A Method for Determining the Parallel Capacitance Value When Testing the PVDF Charge Mode under Explosion Shock Loads

Abstract

The present invention discloses a method for determining the parallel capacitance value when a PVDF charge mode tests an explosive shock load. This method uses an impact test device to simulate the explosive load and applies the same impact load to the PVDF. In each test, the PVDF is respectively connected in parallel with different capacitors. Secondly, the repeatability and variability of the test results are respectively evaluated by combining the intra-group correlation coefficient and the coefficient of variation. When the evaluation results meet the pre-determined test accuracy requirements, it can be considered that the influence of the electrical characteristics of the test circuit on the charge distribution can be ignored, that is, the reasonable value range of the parallel capacitance is determined. At this time, the explosive shock load obtained by the traditional charge mode calculation method is accurate. This method avoids the defects brought about by using a fixed parallel capacitance in the past and ignoring the influence of the test circuit and the test environment, and can obtain more accurate output charges, thereby improving the accuracy and applicable range of PVDF impact load testing.

Description

Technical Field

[0001] The present invention relates to the field of explosive shock load measurement, and particularly to a method for determining the parallel capacitance value when measuring explosive shock load in the PVDF charge mode. Background Art

[0002] The measurement of explosive shock load can completely reflect the entire loading process applied to a specific object, which is of great significance for understanding the dynamic mechanical properties of various materials or structures. Therefore, it is of great importance for the research in related fields such as rock blasting engineering and terminal effect analysis. Due to its high sensitivity, good flexibility, and ability to adapt to various complex test environments, polyvinylidene fluoride (PVDF) piezoelectric film is one of the most commonly used sensors in the field of shock load measurement.

[0003] When conducting explosive shock load tests, due to advantages such as intuitive output signals, the ability to reflect stress changes without integration, and easy estimation of the range, the most commonly used method is to form a circuit by connecting a capacitor of appropriate size in parallel with the PVDF, namely the so-called "charge mode". At this time, it is considered that the charge on the two plates of the parallel capacitor is the charge released by the PVDF under the action of the shock load.

[0004] Currently, when conducting explosive shock load tests, the influence of the parallel capacitance value on the test results has not been seriously considered. Usually, a relatively fixed 10 - 100 nF is simply adopted according to past experience. In fact, due to factors such as the capacitance of the PVDF itself, parasitic capacitance in the test circuit, and external interference, the charge generated by the PVDF under the action of the explosive shock load will be widely distributed in the test circuit. As one of the most critical components in the charge mode, the parallel capacitance value will significantly affect the charge distribution characteristics in the test circuit, thereby affecting the test results. And since the interference capacitance exists in the test circuit in parallel form, choosing a larger parallel capacitance value can significantly reduce its influence. However, when its value is too large, it will result in a smaller test signal, which will cause the test results to be vulnerable to environmental noise interference on the one hand, reducing the test accuracy, and on the other hand, the low-voltage signal will lead to a reduction in the resolution of the test results.

[0005] Therefore, in order to improve the accuracy and reduce the influence of other factors, a suitable parallel capacitance should be selected according to specific test conditions during actual tests, so as to improve the accuracy and reliability of the test results. Summary of the Invention

[0006] To solve the above problems, the present invention proposes a method for determining the parallel capacitance value when measuring strong shock load in the PVDF charge mode.

[0007] To achieve the above objectives, the present invention adopts the following technical solutions:

[0008] A method for determining the parallel capacitance value when a PVDF charge mode tests an explosion shock load, comprising the following steps:

[0009] The first step: Determine the required precision requirement P for the test, select a PVDF sensor with a certain thickness and area, and test its capacitance value C P , and then connect the parallel capacitance C 1 in parallel to both ends thereof through a wire, and at the same time connect an oscilloscope in parallel to C 1 both ends;

[0010] The second step: Determine the shock test device for simulating the explosion load, install the PVDF sensor at the position to be measured, apply a certain shock load to the PVDF, and collect the time history curve V-t of the voltage at both ends of the parallel capacitance under the action of the shock load through the oscilloscope. Before using the oscilloscope, its input impedance should be set to 1 MΩ, and the range should be adjusted accordingly according to C 1 to reduce the error caused by zero drift, and the sampling rate should satisfy the Shannon sampling theorem;

[0011] The third step: Combine the traditional charge mode calculation method to obtain the change process of the accumulated charge Q on the two plates of the parallel capacitance C 1 with time t and the maximum charge quantity Q max ;

[0012] Q = C 1 V

[0013] The fourth step: Repeat steps 2-3, change the capacitance value C 1 connected in parallel with the PVDF, keep the test environment and the applied shock load unchanged, and obtain the Q-t curve and Q 1 under different C max conditions;

[0014] The fifth step: Use the intra-group correlation coefficient ICC to calculate the repeatability of the Q-t curve, and use the coefficient of variation C V to evaluate the variation degree of the maximum charge Q max . If the two evaluation results do not meet the test precision requirement P, the data groups with high dispersion degree should be excluded and the ICC and C V of the data groups after exclusion should be recalculated;

[0015] The sixth step: Repeat step 5 to gradually exclude data until ICC and C V meet P, so as to determine the reasonable parallel capacitance value range C L < C 1 < C U, at this time, the charges generated by PVDF under the action of explosion shock load can be approximately considered to be concentrated on the two plates of the parallel capacitor, and the charge amounts distributed in other parts of the test circuit can be ignored. The explosion shock load applied to PVDF can be accurately obtained through the traditional charge mode calculation method.

[0016] Further improvement: In the second to fourth steps, it is necessary to obtain the charge output characteristics of PVDF when connected to different parallel capacitors. Therefore, the discreteness of the applied shock load should be minimized. However, for some test devices, the repeatability of the shock load may be relatively poor. For example, for the Split Hopkinson Pressure Bar (SHPB), it is very difficult to keep the peak stress consistent in repeated experiments. At this time, the peak stress can be measured by devices such as strain gauges, and then the output charge can be normalized with the peak stress, and the ICC and C V are evaluated.

[0017] Further improvement: In the charge mode, both too large and too small parallel capacitors will reduce the accuracy of Equation (1). Therefore, in the fifth step, the approximate reasonable capacitance range should be qualitatively determined first according to the Q-t curves and Q max of all the data, and then, in the order of size, the test results with too large and too small capacitance values in the data group should be excluded first, and finally the reasonable parallel capacitance value range is gradually determined.

[0018] Further improvement: The ICC used to evaluate the repeatability of the Q-t curve in the fifth step should adopt a two-way random model, the absolute agreement type, and the single measurement estimation result.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The present invention takes into account the influence of factors such as PVDF capacitance, parasitic capacitance of the test circuit, and external interference on the distribution characteristics of PVDF charges in the test circuit, and jointly uses statistical parameters to quantitatively evaluate the magnitude of the influence. Based on this, the reasonable parallel capacitance value range is determined. The method provided by the present invention avoids the errors caused by simply using a fixed parallel capacitance while ignoring the test conditions in the past, and is of great significance for improving the accuracy and reliability of test results.

[0021] 2. The present invention simulates the generation of explosion shock load by using an impact test device, avoiding the disadvantages of complex explosion test environment and poor repeatability of test results. At the same time, the present invention examines the repeatability and variability of the output signals when connected to different parallel capacitors. Therefore, it only requires that the applied shock load has a similar loading waveform. When the peak load of repeated shocks can be kept unchanged, it does not require it to be known. However, when the peak load is difficult to keep unchanged, the output signal can also be normalized first with the load peak, and then the reliability and stability of the normalized result can be evaluated. The present invention has the advantages of being easy to operate and having wide applicability. Description of the Drawings

[0022] Figure 1 It is a schematic diagram of the PVDF charge mode;

[0023] Figure 2 It is the unit stress charge-time curve on a typical parallel capacitor under the action of SHPB impact load;

[0024] Figure 3 It is the maximum charge per unit stress on different parallel capacitors under the action of SHPB impact load;

[0025] Figure 4 It is the change diagram of the within-group correlation coefficient of the charge-time curve of different data groups and the coefficient of variation of the maximum charge amount during the elimination process. Detailed Implementation Modes

[0026] The present invention will be further explained below in conjunction with the embodiments and the drawings, but this is not used as a limitation to the protection scope of the present application.

[0027] Considering that on the one hand, the on-site environment of the explosion test is complex and the test conditions are difficult to control; on the other hand, commonly used impact test devices such as Hopkinson pressure bars and light gas guns have been proven to be able to well simulate the explosion shock load. Therefore, the present invention intends to use a reasonable impact test device to simulate the generation of the explosion load, and based on this, compare the influence of the parallel capacitor value on the test circuit signal output under the same impact conditions, and use relevant statistical parameters, such as: coefficient of variation CV and within-group correlation coefficient ICC, to conduct an overall evaluation of the test results of different capacitor values, so as to determine the range of reasonable parallel capacitor values; the method provided by the present invention avoids the errors caused by simply using a fixed parallel capacitor while ignoring the test conditions in the past, and has great significance for improving the accuracy and reliability of the test results.

[0028] Embodiment:

[0029] (1) Use a Hopkinson pressure bar SHPB with a diameter of 50 mm to simulate the generated explosion shock load. According to the test requirements, determine the accuracy requirement P≥0.95, select a PVDF piezoelectric sensor with a thickness and diameter of 30 μm and 50 mm respectively, and use a multimeter to measure its capacitance value C p =6.68 nF, and connect it in parallel with a 100 nF capacitor and an oscilloscope as shown in Figure 1 . The actual capacitance value of the parallel capacitor is measured with a multimeter before use;

[0030] (2) Install PVDF on the end face of the SHPB transmission rod, adjust the nitrogen gas pressure to 0.4 MPa for the impact test, collect the voltage signal V-t released by PVDF through the parallel capacitor under this test condition by an oscilloscope, adjust the oscilloscope range to 50 V, the sampling rate to 1 M / s, and obtain the time history curve σ-t of the impact load applied by SHPB through the strain sensor pasted on the transmission rod;

[0031] (3) Calculate the amount of charge Q(t) and the maximum charge Qmax accumulated on the two plates of the parallel capacitor C1 during the impact process by combining formula (1);

[0032] Q = C 1 V(1);

[0033] (4) Repeat steps 2-3, use the same PVDF to be connected in parallel with capacitors of 1 nF, 5 nF, 10 nF, 50 nF, 100 nF, 500 nF, 1×103 nF, 5×103 nF, 1×104 nF, and 5×104 nF respectively, combine the oscilloscope to collect the voltage-time curve V-t at both ends when connecting different capacitance values; calculate the Q-t curve and Qmax on each parallel capacitor by combining formula (1), and then standardize them with the peak value σmax of the impact load, and the results after standardization are recorded as Q1-t and Q1max. Typical results are as Figure 2 、 3 shown. The actual capacitance values of all parallel capacitors are measured by a multimeter before use;

[0034] (5) By combining Appendix Figure 2 、Appendix Figure 3 It can be preliminarily determined that the repeatability of the test results is better when the parallel capacitor is in the range of 500 - 5×103 nF. The dispersion degree of the test results is larger when the parallel capacitance value is above 104 nF and below 10 nF. Subsequently, use the intraclass correlation coefficient ICC, two-way random model, absolute agreement type, single measurement estimation result, and coefficient of variation CV to comprehensively evaluate the repeatability of all Q1-t curves and the variation degree of Q1max;

[0035] (6) The calculation results of the fifth step show that the overall results do not meet the requirements of the test accuracy P. Combining the qualitative observation of the test results and the order of the parallel capacitor size in the fifth step, gradually eliminate the test results with larger dispersion degrees in the data group, and calculate the ICC and CV of the data group after elimination respectively. The results are as Figure 4 shown. When ICC≥0.95 and Cv≤0.05, it can be considered that a reasonable range of parallel capacitor values is obtained. At this time, it is considered that the charges generated by PVDF under the impact load are concentrated on the two plates of the parallel capacitor, and the distribution of charges in other parts of the test circuit can be ignored. The accurate explosion shock load can be obtained through formula (1); The elimination results are asFigure 4 As shown, for the convenience of expression, the condition Cv≤0.05 is equivalent to 1 - Cv≥0.95. It can be seen that in this embodiment, the reasonable value range of the parallel capacitance is 500 - 5×103 nF, and when the parallel capacitance is 500 - 1000 nF, the accuracy will be further improved.

[0036] The above embodiments are only used to illustrate the basic principles, main features and advantages of the present invention. Those of ordinary skill in the art can still modify or equivalently replace the specific implementation manners of the present invention. Any such modification or equivalent replacement that does not depart from the spirit and scope of the present invention is within the scope of protection of the claims of the present invention.

Claims

1. A method for determining the parallel capacitance value when testing the explosion shock load in the PVDF charge mode, characterized in that it includes the following steps: 1) Determine the required precision requirement P for the test, select a PVDF sensor with a certain thickness and area, and test its capacitance value C P , and then connect the parallel capacitor C 1 in parallel to its two ends through a wire, and at the same time connect the oscilloscope in parallel to both ends of C 1 ; 2) Determine the impact test device for simulating the explosion load, install the PVDF sensor at the position to be measured, apply an impact load to the PVDF, and observe the voltage V across the parallel capacitor under the impact load through an oscilloscope. Before using the oscilloscope, set its input impedance to 1 MΩ, and the range should be adjusted accordingly according to C 1 to reduce the error caused by zero drift, and the sampling rate should satisfy the Shannon sampling theorem; 3) Combine with the traditional charge mode calculation method to obtain the parallel capacitance C during the impact loading process 1 The change process of the cumulative charge Q on the two electrodes with time t and the maximum charge amount Q max , Q = C 1 V; 4) Repeat steps 2 - 3, changing the capacitance value C in parallel with PVDF 1 , keeping the test environment and the applied impact load unchanged, and obtaining the Q-t curves and Q 1 under different C max ; 5) Calculate the repeatability of the Q-t curve using the intraclass correlation coefficient ICC, and use the coefficient of variation C V to evaluate the maximum charge Q max variation degree. If the two evaluation results do not meet the test accuracy requirement P, the data group with high dispersion degree should be excluded and the ICC and C of the data group after exclusion should be recalculated V ; 6) Repeat step 5 to gradually eliminate data until both ICC and C V satisfy P, thereby determining the reasonable range of parallel capacitance value C under the test conditions of steps 1 - 3 L <C 1 <C U , at this time, the charges generated by PVDF under the action of impact load can be approximately considered to be concentrated on the two plates of the parallel capacitance, and the electric charges distributed in other parts of the test circuit can be ignored. The explosion shock load applied to PVDF can be accurately obtained through the traditional charge mode calculation method.

2. The method for determining the parallel capacitance value when testing the explosion shock load in the PVDF charge mode according to claim 1, characterized in that: In the second to fourth steps, it is necessary to obtain the charge output characteristics of PVDF when connecting different parallel capacitors, reduce the discreteness of the applied impact load, test the peak stress through a strain gauge device, and then standardize the output charge with the peak stress, and evaluate it with the ICC and C per unit stress. V Evaluate.

3. The method for determining the parallel capacitance value when testing the explosion shock load in the PVDF charge mode according to claim 1, characterized in that: Therefore, in the fifth step, based on the Q-t curves of all the data and Q max First, qualitatively determine the reasonable capacitance range. Then, in the order of capacitance magnitude, first eliminate the test results with too large or too small capacitance in the data set. Finally, gradually determine the reasonable range of the parallel capacitance value.

4. The method for determining the parallel capacitance value when testing the explosion shock load in the PVDF charge mode according to claim 1, characterized in that: In the fifth step, the ICC used to evaluate the repeatability of the Q-t curve should adopt a two-way random model, consistent type, and single measurement estimation result.

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