Spring-like Triboelectric Nanogenerator for Monitoring Body Vibration State of the Ship Power Equipment

Abstract

In the navigation process, monitoring the running state of ship power plant equipment is crucial. In bad weather, when the critical equipment is abnormal, it is especially necessary to find out the root cause of the failure as soon as possible. In this case, it is required to use rapid detection equipment to detect and judge the key parameters. This paper proposes a vibration sensor (VS-TENG) of triboelectric nanogenerators based on spring vibration. The sensor adopts the spring structure inside and vibrates with the ship power equipment to collect the low-frequency vibration energy. This paper uses the VS-TENG sensors of two different spring parameters to study the electrical signal output under the excitation conditions of varying vibration frequencies. The results show that in the frequency range of 3–500 Hz, the efficient processing of different vibration excitation frequency signals can be realized, and the vibration frequency can be accurately identified. The error of medium-high frequency identification in VS-TENG is less than 1%. Especially at the resonant frequency, the maximum voltage output value can be achieved. On the PT500 Mini test bench, VS-TENG can reasonably identify the motor frequency and shutdown state. Therefore, VS-TENG can be applied to the condition monitoring of the vibration of the ship’s power plant and has a broad application prospect.

1. Introduction

Under standard sailing circumstances, monitoring the ship’s power equipment to understand its operation status is imperative. Marine engineers can judge the running state of the device according to the pressure gauge, flowmeter, and thermometer in the power plant system. However, in bad weather, it is vital to find out the root cause of the failure as soon as possible. Vibration monitoring data are essential for mastering the equipment’s vibration state. It can provide the most intuitive operating state parameters and reflect information on equipment in abnormal or fault conditions [1]. The existing application monitoring sensors mainly include piezoelectric [2,3], magnetoelectric [4], electromagnetic [5], and fiber optic sensors [6]. Piezoelectric transducers have a simple structural design but meager output power. Magnetoelectric and electromagnetic sensors are large and unsuitable for low-frequency vibration energy harvesting. Fiber optic sensors are expensive. Most of the natural frequencies of environmental vibration are below 50 Hz. It is difficult for the sensors of these technologies to collect the environmental energy of low-frequency vibration for self-powered, and they need to rely on external power sources [7].

The triboelectric nanogenerator (TENG) is a new type of power generator. Using triboelectric nanogenerators as vibration sensors can achieve a self-power supply, effectively solving the problem of using external power sources. Based on the principle of the Maxwell displacement current, TENG technology can effectively convert low-frequency and low-amplitude mechanical energy into electrical energy [8,9,10]. In recent years, triboelectric nanogenerators have aroused the interest of many researchers due to their excellent performance in self-powered sensing in different fields. Compared with traditional vibration sensors, triboelectric nanogenerators have lower manufacturing costs and a longer service life, which can significantly reduce maintenance costs [11,12]. As a self-powered sensor, TENG has been widely studied in the fields of sensing amplitude [13], acceleration [14], angle [15], wind speed [16], sound [17], human motion [18], and human pulse [19]. The vibration sensor of the triboelectric nanogenerator is small in size and light in weight. It can be installed in various parts of the body vibration of the ship power plant without affecting the regular operation of the machine. In addition, in the frequency range of environmental vibration, TENG, as a body vibration sensor, has a fast response speed and can accurately measure the body vibration signal. As a self-powered sensor, the triboelectric nanogenerator can be used to light LED lights and drive small electronic devices such as thermometers, hygrometers, and speedometers [7,20,21].

In the research of self-powered sensors using spring-structured triboelectric nanogenerators, the triboelectric nanogenerators using springs [22], elastic steel [23], and spring spiral structures [24], which are studied to effectively capture energy from the environmental vibration. Triboelectric nanogenerators can also be used further for fault warning, which can convert pipeline vibration into the sensor’s power [25]. Vibration characteristics can be monitored continuously in real-time, and the danger can be warned [26]. They can also be used as a self-powered dynamic displacement monitoring system to accurately sense the vibration acceleration and send an alarm signal [27]. At present, there are few studies on the application of TENG in self-powered sensor of marine power plants.

Each fault has different characteristic frequencies, which can intuitively represent the distribution of vibration sources in the frequency domain [28]. Frequency analysis is a diagnostic method that helps to identify failure modes. This paper proposes the vibration sensor, the triboelectric nanogenerator (VS-TENG), based on spring vibration, which can realize the collection of vibration energy of ship power plant body, especially in the vibration energy collection below 50 Hz. The VS-TENG is experimentally studied at different vibration frequencies, which can realize the efficient identification of different body vibration frequency signals. The results show that VS-TENG can be effectively identified in the 3–500 Hz frequency range, and the medium-high frequency error is less than 1%. VS-TENG is suitable for the detection and early warning of the vibration in the marine power plant’s body and has broad application prospects.

2. Structure and Working Principle

2.1. Structure of VS-TENG

For various common types of ships, mainly including container ships, bulk cargo ships, ordinary cargo ships, teaching boats, the predetermined base frequency signal (f) for diesel engine vibrations falls within the range of 10–40 Hz. This frequency is determined based on the essential diesel engine equipment installed on the ship [29]. Please refer to

Table 1. Parameters of Main Engine Equipment for Different Ship Types.

Ship Type	Equipment Model	Power (kW)	Cylinder Number	Rotational Speed (rpm)	Vibration Base Frequency (Hz)
Container Ship	12K98ME-C (MAN B&W, Munich, Germany)	72,240	12	104	20.8
Bulk cargo Ship	6S50MC-C (MAN B&W, Munich, Germany)	9480	6	127	12.7
Ordinary Cargo Ship	6RT-flex58T-D (Wärtsilä China, Shanghai, China)	10,850	6	105	10.5
Teaching Boat	6S35MC Mk7 (MAN B&W, Munich, Germany)	4440	6	173	28.83

for specific parameters.

Equation (1) indicates that the frequency of the base signal (f) of diesel engine vibrations is determined by the engine speed (n), the number of cylinders (i), and the range number ($\mathsf{\tau }$).

$$\mathrm{f}=\frac{\mathrm{i}\times \mathrm{n}}{60\times (\mathsf{\tau }/2)}$$

(1)

Springs possess excellent elastic recovery ability, strength, and durability. They exhibit particular sensitivity to frequencies within the intermediate range. Thus, this paper proposed a self-powered vibration sensor named VS-TENG, utilizing two different spring parameters for comparison experiments. See

Table 2. Parameters of Different Springs.

Spring Parameter	Spring Wire Diameter (mm)	Spring Outer Diameter (mm)	Spring Length (mm)	Circle Number
0.8-13-25-06	0.8	13	25	6
0.8-13-24-08	0.8	13	24	8

for specific parameters.

Figure 1a displays the application scenario diagram of the VS-TENG, which is placed on the shell of ship power equipment to collect signals through body vibration. The output signal from the VS-TENG undergoes initial filtering to eliminate irregular noise signals. Subsequently, the signal is subjected to Fourier transformation processing to extract frequency signal information. Analyzing the distribution of frequency signals offers insight into the operation of the device. Figure 1b depicts the structure diagram of the VS-TENG, including the shell and internal detection components. The detection component comprises a spring that deforms under vibration. The top of the spring is firmly attached to the acrylic plate containing the triboelectric nanogenerator sensor. Moreover, the lower section of the spring is fixed within the inner shell using a convex attachment. The lower part of triboelectric nanogenerator sensor consists of the bottom electrode Al (Aluminum) affixed with PTFE (Polytetrafluoroethylene) material on its top surface using double-sided glue. Another component of the sensor, the bottom surface of the top cover is equipped with a top electrode (Al), while a gap exists between the PTFE material and the top electrode.

2.2. Theoretical Analysis

The initial step of VS-TENG spring vibration is collecting external energy as input. When vibrating externally, VS-TENG converts the vibration energy into internal electrostatic energy, eventually converted into electrical energy as output. The essence of VS-TENG vibration is the elastic motion of an inertial block under mechanical vibration. It is assumed that the VS-TENG vibration is two-dimensional, and only the vertical displacement of the inertia block is considered to simplify the vibration process. As a result, the VS-TENG system can be modeled as a single-degree-of-freedom particle forced vibration system, represented by the following dynamic Equation (2) [30].

$$\mathrm{m}\ddot{x}+c\dot{x+kx=F_a}\cos \omega t$$

(2)

where $t,$ m, $c$ and $k$ are time, equivalent mass, damping and stiffness, $x$ is the relative displacement, $F_a$ is the external excitation amplitude, and $\omega$ is the external vibration angular frequency.

Under external excitation, the TENG generates periodic charge and voltage signals in vertical separation mode. In the dielectric materials PTFE and air gap, the corresponding electric fields are given by Equations (3) and (4) [31,32].

$$E_2=-\frac{\mathrm{Q}}{S{\epsilon }_0{\epsilon }_{r2}}$$

(3)

$$E_{air}=\frac{-\mathrm{Q}/S+\sigma \left(t\right)}{{\epsilon }_0},$$

(4)

where $E_2$, $E_{air}$ is the corresponding electric field inside PTFE and air gap, respectively; $S$ is the contact area between two materials; ${\epsilon }_0$ is the dielectric constant of vacuum and ${\epsilon }_{r2}$ is the dielectric constant of PTFE. The relationship between voltage (V), charge ($\mathrm{Q}$), and displacement ($x$) of the model can be obtained by using the following equation [31,32,33].

$${\mathrm{V}={\mathrm{E}}_2{\mathrm{d}}_2+\mathrm{E}}_{\mathrm{a}\mathrm{i}\mathrm{r}}x=-\frac{\mathrm{Q}}{\mathrm{S}{\mathsf{\epsilon }}_0}(\frac{{\mathrm{d}}_2}{{\mathsf{\epsilon }}_{\mathrm{r}2}}+x(\mathrm{t}))+\frac{\mathsf{\sigma }x(\mathrm{t})}{{\mathsf{\epsilon }}_0}$$

(5)

$${\mathrm{V}}_{\mathrm{O}\mathrm{C}}=\frac{\mathsf{\sigma }x\left(\mathrm{t}\right)}{{\mathsf{\epsilon }}_0}$$

(6)

$$\mathrm{C}=\frac{\mathrm{S}{\mathsf{\epsilon }}_0}{\frac{{\mathrm{d}}_2}{{\mathsf{\epsilon }}_{\mathrm{r}2}}+x(\mathrm{t})}$$

(7)

$$\mathrm{V}=-\frac{\mathrm{Q}}{\mathrm{C}}+{\mathrm{V}}_{\mathrm{O}\mathrm{C}}$$

(8)

where ${\mathrm{d}}_2$ is the thickness of the PTFE material, $x$ is the gap distance between the two materials, and $\mathsf{\sigma }$ is the surface charge density. In Equation (6), ${\mathrm{V}}_{\mathrm{O}\mathrm{C}}$ is the open circuit voltage of VS-TENG, and $\mathrm{C}$ is the capacitance of VS-TENG.

The triboelectric nanogenerator includes the following two parts: contact electrification and electrostatic induction. From Equation (8), it can be seen that the magnitude of the voltage output from the TENG consists of two main components. One is from the frictional contact charge, which is a function related to the spacing $x$. The other component is the transferred charge generated by electrostatic forces $\mathrm{Q}$. Since the most fundamental device based on electrostatics is a capacitor, the TENG has capacitive behavior. According to the principle of electric potential superposition, the total voltage difference between the two electrodes can be made up of ${\mathrm{V}}_{\mathrm{O}\mathrm{C}}$ and $-\frac{\mathrm{Q}}{\mathrm{C}}$ together. The equivalent circuit of the V output is shown in Figure 1c and is equivalent to a voltage source and a variable capacitor in series. The output voltage variation is mainly related to the distance between the two friction materials.

2.3. Working Principle of VS-TENG

Due to the different binding forces of material nuclei on electrons, when the two materials contact each other and produce friction, the strength of the binding force determines the type of charge. The material with a weak binding force will lose electrons and be positive, and the material with a solid binding force will get electrons and be negative. VS-TENG works in vertical separation mode, and the internal structure mainly adopts the spring-type structure form. When the spring vibrates, it drives the periodic contact and separation between the PTFE material and the top electrode Al. When the distance between them changes, an inductive charge is created, thus generating periodic voltage and current signals. The working principle of power generation is shown in Figure 2a.

In the initial state (stage I), the spring does not vibrate and there is no relative motion between the PTFE material and the top electrode Al in the VS-TENG. No charge is generated by relative friction; only the same amount of positive and negative charges are generated by electrostatic induction. The Al is positively charged, and PTFE is negatively charged.

When the external vibration excites, the spring vibrates upward, driving the PTFE material above upward and causing friction with the top electrode Al (stage II). Under the action of an external force, the gap distance $x$ between two friction materials (PTFE and Al) changes. As a result of the friction charge, friction charges of opposite polarity are generated on the surfaces of the two materials. The PTFE is negatively charged, and the Al electrode on top is positively charged. In order to balance the potential difference between the top and bottom AL electrodes, electrons are transferred through an external circuit. When the two electrodes are connected to the load through the wire, a current is formed in the external circuit. The transfer will stop once an electrostatic equilibrium is reached.

Electrostatic equilibrium is reached when the Al electrode above and the PTFE material are in complete contact (stage III), while the gap is zero. At this time, there is only an equal amount of positive and negative charges generated by induction, Al positive charge, PTFE negative charge.

When the spring returns to normal downward, vibration drives the PTFE downward movement above the top Al electrode separation (IV stage). In order to equalize the reappearing potential difference, electrons are transferred in the opposite direction through an external circuit. At this point, a voltage difference in the opposite polarity is generated between the electrodes.

When a new cycle begins, and the VS-TENG continues to move upward, it will create the same inductive charge as the previous motion cycle. This cycle repeats itself, creating a periodic AC electrical signal.

The charged VS-TENG open circuit voltage was analyzed in vertical separation mode using COMSOL Multiphysics software (V5.3). The initial gap between the top electrode Al and PTFE is 10 mm, which gradually decreases and then gradually increases to return to the initial position. Figure 2b shows that the output open-circuit voltage gradually increases as the gap decreases; conversely, if the gap increases, the open-circuit voltage decreases subsequently. The result is consistent with the above principal analysis.

3. Experimental Method

VS-TENG mainly uses Al and PTFE friction materials. The upper friction material is Al material with a mass of 0.7 g. The lower friction material is made of PTFE and then bonded with Al film material, with a mass of 0.7 g. The acrylic plate was fixed after the lower Al film, and the mass was 1.2 g. Two different spring parameters, 0.8-13-25-06 and 0.8-13-24-08, are connected under the acrylic plate. The mass of the 0.8-13-25-06 spring is 0.9 g, and that of the 0.8-13-24-08 spring is 1.2 g. The specific parameters of the two springs are shown in Table 2. The VS-TENG comprises the above two kinds of friction materials, acrylic plate, spring, and external shell, and was tested at different excitation frequencies.

This study proposed to build a VS-TENG sensor electrical signal test device, as shown in Figure 3. The system uses an external exciter and a linear motor to excite the VS-TENG sensor to produce forced vibrations in the spring and shell. A signal generator (YE1311) and a power amplifier (YE5873A) regulate the external excitation signal to adjust the frequency and amplitude. An electrostatic meter (Keithley 6514) captures the electrical signal output from the VS-TENG.

The electrical signal of VS-TENG is output to the signal processing unit. The unit filters the input signal through the program and then converts the electrical signal in the time domain into the frequency domain using the Fourier transform method. Analyze the amplitude voltage at different frequencies in the frequency domain, find the frequency information under the maximum amplitude voltage, and output the frequency. This output frequency is approximately equal to the input excitation frequency value. Under different excitation frequency signals, by converting the frequency information under the maximum amplitude voltage, the operating state of the VS-TENG sensor in the device can be fully obtained.

4. Results and Discussion

4.1. Output Performance Characteristics at 10–100 Hz

The electrical signals output from the VS-TENG were tested on a fixed shaker amplitude of 1 mm and collected at 10 Hz intervals. Blue represents the filtered signal for each frequency of the spring parameters 0.8-13-25-06, while red represents the filtered signal for each frequency of the spring parameters 0.8-13-24-08.

Figure 4a shows the open circuit voltage at 10–100 Hz frequency. The comparison of two different spring parameters shows that the voltage output signal does not increase gradually with the increase in the input signal frequency. The spring parameters of 0.8-13-26-06 reach the maximum value at about 80 Hz, while the spring parameters of 0.8-13-24-08 reach the maximum value at around 70 Hz. This frequency is the resonance frequency at this spring stiffness factor. Figure 4b,c are the filtered charge and current signals of different spring parameters at frequencies of 10–100 Hz, respectively. As the input signal frequency increases, the two spring parameters indicate that the charge output signal does not show a gradual increase in the law of change, and the voltage signal is similar.

Figure 4d illustrates the correlation between input and output frequency after filtering. The output frequency is obtained by VS-TENG electrical signal processing. Firstly, the electrical signal of VS-TENG is filtered, and then the electrical signal in the time domain is converted into the frequency domain by Fourier transform. Analyze the amplitude voltage at different frequencies in the frequency domain, find the frequency information at the maximum amplitude voltage, and finally output this frequency value. The two straight lines in Figure 4d represent the VS-TENG output frequency values of the two spring parameters, obtained by fitting the output frequency values at different excitation frequencies. The filtered frequency signal of the two different spring parameters exhibits an exceptionally high correlation (correlation coefficient = 0.999) with the vibration frequency of the excitator. This robust correlation confirms that the voltage signal collected by the VS-TENG effectively tracks changes in the frequency of the input excitation signal, enabling precise monitoring of the vibration state of the ship’s power equipment body.

Figure 4e shows each frequency’s maximum amplitude voltage and error distribution after filtering under 0.8-13-25-06 and 0.8-13-24-08 spring parameters. The left y-axis is the maximum amplitude voltage, while the right y-axis is the percentage of output frequency error. It can be seen from the figure that the spring parameters of 0.8-13-25-06 reach the maximum value at about 80 Hz, and the spring parameters of 0.8-13-24-08 reach the maximum value at about 70 Hz, which further verifies the influence of resonance frequency on the output voltage. In addition, the absolute value of the relative error of the frequency is less than 1%, and the error decreases with the increase in the input signal frequency.

Lastly, Figure 4f is 0.8-13-24-08 continuous outputting stability test diagram. It can be seen from the figure that VS-TENG outputs an alternating voltage signal, and the voltage peak-to-valley value is between −0.5 V and 0.5 V. Although there is a slight fluctuation, the output voltage amplitude is constant, which can maintain a stable output for a long time.

4.2. Output Performance Characteristics at Low Frequencies (3–10 Hz)

To test the response characteristics of the VS-TENG in the low-frequency range, based on different fixed amplitudes of the exciter of 1–6 mm. The output frequency response of the VS-TENG was tested at 3–10 Hz excitation frequency, respectively. The frequency acquisition was performed at 1 Hz intervals, and the frequency acquisition range was 3–10 Hz.

Figure 5a depicts the open circuit voltage of VS-TENG after filtering with two spring parameters at a fixed amplitude of 5 mm. Blue represents the voltage signal after filtering at each frequency of 0.8-13-25-06 spring parameters, and red represents the voltage signal after filtering at each frequency of 0.8-13-24-08 spring parameters. It can be seen that after Butterworth digital filtering, the voltage output signals under the two parameters are more regular, and there is no clutter interference, which is conducive to extracting of frequency signals. As the frequency increases, the output open circuit voltage gradually increases. The output voltage of the VS-TENG with 0.8-13-25-06 spring parameters is greater than that of the VS-TENG with 0.8-13-24-08 spring parameters. Figure 5c is the fast Fourier transform (FFT) frequency distribution diagram of VS-TENG before and after filtering at the excitation signal frequency of 7 Hz. The voltage signal is distributed at 7 Hz and its integer times before filtering, and only 7 Hz has voltage signal distribution after filtering. It can be seen that after Butterworth filtering, frequency waveforms other than the excitation frequency can be effectively filtered out.

Figure 5b offers a correlation coefficient to analyze the relationship between input and output frequency after filtering. The correlation between the filtered voltage signal and the vibration frequency of the shaker for two different spring parameters reaches more than 0.999. Therefore, the voltage signal acquired by VS-TENG can also reflect the frequency variation in the input excitation signal in the low-frequency range of 3–10 Hz.

Figure 5d,e show the output voltages at different 1–6 mm amplitudes for two spring parameters at different frequencies. The results show that the VS-TENG can detect vibration after an amplitude of 3 mm from a frequency of 3 Hz. At the same frequency, the higher the amplitude, the higher the open-circuit voltage. The graph shows that the 0.8-13-24-08 spring parameter has a weaker ability to detect vibration than the 0.8-13-25-06 spring parameter.

Figure 5f shows the error distribution of each frequency after filtering under 0.8-13-24-08 and 0.8-13-25-06 spring parameters. The upper and lower broken lines are 0.8-13-25-06 and 0.8-13-24-08 spring parameters VS-TENG ‘s error values of each output frequency under 3–6 mm vibration amplitude. As its lessor excitation frequency, the absolute value of the relative error of the frequency is more smaller than that at the excitation frequency of 10–100 Hz.

4.3. Detection of Input Frequencies in the 100–500 Hz Range

The above two parts of the analysis show that VS-TENG can reflect the frequency signal very well when the input frequency is the mid-frequency 10–100 Hz region. The maximum output amplitude is reached at the resonance frequency, and the 0.8-13-25-06 spring parameter VS-TENG output is superior to the 0.8-13-24-08 spring parameter. In the low-frequency region of 3–10 Hz, VS-TENG also reflects the frequency information better, but the output voltage amplitude is smaller than that of 10–100 Hz.

We systematically raised the input signal frequency to assess the competence of VS-TENG at higher frequency levels. We recorded the VS-TENG’s output frequency signal response, which was set at 0.5 mm amplitude while varying input signals from 100 Hz to 500 Hz using a 50 Hz sampling rate.

Figure 6a displays the open-circuit voltage at a fixed amplitude of 0.5 mm within the frequency range of 100–500 Hz. The filtered voltage signal at each frequency is depicted in blue for the 0.8-13-25-06 spring parameter. The graph reveals that the voltage output signal gradually decreases as the input signal frequency increases.

Figure 6b illustrates the correlation between the input frequency and the frequency of the filtered output. Remarkably, the correlation between the filtered voltage signal frequency and the exciter’s vibration frequency under both spring parameters equals one. It demonstrates that the voltage signal collected by VS-TENG preciously captures frequency variations in high-frequency input excitation signals.

Figure 6c,d present the error distributions for each frequency after filtering with the two different spring parameters. In both cases, the absolute relative errors remain below 0.072%. Thus, VS-TENG effectively gathers frequency signals.

5. The Practical Application of VS-TENG

The application of VS-TENG on ships was preliminarily explored by using PT500 Mini ship power system transmission test bench. The test bench is composed of a three-phase asynchronous motor, frequency converter, coupling, friction support kit, speed regulation module, and so on, as shown in Figure 7a. The VS-TENG with 0.8-13-24-08 spring parameters was used to test the inverter of the three-phase asynchronous motor at 17 Hz and shutdown. The experimental results are shown in Figure 7b,c. In the experiment, VS-TENG is attached to the shell of the three-phase asynchronous motor, and the inverter controls the speed of the motor. Figure 7b is the voltage data collected on the surface of the motor shell under the input of 17 Hz frequency converter. The data encompass the frequency distribution map of the filtered data after Fourier transform. Only the 17 Hz voltage has a significant amplitude, and the voltage value of the other frequency bands is basically zero. The filter frequency distribution diagram after the shutdown is shown in Figure 7c. At this time, the three-phase asynchronous motor was stopped, and only the power frequency of 50 Hz in the power supply input of the system affected the VS-TENG. VS-TENG has great application potential in ship power systems.

6. Conclusions

This paper introduces a self-powered sensor, the spring-structured VS-TENG, which operates based on the principle of triboelectric generation. The structure of the TENG involves an acrylic shell and an internal TENG detection component, where the spring structure plays a vital role in sensing the body’s vibration signal. The electrode is made of Al, and the friction between PTFE and Al generates the electrical signal. Filter analysis and fast Fourier transform (FFT) are employed to determine the excitation frequency range, facilitating the assessment of the standard operating condition of the ship’s power system. At different excitation frequencies of 3–500 Hz, VS-TENG can correctly reflect different input frequencies. The output frequency error of medium-high frequency is within 1%, especially in the range of 100–500 Hz, the error range is less than 0.072%. This work provides an effective frequency test method for monitoring the operating status of ship power units, which has a wide range of application prospects.