Novel, lightweight TENG performs as well as heavier commercial systems
Improvements in wireless communication and a reduction in the cost of sensor networks mean that it’s now possible to continuously monitor the structural health and performance of large-scale structures. As a result, it’s becoming increasingly common for civil infrastructure such as dams, tunnels, high-rise buildings and bridges to be highly-instrumented. A limitation of these wireless sensor networks is their reliance on external power supplies. New research from a group of Chinese engineers suggests that self-powered structural sensors might be a step closer.
Writing in the latest issue of Nano Energy [DOI: 10.1016/j.nanoen.2023.108960] they report on their design of a novel acceleration TENG sensor (A-TENG) that they use to monitor cable bridges. The A-TENG has two main components: an external cylindrical shell and an internal mass-spring-damper mechanical system. The latter comprises a cylindrical mass block, and two springs (with one on each end of the cylinder) mounted on a central pillar. There is a stable 1 mm gap between the mass and the shell. Such non-contact or free-standing TENGs tend to be especially sensitive to small vibrations, which makes them of interest to those working in structural health monitoring.
The triboelectric pair of materials are aluminium – in the form of two electrodes on the inner surface of the shell – and a polytetrafluoroethylene (PTFE) film on the outer surface of the mass. When subjected to axial stimulation (e.g. via cable vibrations), the mass freely slides within the shell, oscillating between the electrodes. The presence of the triboelectric pair on these moving objects means that if attached to a structure (e.g. a bridge cable) the electrical output signals generated by the motion can be used to obtain information about the vibration and acceleration of that structure.
The researchers first established a theoretical model for the TENG, finding that the acceleration of the host structure can be obtained by collecting the A-TENG’s output short-circuit current signal. To test this experimentally, they placed an A-TENG onto a modal shaker, and measured its electrical output as the shaker operated at different loading frequencies and accelerations. They found “an excellent linear relationship between the amplitude of output short-circuit current and the vibration acceleration” (R2 = 98.08%). To further demonstrate the reliability of their theoretical model, the team carried out the same experiment with a commercial accelerometer. They found close alignment between the responses of the A-TENG and the commercial device when subjected to sine waveforms. Under arbitrary waveforms, they showed that measured vibration frequencies were “highly consistent, but differ somewhat in amplitude.” At lower frequencies (<15 Hz), the A-TENG exhibited higher output than the commercial sensor, while at higher frequencies, the opposite occurred. They attribute this to the A-TENG being mainly dominated by low-order resonance modes, while the commercial sensor – which relies on the piezoelectric effect – is strongly dominated by high-order harmonics. In addition, they showed that the A-TENG was sensitive to a wide range of accelerations; from 0.1 m/s2 to 70 m/s2. Its sensitivity was shown to be unaffected by temperature and humidity, and it did not degrade after 30,000 cycles.
Cable tension is an important metric in ensuring the safe operation of cable bridges, and it is typically determined from the average of the first three natural frequencies of that cable. In the next set of experiments, the modal shaker was replaced by a stainless-steel cable; typical of those used in structural applications. The team showed that their A-TENG was “capable of accurately measuring the time-history vibration acceleration of the cable, even under different loading conditions with varying magnitudes and directions of applied forces.” They attributed this to the low mass (~8g) of the A-TENG. At 142g, the commercial sensor altered the natural frequency of the cable, reducing the accuracy of cable tension values.
Finally, the team tested their A-TENG in the field; specifically on an existing cable-supported arch bridge in Quzhou. In all cases, the A-TENG’s performance was in close agreement with the commercial sensor. The authors conclude that, “These findings not only confirm the feasibility and long-term stability, but also highlight the good sensing accuracy of the proposed sensing theory and A-TENG sensor for real-time quantitative monitoring of bridge acceleration.”
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Kangxu Huang, Yuhui Zhou, Zhicheng Zhang, He Zhang, Chaofeng Lü, Jikui Luo, Libin Shen. “A real-time quantitative acceleration monitoring method based on triboelectric nanogenerator for bridge cable vibration,” Nano Energy 118 (2023) 108960. DOI: 10.1016/j.nanoen.2023.108960