[ Article ]

JOURNAL OF SENSOR SCIENCE AND TECHNOLOGY - Vol. 29, No. 5, pp.312-322

ISSN: 1225-5475 (Print) 2093-7563 (Online)

Print publication date 30 Sep 2020

Received 22 Sep 2020 Revised 28 Sep 2020 Accepted 29 Sep 2020

DOI: https://doi.org/10.46670/JSST.2020.29.5.312

A Triboelectric Nanogenerator Design for the Utilization of Multi-Axial Mechanical Energies in Human Motions

Hee Jae Ryoo¹^{, *} ; Chan Woo Lee¹^{, *} ; Jong Won Han¹^{, *} ; Wook Kim¹^{, +} ; Dukhyun Choi¹^{, +}

1Department of Mechanical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin, Gyeonggi-do 17104, South Korea

Correspondence to: ⁺ choice124@khu.ac.kr (W. Kim) and dchoi@khu.ac.kr (D. Choi) Contributed by footnote: * These authors equally contributed to this work.

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(https://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

As the use of mobile devices increase, there is public interest in the utilization of the human motion generated mechanical energy. The human motion generated mechanical energies vary depending on the body region, type of motion, etc., and an appropriate device has to be designed to utilize them effectively. In this work, a device based on the principles of triboelectric generation and inertia was assessed in order to utilize the multi-axial mechanical energies generated by human motions. To improve the output performance we confirm the changes in the output that vary with the structural design, the reasons for such changes, and variations in performance based on the parts of the human body. In addition, the level of electrical energy generated based on motion type was measured; a maximum voltage of 30 V and a current of 2 μA were generated. Finally, the proposed device was utilized in LEDs used for lighting, thus demonstrating that multi-axial mechanical energies can be harvested effectively. Based on the results, we expect that the developed device can be utilized as a sensor to detect mechanical energies, to sense changes in motion, or as a generator for auxiliary power supply for mobile devices.

Keywords:

Triboelectric nanogenerator, Human motion, Multi-axis, Mechanical sensor

1. INTRODUCTION

The rapid development of Internet of Things technology and the wide presence of personal electronic communication devices in our daily lives have led to the consideration of a new source of electrical energy. The development of mobile electronic communication devices has led to the evolution of power sources utilizing the mechanical energies generated by human motion.

In addition to the various levels of mechanical energy generated according to the type of human motion and the different categories of moving parts, another advantage of such a power source is that mechanical energies can be supplied continuously throughout the day. However, it is difficult to use the human motion itself as an input energy source with the existing power-generation technology. Nevertheless, with the emergence of a new type of power-generation technology known as piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs), it has become possible to convert low-level mechanical energies produced by human motion into electrical energy [1-6].

The TENGs has expanded the development direction of a human motion-based power-generation system, based on their novel working principle. The basic operation of a TENG is based on contact electrification and electrostatic induction phenomena [7-9]. Contact electrification or triboelectrification affects the formation of a surface charge above tribo-materials, and this causes the electrostatic induction phenomenon to contribute to the generation of current between electrodes. TENG has various operation modes that are classified into two broad categories—the vertical contact separation mode (VCS-mode) or lateral sliding mode (LS-mode) depending on the loading methods, and into the single electrode mode (SE-mode) or freestanding mode (FS-mode) depending on the methods of connecting the upper and lower electrodes of the tribo-material [10-13]. The presence of various operation modes implies that the most appropriate mode can be selected depending on the location of the device. In other words, a suitable structure can be designed to increase the utilization of mechanical energies.

Based on this principle, devices or apparatus that can be attached to the skin or worn [14-16], and generators utilizing vibration structures that can be attached to the human body have been developed [17, 18]. Devices that can be attached to the skin or worn are applied to those parts of the human body where a large deformation can occur (elbows, knees, etc.). Although it has the advantage of being light and the user does not sense its presence, such a device has difficulty in utilizing the kinetic energy of the human body. An attached generator based on the previous apparatus or vibration structure has a limitation that it mainly utilizes mechanical energies applied along a single or twin axis. The mechanical energies generated through human motions, however, act along multiple axes. Therefore, to utilizing multi-axial motions, various pendulum based TENGs were proposed [19-21]. The previous researchers utilized the relative motion of pendulum at the incident of input mechanical energies. The simple reciprocal pendulum motion, rotation of pendulum, and chaotic motion of series connected double pendulum were utilized. However, the collision of pendulum itself as a mechanism of electric generation is not considered in common.

Thus, this study presents the TENG structure to effectively harvest mechanical energy by human motion along multiple axis and to utilize the collision of pendulum during the incidence of mechanical energies. For this purpose, a pendulum that can be deformed mechanically was manufactured in order to improve the output. Moreover, in order to select a suitable operation mode, TENG devices based on the SE-mode and FS-mode were implemented to compare their performances. In addition, a suitable structural criterion was selected based on changes in the output caused by the different diameters of the cylinder that constitute the TENG, and based on the mass of the pendulum. Furthermore, the degree of the change in output according to the attachment site of the device on various areas of the human body was confirmed, and a suitable body part was selected for electrical energy conversion. Finally, changes in output behavior depending on the type of motion were confirmed, and LEDs powered by the device were lit to demonstrate that mechanical energies by human motions could be converted to electrical energy and utilized as necessary.

2. EXPERIMENTAL

2.1 Production of TENG Device

As shown in Fig. 1, the TENG device consists of a pendulum and a cylinder wrapped around it. A rubber-based sphere with a hollow core and polyvinyl alcohol (PVA)- borax were utilized as the positive tribo-material. The PVA-Borax composite was produced by mixing its composite elements for 15 min at a weight ratio of 10:1. The composite produced was later inserted into a rubber sphere, and four different pendulums with masses of 10, 12, 14, and 16 g were prepared. To connect the pendulum and the top plate of the cylinder, a polyurethane single string of 0.3 cm diameter was used.

Fig. 1.

Structure and power-generation performance of TENG device for utilizing multi-axis mechanical energies; (a) Basic concept and form, (b) Structure and composition parts of the device, (c) Schematic diagram of multi-axis movement of the pendulum on i) x-z and ii) x-y planes, ii) changes in output based on types of motion. The output voltage was measured with a voltage probe with an internal resistance of 40 MΩ.

The external form of the cylinder was produced with polylactic acid (PLA) using a 3D printer. To confirm the effect of the design variables, four cylinders having diameters ranging from 4 to 7 cm, and an interval of 1 cm, were prepared. The cylinders were configured to allow electrostatic generation to occur on the wall and floor surfaces of the cylinders. Initially, an aluminum (Al) electrode was fabricated, and a single electrode without a pattern and another electrode with a grating pattern were attached. The electrode with the grating pattern was manufactured so that there is a 0.4 cm gap between the bars of the grate.

Similarly, two different types of cylinder floors were produced, one that is suitable for a single electrode and the other for an electrode with a pattern. The pattern was repeated at an angle of 30^o with a 0.2 cm gap. The single electrode mode (SE-mode) was set as the basic operational mode for the cylinder configured with a single electrode, while the freestanding mode (FS-mode) was the basic operational mode for the cylinder with the grating pattern. A negative tribo-material was attached after the attachment of the electrodes, utilizing a 130 μm thick polytetrafluorethylene (PTFE) film. The PTFE was attached to the top surface of the electrodes and was configured to cover the entire area. Finally, the cylinder and pendulum were combined to produce the TENG device.

2.2 Assessment of TENG Device

In order to verify the output behavior of the TENG device that we implemented, an oscilloscope (MDO3052, Tektronix), a voltage probe (P5100A,Tektronix) with internal resistance of 40 MΩ, and a low-noise current preamplifier (SR570, Stanford Research Systems) were utilized. Four different types of experiments were conducted by varying the following parameters to analyze the changes in output behavior: (1) diameter of the cylinder, (2) mass of pendulum, (3) attachment site (4) type of motion.

In order to confirm the effects of the diameter of the cylinders and pendulum mass, a frequency of 3 Hz and a mechanical load moving at a speed of 0.9 m/s were applied in the horizontal direction. As shown in Fig. 4a, in the experiments conducted on different areas of the human body, the device we designed was attached to the subject’s arm, back, and ankle. Next, the voltage and current were measured while the subject walked for 10 m at a speed of 1.2 m/s. After selecting a suitable area for attachment, changes in output behavior based on human motion were verified.

As indicated in Fig. 5a, the output due to electrostatic generation occurred in three different types of motions: walking, running, and jumping. The walking experiment was conducted under the same conditions as previously mentioned, and the running experiment was conducted while the subject was moving the same distance at a speed of 1.8 m/s. Furthermore, the jumping experiment was conducted at the same place with a speed of 0.75 m/s and a frequency of 1 Hz. Finally, sufficient power to light 13 LEDs was generated using the same load, which confirmed the possibility of utilizing electrical energy generated via human motions.

3. RESULTS AND DISCUSSIONS

3.1 Changes in TENG Output Based on Operation Mode

As mentioned previously, a pendulum-based structure was fabricated, as shown in Fig. 1a, in order to effectively utilize mechanical energies acting along multiple axes in this study. Fig. 1b shows an Al electrode and a tribo-material layer configured with a PTFE film that acts as a negative tribo-material, formed inside the 3D printed cylinder structure. The outer surface of the pendulum was configured with a positive tribo-material of a rubber film, and the inner surface was filled with PVA/Boras composite. Because the form of the pendulum is circular, the use of relatively rigid materials can cause problems at the point of contact resulting in lower energy conversion efficiencies due to small contact surfaces. Therefore, in order to increase the contact area when attaching the tribo-materials, a deformable material (PVA/borax composite) was utilized. As indicated in Fig. 1c, multiple axes can be utilized because this design has a pendulum structure, and a wider contact area can be obtained through mechanical deformation of the pendulum. This design is expected to improve the power generation performance because all the three-dimensionally applied mechanical energies can be utilized, and all the mechanical energies generated by various types of motion can be harvested and converted into electrical energy. Generally, a TENG device is assembled and an electrode is placed on each tribo-material, allowing electrons to move between two electrodes. However, in the case of pendulum-based structures, because it is difficult to place an electrode on a pendulum, the SE-mode or FS-mode should be utilized. As shown in Fig. 2a, a SE-mode-based device was produced by replacing the electrode on the pendulum with a ground connection, which is connected to the electrode of the negative tribo-material. Contact electrification occurs between the tribo-materials in the initial phase of Figs. 2a-i. Once both tribo-materials are separated by the mechanical energies generated (Fig. 2a-ii), an electric field is created by the surface charge, which is generated because of triboelectrification. Next, an electrostatic induction phenomenon occurs between the tribo-material and the electrode due to this electric field. Owing to the electrostatic induction, the electron migrates from the electrode of the negative tribo-material to the ground, and the electrons continue to flow until the TENG device is electrostatically stable. In contrast, if the pendulum gets closer again, the electrons flow into the electrode on the negative tribo-material through the ground.

Fig. 2.

Changes in power-generation performance based on operation mode; Power-generation process of the device in (a) SE-mode and (b) FS-mode, (c) Voltage and (d) Current measured by operation mode. The output voltage was measured with a voltage probe with an internal resistance of 40 MΩ.

The FS-mode depicted in Fig. 2b has a feature in common with the SE-mode that it has no electrode on a single tribo-material. However, there are differences wherein a pattern exists at the lower electrode of the negative tribo-material, and electrons move between the lower electrodes. Because of this, the FS-mode has an advantage that it can be applied to the lateral sliding mode (LS-mode) as well as to the vertical contact separation mode (VCS-mode). This indicates that compared to the SE-mode, the FS-mode can convert more diverse types of mechanical energy and different categories of pendulum motion into electrical energy.

Based on this conclusion, an experiment to compare the power generation performance was performed. To accomplish this, a horizontal mechanical load with a frequency of 3 Hz and speed of 0.9 m/s was applied. As can be seen in Figs. 2c and d, when the SE-mode is utilized, voltages of 11.5 V and current of 0.66 μA were generated. In contrast, the FS-mode was able to generate approximately twice the amount of electrical energy at a measured voltage and current of 20.9 V and 1.21 μA, respectively. This implies that pendulum motion is not simple under the same horizontal load, and the FS-mode is able to convert the pendulum motion energy into electrical energy more efficiently than the SE-mode. Thus, in this study, an FS-mode-based device was fabricated, and changes in performance based on certain design variables were confirmed.

3.3 Changes in TENG Behavior Based on Design Variables

Based on the conclusion that the FS-mode-based device is better at energy conversion, the changes in performance based on design variables were confirmed. The diameter (d) of the cylinder with the attached PTFE, and the mass of the pendulum were designated as the major variables; the changes in output were confirmed under the same conditions as in the previous experiment (frequency 3 Hz and lateral velocity 0.9 m/s). In addition, because energy conversions occur in the cylinder’s wall and floor surfaces, the output voltage and current were measured on each surface. To begin with, in order to confirm the effect of the diameter (d) of the cylinder, a comparative experiment was performed with diameters from 4 to 7 cm, with an interval of 1 cm.

As indicated in Figs. 3a–d, we verified that the output differed depending on the change in the diameter of the cylinder. Figs. 3a-i and 3a-ii indicate the values of voltage and current measured on the cylinder wall; a voltage of 1.8 V and current of 0.02 μA were measured when the diameter was 4 cm. When the diameter of the cylinder was 6 cm, a voltage of 2 V and current of 0.08 μA were generated. In the case of a cylinder with a diameter of 6 cm, the output energy was three times more than in the previous case, and a voltage of 6.2 V and current of 0.28 μA were measured. The maximum output was measured on the device with a cylinder diameter of 7 cm, and the voltage and current at this point were 9.8 V and 0.36 μA, respectively.

Fig. 3.

Changes in power-generation performance based on design variables; Changes in power-generation performance on (a) cylinder wall surface and (b) floor surface based on cylinder diameter, Power generation performance on (c) cylinder wall surface and (d) floor surface based on the pendulum mass, i) and ii) refer to voltage and current, respectively, Inset is indicating the diameter, d, of the cylinder. The output voltage was measured with a voltage probe with an internal resistance of 40 MΩ.

The cause of these changes in output can be found in the characteristics of electrostatic power generation. Electrostatic power generation requires two processes: contact and breakaway. If a clear breakaway is not made after contact, the electric field generated by the surface charge on the tribo-material is weakened, and thus, it is difficult for a clear electrostatic induction phenomenon to occur. Therefore, if the diameter becomes less than 7 cm, the gap between the rubber-based pendulum and PTFE is reduced, and the decreased strength of the electric field can result in lower output energy.

Figs. 3b-i and 3b-ii exhibit the change in electrical energy generated on the cylinder floor. A cylinder with a diameter of 4 cm was used to generate a voltage of 0.43 V, and current of 0.01 μA. In the case of the cylinder with a 5 cm diameter, an improved voltage (1.4 V) and current (0.06 μA)as compared to the previous case were measured. On the floor of the cylinder with a 6 cm diameter, a voltage of 3.3 V and current of 0.12 μA were converted, and in the case of the cylinder with a 7 cm diameter, a voltage of 2.81 V and a current of 0.107 μA were generated. It was confirmed that while the maximum power generation performance on the wall surface occurred on the cylinder with a 7 cm diameter, the same level of performance on the floor surface occurred on the cylinder with a 6 cm diameter. The improved performance demonstrated because of the increase in diameter can be considered to be due to the formation of a suitable gap between tribo-materials and the activation of the electrostatic induction phenomenon.

Because the power generation on the cylinder floor surface relied on contact sliding, if the sliding gap was narrow, the electrostatic induction by surface charge might be weakened. Electrostatic induction relies on the strength of the electric field, and the strength of the electric field varies depending on the amount of surface charge and its interference status. Thus, a suitable gap with an uninterrupted electric field is required, and having a cylinder diameter of 6 or 7 cm could prevent the interference of the electric field and activate the maximum electrostatic induction. In addition, because the electrical energy generated on the cylinder wall surface is three times greater than that on the floor surface, we can state that a cylinder diameter of 7 cm demonstrating the maximum performance on the cylinder wall surface is suitable.

Fig. 3c-i and 3c-ii display the changes in the output on the cylinder wall surface caused by changes in the pendulum mass. When the pendulum mass was 10 g, the measured voltage and current were 4.77 V and 0.43 μA, respectively. The maximum performance was measured when using a 12 g pendulum. The voltage and current in this experiment were 11.38 V and 0.68 μA, respectively, which are each approximately twice the magnitude of that observed under other conditions. When the mass was increased to 14 g, a voltage of 6.98 V and a current of 0.55 μA were generated. When the mass was increased to 16 g, a voltage of 5.23 V and a current of 0.28 μA were generated, demonstrating the worst performance. When the pendulum mass is increased, the inertia of the pendulum increases, and a relatively stronger load can be applied to the wall surface. Moreover, because the device utilizes a pendulum that possesses excellent deformation characteristics, it has an advantage that the contact area can be increased. However, because the increased mass of the pendulum implies that the volume occupied by the pendulum inside the cylinder increases, the gap between the tribo-materials might become narrow. In other words, the decreased gap between the tribo-materials causes problems where interference between electric fields and the reduction of electrostatic induction phenomenon occurs, and the output is reduced. Therefore, we conclude that when the pendulum mass is 12 g the contact load and area increase and a suitable gap between the tribo-materials is created.

Figs. 3d-i and 3d-ii display variations in output behavior on the cylinder floor surface caused by changes in the pendulum mass. It was confirmed that when the pendulum mass was 10 g, the measured voltage and current were 4 V and 0.31 μA, respectively. It was found that the output increases when using a pendulum with a mass of 12 g, and that the voltage and current with these parameters were 12.8 V and 0.63 μA, respectively, which are approximately three times greater than those under other conditions. A voltage of 12.9 V and current of 0.62 μA can be generated when the mass is increased to 14 g. On the average, our system could generate electrical energy at a similar level when the mass is 12 g. However, a higher level of electrical energy can be generated when a margin of error has to be considered. In the case where the mass was increased to 16 g, the voltage and current were 3.45 V and 0.42 μA, respectively, this displaying the worst performance. This trend can also be regarded as a change in performance due to increased mass and volume. It was possible to achieve positive effects with a mass of up to 16 g by utilizing an increased contact area and a higher load. In the case of a mass of 16 g, it was determined that the decreased gap between the tribo-materials caused a degradation in the power-generation performance. Although a mass of 14 g is effective considering the output on the floor surface, a mass of 12 g can be regarded as sufficiently suitable, considering the sum of the floor surface output and the cylinder wall surface output.

3.4 Changes in Power-generation Performance Based on Contact Area on Human Body

After confirming the changes in output based on design variables and selecting a suitable set of initial conditions, the output behavior based on the contact area when the device is attached to the human body was verified. As indicated in Figs. 4a-i, 4a-ii, and 4a-iii, the device was attached to the arm, back, and ankle, and the average values of voltage and current generated while the subject walked for 10 m at a speed of approximately 1.2 m/s were calculated. Similar to the previous experiment, the voltage and current generated on the cylinder wall and floor surfaces were measured separately. Figs. 4b and c display the changes in the power-generation performance at the cylinder wall surface depending upon the contact areas. When the device was attached to the arm, a voltage of 5.6 V and current of 0.18 μA were generated. When the device was attached on the back, a voltage of 2.2 V and current of 0.067 μA were generated, which are about three times less than that generated when the device attached to the arm. The maximum electrical energy was measured when the device was attached to the ankle, where the measured voltage and current were 9.36 V and 0.32 μA, respectively. The same output characteristics were also confirmed on the cylinder floor surface, which can be seen in Figs. 4d and e. The lowest electrical energy was generated when the device was attached to the back, at which point the voltage and current were confirmed to be 0.89 V and 0.06 μA, respectively. The second highest energy was generated when it was attached to the arm, and a voltage of 1.6 V and current of 0.12 μA were measured. The highest electrical energy was generated when it was attached to the ankle, where a voltage of 1.8 V and current of 0.18 μA were observed.

Fig. 4.

Changes in power-generation performance based on attached area on human body; (a) Images of attached areas i) arm, ii) back and iii) ankle, (b) Voltage and (c) current generated on the cylinder wall surface, (d) Voltage and (e) current measured on the cylinder floor. The output voltage was measured with a voltage probe with an internal resistance of 40 MΩ.

In order to identify the reason for the variations in output, we referred to the results of an existing biotrepy. Via this reference, it was confirmed that there was a difference in acceleration generated on the arm and ankle when walking [22]. The arm and ankle move in such a way that certain angles and trajectories occur. Therefore, a relatively higher acceleration can result. On average, the ankle has approximately twice the magnitude of acceleration than that of the arm; thus, a higher mechanical load can be applied, and the output can become proportionally higher. Because the motion of the back has a lesser displacement than the trajectories of the arm and ankle, attachment of the device to the back is determined to have a low output characteristic. Thus, if the device is to be attached to the human body and used, attaching it to the ankle seems to be the best option to improve the power-generation efficiency.

3.5 Changes in Power-generation Performance Based on Type of Motion

After confirming the changes in output behavior depending upon the area of the human body to which the device is attached, the changes in power-generation performance based on the type of motion were verified. As shown in Figs. 5a-i, 5a-ii, and 5a-iii, the electrical energies generated by the three types of motion, including walking, running, and jumping, were measured while the device was attached to the ankle. The walking experiment was conducted at a speed of 1.2 m/s, which is the same as that used in the previous experiment. The speed of motion during the running experiment was set at approximately 1.8 m/s. Finally, the speed during the jumping experiment was set at about 0.75 m/s, and the experiment was carried out by limiting the motion to occur only in the vertical direction. Figs. 5b and c show the changes in power-generation performance on the cylinder wall surface depending upon the types of motion. Typically, a voltage of 8.1 V and a current of 0.44 μA were generated while walking. While running, a voltage of 11.5 V and current of 0.69 μA were generated, which were about 1.4 times greater than those generated during walking. The maximum electrical energy, measured while jumping, was at a voltage of 17.3 V and a current of 1.18 μ. The same output trends were confirmed on the cylinder floor surface, as can be seen in Figs. 5d and e. The lowest magnitude of electrical energy was generated while walking at a measured voltage and current confirmed to be 1.5 V and 0.24 μA, respectively. The second highest electrical energy was generated while running, with a measured voltage of 1.7 V and current of μA. The highest electrical energy was generated while jumping, with a voltage of 2.0 V and electrical energy of 1.22 μA. The list of movements—walking, running, and jumping provide different mechanical loads, and the mechanical load increases in the order of that listing. The time taken to induce the load, however, is reduced to a greater degree, and in the same order [23]. The mechanical load increased the contact area between the tribo-materials, because of which, the voltage and current were improved. In addition, a reduced time of load application implies that the contact and breakaway time between the tribo-materials are reduced. This means that the moving time of charge between the electrodes is reduced, and an additional improvement of current can be pursued. In particular, because an instantaneously heavy load occurs while jumping in the vertical direction, such a motion could demonstrate the highest power-generation performance. Thus, based on this result, it can be concluded that the device in this study can be utilized as a self-powered acceleration sensor.

Fig. 5.

Changes in power-generation performance based on types of motion; (a) Images of human motion i) walking, ii) running, and iii) jumping in place, (b) Voltage and (c) current generated on the cylinder wall surface, (d) Voltage and (e) current measured on the cylinder floor surface. The output voltage was measured with a voltage probe with an internal resistance of 40 MΩ.

Fig. 6 shows the light LEDs utilizing the produced device. Figs. 6a and b show the electrical energies generated on the cylinder wall and floor surfaces as well as the result of measuring both at the same time. These are the average values of voltage and current generated while the experimental subject is jumping at a speed of 0.75 m/s and frequency of 1 Hz. The simultaneous measurement of the value of these two parameters was performed after assessing them on the wall or floor surface individually. When the load was applied, the floor-generated voltage of 11.5 V and current of 0.66 μA, and a wall-generated voltage of 20.8 V and current of 1.2 μA were observed. When these were measured simultaneously by making a parallel connection, the average values of voltage and current were 26.8 V and 1.9 μA, respectively. The output can be increased if the power is generated cumulatively and at the same instant on the wall and floor surfaces during the simultaneous measurement. However, if the instant at which the power is generated differs, the electron movement is offset, which may reduce the output. This can be confirmed through the error range. Fig. 6c shows the circuit used for lighting the LEDs. Because of an offset of electrons, movement could occur if the generated outputs from the cylinder wall and surface were used simultaneously, and therefore, the electricity from the wall and floor surfaces was separated to light the LEDs. Ten LEDs were connected to the wall surface with a relatively higher output, and three LEDs on the floor surface. Utilizing this connection, LEDs were lit while walking, running, and jumping. As indicated in Fig. 6d, it was confirmed that six LEDs were fully lit while walking, eight to ten LEDs were lit while running, and 11 to 13 were LEDs were lit while jumping. It is confirmed that this device can be utilized as a self-powered acceleration sensor that utilizes a power-generating signal, and as a signal sensor to interpret and analyze human motions. Furthermore, we expect that this device can be employed as a high-power generator that utilizes human motions if it is combined with the development of more suitable materials that can be used in its fabrication.

Fig. 6.

Power-generation performance of the produced device and lighting of LED through utilizing the performance; (a) Voltage and (b) Current measured individually and after in a parallel connection, (c) A schematic of the circuit used for lighting LED, (d) Images of lighted LED by types of motion i) walking, ii) running, and iii) jumping. The output voltage was measured with a voltage probe with an internal resistance of 40 MΩ.

4. CONCLUSIONS

In this study, a pendulum-based TENG that utilizes inertia to convert the mechanical energy generated by human motion into electrical energy is proposed and implemented. To improve the power generation performance, a pendulum that could give rise to a large mechanical deformation was assembled, and changes in output based on the pendulum mass and the cylinder design were verified. The pendulum mass and the cylinder diameter commonly affect the gap between tribo-materials; if a suitable gap is maintained (a pendulum mass of 12 g and a cylinder diameter of 7 cm), the offset of electric fields, which is generated by surface charge, can be prevented to improve the triboelectric performance. In addition, as the mass of the pendulum increases the mechanical load, it could increase the contact area and induce an additional improvement in the output. To utilize the energy of human motions, it was confirmed that the device attached to the ankle demonstrated the highest power-generation performance, and we determined that this was due to a high acceleration and mechanical load. Finally, by powering LEDs and lighting them using electrical energy generated by three types of motion—walking, running, and jumping—it was demonstrated that mechanical energies generated by human motions can be harvested. Therefore, we expect that a self-powered sensor for analyzing and interpreting human motions as well as a power generator that utilizes human motions can be produced in the future by using the results of this study.

Acknowledgments

H. J. Ryoo, C. W. Lee, and J. W. Han equally contributed to this work. This research was financially supported by the Mid-career Research Program (NRF-2019R1A2C2083934) and Post-Doctoral Domestic Training Program (NRF-2020R1A6A3A01096572) through the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT and the Ministry of Education.

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