Design and experimental study of acceleration sensor based on PVDF piezoelectric film

Abstract

PVDF piezoelectric film is a piezoelectric polymer material with excellent properties. In this paper, the research status of PVDF piezoelectric thin film was summarized, and the working principle of PVDF piezoelectric thin film accelerometer was introduced. On the basis of optimizing parameters, a cantilever PVDF piezoelectric film accelerometer and signal processing circuit were designed. The signal processing circuit which was composed of amplifier circuit and filter circuit was analyzed experimentally,and the frequency characteristic diagram of the filter circuit was obtained.The amplitude-frequency and phase-frequency characteristics of the filter circuit were better in a certain range,which can meet the design requirements.Experiments on the designed PVDF piezoelectric film accelerometer were carried out, including the calibration experiment of the PVDF accelerometer and the performance test of the sensor.The analysis results showed that the sensor has high sensitivity and good linearity, and can meet the measurement requirements.

Keywords

Polyvinylidene Fluoride (PVDF)acceleration sensor signal processing circuit preferred parameters analysis

1. Preface

In 1969, Japanese scientist Kawai discovered a polymer material named polyvinylidene fluoride (PVDF), which had a strong piezoelectric effect making the study of piezoelectric polymers a historic turning point. PVDF film has aroused the great interest of researchers in various countries because of its advantages of strong piezoelectric performance, low acoustic impedance, wide frequency response, light quality, flexibility, good processing performance and low price [1, 2, 3]. Japan, The United States and Europe countries have made good progress in the preparation process, performance and practical application. Products have been used in mechanical measurement, structural modal measurement, medical measurement, tactile measurement and energy collection [4, 5, 6]. Since the development of PVDF thin films in China in the 1980s, some universities and research institutes have carried out research on PVDF piezoelectric sensors and developed relatively rapidly. At present, some research results have been put into production and application.

2. Working principle of PVDF thin film acceleration sensor

The acceleration sensor designed in this paper adopted a cantilever beam structure which was a free elastic element with one end fixed to the other. The sensor attached the PVDF film to the cantilever beam structure which was fixed on the shell, and the shell was bonded with the vibrator to vibrate. The acceleration sensor vibration cantilever beam was designed as shown in Fig. 1. The PVDF thin film material has a specification of 10 $\times$ 15 mm and the film thickness was 200 $\mu$ m. The electrode was printed on the PVDF film, the film layer was pressed on the polyester substrate, and two pressed terminals were riveted, and the pressed terminals were connected to the two electrodes respectively. Vibrating cantilever beams were pasted on rectangular support blocks, and the supporting materials were organic glass with an overall mass of less than 2 grams. Its structure was shown in Fig. 2.

Figure 1.

Structure of PVDF accelerometer cantilever.

Figure 2.

Structure diagram of the cantilever beam sensor.

The PVDF film acceleration sensor measures the motion parameters.Firstly, the acceleration was converted into a charge by means of an inertial system composed of mass, spring and damper, etc.., and then the charge was converted into a voltage output by subsequent signal processing electricity [7, 8, 9]. When the position of the piezoelectric film on the cantilever beam was fixed, the value of the sensitivity coefficient K was constant. Then the amount of charge generated by the PVDF sensor was linearly related to the acceleration.

$\displaystyle Q=ka$ (1)

Therefore, under the condition that the sensor material and structure were determined, piezoelectric thin film cantilevered beam sensors can be used to measure the vibration acceleration. The sensitivity coefficient can also be changed by changing the structural parameters of the beam such as the cantilever length, the width and the thickness of the beam.

3. Design of signal processing circuit for PVDF acceleration sensor

The PVDF piezoelectric film sensor needs to be connected to the measuring instrument to output the signal. Due to the high impedance of the sensor and the weak output signal, there was external interference that distorts the signal at the same time [10, 11, 12, 13]. In order to measure accurately, the signal processing circuit of the sensor was designed which consisted of an amplifier circuit and a filter circuit.

3.1 Design of amplifier circuit

The designed amplifier was an amplifier whose output voltage was proportional to the amount of input charge. The amplifier was a high-gain operational amplifier with feedback capacitance, which can convert weak charge signal output of the sensor into an amplified voltage signal, and can convert the high impedance output of the sensor into a low impedance output at the same time [14, 15, 16, 17]. The design of the amplifier circuit required low drift, broadband, high gain and high impedance input. According to the requirements, the operation amplifier was selected as CA3140, the feedback resistance $R_{1}$ was 100 M $\Omega$ , the input resistance $R_{2}$ was 1 M $\Omega$ , and the feedback capacitance C1 was 1000 pF. The circuit diagram of amplifier was shown in Fig. 3.

Figure 3.

Amplifier circuit.

In order to reduce experimental errors and follow-up experimental needs,the adjustable amplifier circui twas added on the basis of the amplifier circuit Fig. 3. The circuit diagram was shown in Fig. 4 below: where Rp was used as a potentiometer to adjust the gain, and the access part was blocked. The input part increases the capacitance in order to filter the polarization voltage. The circuit magnification was taken 6 times, $R_{1}=$ 100 K $\Omega$ , $R_{2}=$ 600 K $\Omega$ , Rp maximum resistance was 200 K $\Omega$ , and the magnification was about 6 to 8.

Figure 4.

Adjustable amplifier circuit.

3.2 Design of filter circuit

The Frequency signal in room and noise signal outside have a great influence on the sensor output during the research. Therefore, the filter circuit was designed to eliminate the influence of the interference signal on the sensor. A band pass filter was designed with a band pass frequency range of 50 Hz to 3000 Hz and a band stop edge frequency range of 20 Hz and 7500 Hz. The design process was as follows.

3.2.1 The bandpass filter implemented by using a low-to-high pass filter circuit cascade

The technical indexes of bandpass filter were divided into two independent technical indexes: low-pass filter circuit and high-pass filter circuit. respectively, low-pass filter circuit and high-pass filter circuit were designed, and the bandpass filter was obtained by cascade.

1)
The technical specifications of the low-pass filter circuit were set as follows:

$\displaystyle A_{p}=3∼{}\textit{dB},A_{s}=30∼{}\textit{dB},f_{P}=3000∼{}% \textit{Hz},f_{S}=7500∼{}\textit{Hz}.$

Among them, $A_{p}$ : the number of decibels attenuated in the pass band, $A_{s}$ : the number of decibels attenuated in the stop band, $f_{p}$ : the corresponding frequency for $A_{p}$ , $f_{s}$ : the corresponding frequencys for $A_{s}$ .
2)
The technical specifications of the high-pass filter circuit were set as follows:

$\displaystyle A_{p}=3∼{}\textit{dB},A_{s}=30∼{}\textit{dB},f_{P}=50∼{}\textit{% Hz},f_{S}=20∼{}\textit{Hz}$

Among them, the meaning of the symbol was the same as that of the low-pass filter circuit.
3.2.2 Use butterworth to calculate the order

1)
Order $N_{1}$ of the low-pass filter circuit

$\displaystyle N_{1}\geqslant\frac{\log\left[\sqrt{(10^{0.1A_{S}}-1)/(10^{0.1A_% {P}}-1)}\right]}{\log(f_{s}/f_{p})}$ (2)
2)
Order $N_{2}$ of the high-pass filter circuit

$\displaystyle N_{2}\geqslant\frac{\log\left[\sqrt{(10^{0.1A_{S}}-1)/(10^{0.1A_% {P}}-1)}\right]}{\log(f_{p}/f_{s})}$ (3)

After the substitution numerical calculation, $N_{1}=$ 4 and $N_{2}=$ 4. So the selected low-pass and high-pass filter circuits were fourth-order Butterworth filters.
3.2.3 Circuit selection and calculation of component parameters

Voltage Controlled Voltage Source (VCVS) filter circuit was one of the active filter circuits. Its operational amplifier was the same connection method, the advantages of this filter were high input impedance, low output impedance, strong load capacity, circuit performance stability and gain easy to adjust.Therefore, VCVS circuit was chosen here. The low-pass filter circuit and the high-pass filter circuit use the fourth-order Butterworth active filter circuit as shown in Figs 5 and 6. They were cascaded by two second-order Butterworth filter circuits.

Figure 5.

Low-pass filter.

In Fig. 5, the parameters of the designed low-pass filter circuit components were calculated as follows: $C=$ 330 pF, $R=$ 160 K $\Omega$ , At this point, the filter cut-off frequency $f$ was 3012 Hz. After calculation and analysis: $R_{1}=$ 576 K $\Omega$ and $R_{2}=$ 715 K $\Omega$ , and the nominal resistance values were 550 K $\Omega$ and 700 K $\Omega$ respectively; $R_{3}=$ 2425 K $\Omega$ and $R_{4}=$ 368 K $\Omega$ , and the nominal resistance values were 2.5 M $\Omega$ and 350 K $\Omega$ respectively. The voltage gain A0 of the fourth-order low-pass filter circuit is 2.55.

The fourth-order Butterworth high pass filter circuit was shown in Fig. 6. The design method was the same as the fourth-order Butterworth active low pass filter circuit. After calculation, the parameters of the high-pass filter circuit element were as follows: $C=$ 0.1 $\mu$ F, $R=$ 30 K $\Omega$ , $f=$ 53 Hz, $R_{1}=$ 110 K $\Omega$ , $R_{2}=$ 130 K $\Omega$ , $R_{3}=$ 500 K $\Omega$ , $R_{4}=$ 70 K $\Omega$ , $A_{0}=$ 2.45.

Figure 6.

High-pass filter.

3.2.4 Experimental analysis of filter circuit

Measurement of 100 mV input to filter circuit selected in experiment. The experimental data were fitted with Origin software, and the filtering effect was shown in Fig. 7. It shows the relationship between the voltage and frequency of the output when the filter input is 100 mv. Through the above graphic analysis, it can be found that the selected circuit has better amplitude frequency characteristics and phase frequency characteristics at frequencies 50 Hz to 3000 Hz, and can meet the requirements for the use of experimental sensor at frequencies 100 Hz to 2500 Hz.

Figure 7.

Frequency characteristic curve of the filter circuit in experiment.

4. The calibration experiment of PVDF acceleration sensor

According to the laboratory situation, the PVDF acceleration sensor was calibrated by the voltage comparison method. Its installation structure was shown in Fig. 8.

Figure 8.

Diagram of the calibration experiment.

Two acceleration sensors were mounted back to back on a rigid stand. The following was a reference standard acceleration sensor whose sensitivity and all technical properties were known. The upper one was the calibrated sensor. The vibration table was used to stimulate the sensor. After the signal was amplified, the impedance transformation, adjustment, and amplification were connected to the conversion switch. The voltmeter or oscilloscope were used to measure the output signal. Using the same acceleration a to stimulate them, their output voltages were:

$\displaystyle U_{s}=K_{s}a,U_{t}=K_{t}a$ (4)

In the formula, $U_{s}$ , $K_{s}$ were the output voltage and sensitivity of the standard sensor respectively. $U_{t}$ , $K_{t}$ were the output voltage and sensitivity of the calibrated sensor respectively. Therefore, it was available:

$\displaystyle K_{t}=\frac{U_{t}}{a}=\frac{U_{t}K_{s}}{U_{s}}$ (5)

The specific calibration method was as follows: Firstly, the conversion switch was placed in K ${}_{2}$ , and the sensitivity adjustment switch of the signal processing circuit was placed in the sensitivity position of the standard sensor, and the gain was 1 V/g (g represents 1 times of the gravitational acceleration). Set the excitation frequency of the vibration table to a certain frequency (usually 150 Hz), observed the voltmeter and oscilloscope output and adjust the excitation signal range of the signal source so that the voltmeter was indicated as an integer value (such as peak 1 V). So the voltmeter indicator was the vibration acceleration of the table. Change the conversion switch from K ${}_{2}$ to K ${}_{1}$ , which was then switched on by the calibration sensor to the voltmeter. By adjusting the signal processing circuit, the voltmeter was the same as the output of the standard sensor (note the gain file position). For example, if the voltmeter output was 1 V (peak), the charge amplifier gain was 1 V/g at the moment.

4.1 Process of experiment

The structural diagram of the experiment was shown in Fig. 8. The ES-2 electric vibration test system was used in the experiment which namely the electric vibration table. The signal processing circuit converted the charge signal of the sensor into a voltage signal and entered it into a digital oscilloscope or voltmeter. The standard sensor was JF-100C of JF Sensing Technology. Its charge sensitivity was 1.89 pC/ms ${}^{-2}$ , the frequency range was 1 Hz $\sim$ 12000 Hz, the temperature range was $-$ 20 ${}^{\circ}$ C $\sim$ 120 ${}^{\circ}$ C, the weight was 13 grams, and the central compression piezoelectric sensor. The excitation signal was generated by the RVC-2 vibration control instrument, which used the device to transmit the vibration sweep frequency to the vibration table, and received the feedback signal of the vibration table to make the vibration table sinusoidal vibration.

The length of the free end of the PVDF cantilever beam sensor was 5 mm. The dynamic calibration experiment was carried out according to the connection circuit of Fig. 8. The sensitivity of standard sensors and PVDF acceleration sensors at different accelerations were recorded according to the test principle. Set the vibration acceleration and vibration frequency of the vibration table, set the time to 5 to 6 minutes, and recorded the relevant data of the experiment. According to the preliminary calculation of the cantilever beam sensor with a length of 5 mm free end, its first-order natural frequency was 1500 Hz, so the selected test frequency was 50 Hz $\sim$ 2000 Hz. The operating frequency and sensitivity were calculated based on experimental data. Finally, the voltage value at different accelerations in the general environment (frequency 150 Hz, room temperature 20 ${}^{\circ}$ C $\sim$ 25 ${}^{\circ}$ C) were measured to obtain the relationship between the output voltage and the acceleration of the PVDF acceleration sensor.

4.2 Analysis of experiment result

At frequency of 150 Hz and room temperature 20 ${}^{\circ}$ C $\sim$ 25 ${}^{\circ}$ C, the relationship between output voltage and acceleration of PVDF sensor system at different vibration accelerations was shown in Table 1 (Due to experimental condition, the highest vibration acceleration was less than 10 g), and the actual acceleration was measured by the feedback signal of the vibration console.

Table 1
Output of PVDF in different accelerations

Preset acceleration (g)	Actual acceleration (g)	Voltage (mv)	Preset acceleration (g)	Actual acceleration (g)	Voltage (mv)
0	0	0	5	4.920	668.2
1	0.998	135	5.5	5.422	737.1
1.5	1.492	206.3	6	5.911	801.6
2	1.970	272.7	6.5	6.418	866
2.5	2.463	344.2	7	6.908	933.2
3	2.945	407.3	7.5	7.432	1001
3.5	3.470	476.2	8	7.892	1066
4	3.941	543.6	8.5	8.420	1136.2
4.5	4.437	607.6	9	8.873	1207.6

Analysis and processing of data in Table 1 using Origin software, and the method of linear minimum square fitting was adopted to approximate the actual data distribution. Finally, its voltage sensitivity was obtained as shown in Fig. 9.

Figure 9.

Voltage sensitivity of the PVDF film sensor.

Its fitting equation was:

$\displaystyle y=134.591x+6.663$ (6)

The performance of the sensor was as follows:

Full range output YFS:

$\displaystyle Y_{\textit{FS}}=1207.8∼{}\textit{mV}$ (7)

Sensitivity $K$ :

$\displaystyle K=\frac{\Delta y}{\Delta x}=134.59∼{}\textit{mV / g}$ (8)

Nonlinear error $e_{L}$ :

$\displaystyle e_{L}=\pm\frac{(\Delta y)\max}{Y_{\textit{FS}}}\times 100\%=\pm∼% {}0.571\%\textit{FS}$ (9)

From Fig. 9, the fitting curve basically passed through the coordinate origin, and the voltage sensitivity of the PVDF sensor was 134.59 mV/g. In the general environment, the vibration acceleration was in the range of 0 to 9 g. The output of PVDF film acceleration sensor had a good linear relationship with acceleration, and it was suitable for measuring acceleration less than 9 g.

5. Conclusions

PVDF film is an excellent piezoelectric sensor sensitive element material because of the advantages of strong piezoelectric performance, low acoustic impedance, wide frequency response, light quality, good processing performance and low price. A cantilevered beam acceleration sensor based on PVDF film was designed and manufactured in the paper. The designed sensor signal processing circuit consisted of an amplifier circuit and a filter circuit. In order to solve the external interference problem, the designed filter circuit adopted low-to-high pass filter circuit to realize bandpass filter. The experimental analysis of the band results were in a band frequency range of 50 Hz to 3000 Hz, and the effect was good.

Finally, the PVDF piezoelectric sensor was tested and studied by means of voltage comparison. The result showed that the signal processing circuit can meet the demand for PVDF sensor output signal, and the measurement system was feasible. It was concluded that its high sensitivity and good linearity can satisfy the vibration acceleration measurement of 0 $\sim$ 9 g in frequency range.

Footnotes

Acknowledgments

This paper was supported by the Young Innovative Talents Project of Guangdong Provincial Department of Education (2017KQNCX132), the Natural Science Foundation of Guangdong Province (2018A0303070004) and Maoming Science and Technology Plan Project (201613).

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Design and experimental study of acceleration sensor based on PVDF piezoelectric film

Abstract

Keywords

1. Preface

2. Working principle of PVDF thin film acceleration sensor

3.1 Design of amplifier circuit

3.2.1 The bandpass filter implemented by using a low-to-high pass filter circuit cascade

4.2 Analysis of experiment result

Table 1 Output of PVDF in different accelerations

Footnotes

Acknowledgments

References

Table 1
Output of PVDF in different accelerations