An energy harvesting shock absorber for powering on-board electrical equipment in freight trains

Summary To realize smart detection and safe operation of freight trains, a continuous and stable energy source is required for electrical equipment on the train. It is a feasible scheme to harvest the vibration energy of train suspension to supply power for on-board electrical equipment. This paper presents an energy-harvesting shock absorber (EHSA) based on the slider-crank mechanism and ratchet-pawl mechanism, which provide a vibration reduction effect and renewable electricity. To determine the damping performance and the power generation performance of EHSA, a dynamic model is established based on MATLAB. According to the tests on the mechanical testing and sensing (MTS) bench, the maximum power generation mechanical efficiency of the EHSA is 67.75%, and the maximum output power of the EHSA is 1.65W. In addition, the charging tests on the MTS bench show that the proposed device is applicable to power on-board electrical equipment on freight trains.


INTRODUCTION
Freight trains are one of the most important modes of modern logistics. 1 With the widespread applications of Internet of Things (IoT) technology in trains, including train management, operation, maintenance, video monitoring systems and train control systems, 2 freight trains' efficiency, economy, and safety are further developed. 3 Based on the IoT technology, in the daily operation of modern freight trains, a large number of electrical equipment, such as various sensors, 4 monitors, 5 and controllers, 6 are required to monitor and control freight trains to ensure their safe and efficient operation. At present, standard freight trains are only equipped with brake pipes, not electrical pipes, 7 so the power supply of these on-board electrical equipment mainly comes from chemical batteries. However, due to their limited working life, chemical batteries have become increasingly challenging to meet the energy needs of modern freight trains. Frequent battery replacement will not only waste human and financial resources, but also pollute the environment. 8 Therefore, the development of an efficient, stable, and sustainable environmental energy harvesting system is a promising solution for the power supply of onboard electrical equipment in freight trains.
There are various forms of energy in the environment around the trains, 9 including solar, 10 wind, 11 and vibration energy. 12 Various researchers have proposed and designed railway energy harvesters based on various energy conversion mechanisms. Hao et al. designed a miniature solar collector with foldable wings to power low-power equipment on the trackside, which integrates optical and mechanical sensors to control the switch of the wings depending on weather conditions to reduce dust buildup. 13 Pan et al. proposed a novel structural energy harvesting device, generating electricity with the wind in the tunnel by two specially designed rotors to supply power for low-power sensors inside rail tunnels. 14 Wang et al. designed a solar-wind hybrid energy harvesting device for the Qinghai-Tibet Plateau. According to the experimental results, when the wind speed is set at 13 m/s, and the load resistance is set at 8U, its output power reaches 1.08W. 15 However, whether solar energy, wind energy, or a combination of the two, their energy harvesting effect depends on the weather and is unstable. This is contrary to the energy requirements in the modern railway system and may affect the safety of trains in extreme cases. Therefore, the vibration energy around the railway has attracted the attention of researchers due to its universality.
The vibrations in the railway system do not depend on the environment and weather and are widespread and stable energy sources. Usually, this vibration energy is eventually transformed into heat energy and dissipated. 16 In the existing research, environmental vibration energy harvesters can be divided mainly into piezoelectric and electromagnetic systems. 17 Based on the piezoelectric mechanism, piezoelectric energy harvesters generate electricity by absorbing and converting mechanical movements, 18 with high sensitivity, service life, and energy density. 19 Gao et al. developed a piezoelectric cantilever beam to increase the strain in rail deflection and improve the power generation performance. Tests showed that its peak power was 4.9 mW, enough to power low-power appliances. 20 In order to solve the problem of insufficient power of piezoelectric transducers, Yuan et al. used an array of drum-type piezoelectric transducers placed under sleepers, whose power generation capacity under the real rail can reach 100 mW. 21 Wang et al. proposed using piezoelectric transducers in a stacked and patched arrangement to harvest vibrations from the rail and generate electricity for sensors to monitor the train's real-time operation status. 22 On the other hand, based on the electromagnetic conversion mechanism, 23 vibration energy generators in this field consist of linear and rotary types. Gao et al. first developed a rail energy harvester based on the linear electromagnetic generation mechanism with a peak output voltage of 2V. 24 Later, Gao et al. designed a prototype of an energy harvester based on the rotary electromagnetic power generation mechanism, which is used to harvest the vibration energy of wheel and rail and integrates the charging management system and wireless sensor nodes (WSN). 25 Compared with the linear electromagnetic generator, the rotating electromagnetic generator has a more compact structure and is more suitable for layout in the railway system. Meanwhile, their higher energy density can support larger-scale WSNs, monitors, and controllers in the future. 26 However, most of the energy harvesters in the existing research are designed to supply power to the equipment along the track, ignoring the electricity demand of the on-board electrical equipment.
Currently, the vibration energy harvesting (VEH) technology in railway systems can still be improved from these three aspects: (1) optimizing the structure, size, and layout of the vibration energy harvester to improve its stability, durability, and economy, (2) making the energy-harvesting shock absorber (EHSA) universal in the field of a freight train, easy to assemble and disassemble, and not changing the structure of the original train, and (3) improving the mechanical efficiency and generating power of vibration energy generator to supply power for a wider variety of on-board self-powered equipment.
In this study, an EHSA based on the slider-crank mechanism and ratchet-pawl mechanism is designed, and its modeling, simulation, in-lab tests, and field installations are carried out. The kinematics model of vibration rectification mechanisms and the dynamics model of the system is developed to predict generator speed and system damping under different excitation, and simulation experiments are carried out as preliminary verification. In the MTS bench test, the sinusoidal displacements with the amplitude of 7mm and frequencies of 1-3.5mm are used as input, and the highest power output of 1.65W can be obtained through EHSA, and the maximum mechanical efficiency of 67.75% is achieved. Compared with the existing on-board electromagnetic train energy harvesters, the proposed EHSA has the advantages of small size, simple manufacture, reliable transmission, excellent power generation performance and easy installation on different types of freight train suspensions. In practical application, the EHSA is installed on the train suspension via the clamps, used to harvest the vibration energy of the train suspension to supply power for the on-board equipment on the freight train. When freight trains are traveling at a certain speed, the suspensions of the trains will continuously vibrate vertically. The EHSA converts vibration energy into electrical energy. After rectification, voltage regulation, and storage, it is used to supply power for on-board electrical equipment such as sensors, monitors, and controllers on freight trains. With the help of the IoT, these devices can feed real-time information about trains to a remote monitoring system and, if necessary, carry out corresponding instructions to avoid severe accidents. As the scale of on-board electrical equipment continues to expand, the use of battery power will lead to maintenance difficulties, waste of human and material resources, and environmental pollution problems. On the contrary, the vibration energy of the train suspension always exists with the train operation. Through the presented EHSA in this paper, a steady stream of electricity will be provided to the equipment on the train. Therefore, the research of the EHSA for freight trains in this paper is highly beneficial to the environment and energy.
The rest of the paper is arranged below. The section ''system design'' introduces EHSA, including vibration rectification, generator, and energy storage modules. The section ''modeling and analysis'' elaborates on the modeling and theoretical analysis of the presented EHSA, mainly including kinematic analysis of the vibration rectification mechanism, dynamic analysis, and simulation testing of the system. The section ''bench test details'' describes the details of the experiments of the generation performance and efficiency

System design
The EHSA presented in this paper is mainly composed of three modules -vibration rectification, generator, and energy storage module-and its complete architecture is shown in Figure 1. The primary function of the EHSA is to harvest the vibrations generated by freight train suspensions and provide damping force to reduce the vibration amplitude of the suspensions. During the movement of the freight train, the iScience Article reciprocating vertical vibration generated by its suspension is transmitted to the EHSA. These vibrations have a strong, rapidly changing impact force and cannot be directly input into the generator module. The mechanical motion rectifier (MMR) can reduce the impact force in train vibration and improve the reliability and durability of the EHSA. In addition, the bidirectional vibration of the train suspension is converted into the unidirectional rotation of the generator by the MMR, which also eliminates the inertia loss when the rotating bidirectional switching and further improves the power generation efficiency. Therefore, the MMR-based vibration rectification module is essential in EHSA systems. The vibration rectification module rectifies the bidirectional vibrations into one-way rotary motion through the slider-crank mechanism and the ratchet-pawl mechanism, and the rectified motion will serve as the power source for the generator module. Then, with the one-way rotary motion, electricity is generated from the generator module. The output energy is stored in supercapacitors of the energy storage module, which supplies power for various electrical equipment on the freight train, such as on-board sensors, monitors and controllers.
Vibration rectification module Figure 2 shows the structural design of the EHSA, and the vibration rectification module is its core component, composed of the slider-crank mechanism and ratchet-pawl mechanism. The specially designed double-cranks structure can absorb bidirectional vibrations. Moreover, the specially designed double-pawls structure can rectify bidirectional vibrations into one-way rotations. To capture the suspension vibrations, the EHSA was mounted on freight train suspensions, parallel to the coil spring, as shown in Figure 3. The slider is fixed on the top of the spring, the connecting rod and the crank are connected by bolts, and the transmission between the ratchet and the main shaft is accomplished through a flat key connection. One end of the main shaft is mounted on the base plate by the bearing and bearing seat, and the other is connected to the generator module, which is installed on the base plate through a special bracket.
When the train is in operation, the telescopic motion of the suspension is absorbed by the vibration rectification module by the fixed devices of the slider and the base plate. The vibration rectification module rectifies the bidirectional linear motion into one-way rotary motion, and the resulting one-way rotary motion is then transferred to the generator module. The detailed motion rectification demonstration of the vibration rectification module of the EHSA is shown in Figure 4. Figure 4A demonstrates the compression stroke of the EHSA. When the suspension compresses, the slider at the top moves vertically downward and drives the connecting rods and cranks to rotate. At the same time, the pawls attached to the crank and arranged on either side of the ratchet move down. The claw-shaped pawls slide down the edge of the ratchet without transmitting any motion to the ratchet. The pushing pawl drives the ratchet down to rotate anticlockwise, and the rotary motion of the ratchet is also transmitted to the main shaft. Figure 4B demonstrates the extension stroke of the EHSA. When the suspension is extended, the slider moves vertically upward and drives the connecting rods and cranks to rotate. At the same time, the pawls move upward. The pushing pawls slide up along the ratchet's edge without transmitting any motion to the ratchet. The claw-shaped pawls drive the ratchet up to rotate anticlockwise, and the rotary motion of the ratchet is also transmitted to the main shaft.
After such a rectification process, the telescopic motion of the suspension is transformed into the bidirectional vertical motion of the slider and then into the one-way rotation of the ratchet and transmitted to the generator shaft to drive the generator module to output a three-phase current.

Generator module
After the vibration rectification module absorbs and transforms the telescopic motion of freight train suspensions, the one-way rotary motion transferred to the generator module will significantly improve its working conditions, and better power generation performance will be obtained. A matched set of  iScience Article generator and gearbox consists of the generator module, as shown in Figure 5, and is fixedly installed on the base plate by the special bracket. Because of its small rotor inertia, weight, and size, the brushless direct current motor (model 57BL55S06-230TF9) is used as the generator in this study. The gearbox's transmission ratio is 1:24. Table 1 shows the relevant electrical parameters of this module in detail.

Energy storage module
The rotational velocity of the main shaft of the generator mainly depends on the vibration frequency of the train suspension. Generally, a freight train's frequency range of suspension vibration is 1.3Hz-3.5Hz when it travels at a speed of 40-100 km/h. 7 Since the vibration of freight train suspension is irregular, the voltage and current output from the generator module will fluctuate, making it challenging to maintain a constant value. To solve this problem, supercapacitors are selected to store the energy generated by the EHSA, which have the characteristics of faster charging and discharging higher input and output power and longer working life. Supercapacitors are used in combination with voltage regulators and rectifier circuits to form the complete energy storage module. The circuit schematic diagram is shown in Figure 6A.
The rectifier selected in this paper is a three-phase rectifier, which is a bridge circuit composed of 6 diodes with the unidirectional conduction characteristics. The voltage differences between the phases determine whether each diode is conductive. Figure 6B shows the AC-DC conversion principle of the three-phase rectifier. There is a 120-degree phase difference between the three-phase currents generated by the generator, and the three-phase voltages are represented by V1, V2, and V3 respectively. After rectification, the pulsed direct current will be output, and its voltage is represented by VL.

Modeling and analysis
In this section, the kinematics of the MMR mechanism in the EHSA is studied first, and then the system's dynamic model is analyzed. The power generation capacity and damping effect of the EHSA are analyzed based on the system's dynamic model.

Kinematics of the slider-crank mechanism
The vibration rectification module consists of a slider-crank mechanism and a ratchet-pawl mechanism. The slider-crank mechanism converts the reciprocating motion in the vertical direction produced by vibrations in the suspension of freight trains into rotary motion. The ratchet-pawl mechanism restricts the rotary motion to a one-way motion. The suspension is assumed to have sinusoidal harmonic vibration in the vertical direction. Therefore, the motion of the slider fixedly connected to the top of the suspension can be expressed as: where x s is the slider displacement. A s , f and t represent the amplitude, the frequency and the time of the sinusoidal harmonic vibration, respectively.
As shown in Figure 7, the kinematic relationship is analyzed based on the simplified model of the slidercrank mechanism. Referring to the principle of movement synthesis, the relationship between the slider movement velocity (v s ) and the movement velocity of the connection point between the connecting rod and the crank (v c ) can be obtained as follows: where v c 0 is the velocity at which the connection point rotates around the slider.
According to the geometric relationship between the mechanical parts in Figure 7, the following equation can be obtained: where L c and L cr , respectively, represent the lengths of the crank and connecting rod. a c and a cr , respectively, represent the rotation angles of the crank and connecting rod.
According to the transformation principle for kinematics, the following formulas can be obtained: where u c and u cr are the velocities of the crank rotation and connecting rod rotation, respectively.
According to the algorithms of trigonometry, the following formulas can be obtained: By substituting Equations 7 and 8 into Equation 11, the equation with differential factors can be obtained as follows: By integrating both sides of Equation 12, the relationship between a c and t can be expressed by the following formula: where C is an integral correction constant c.
By simulating Equation 13 and plotting time-angle curves with MATLAB, it can be seen obviously that the crank can be regarded as a harmonic swing in Figure 8. By fitting the sine curve with MATLAB, the expression of a c can be obtained as follows: where A c is the amplitude of the harmonic swing, and a 0 is the initial angle of the crank during the harmonic swing. By simultaneously taking the derivative with respect to time for both sides of Equation 14, the expression of u c can be obtained as follows:  , it can be seen that the relationship between the crank's rotational velocity and connecting rod rotation is as follows: )

Dynamics analysis of the EHSA
In order to analyze the dynamic characteristics of EHSA more accurately, the Lagrangian formulation is introduced in this section. The input force F and input displacement x are shown in Figure 9. Easily obtained from the principle of the formulation, in the proposed EHSA, the following dynamic relationship exists: where F is the excitation force, T is the kinetic energy, _ x is the excitation speed, E is the potential energy, x is the excitation displacement, and D is the damping energy, respectively. For the proposed EHSA, x = x s and _ x = v s , so the kinetic energy T is expressed as the following formula: where u g is the generator rotor rotation velocity, m v is the mass of the components moving vertically, J cr , J c , J r , J ms , J gb and J g , respectively, represent the moments of inertia of a single connecting rod, a single crank, a single ratchet, the main shaft, the gearbox and the generator.
For the two other forms of energy E and D, their expressions are given by the following equations: where C L represents the EHSA damping ratio coefficient, which is a linearly varying value. Figure 9 shows the schematic diagram of the movement of the EHSA, which can be used to quantitatively calculate the value of the damping ratio coefficient of the device. The input power P inp is calculated by the following formula: ) iScience Article From the perspective of energy output, the input power P inp can also be expressed as the following formula: where P g represents the generator output power, P f is the power consumed due to friction in the slidercrank mechanism, ratchet-pawl mechanism, gearbox, and generator. According to the definition of the generator output power P g , it can be expressed as follows: where the efficiency of each component, including the slider-crank mechanism, the ratchet-pawl mechanism, the gearbox, and the generator, is respectively represented by h s , h r , h gb and h g .
According to the power balance equation of electrical and mechanical damping, the following relationship can be obtained: ) The following relationship exists between the torque of the generator shaft T g and the generator rotor rotation velocity u g : ) Equations 27 and 28 are combined to obtain the following relation: where C R is the rotation damping ratio coefficient of the EHSA equipped with the circuit, and its expression is as follows: where k e is the back EMF (electromotive force) constant of the generator, k t is the generator torque constant, R is the external load resistance from the circuit, and r is the internal resistance of the generator.

By integrating Equations 24 and 29 into Equation 26
, the following formula can be concluded: In the EHSA, the gearbox's function is to amplify the rotation velocity of the generator rotor. The relationship between the rotational velocity before and after amplification is as follows: where i is the amplification ratio of rotary motion after transmission by the gearbox.
From the reasoning in the previous section, the following equation is known: By solving Equations 31, 32, and 33 simultaneously, the damping ratio coefficient C L of the EHSA can be expressed by the following formula: The equivalent inertial mass m ei of the moving parts of the EHSA can be calculated as follows: 4J cr L c 2 cos 2 a c L cr 2 cos 2 a cr +4J c +2J r +J ms +J gb +i 2 J g A s A c

Simulation calculation
In order to quantify the specific properties of the EHSA, a corresponding mathematical simulation model is established in MATLAB based on the theoretical kinematics and dynamics of the device, which has been deduced above. Firstly, sinusoidal displacements of different frequencies are input into the mathematical model of the EHSA in simulation. As can be obtained from Equations 15 and 32, the rotational velocity of the generator rotor simulated is shown in Figure 10. Obviously, under the excitation of sinusoidal iScience Article displacement, the rotational velocity of the generator rotor is proportional to its frequency. The simulated rotational velocities are further verified for the selection of the gearbox and generator, ensuring that the working speed of the generator module is reasonable and efficient.
Then, before simulating the damping force of the EHSA, it is necessary to estimate the transmission efficiency of each component of the device. In this paper, the transmission efficiency of the slider-crank mechanism, the ratchet-pawl mechanism, the gearbox, and the generator are respectively presented by h s , h r , h gb and h g , and their estimated values are respectively 0.93, 0.95, 0.90, and 0.91. Therefore, the overall transmission efficiency of the EHSA is approximately 0.72. Thus, the damping force of the EHSA can be calculated from Equation 37, and the relationship between displacement and force in the simulation is shown in Figure 11. It can be seen that the maximum damping force of EHSA is positively correlated with the excitation frequency when other parameters are unchanged. In addition, due to the mass inertia of ESHA itself, the damping force change trend at any excitation frequency is negative slope.

Bench test details
The MTS Landmark Servohydraulic Test System of model 370.02 is selected in this paper, and its relevant technical parameters of the equipment are shown in Table 2. The bench test was carried out using the MTS to measure the performance of the EHSA obtained in simulations, as shown in Figure 12. The overview of setup experiments is shown in Figure 12A, including an MTS system for input sinusoidal displacement and measurement of multiple outputs, an experimental model machine of the EHSA, and a computer. Figure 12B shows the assembled full-scale model machine of the EHSA, and Figure 12C shows its bench installation process. A computer linked to the MTS system can record and save the experimental data of displacement and force received by the sensors in the MTS system, as shown in Figures 12D and 12E shows the preliminary analysis of the experimental data of displacement and force. Figure 12F shows the supercapacitor charging circuits, and the charging test is shown in Figure 12G. In addition to the bench tests, the prototype was installed on a real freight train to verify the universal applicability of the EHSA in freight trains, as shown in Figure 13.

Experimental results and analysis
The main parameters that need to be set in the experiments are the mechanical parameters, which include: the amplitude of excitation displacement is 7mm; the excitation frequencies are 1Hz, 1.5Hz, 2Hz, 2.5Hz, 3Hz, and 3.5Hz, respectively. In addition, supercapacitors with capacities of 1mF, 2mF, and 3mF are selected in the charging tests of this experiment. Figures 14 and 15 show the experimental results of the EHSA with the rated load on the MTS bench.
In detail, Figure 14 is the force and displacement data collected by the MTS system. It is known that the displacement-force annular area represents the vibration energy absorbed by EHSA in a period. The results iScience Article of bench tests show that the maximum damping force of EHSA is positively correlated with the excitation frequency, and the energy storage effect of EHSA is improved with the increase of excitation frequency. It is worth pointing out that, with the increased frequency, the moving parts in the EHSA will be affected by the more obvious flywheel effect. 27 Moreover, it is obvious that when the moving parts of the EHSA move near the extreme position, the magnitude of the force is close to none. Based on the mentioned conditions, the displacement-force data recorded in the experiments are shown in Figure 14.  iScience Article Figure 15 is the voltage data collected by the oscilloscope in this experiment. According to the analysis of Figure 15, when the amplitude of the input sinusoidal displacement is 7mm, and the rated value of the selected load resistance is 4U, with the increase of the frequency of the input sinusoidal displacement, the resulting voltage waveforms are similar, but the maximum voltage and the frequency of the voltage waveform output from the EHSA increase.
These data obtained from the MTS system were mutually verified with the results of the simulation experiments in the section modeling and analysis, which also confirmed the generating capacity of the EHSA.
The power input P inp by the MTS system to the EHSA can be calculated by the following formula: iScience Article where DW is the total amount of mechanical energy input by the MTS system, whose value is obtained by integrating displacement and force data in Figure 14, and T is the periodic value of sinusoidal displacement input by the MTS system.
The power output P o from the EHSA can be calculated by the following formula: where V ðtÞ represents the value of the voltage across the load resistor.
In the bench test, the overall efficiency h of the EHSA can be calculated by the following formula: For the mechanical transmission efficiency h m of EHSA, its value can be obtained by the following formula: where h c is the efficiency of the circuit, which can be calculated by substituting the initial conditions of the experiments into the following formula:  Table 3 shows the average power input from the MTS, the average power output by the EHSA, the overall generation efficiency, and the mechanical transmission efficiency of the EHSA under different excitation frequencies. It can be revealed from Table 3 29 The EHSA can obtain better power generation effect and efficiency both theoretically and experimentally if the optimization of the above techniques is adopted in the energy storage module. Figure 16 shows the charging test results of the EHSA on the MTS bench. These results include the charging voltages of supercapacitors with different capacities when the frequency of the sinusoidal excitation is 2Hz, and the amplitude is 7mm. It can be seen that under this excitation, the output voltage of the EHSA is around 8.7V.
In the charging test, because only one rated sinusoidal excitation is set, the voltage output by the EHSA is the same, and theoretically the supercapacitor should also be charged to the same voltage. According to Figure 16, it can be seen that the voltages of the supercapacitors during the charging process rise rapidly and then gradually become constant but not exactly the same, this is because the selected supercapacitors have small capacities and are more sensitive to the impedance existing in each component in the circuit, resulting in some experimental errors, but this does not affect the conclusions obtained. In addition, as the capacity of the supercapacitor increases, the time it takes to be charged to the ultimate voltage also increases. This is because the two poles of the supercapacitor with a larger capacity can hold more positive and negative charges respectively, so it takes more time to reach the constant voltage. It should be noted that supercapacitors are particularly suitable for use with on-board self-powered equipment due to their short charging time and wide range of optional capacities.
In order to reflect the improvement in power generation performance of the proposed EHSA, the existing on-board train electromagnetic energy harvesters are summarized in Table 4, in which all the data are from the corresponding literature, and in which their characteristics, installation positions and power generation performance are introduced and compared. It can be seen from the comparison that the proposed EHSA has higher mechanical efficiency.

Conclusions
In this paper, an energy-harvesting shock absorber for freight trains is proposed to convert the vibration energy of freight trains into electrical energy. The device can be used to provide a continuous and stable iScience Article power supply for on-board sensors, monitors, and controllers, which can timely detect the running status of the freight trains and ensure the safety of the freight trains. The design of the device is based on the slidercrank mechanism and ratchet-pawl mechanism, and the theoretical analysis, simulation testing, modeling, manufacturing, installation, and experimental testing are also carried out in this paper. Among them, the theoretical analysis includes the kinematics of the MMR mechanism and the dynamics of the whole system, and the simulation test mainly includes the mechanical properties of the device. Then, in the MTS bench test, when the excitation amplitude was set at 7mm, the frequency at 3.5Hz, and load resistance at 4U, the measured power generation performance of the device reached the highest, that is, the mechanical efficiency was 67.75%, and the output power was 1.65W. Finally, the charging test proves that the device is suitable for use with on-board self-powered equipment.

Limitations of the study
Further optimization and field testing are still required for the proposed vibration energy harvesting shock absorber. The limitations of this study can be summarized as follows.
(1) Due to the limitations of experimental conditions and equipment, both simulation and experimentation utilized regular sine wave excitation. To accurately assess the power generation performance of the proposed device on a real train, future studies will involve tests conducted on operational freight trains.
(2) By optimizing material selection and enhancing the accuracy of processing and assembly, the mechanical efficiency and durability of the proposed device will be further improved in the future.
(3) The current study concentrates on the mechanical structure and efficiency of the power generation process. Future enhancements include the incorporation of a power optimization circuit to enhance overall power generation efficiency.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following: