylinder 1, such as Figure 4 As shown. At the same time, the acceleration sensor 6 installed on the calibration piston rod 3 can monitor the acceleration a(t) of the piston rod during this process. The equivalent stress curve of the PVDF sensor 4 and the output curve of the PVDF stress sensor can be calculated according to the pressure monitoring result p(t) of the standard pressure sensor 2 and the acceleration a(t) of the calibration piston rod 3 through formula (2), as shown in Figure 3 Figure 4 As shown. By properly adjusting the angle of the connecting rod 8 and the falling angle of the hammer head 7, the magnitude of the force applied to the force transmission mass block 5 can be adjusted. Thus, the magnitude of the load generated in the PVDF stress sensor 4 can be adjusted. By using the above method and multiple tests, the output value of the PVDF sensor 4 under stress load excitation of different amplitudes can be obtained. By fitting the equivalent stress and output value of the PVDF sensor 4 with the least squares method, the working characteristic curve of the PVDF sensor 4 can be obtained as shown in Figure 5 As shown, the working characteristic parameter indicators such as sensitivity and linearity of the PVDF sensor 4 can be calculated to achieve the calibration of the working characteristics of the PVDF stress sensor.

Claims

1. A PVDF target embedded stress sensor working characteristic calibration system, characterized in that: The invention comprises a pressure-generating oil cylinder (1) arranged on a working platform of an excitation load applying device, a calibration piston rod (3) connected to the inside of the oil cylinder is arranged at the upper end of the pressure-generating oil cylinder (1), a PVDF stress sensor (4) to be calibrated is rigidly connected to the upper surface of the mounting platform of the calibration piston rod (3), a force transmission mass block (5) having the same cross section as the sensitive surface of the PVDF stress sensor (4) to be calibrated is rigidly connected to the sensitive surface of the PVDF stress sensor (4), and the force transmission mass block (5) is a column; an acceleration sensor (6) is installed on the lower surface of the mounting platform of the calibration piston rod (3), the inside of the pressure-generating oil cylinder (1) is filled with a pressure-generating medium, and a standard pressure sensor mounting hole connected to the inside of the pressure-generating oil cylinder (1) is provided on the side of the pressure-generating oil cylinder (1) for mounting a standard pressure sensor (2); The excitation load applying device applies nearly half-sine force loads of different amplitudes to the upper surface of the force transmission mass block (5).

2. The calibration system according to claim 1, characterized in that: The area of ​​the mounting platform of the calibration piston rod (3) is larger than the area of ​​the sensitive surface of the PVDF stress sensor (4) to be calibrated.

3. The calibration system according to claim 2, characterized in that: The PVDF stress sensor (4) to be calibrated is mounted on the mounting platform at the upper end of the calibration piston rod (3) by gluing.

4. The calibration system according to claim 3, characterized in that: The working frequency band of the standard pressure sensor (2) is greater than the effective frequency band of the pressure signal in the pressure-generating oil cylinder (1).

5. The calibration system according to claim 4, characterized in that: A plurality of standard pressure sensors are set at the same time, and the average of the test results of the plurality of standard pressure sensors is taken as the final measurement result.

6. The calibration system according to claim 5, characterized in that: The excitation load applying device is a pendulum-type hydraulic generating device (9). During calibration, the installed pressure-generating cylinder is placed on the working platform of the pendulum-type hydraulic generating device (9); and the falling point of the hammer head (7) of the pendulum-type hydraulic generating device (9) is ensured to be on the central axis of the calibration piston rod (3); the pendulum-type hydraulic generating device (9) controls the hammer head (7) installed on the connecting rod (8) to fall at different heights, thereby applying nearly half-sine force loads of different amplitudes to the upper surface of the force-transmitting mass block (5).

7. A method for calibrating the working characteristics of a PVDF target embedded stress sensor using the system according to any one of claims 1 to 6, characterized in that: The steps include: Step (1): The connecting rod (8) is rotated to a specified angle by a pendulum-type hydraulic generating device (9) and the hammer is released; the hammer head (7) will swing freely under the action of gravity and finally hit the force transmission mass block (4), generating a nearly semi-sinusoidal stress load on the sensitive surface of the PVDF sensor (4) to be calibrated, and at the same time, the downward pressure is transmitted to the calibration piston rod (3) through the PVDF sensor (4) to be calibrated, and the calibration piston rod (3) compresses the pressure-generating medium inside the pressure-generating cylinder (1), generating a nearly semi-sinusoidal pressure signal in the pressure-generating cylinder (1); Step (2): The acceleration sensor (6) monitors the acceleration a(t) of the calibration piston rod (3), reads the pressure monitoring pressure value p(t) of the standard pressure sensor (2), and solves the equivalent stress curve of the PVDF stress sensor (4) to be calibrated and the output curve of the PVDF stress sensor (4) to be calibrated according to the dynamic model of the calibration system; Step (3): adjusting the angle at which the connecting rod (8) and the hammer head (7) fall, thereby adjusting the magnitude of the load generated in the PVDF stress sensor (4) to be calibrated; obtaining the output curve of the PVDF stress sensor (4) to be calibrated under stress load excitation of different amplitudes through multiple tests using the method of step (2); Step (4): Using the least square method, the equivalent stress and output value of the PVDF stress sensor (4) to be calibrated are fitted to obtain a working characteristic curve of the PVDF stress sensor (4) to be calibrated, and working characteristic parameter indicators of the PVDF stress sensor (4) to be calibrated are calculated to achieve calibration of the working characteristics of the PVDF stress sensor.

8. The method according to claim 7, characterized in that The kinetic model in step (2) is S2σ(t)-S1p(t)-f=(m1+m2+m3)(a(t)-g) Where, S2 is the sensitive surface area of ​​the PVDF stress sensor, S1 is the cross-sectional area of ​​the calibration piston rod, σ(t) is the stress load on the PVDF stress sensor, p(t) is the internal pressure value of the pressure cylinder detected by the standard pressure sensor, f is the resistance encountered by the calibration piston rod when it moves, m1 is the mass of the calibration piston rod, m2 is the mass of the PVDF stress sensor, m3 is the mass of the acceleration sensor, a(t) is the test result of the acceleration sensor, and g is the acceleration of gravity; Ignoring the influence of the resistance f under ideal lubrication conditions, the relationship between the stress load σ(t) of the PVDF stress sensor (4) to be calibrated, the pressure value p(t) obtained by monitoring the standard pressure sensor (2), and the acceleration a(t) obtained by monitoring the acceleration sensor (6) is: The equivalent stress curve of the PVDF stress sensor (4) and the output curve of the PVDF stress sensor are calculated by the above formula.

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