A triboelectric vibration sensor for detecting loose bolts in automobile

ABSTRACT Recently, the application of flexible sensors in mechanical equipment has attracted widespread attention. However, the design of vibration sensor materials and structures for complex mechanical environments still faces challenges. Hence, we proposed an annular structure CCTO/PDMS-based triboelectric nanogenerator (CP-TENG) as self-powered triboelectric automotive bolt tightness sensor. Due to the high dielectric properties of CCTO/PDMS film, the CP-TENG device can obtain high electrical output performance to achieve high sensitivity sensing. Through the annular structure design, CP-TENG devices can be integrated into bolt fittings with good structural compatibility. The output voltage and current of CP-TENG can monitor the different accelerations borne by the bolts from 9 to 60 m/s2. Moreover, by installing on the gasket, the CP-TENG device can also monitor bolt looseness to achieve early warning of the safety status of automotive components. This research can promote the development of self-powered sensor in the field of automotive bolt vibration monitoring.


Introduction
Recently, vehicle driving safety has become a research hotspot [1,2].The main factor affecting vehicle driving safety comes from the looseness and detachment of components [3].Therefore, sensor systems for monitoring the looseness of automotive bolts are crucial.Conventional monitoring methods rely on manual detection of abnormal bolts in assembly, which is difficult and cumbersome, and has a high rate of misjudgement [4,5].It is noteworthy that the rapid development of flexible electronic devices is gradually merging with the development of sensors [6,7], energy storage devices [8,9], and energy harvesters [10,11].Sensor components based on flexible materials typically have good flexibility and tensile properties, and can be well adapted to complex environments [12].In particular, sensors used for vibration monitoring of mechanical equipment require materials with strong compatibility and structural design with high sensitivity sensing effects [13][14][15].Also, because long-term vibration of mechanical equipment can cause components to heat up, it is important to have sensors with good heat resistance.The most important point is that the structural design of the sensor device needs to be able to integrate with the monitored equipment without affecting the original mechanical equipment and avoiding additional negative interference.Generally, based on the signal amplitude, frequency, and signal shape characteristics of the vibration sensor, the vibration of the monitored equipment can be fed back to achieve fatigue monitoring of mechanical equipment, especially commonly used vehicles such as automobiles.In the past, vibration sensors were mainly based on the principle of piezoresistance [16], capacitive effect [17], and piezoelectric effect [18], and triboelectric effect [19].It is worth noting that triboelectric electrical sensors are the best choice for vibration monitoring of mechanical equipment due to their high response rate and independence from external power sources.
Recently, triboelectric nanogenerators (TENGs) based on triboelectric effects have been extensively studied, and many self-powered vibration sensors have been developed based on TENG devices [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36].Xiao et al. designed a self-powered vibration sensor based on a spring assisted structure [37].When the sensor device is subjected to mechanical impact, the slider located at the spring section will move, generating a voltage output signal to achieve structural health monitoring.However, due to the structural design of the sensor, the oscillator will produce resonance characteristics during use, which will affect the resolution of the sensor.Xiao et al. proposed a new vibration monitoring structure TENG based on plastic balls, which has good frequency self-adaptability [38].However, it is worth noting that contact and collision between rigid triboelectric materials can cause severe wear and noise.According to this, Cheng et al. used a soft sphere filled with liquid as a vibrator, but this soft sphere can cause complex deformation during the collision process, and the manufacturing cost is on the high side [39].In addition, liquid metal is also used to manufacture vibrators for frictional electrical sensors, which has good practical value.Zhang et al. proposed a triboelectric vibration sensor based on liquid metal, which can achieve acceleration sensing [40].However, this liquid metal vibration triboelectric electrical sensor is toxic, and this oscillating structure is difficult to match with automotive bolts.Improving the dielectric constant of triboelectric materials is an important way to improve the output performance of TENG devices.Although researchers have been striving to improve the dielectric constant of polymer dielectrics without losing their flexibility, processability, and other inherent properties, no significant results have been achieved.Significantly, the covalently bonded calcium copper titanate (CaCu 3 Ti 4 O 12 , CCTO) can enhanced dielectric constant (e) and low dielectric loss of polydimethylsiloxane (PDMS), which brings a new provides a new approach to improving the output performance of TENG devices based on PDMS.To achieve the goal of monitoring vibration of automotive bolts, efforts should be made to design flexible triboelectric materials and related matching structures.
In this work, we proposed an annular structure CCTO/PDMS-based triboelectric nanogenerator (CP-TENG) as self-powered triboelectric automotive bolt tightness sensor.The CCTO/PDMS film and copper was used as the triboelectric pairs.Due to the high dielectric properties of CCTO/PDMS film, the CP-TENG device can obtain high electrical output performance to achieve high sensitivity sensing.Through the annular structure design, CP-TENG devices can be integrated into bolt fittings with good structural compatibility.The output voltage and current of CP-TENG can monitor the different accelerations borne by the bolts from 9 to 60 m/s2 .Moreover, by installing on the gasket, the CP-TENG device can also monitor bolt looseness to achieve early warning of the safety status of automotive components.In detail, when the vehicle bolts are tightened, the CP-TENG installed on the gasket will generate a periodically stable voltage signal.When a bolt is loosened, CP-TENG can produce a high impact pulse signal to achieve monitoring of loose bolts.CP-TENG can still maintain good  reliability under nearly 10,000 working times.In addition, due to the excellent temperature stability of the CP-TENG electrical output, it is very suitable for monitoring complex mechanical vibration environments such as automobiles.

Experiments
The detailed preparation process of CCTO/PDMS film is shown in Figure 1(c1).Firstly, add mineral oil and CCTO particles into PDMS mixture (10:1) for mechanical mixing, and control the mass ratio of CCTO to PDMS at 20%.Besides, silica gel and curing agent (10:1.2) are used to accelerate the curing and stability of PDMS.A ring structure Kapton film is used as the material substrate, as present in Figure 1(c2).Then, paste copper foil on the surface of the annular structure Kapton film, as shown in Figure 1(c3,c4).Apply the mixed colloid on the surface of copper foil to form CCTO/PDMS film, and then cure the sample at 100°C for 1 hour, as illustrated in Figure 1(c4,c5).After complete curing, CCTO/PDMS composite film is obtained, and a triboelectric pair is formed with conductive copper foil, as shown in Figure 1(c6).The the annular structure CP-TENG can be installed on the gasket.For CCTO/PDMS, the patterns are the same as that of CCTO, manifesting the intact crystal structure without being broken by the linkage with the combination with PDMS.

Results and discussion
To evaluate the sensing performance of CP-TENG device from a quantitative perspective, we use linear motors to provide mechanical excitation with different vibration frequencies, vibration amplitudes, and accelerations, thereby driving CP-TENG to generate electrical output signals.As shown in Figure 2(a), fix the gasket attached to the CCTO/PDMS film on the moving end, and then fix the gasket attached to the copper foil on the fixed end.The electrometer 6514 was installed in the circuit to test for V oc and I sc of CP-TENG.Furthermore, Figure 2(b) depicts the detailed output signal waveform of V oc and I sc of CP-TENG device.The mechanical vibration state of CP-TENG can be fed back from the peak value and frequency of the output waveform.Figure 2(c1,c2) presents the CP-TENG working principle driven by external forces.During the collision between CCTO/ PDMS film and copper, electrons migrate from the copper foil surface to the CCTO/PDMS film surface.When the two pads move with each other, driven by the potential difference between CCTO/PDMS film and copper foil, the driving electrons periodically flow between the two copper electrodes, generating an induced current.Due to the fact that mechanical equipment can generate heat between bolts and gaskets during vibration, it is crucial to study the effect of temperature on the output performance of CP-TENG. Figure 2(d,e) illustrates the V oc and I sc of CP-TENG under different temperature.It can be observed that the output performance of CP-TENG remains stable despite temperature changes.In addition, the thickness of CCTO/PDMS film is also a key factor affecting the output performance of CP-TENG.Therefore, we also investigated the output performance of CP-TENG with different thicknesses of CCTO/PDMS films under different accelerations, as shown in Figure 2(f,g).The CP-TENG device with thicker CCTO/PDMS film will produce high output performance, but the V oc of CP-TENG device tends to stabilize as the acceleration increases.Also, we measured the output voltage of CP-TENG (size: 2 cm × 2 cm), as shown in Figure S4.Furthermore, according to Ohm's law, we calculated the output current and power of CP-TENG under different loads.From the results, when the resistance of the load increases, the output voltage increases and the output current decreases.Besides, when the resistance value of the external load is 20 MΩ, the output power of CP-TENG reaches its maximum value of 175 μW.
Considering the influence of the thickness of CCTO/PDMS film on the CP-TENG sensing performance, we chose CCTO/PDMS film with a thickness of 1.5 mm as the experimental object.Figure 3(a,b) shows the output signal waveform of the V oc and I sc of CP-TENG device.It can be observed that as the acceleration increases, the output performance of CP-TENG gradually increases, and its output performance signal can be used as a sensor signal to monitor the acceleration received by the device.During the driving process of a vehicle, the fastening of bolts is very important, which will directly affect the safety factor of the vehicle.The monitoring of bolt tightness and impact force by CP-TENG can effectively prevent the failure of important components.Furthermore, considering the long-term reliability of CP-TENG, we have tested its output performance stability of CP-TENG under continuous operation.Figure 3(c,d) illustrates the V oc and I sc of CP-TENG under continuous working conditions of nearly 10,000 times.It can be observed that the output performance of CP-TENG is very stable under multiple consecutive operating conditions.Moreover, we developed the output performance of CP-TENG under various humidity (from 30% to 50%), as shown in Figure S2.From the results, the changing humidity did not have a significant impact on the output performance of CP-TENG.Furthermore, we also measured the electrical output of CP-TENG under long-term continuous working conditions (frequency: 10 Hz, time: 6000 s), as illustrated in Figure S3.
To simulate the bolt vibration monitoring scenario, we fixed the bolts on the prefabricated components, and arranged the gaskets with CP-TENG devices between the bolts and the components to maintain a certain gap, as shown in Figure 4(a).Figure 4(b) illustrates that after the vibration system is started, the vibration amplitude of the CP-TENG device gradually increases, and its voltage signal also increases, reflecting the loosening information of the bolt.Furthermore, Figure 4(c) presents a bolt vibration scenario, where it can be observed that two CP-TENG mounted gaskets are arranged between the nut and the component.To visually evaluate the rapid response of CP-TENG to acceleration, we use a linear motor to provide shock vibration, and feedback the vibration effect through the output signal of CP-TENG.The fast Fourier transform (FFT) spectrum of the output voltage signal of the CP-TENG device clearly shows that the natural frequency varies from 23 Hz to 35 Hz, as present in Figure 4(d).
In addition to monitoring different accelerations, CP-TENG can also monitor sudden loosening of bolts and evaluate equipment safety.As shown in Figure 5(a), when the vibration system is started, CP-TENG will generate a small stable voltage signal due to the existence of small gaps between the shims.When the bolt loosens significantly, the output voltage of CP-TENG will produce abrupt spikes, as shown in Figure 5(b).When the vehicle is running, the overall vibration can be very large, leading to damage and detachment of components.Real-time monitoring of bolt vibration information will help to solve automotive safety issues.Figure 5(c) shows the output voltage signal of the PC-TENG device under the normal vibration state of the bolt, and a stable periodic waveform can be observed.When the bolts become loose, there will be a gap between the shims, as shown in Figure 5(d).Under the action of component vibration, CP-TENG generates a relatively high output voltage, which can be used to evaluate the pre-tightening effect of bolts.

Conclusion
In summary, we designed an annular structure CP-TENG as self-powered triboelectric automotive bolt tightness sensor.The CCTO/PDMS film and copper was used as the triboelectric pairs.Through the annular structure design, CP-TENG devices can be integrated into bolt fittings with good structural compatibility.The output voltage and current of CP-TENG can monitor the different accelerations borne by the bolts from 9 to 60 m/s 2 .Moreover, by installing on the gasket, the CP-TENG device can also monitor bolt looseness to achieve early warning of the safety status of automotive components.CP-TENG can still maintain good reliability under nearly 10,000 working times.In addition, due to the excellent temperature stability of the CP-TENG electrical output, it is very suitable for monitoring complex mechanical vibration environments such as automobiles.

Disclosure statement
No potential conflict of interest was reported by the authors.

Figure 1 .
Figure 1.(a) Conceptual diagram of bolts in automobiles.(b1, b2) the schematic of inserting the TENG sensor into the loose bolt.(c1-c6) the fabrication process of CCTO/PDMS film and CP-TENG device.(d) the picture of gasket with copper foil attached.(e) the picture of gasket with CCTO/PDMS film attached.(f) the SEM image of CCTO/PDMS film surface.

Figure 2 .
Figure 2. (a) Structural diagram of mechanical system for CP-TENG device.(b) the output signal waveform of V oc and I sc of CP-TENG device.(c1, c2) the working principle of CP-TENG device.The (d) V oc and (e) I sc of CP-TENG under different temperature.(f, g) the influence of CCTO/PDMS film thickness on CP-TENG output performance under different acceleration conditions.

Figure 3 .
Figure 3. (a) the V oc of CP-TENG under different acceleration.(b) the I sc of CP-TENG under different acceleration.(c, d) the reliability testing of CP-TENG device.
Figure 1(d) shows the picture of gasket with copper foil attached, and Figure 1(e) illustrates the picture of gasket with CCTO/PDMS film attached.Furthermore, Figure 1(f) shows the scanning electron microscope (SEM) image of CCTO/PDMS film surface.Besides, we provided the XRD patterns of CCTO, PDMS, and CCTO/PDMS, as illustrated in Figure S1.

Figure 4 .
Figure 4. (a) System overview diagram of CP-TENG device for vibration monitoring.(b) the output voltage signal of CP-TENG device during vibration process.(c) the picture of bolt vibration test scenario.(d) the time domain signal and FFT spectrum of the CP-TENG device.

Figure 5 .
Figure 5. (a) the output voltage signal of CP-TENG under normal operation.(b) the output voltage signal of CP-TENG under breakdown state.The output voltage signal of CP-TENG under (c) compression state and (d) loose state.(e) the output voltage signal of CP-TENG under low-frequency impact conditions.

Figure 5 (
e) shows the output signals of PC-TENG subjected to lowfrequency shock vibration under different bolt loosening states.