Research on the Folding Spring Triboelectric Nanogenerator for Rock Climbing Trajectory and Time Monitoring

The monitoring data of climbing trajectory and climbing time in rock climbing training are the basis for coaches to select athletes and formulate the next training plans. This article proposes a folding spring triboelectric nanogenerator that can be used for trajectory training and time training in rock climbing training. The folding spring triboelectric nanogenerator, made of PMMA (polymethyl methacrylate), Kapton, PTFE (Polytetrafluoroethylene), Cu (copper) and PLA (polylactic acid), realizes the detection functions based on the voltage pulse generated by the vertical friction between each two adjacent friction layers, and then the width of the output voltage pulse is the training time and the envelope of the space area formed by all the contacted folding spring triboelectric nanogenerator is the climbing trajectory. Test results show that the output voltage amplitude are between 7.5 V and 10.8 V, the output current will drop sharply from <inline-formula> <tex-math notation="LaTeX">$24.5 \times 10^{-7}$ </tex-math></inline-formula> A when the load resistance exceeds 47k ohms, the time interval between two consecutive uses should be greater than 0.2 s, and the maximum output power is <inline-formula> <tex-math notation="LaTeX">$163.6 \times 10^{-8}$ </tex-math></inline-formula> W when a <inline-formula> <tex-math notation="LaTeX">$4.7\times 10^{6}$ </tex-math></inline-formula> ohm load resistance is connected in series, which shows a strong signal-to-noise ratio and anti-interference ability. In addition, the trajectory training and the time training tests results show that the folding spring triboelectric nanogenerator can be used for real-time monitoring in rock climbing training, but it needs to be pressed twice in succession when used for time training limited by the structural and principle.


I. INTRODUCTION
Rock climbing is a sport that relies on hands and feet to climb rock points on natural or artificial climbing walls to achieve the purpose of upward climbing [1], and its basic training contents include trajectory training and time training. Trajectory training means the routes from the bottom to the top of the climbing wall formed by the athletes when climbing, and it can test the ability of athletes because different routes represent different difficulties [2]. Time training means the time in which athletes rely on their fingers to grab a rock point and hang their entire body, and it can also test the ability of athletes because the longer the suspension time, the stronger the athletes [3], [4]. Data of trajectory training and time training are the basis for coaches to select athletes and formulate the The associate editor coordinating the review of this manuscript and approving it for publication was Gongbo Zhou . next training plans, but there are no professional measuring instruments at present. The data recording methods still relies on manual recording after visual observation, which means high labor intensity, high error and low efficiency. Therefore, there is an urgent need to develop professional instruments to meet the requirements of rock climbing trajectory training and time training.

II. MANUFACTURING AND WORKING PRINCIPLE
As shown in figure 1(a), the rock points fixed on the indoor climbing wall by bolts are artificially manufactured, so our design idea is to expand the size of the bolt mounting hole to install the FS-TENG, and then replace the bolt with hollow bolt to make the signal wires to pass through. As one of the main components of the FS-TENG, the folding spring, with a size of ϕ25 × 20 mm, is made of a 0.05 mm thick Kapton (PY11YG, Lingmei Co., LTD, Dongguan, China) to ensure elastic deformation, and any two adjacent surfaces of the folding spring are pasted with different nanomaterials to form the friction layers. One of the friction layers is made of PTFE (CTF30, Bench Co., LTD., Suzhou, Jiangsu, China) with a thickness of 0.03 mm, and the other is PMMA (0703, Chuangyou Co., LTD, Suzhou, China) with a thickness of 0.2 mm. Cu (C1100, ZYTLCL Co., LTD., Dongguan, Guangdong, China) with a thickness of 0.05 mm is used as electrode layer under both friction layers. VOLUME 8, 2020 The folding spring, embedded in the spring, fixed in the rock point together with the hollow bolt, and all the same friction layers in the folding spring are connected by wires. The folding spring and the spring will be deformed together when the FS-TENG is pressed by athletes, thus causing frictional electrification on any two adjacent surfaces of the folding spring. Subsequently, the contacted surfaces will gradually separate due to the restoring force of the spring when the pressing force disappears, thus generating charges transfer, so the detection function of whether the athletes pressure the rock point can be achieved by further analyzing the rule of transferred charges. Then the envelope of the formed area in the software is the training trajectory when all the pressed FS-TENG are spatially located to the corresponding coordinates in the software, and the duration of the pressing signal of the single FS-TENG is the training time.
As shown in figure 1(b), the working principle of the FS-TENG can be further explained by taking any adjacent two surfaces of the folding spring as an example.
Step 1 is the initial state where there is no contact between the two surfaces.
Step 2 is the state where the two surfaces are in contact with each other under the pressure of the athletes, and the friction layer with PMMA is positively charged while the PTFE is negatively charged because PMMA is more likely to lose electrons.
Step 3 is the state where the two surfaces are gradually separated under the spring restoring force, and a short-circuit current is generated and the open-circuit voltage gradually expands with the separation distance expands.
Step 4 is the state where the two surfaces are separated to the maximum distance, and the open-circuit voltage thus reaches the maximum value.
Step 5 is the state where the two surfaces are gradually approach each other when the athletes presses again, and a reverse short-circuit current is generated and the open-circuit voltage gradually decreases with the separation distance decreases. Then the open-circuit voltage is reduced to zero when finally returning to Step 2. In the above steps, the theoretical output voltage and current signal are shown in figure 1(c). As a voltage pulse signal will be generated no matter the FS-TENG is pressed for the first time or more than the second time, a single voltage pulse is selected as the detection signal in this article.

III. TESTS
Tests are divided into two parts, one is to test the output performance of the FS-TENG, and the other is to test the functions of trajectory training and time training, which are introduced separately as follows. Figure 2 shows the output signal of the FS-TENG, and we can obtain that the FS-TENG will output a voltage or current pulse signal no matter it is pressed by hands or feet, so the contact detection function can be realized by connecting the output signal wire to the pulse counting pin of any MCU (Microprogrammed Control Unit). In addition, the reason why the waveform of the hands are smoother than the feet is that there are strong jitter when pressed by feet. Figure 3 shows the test results of the FS-TENG output signal, and we can obtain the following conclusions. (1) The number of output voltage tests are 1000, in which hands and feet account for half, and parts of the test data are shown in figure 3(a). As shown in figure 3(a), the output voltage distribution are irregular, but the amplitude are concentrated between 7.5 V and 10.8 V, which are much larger than the noise signal and shows a strong signal-to-noise. (2) As shown in figure 3(b), the FS-TENG can still work normally at a working frequency of 5 Hz, which indicates that the time interval between two consecutive uses should be greater than 0.2 s. (3) As shown in figure 3(c), the output voltage will gradually increase from 7.65 V to 8.7 V as the ambient temperature rises to 100 degrees Celsius. The larger the signal amplitude, the easier to be detected, so the higher the temperature within 100 degrees Celsius, the stronger the signal-to-noise ratio and anti-interference ability of the FS-TENG. (4) We did 10000 tests to test the reliability of the FS-TENG, and each test is the averaged for 10 seconds measurement. As shown in figure 3(d), the output voltage increases to 8.12 V and then gradually decreases to 7.62 V as the number of tests increases, but it still shows a high signalto-noise ratio even if the output voltage reduce to 7.62 V, which shows a high reliability. However, the output performance will be affected by long-term wear, so wear-resistant materials or metal materials will be selected as friction layers in the next study to improve the long-term stability. Figure 4 shows the test results of the FS-TENG power generation performance, and we can obtain the following conclusions. (1) As shown in figure 4(a), the output current is different because the pressure of fingers, hands and feet VOLUME 8, 2020 are different, so we tested the output current under different pressures, and the results are shown in figure 4(b). As shown in figure 4(b), the output current gradually increases with the increase of pressure, and it will reach a maximum value when the pressure exceeds 8 N. Because the climbing pressures in actual rock climbing are much greater than 8 N, the FS-TENG will output the maximum current no matter it is pressed by hands or feet. (2) As shown in figure 4(c), the output current gradually decreases with the increase of resistance, and it will drop sharply from 24.5 × 10 −7 A when the resistance exceeds 47k ohms, so a resistance less than 47k ohms can be connected in series to obtain a larger output signal when the output current is selected as the detection signal. (3) As shown in figure 4(d), the FS-TENG can output a maximum power of 163.6 × 10 −8 W when the load resistance is about 4.7 × 10 6 ohms, so a resistance about 4.7 × 10 6 ohm can be connected in series to obtain a larger output power when the FS-TENG is used as a power source. Figure 5 shows the test results of the FS-TENG used for trajectory training and time training, and we can obtain the following conclusions. (1) As shown in figure 5(a), the FS-TENG is installed on the rock point, and it's output signal are input into the software after being processed by the electrometer (6514, Keithley Co., LTD., Solon, America) and the MCU (USB5632, ART Technology Co., LTD., Beijing, China) in turn. As shown in figure 5(b), each rock point with a specific spatial coordinate on the climbing wall can be mapped to the indicator light at the corresponding coordinate in the software by programming in LabView language. The indicator light at the corresponding coordinate in the software will be lit red when the athletes contact the rock point, and then the envelope of the area formed by all red indicators is the climbing trajectory.

IV. CONCLUSION AND DISCUSSIONS
The FS-TENG can meet the needs of trajectory training and time training in rock climbing. The width of the output voltage pulse is the training time, and the envelope of the space area formed by all the contacted FS-TENG is the climbing trajectory. Test results show that the output voltage amplitude are between 7.5 V and 10.8 V, the output current signal are lager than 24.5 × 10 −7 A when the series resistance is less than 47k ohms, the interval between two consecutive uses should be greater than 0.2 s, and has a strong anti-interference ability and reliability. In addition, the FS-TENG can output a maximum power of 163.6×10 −8 W when the load resistance is about 4.7 × 10 6 ohms, so a resistance about 4.7 × 10 6 ohm can be connected in series to obtain a larger output power when used as a power source.
But there are still two aspects that need to be further improved. One is that the bolt mounting holes of the rock point need to be enlarged to install the FS-TENG, so the next step is to change the structure into a bolt shape to replace the original one for installation, thereby avoiding changing the rock point. The other is that the athletes need to press the FS-TENG twice in a row when used for time training, so the next step is to change the triggering mode to realize the function that the friction layer of the FS-TENG contact twice when pressed once, thereby simplifying the operation.