Energy Conversion and Management 299 (2024) 117873
0196-8904/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
A contra-rotating free-spinning energy harvester
Haopeng Ma
a, Lihua Tang
a,*, Haipeng Liu
b,*aDepartment of Mechanical and Mechatronics Engineering, The University of Auckland, 5 Grafton Road, Auckland 1010, New Zealand
bSchool of Mechatronical Engineering, Beijing Institute of Technology, Beijing, 100081, China
A R T I C L E I N F O Keywords:
Energy harvesting Contra-rotating Free-spinning One-way bearing Ultralow frequency
A B S T R A C T
In the field of energy harvesting from motions or vibrations, enhancing the performance of a harvester under ultralow frequency and intermittent input sources remains challenging. To address this issue, this paper presents an electromagnetic energy harvester design featuring a novel design and compact arrangement of dual one-way bearings in the magnet and coil layers. The unique one-way bearing design is capable of significantly enhancing the energy harvesting performance by eliminating springs to reduce friction for contra-rotating high-speed free- spinning motions. The devised harvester is characterised under various controlled low-frequency excitation conditions, yielding a maximum average power of 8 mW at the excitation frequency of 1 Hz. A finite element simulation is then performed to validate the performance of the energy harvester. Furthermore, with a mini- mized form factor, the energy harvesting capability of the proposed harvester in real-life applications of pow- ering electronic devices is demonstrated. The innovative approach from this work holds substantial promise for high-performance energy harvesting from reciprocating motions in ultralow frequency scenarios.
1. Introduction
Driven by the recent advances in semiconductor industries and microelectromechanical systems (MEMS) technologies, portable sensing devices have seen significant growth in the health monitoring and data acquisition for personal, machinery, structural, and environmental ap- plications [1]. The majority of the developed devices feature low power consumption, requiring a power supply in the milliwatts scale [2].
Conventional batteries are often favoured as the primary power solution as the elimination of wiring system reduces installation costs and en- hances portability of the devices. However, the reliance on batteries brings inherent drawbacks, including limited lifespan, additional structural design required, potential pollution issue and high mainte- nance cost due to the need for replacement or recharging [3]. As a result, various energy harvesting methods that convert ambient energy into electricity as alternative power sources have gained enormous interest in recent years.
Kinetic energy derived from motions or vibrations is ubiquitous in our environment [4,5]. Many of these kinetic energy sources exhibit ultralow frequency, intermittent or random characteristics such as human body movements during daily activities [6], ocean wave heave motions [7], and vibrations from railway track deflections induced by passing trains [8]. Hence, researchers proposed a variety of mechanisms
that accommodated the unique characteristics of various motion inputs.
For piezoelectric energy harvesters, Zhou et al. [9] proposed an energy harvester that utilised rotatable magnets to introduce nonlinearity into piezoelectric energy harvesters. By manipulating the angular orientation of the magnets, the frequency bandwidth was broadened to 4 – 22 Hz.
Saha et al. [10] proposed the use of magnetic springs to develop a low- cost, high tunability, and robust nonlinear electromagnetic generator that could generate the electrical power of 300 μW to 2.5 mW from human body motions. Other than nonlinear designs, non-resonant rotational electromagnetic energy harvester was also investigated in [11,12]. Through the utilization of permanent magnets in both the rotor and stator of the device, the device was capable of harvesting energy from human motions with different amplitudes, frequencies, and directions.
Another promising technique for enhancing the power output of energy harvesters is frequency up-conversion. In rotational piezoelectric energy harvesters, this can be accomplished through mechanical plucking [13–15] or magnetic plucking [16], where the piezoelectric bimorphs are first deflected and then released to vibrate freely. This technique allows the piezoelectric bimorphs to operate at resonance frequencies with significantly boosted power output, even when the excitation frequency is ultralow. However, successful implementation of the plucking technique necessitates careful design and tuning of the
* Corresponding authors.
E-mail addresses: [email protected] (L. Tang), [email protected] (H. Liu).
Contents lists available at ScienceDirect
Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman
https://doi.org/10.1016/j.enconman.2023.117873
Received 13 April 2023; Received in revised form 20 October 2023; Accepted 9 November 2023
plectra and magnets in mechanical and magnetic plucking, respectively.
In addition to magnetic plucking, magnetic gearing has also been investigated as a frequency up-conversion mechanism in linear-to- rotational electromagnetic energy harvesting [17–19]. In [18], a concentric magnetic gear with a gear ratio of 26:3 was integrated in an embedded electromagnetic generator. The frequency up-conversion ef- fect enabled the generator to output 1.74 mW at an ultralow input fre- quency of 0.65 Hz. However, the magnetic gear used in such a system only served as a frequency up-conversion mechanism; the energy transduction mechanism was achieved through electromagnetic induc- tion using another set of magnets that was connected to the output layer of the magnetic gear. This resulted in cumbersome structural design of the proposed energy harvester. Furthermore, although frequency up- conversion mechanisms increased the power output, the output voltage waveform was exactly in sync with the input source. For an input source that is linear reciprocating, the output power inevitably de- creases to zero during reversion of the input motion directions, leading to losses during the rectification of AC voltage to usable DC voltage, particularly in low voltage ranges.
Recently, mechanical motion rectifiers (MMRs) were adopted to convert the bi-directional input rotations or linear reciprocating motions into unidirectional output rotations [8,20]. In contrast to previous power enhancing techniques, MMR tends to yield higher efficiency as it is capable of generating sustained output even during the reversion of input motion directions. In [21], MMR was achieved through the implementation of a spring-loaded pawl-ratchet clutch system that converted pendulum oscillations into unidirectional rotations of a flywheel connected to a generator through a gear system. However, the problem of the spring-loaded pawls led to additional friction during rotation in the disengaged direction as the pawls were repeatedly compressed and released by the ratchet teeth. To address this issue, Luo et al. [22] developed a pawl-ratchet mechanism that eliminated the use of springs, which relied solely on centrifugal force during the rotation of the centre rod to extend the pawls outward and engage the grooves on the flywheel. The removal of springs in the pawl-ratchet increased ef- ficiency, resulting in an average power output of 85 mW at a rotation speed of 140 revolutions per second (RPS). However, these pawl-ratchet designs only utilised a single stroke out of the complete two-stroke reciprocating motion to harvest energy, resulting in approximately half of the input source energy being wasted. In [23], two opposite pawl- ratchet mechanisms were affixed to both sides of a flywheel with mag- nets. Thus, irrespective of the input motion direction, the flywheel could always be accelerated to induce voltage in the relatively stationary coil layer.
One alternative approach to achieving motion rectification is through the implementation of one-way bearings, which are also referred to as one-way clutches or mechanical diodes [24]. Similar to ratchets, one-way bearings permit the transmission of input race rota- tions to the output race by locking the rollers between the inner and outer races. Conversely, the bearing is disengaged in the other input direction to allow freewheeling of the output race. Liang et al. [25]
designed an MMR-based pendulum-based energy harvester, where two oppositely rotating one-way bearings were incorporated in a miter gear system to rectify the bi-directional pendulum swing motions. From a comparative analysis against a non-MMR pendulum energy harvester, it was found that despite having a higher mechanical damping, the MMR pendulum energy harvester could yield higher efficiency and bandwidth than the non-MMR design. In [26], MMR was achieved through the implementation of two oppositely rotating one-way bearings in a spur gear system and the MMR energy harvester exhibited an astonishing power output of 0.72 W and a power density of 0.43 W/kg. Despite generating substantial power output, the proposed one-way bearing energy harvesters had limitations: the one-way bearings were only used to rectify motion and the energy transduction was achieved through transmitting the rectified motion to additional mechanisms such as generators. These additional mechanisms resulted in cumbersome
structural design that limited the application scenarios of such kind of technology. Furthermore, the existing one-way bearing designs contain springs that constantly force the bearing balls to come into contact with the inner and outer races of the bearing, which causes significant friction during rotation.
In this work, we propose a rotational electromagnetic energy harvester that can achieve high performance under ultralow frequency reciprocating excitations through the implementation of low-friction contra-rotating free-spinning motions. The innovative configuration of the developed harvester can achieve motion rectification with reduced structural complexity by integrating dual one-way bearings directly in the magnet and coil layers. A novel one-way bearing design is proposed to completely eliminate the use of springs and allow free-spinning mo- tions for performance enhancement. The remainder of this paper is organised as follows. Section 2 details the system design of the proposed energy harvester. Section 3 presents the experimental setup and char- acterization results of the proposed energy harvester. The superior performance of implementing contra-rotating free-spinning design is demonstrated through a comparative study with the designs using no one-way bearing or using one-way bearing in one layer only. In Section 4, finite element simulation is conducted to confirm the performance of the proposed harvester and investigate the influence of the magnet ar- rangements and dimensions. The power generation capability of the developed energy harvester is demonstrated through powering various electronic devices in Section 5. In Section 6, a comparative analysis additionally highlights the superior performance of the proposed energy harvester design when compared to other MMR-based devices. Con- clusions from this work are drawn in Section 7.
2. System design
The overall design of the contra-rotating free-spinning electromag- netic energy harvester is illustrated in Fig. 1. It is mainly composed of an input shaft, a magnet layer, a coil layer, two customised one-way bearings, a slip ring, and an outer casing. For the magnet and coil layers, each of them is mounted onto the input shaft with a one-way bearing to enable free-spinning in opposite directions.
As discussed previously, the main disadvantage of the designs in the literature is that one-way bearings were only used as motion rectifica- tion mechanisms, hence additional mechanisms were required for en- ergy transduction. In the proposed design, through fitting one-way bearings into the magnet and coil layers, the one-way bearings are directly involved in both the motion rectification and energy trans- duction, and the cumbersome output shaft and gearing systems can all be eliminated to achieve a much simpler design. The input shaft, which can be driven by a spiral torsional spring or strings wound onto the grooves at its end, converts linear reciprocating motions into rotational motions that drive the magnet and coil layers. The number of magnets is matched with the number of coils to avoid any cancellation in the induced voltage. The slip ring is capable of transmitting the generated electricity out from the rotating coil layer. All components of the energy harvester are encapsulated in the 3D printed case. The design of each component will be detailed later. The design parameters of the energy harvester are summarised in Table 1.
2.1. Input shaft
The design of the input shaft is illustrated in Fig. 2(a). There are two methods implemented for transforming the reciprocating motions into contra-rotating motions of the magnet and coil layers of the harvester.
The first method involves designing two grooves at one end of the shaft (Fig. 2(a)), with each groove having a diameter of 10 mm at its inner rim, a diameter of 25 mm at its outer rim, a height of 2 mm, and holes with a diameter of 1.5 mm drilled through the inner wall. One string is wound around each groove, with the beginning of the string secured within the drilled hole. The strings of two grooves are wound in opposite
directions, meaning that when one of the strings is being pulled away to unwind it from the groove, the other string will get wound onto the other groove simultaneously. This setup makes it suitable for testing on a shaker generating linear reciprocating motions, as demonstrated in the experiment section (Section 3). The second method employs a spiral torsional spring as illustrated in Fig. 2(b). The centre of the torsional spring is clamped at the fins of the torsional spring housing, which is
attached onto the end plate of the energy harvester. The spool of the torsional spring has an extrusion which can be inserted into the dedi- cated slot opening at the end of the input shaft. A strand of string is connected to the end of the torsional spring. When the spring is pulled, the spool rotates as the spring stores elastic potential energy. Since the spool is directly connected to the input shaft, the rotation is transmitted to the input shaft. When the string is released, due to the release of stored potential energy in the torsional spring, the input shaft rotates in the other direction automatically. With this input method, the bi- directional reciprocating input with a single string offers a simpler implementation suitable for real-life applications. All the components for this input method are detachable from the input shaft (Fig. 2(b)). In addition, this detachable design has the advantage of offering inter- changeable input methods at a low assembly cost.
Beside the grooves of the input shaft is an adapter for attaching a plastic radial load bearing that is connected to the end plate. The left end plate (Fig. 1) constitutes a part of the outer casing of the energy harvester which is stationary. Therefore, a bearing is needed to support the rotating input shaft. The shaft key is designed with a sliding clear- ance fit with the keyway of the one-way bearings, thereby facilitating the successful transmission of torque from the input shaft to the output layers. At the right end of the shaft key (Fig. 2(a)), there is a 3D printed thread that allows for the connection of a slip ring bearing adapter, which supports the bearing that is linked to the stationary slip ring housing. Additionally, this slip ring bearing adapter serves the purpose Fig. 1. Prototype and exploded view of the contra-rotating free-spinning energy harvester.
Table 1
Design parameters of the contra-rotating free-spinning energy harvester.
Component Description Value
Magnets Material Neodymium
Magnet grade N45
Cylinder outer diameter / height (mm) 10 / 6.5 Air gap to coil layer (mm) 1.5
Quantity 6
Coils Material Single core copper
wire
Wire diameter (mm) 0.1
Sector outer diameter / inner diameter /
height (mm) 17 / 6 / 3
Quantity 6
Total number of turns 6000
Total internal resistance (Ω) 390
Prototype Volume (cm3) 232
Mass (g) 127
of securely compressing the magnet and coil layers together to minimize any axial movements that might affect the air gap between the two layers.
2.2. One-Way bearing
The design of the one-way bearing is shown in Fig. 3. It consists of two main components: a plastic radial bearing and a one-way rotation mechanism fabricated by 3D printing. The one-way rotation mechanism contains an input race, an output race, and eight plastic bearing balls.
The inner ring of the radial bearing is connected to the input race of the one-way mechanism, which is then connected to the input shaft via a keyway to transmit bi-directional rotations; the outer ring of the radial bearing is connected to the output race of the one-way mechanism, which transmits the unidirectional rotation to the magnet or coil layer in which the one-way bearing is inserted.
As shown in Fig. 3, the input and output races are designed with novel curvature patterns that form eight channels for the bearing balls to reside in. These channels are designed and adjusted to provide two working modes for the one-way bearing: engaged and disengaged. To
enter the engaged mode, the input race needs to rotate in the engaging direction, as shown in Fig. 4(a). As the input race rotates clockwise, the bearing balls are forced to travel towards the narrower end of the channel formed by the vertices of the input race and extrusions on the output race’s sidewall. This action generates a ‘wedging’ effect that prevents further rotation of the input race, ‘engages’ the two races and allows torque to be transmitted from the input race to the output race. In the engaged mode, both races must rotate at the same angular velocity.
By looking closely at the section of the one-way bearing in Fig. 3, one can observe that there are curvatures designed on the horizontal plane of the output race, which help the bearing balls to return to their neutral po- sitions where successful engagement can be achieved. Without this design, as the output race rotates, the bearing balls would tend to travel outwards to the sidewall of the output race due to inertia, making the next engagement impossible since the bearing balls would no longer contact the input race. Alternatively, the one-way bearing can enter the disengaged mode when the input race rotates towards the disengaging direction, as shown in Fig. 4(b). As the race rotates, the bearing balls are forced to travel towards the wider end of the channel and stopped by the sidewall of the adjacent channel. When the vertices of the input race Fig. 2. (a) Input shaft design and (b) torsional spring input method.
Fig. 3. Exploded view of the one-way bearing design.
rotate towards the bearing balls, the balls are pushed up to the clearance above it, rather than generating a wedging effect like in the narrower end of the channel. This enables the input race to keep on rotating with minimal resistance. In this mode, the input race can also rotate in the clockwise direction, but the one-way bearing can remain disengaged as long as the output race is rotating in the same direction at a higher angular velocity, i.e., the angular velocity of the input race relative to the output race is in the disengaging direction (counter-clockwise).
Compared to the conventional one-way bearings that use springs to force bearing balls to come into contact with inner and output races, the proposed design offers much lower friction, which enables free-spinning motion and enhances overall energy harvesting performance significantly.
2.3. Magnet layer
The magnet layer comprises a one-way bearing, six N45 graded neodymium cylinder magnets with polarities vertically upwards, and a 3D printed housing, as shown in Fig. 5. Each magnet has a diameter of 10 mm and a thickness of 6.5 mm and they are evenly spaced along a pitch circle with diameter of 40 mm. The one-way bearing inserted has an engaging direction in the clockwise direction.
2.4. Coil layer
The coil layer comprises a 3D printed housing, a one-way bearing, and six coils wound in series using 0.1 mm enamelled copper wires, as shown in Fig. 6(a). The coils are wound onto 6 coil adapters first before being assembled into the coil housing to make the winding process easier. Each coil with sector shape has 1000 turns that are wound using the self-designed coil winding machine (Fig. 6(b)). The two ends of the coil windings pass out through two stands on the opposite side of the coil layer to connect with the slip ring (see Section 2.5). The one-way bearing inserted has an engaging direction in the counter-clockwise direction, which is opposite to the engaging direction of the magnet layer.
2.5. Slip ring
The operation of the rotating coil layer necessitates the utilization of a stationary slip ring section to transmit electrical power. The selection of the appropriate slip ring is contingent upon two crucial factors:
minimising rotating friction for enhanced energy harvesting perfor- mance and minimising size for a compact overall design. In light of these requirements, two slip ring designs are proposed and evaluated. The first design uses a platter shaped configuration where copper rings are placed on the same plane separated by thin walls as shown in Fig. 7(a). The ends of the coil winding are soldered with two stranded wires which act as brushes that contact with the two copper rings respectively. As the coil layer rotates, the stranded wires brush along the stationary copper rings to transmit electricity. The platter configuration is advantageous in reducing the axial length of the device, but prone to brush wear, which requires continuous maintenance since the worn-down copper rings can disrupt smooth electricity transmission, as shown in the measured voltage waveform in Fig. 8(a). Furthermore, stable electricity trans- mission relies on stable contact between the stranded wires and the copper rings, which significantly increases the rotational friction of the coil layer, causing it to stop almost immediately when the engagement of the one-way bearing finishes. The second slip ring design uses a Fig. 4. Processes of one-way bearing during (a) engagement and (b) disengagement.
Fig. 5.Magnet layer design.
cylindrical configuration. The two ends of the coil are wound onto the ring section and sleeve section of a 3.5 mm headphone plug using screws and nuts, as shown in Fig. 7(b). Then, two insulated wires are soldered onto the corresponding parts of the headphone plug, enabling the
successful conductance of electricity. This design inevitably increases the axial length of the energy harvester; however, it minimizes the contact area between the coil wire and the headphone plug, thereby reducing the rotational friction significantly while providing stable Fig. 6. (a) Coil layer design and (b) self-designed coil winding machine.
Fig. 7. Slip ring designs: (a) platter shaped configuration and (b) cylindrical configuration.
Fig. 8.Voltage waveform comparison with two slip ring designs: (a) platter shaped configuration and (b) cylindrical configuration.
power output. Further, the surface of the headphone plug is nickel plated, which offers better resistance to wear and oxidation compared to the copper rings used in the platter configuration. The voltage waveform measured using the cylindrical slip ring configuration is shown in Fig. 8 (b). It is evident that the waveform is much more stable compared to that of the platter configuration. Thus, the cylindrical slip ring configuration is used in the final prototype of the energy harvester.
3. Experiment and results 3.1. Experimental setup
To assess the efficacy of the proposed design of the energy harvester, a comparative analysis of three distinct operation modes is conducted:
Mode 1 is the contra-rotating free-spinning mode as intended, employing customized one-way bearings in both the magnet and coil layers; Mode 2, in contrast, utilizes one one-way bearing in the magnet layer only, while the coil layer remains stationary, thereby examining the performance of utilizing only a single stroke of the reciprocating motions for energy harvesting; Finally, Mode 3 is implemented with one-way bearings entirely eliminated and the magnet layer directly connected to the input shaft. Therefore, the characteristics of the elec- trical output of rotational energy harvesters match the input motion.
The experimental setup is shown in Fig. 9. The strings wound on the input shaft of the energy harvester are connected through a pulley to an electrodynamic shaker (APS 113 ELECTRO-SEIS), which is paired with an amplifier (APS 125) and a shaker controller (Vibration Research VR 9500) that takes the feedback from an accelerometer mounted on the shaker and controls the input displacement via VibrationView software installed on a desktop computer. This allows for conducting highly customizable tests with linear reciprocating motions in a range of am- plitudes and frequencies. The electric output of the energy harvester is measured using an oscilloscope and the acquired data is analysed using MATLAB. This experimental setup enables a quantitative characteriza- tion of the energy harvester by conducting sinusoidal tests with excita- tion amplitudes ranging from 30 mm to 90 mm and frequencies from 0.5 Hz to 1 Hz.
3.2. Comparative study 3.2.1. Open-circuit voltage
We first compare the open-circuit voltage characteristics of the de- vice in three different modes. Fig. 10 shows the open-circuit voltage
waveform acquired in the duration of 6 s under the excitation amplitude of 50 mm and frequency of 0.5 Hz.
For Mode 1, in one complete cycle of the input with a period of 2 s, the output waveform exhibits a sinusoidal wave that gradually decays in terms of both amplitude and frequency for every second. This decaying behaviour is due to the fact that small mechanical damping exists in the system that reduces the rotating speed. When the voltage output is at its highest point of about 2 V (i.e. the last moment of engagement), the time period of one complete cycle of the voltage waveform is around 0.06 s, meaning that the relative angular velocity between the magnet and the coil layers is around 167 revolutions per minute (RPM). Conversely, the time period of the one complete cycle of the voltage waveform decreases to approximately 0.1 s when the voltage output reaches its lowest, which is equivalent to a relative angular velocity of 100 RPM. The decaying behaviour occurring twice in one cycle of excitation (2 s) is a result of the input shaft engaging with both one-way bearings in the magnet layer and coil layer once, which exerts an impulsive torque that quickly ac- celerates either the coil or magnet layer, leading to an increase in the
Fig. 9. Experimental setup for comparative study using shaker.
Fig. 10.Comparison of typical open-circuit voltage waveforms of the energy harvester in three modes under the excitation displacement of 50 mm and frequency of 0.5 Hz.
induced voltage followed by the decaying behaviour associated with free-spinning. The engagement of the one-way bearing is transient. It only occurs when the input race is accelerated by the input source and the output race is rotating at a lower velocity than the input race. The input race consequently forces the output race to rotate at the same angular velocity. After the input race starts to decelerate, the output race disengages with the input race and decelerates at a much lower rate than the input race due to the minimal damping with the customised one-way bearing design. Similarly, when the input source starts to accelerate towards the other direction, the one-way bearing located in the other layer will get engaged in the same procedure. In addition, it can be observed that in one excitation cycle, the amplitude of the second part of the decaying oscillations of the voltage waveform is slightly lower than the first part. This phenomenon is due to the fact that for the coil layer, the constant contact between the headphone plug and the slip ring re- sults in extra mechanical loss and thus a slightly reduced maximum angular velocity and faster decaying rate as compared to the magnet layer.
In Mode 2, the utilization of a single one-way bearing clearly in- dicates that only one gradually decaying oscillation waveform is pro- duced during each excitation cycle (2 s). This is because the one-way bearing inserted in the magnet layer can only be engaged once in one cycle of the two-stroke reciprocating motion. The relative angular ve- locities of the energy harvester at the highest and lowest voltages are 90.9 RPM and 58.8 RPM, respectively, which are 45.6 % and 41.2 % lower than those of Mode 1. The highest voltage is about 1.3 V, which is also much lower than that of Mode 1. This result demonstrates that although utilizing a single one-way bearing can still produce free- spinning motion and sustained outputs with a similar decaying rate as the design with two one-way bearings (i.e. Mode 1), the voltage output and relative angular velocity are significantly lowered due to only a single stroke of the complete two-stroke reciprocating motion is utilized.
Hence, the overall energy harvesting performance of Mode 2 is signifi- cantly reduced as compared to Mode 1.
In Mode 3, the direct connection of the magnet layer to the input shaft dictates the output waveform, resulting in the varying envelope of the voltage waveform at the exact same rate as the input source, as shown in Fig. 10. The relative angular velocity of the energy harvester at the highest and lowest voltages are 83.3 RPM and 42 RPM, respectively, which are around 50.1 % to 58 % lower than that of Mode 1. In addition, in the first half of the excitation cycle, Mode 3 is less effective than Mode 2. This is because the magnet layer has to undergo acceleration and deceleration process in Mode 3 while Mode 2 experiences an impulsive acceleration during engagement of the one-way bearing, followed by the free-spinning and gradual decaying motion. However, in the second half
of the excitation cycle where the input motion direction is reversed.
Mode 3 exhibits the same waveform, while the relative angular velocity of the magnet layer in Mode 3 continues to decay, resulting in a lower voltage output than that of Mode 2. As a result, Mode 3 outperforms Mode 2 in the second half of the excitation cycle, compensating the disadvantage in the first half compared to Mode 2.
Fig. 11 summarises the measured RMS open-circuit voltage charac- teristics of the device under different excitations. In general, the voltage output decreases almost linearly as the frequency of excitation displacement decreases. This trend is observed across all modes at all frequencies and excitation displacements. It is noted that Mode 1 (i.e.
the intended mode) produces the highest open-circuit RMS voltage across all frequencies and excitation amplitudes. The highest RMS open- circuit voltage value of 5.56 V is recorded under the excitation displacement of 90 mm and frequency of 1 Hz. The lowest RMS open- circuit voltage of 0.78 V is observed under the excitation displacement of 30 mm and frequency of 0.6 Hz. Mode 2 witnesses a substantial decrease in RMS open-circuit voltage as compared to Mode 1, with the highest value of 2.89 V recorded under the excitation displacement of 90 mm and frequency of 1 Hz, and the lowest value of 0.52 V under the excitation displacement of 30 mm and frequency of 0.6 Hz, resulting in 48 % and 33.3 % decreases as compared to Mode 1. Similarly, for Mode 3, which eliminates one-way bearings, exhibits a lower RMS open- circuit voltage as compared to Mode 1, with the highest value of 2.27 V recorded at the excitation displacement of 90 mm and frequency of 1 Hz, and the lowest value of 0.45 V at the excitation displacement of 30 mm and frequency of 0.6 Hz, resulting in 59.2 % and 41.8 % decrease as compared to Mode 1.
The superior performance of Mode 1, particularly under the higher excitation frequencies and displacements is more evident by directly comparing the RMS open-circuit voltage measurements in different modes in Fig. 12. It is noted that Mode 2 outperforms Mode 3 but only marginally, indicating that although the implementation of one-way bearing is still effective, the overall performance is compromised since only a single stroke of the complete two-stroke reciprocating cycle is utilised.
Furthermore, it should be mentioned that although Mode 3 only presents a slightly lower RMS open-circuit voltage as compared to Mode 2, the increasing and decreasing envelope associated with the acceler- ation and deceleration (the bottom plot in Fig. 10) is less efficient and could lead to more loss during the rectification of AC voltage to usable DC voltage in the realistic interface circuit, particularly for the low voltage ranges. As a result, Mode 3 offers the poorest energy harvesting performance among the three working modes.
Fig. 13 shows the typical open-circuit voltage waveform of the
Fig. 11.RMS open-circuit voltage of the energy harvester under different excitation displacements and frequencies.
energy harvester under the excitation displacement of 90 mm and fre- quency of 1 Hz in Mode 1, which gives the highest RMS value. The waveform in one excitation cycle (1 s) is separated into 2 sections by the coloured rectangles, where the blue and purple rectangles represent the outputs during free-spinning motions induced by the engagements of the coil layer and the magnet layer, respectively. The lowest relative angular velocity is approximately 484 RPM, which occurs at about 0.48 s. The highest relative angular velocity is approximately 600 RPM which oc- curs at about 0.52 s during the engagement of the magnet layer. In Section 4, a relative angular velocity of 600 RPM between the magnet and coil layers will be used to simulate the open-circuit voltage and compared with the measurement.
3.2.2. Power output
To investigate the optimal external load resistance and the maximum power generation of the energy harvester, a variable resistor is con- nected to the energy harvester working in Mode 1 and the load resis- tance is varied from 20 Ω to 4000 Ω in the experiment. The experiment is conducted under two excitations: (1) excitation displacement of 90 mm and frequency of 1 Hz; (2) excitation displacement of 50 mm and fre- quency of 0.5 Hz. With the measured voltage output VL across the load resistance RL, the average power pavg is calculated by pavg =
∫T
0V2Ldt RLT , where T is the time for data collection. The experimental results are presented in Fig. 14. In both excitation scenarios, it is revealed that an optimal load resistance is approximately 800 Ω, providing a maximum average power of 8.0 mW and 0.51 mW, respectively. This optimal resistance is higher than the internal resistance of the coils (390 Ω), which could be attributed to the electromechanical coupling and the associated electrical damping, which will be explained later.
3.2.3. Characteristics at optimal load
With the determined optimal load, we further explore the output characteristics of the energy harvester by looking at its voltage across the load resistance of 800 Ω. The excitations are kept the same as those in the open-circuit voltage measurements in Section 3.2.1 except that the case of excitation displacement of 30 mm is omitted since the voltage is too low to give meaningful results. It can be noted in Fig. 15 that the overall trend of the voltage output at the load of 800 Ω is similar to that of the open-circuit results. The energy harvester in Mode 1 produces the highest output across all frequencies and excitation amplitudes. In Mode 1, the highest RMS value of 2.52 V is recorded under the excitation displacement of 90 mm and frequency of 1 Hz and the lowest value is 0.64 V under the excitation displacement of 50 mm and frequency of 0.5 Hz. Interestingly, different from the open-circuit results observed for Mode 2 and Mode 3 results (Fig. 12), the performances of Mode 2 and Mode 3 are very similar, and Mode 3 even performs better under lower excitation amplitudes and frequencies. This can be explained by the electromechanical coupling involved in Mode 2. At the optimal load, the maximum power output also induces large amount of electrical damping Fig. 12.Comparison of RMS open-circuit voltage of the energy harvester in three modes.
Fig. 13. Open-circuit voltage waveform with the highest RMS value of the energy harvester in Mode 1 under the excitation displacement of 90 mm and frequency of 1 Hz.
to the free-spinning motion so that the relative angular velocity and the voltage output decay quicker than that in the open-circuit condition.
This is evident by comparing the voltage waveforms of Mode 1 or Mode 2 in the open-circuit case (Fig. 10) and optimal load case (Fig. 16). On the other hand, since the one-way bearing is directly connected with the input shaft in Mode 3, the electrical damping force has no effect on the motion that is completely controlled by the input shaft.
4. Finite element simulation
To further investigate the effect of varying design parameters of the developed harvester such as magnet polarities and dimensions, finite element simulation using COMSOL Multiphysics is conducted. The open- circuit voltage output from the rotation of the magnet and coil layers is simulated using COMSOL’s rotating machinery physics interface. The device is represented as a 3D model, as depicted in Fig. 17, with its key parameters listed in Table 1. The model comprises a single coil in a cylindrical coil layer, and a cylindrical magnet layer containing six magnets. The coil’s geometry is simplified by slightly modifying the hollowed sector block to a hollowed rectangle block. To save the computational cost and streamline the simulation process, only one coil is considered in the modelling. Consequently, the simulated open-circuit voltage output should be multiplied by a factor of six to compare with
Fig. 14.RMS voltage and power output with varying load resistance under two excitation scenarios: (a) excitation displacement =90 mm and frequency of 1 Hz and (b) excitation displacement =50 mm and frequency of 0.5 Hz.
Fig. 15.Comparison of RMS voltage of the energy harvester with the load resistance of 800 Ω in three modes.
Fig. 16.Comparison of typical voltage waveforms of the energy harvester with the load resistance of 800 Ω in three modes under the excitation displacement
=90 mm and frequency of 1 Hz.
the results from the experiment. The magnet’s material properties are defined by using COMSOL’s built-in properties for a N45 (Sintered NdFeB) magnet, while the coil’s material properties are determined using COMSOL’s built-in properties for copper and modelled as ho- mogenized multiturn.
The relative angular velocity used in the simulation is obtained from the highest relative angular velocity observed in the experimental testing (approximately 600 RPM occurring under an excitation displacement of 90 mm and a frequency of 1 Hz). In the simulation, only the magnet layer is set to rotate about the centre of the magnet layer at the specified constant angular velocity of 600 RPM, while the coil layer remains stationary. This setup can also simplify and streamline the simulation process. Since the electrical output is related to the relative angular velocity between the magnet and coil layers, conceptually there is no difference between this setup in the simulation and the contra- rotating motion with the same relative angular velocity in the experi- ment. The simulation time is set to 0.1 s, which corresponds to approximately one complete rotation cycle of the magnet layer. To simplify the simulation, friction/damping effects are omitted during this relatively short simulation interval.
The simulated open-circuit voltage output is shown in Fig. 18. It can be observed that the simulation produces generally consistent voltage output waveforms as the measured one from experiment despite the slightly lower amplitude observed in the simulated waveform. This discrepancy could be attributed to some simplifications made in the finite element model (e.g., the hollowed rectangle-shaped coil).
4.1. Changing magnet polarities
We further perform the finite element simulation to understand the effect of magnet arrangements. The analysis in this section involves modifying the magnet polarities of the designed device. Instead of
orientating all magnet polarities consistently facing one direction in the original design, the magnet polarities are alternated as shown in Fig. 19.
The simulated open-circuit voltage waveform of this design with alter- nating polarities is compared with that of the original one in Fig. 20. The results indicate that the alternating polarities show interference in the electromagnetic induction between neighbouring magnets, resulting in a complex waveform instead of a quasi-sinusoidal waveform from the original design. As compared to the RMS open-circuit voltage of the original consistent polarity design (4.11 V), the alternating polarity design yields a lower RMS value of 3.39 V.
4.2. Changing magnet dimensions
To examine the effect of magnet height on the open-circuit voltage, the magnet height is increased from 6.5 mm to 9 mm in the simulation.
The voltage waveforms are compared in Fig. 21(a). It is noted that the magnet height has no significant impact on the induced voltage wave- forms, except that the peak voltage values are slightly increased to around 6.9 V with an RMS value of 4.64 V. In Fig. 21(b), the magnet diameter is increased from 10 mm to 14 mm. However, no apparent impact in the induced voltage waveform is observed. These results indicate that the effect of increasing magnet dimensions is marginal. A 38.5 % increase in the magnet height can only increase the voltage output by 12.9 %. On the other hand, physically altering the magnet dimensions would require a redesign of the magnet layer and increase the overall dimensions and weight of the device. Hence, the alterations are deemed unnecessary.
5. Applications
To examine the potential real-life applications of the energy harvester, various sensors and devices are connected through the power management circuit to the energy harvester for demonstration.
5.1. Power management circuit
The power management circuit is composed of two components, as depicted in Fig. 22(a). The first component is a voltage multiplication section, which rectifies the AC input voltage while simultaneously increasing its peak value by a maximum factor of two. The second component is a voltage regulation section, which maintains a stable 3.3 V DC output for powering electronic devices. The detailed circuit dia- gram is shown in Fig. 22(b). The voltage multiplication is achieved by Villard voltage doubler design. When the sinusoidal input voltage at VIN1 and VIN2 is positive, diode D2 becomes forward-biased, thereby Fig. 17.Finite element simulation model.
Fig. 18. Comparison of open-circuit voltage waveforms of the energy harvester
from finite element simulation and experiment. Fig. 19.Finite element simulation model of alternating magnet polarities.
charging capacitor C1 to the input voltage. Conversely, when the sinu- soidal input voltage becomes negative, diode D1 becomes forward- biased, thereby charging capacitor C2. After one complete cycle of the sinusoidal voltage input, the output voltage measured across the two capacitors is equal to double the peak input voltage and is stored in the storage capacitor C3 for subsequent regulation. As compared to tradi- tional full-wave rectifiers, the full-wave voltage doubler offers two
distinct advantages. First, it significantly broadens the usable voltage range generated by the energy harvester, especially under low voltages.
Second, it reduces the forward voltage drop by 50 % compared to full- wave rectifiers, since under each half cycle of the sinusoidal voltage input, the current passes through only one diode instead of two. These advantages greatly improve the efficiency of the power management module, particularly in low frequency environments where the voltage generated is limited.
Once the rectified voltage is stored in C3 (4700 µF), it then passes through a regulator to output a 3.3 V DC voltage. Additionally, two capacitors are connected at the input and output pins of the regulator to minimize voltage ripples and ensure a stable voltage supply to the connected devices.
5.2. Demonstration of powering electronic devices
Fig. 23 demonstrates the energy harvester powering electronic de- vices driven by shaker and by hand pulling. In the first demonstration (Fig. 23(a)), two devices can run simultaneously under the excitation of displacement of 90 mm and frequency of 1 Hz controlled by the elec- trodynamic shaker. The devices connected are one digital calculator and one Xiaomi temperature and humidity sensor with Bluetooth connec- tion, both of which are modified by the removal of their batteries and direct connection to the output pins of the power management module using jumper wires. The temperature and humidity sensor has an average power requirement of approximately 2 mW, with a higher power requirement (4–––5 mW) during the start-up stage as the E-ink display needs to be refreshed and Bluetooth connection needs to be established. Starting with a completed depleted storage capacitor, the energy harvester needs to be exited for approximately 10 s to start up the digital calculator and 20 s to start up the temperature and humidity sensor. Once started up, both devices can remain functional as long as the excitation from the input source continues.
In Fig. 23(b), the energy harvester is manually driven through the spiral torsional spring attached onto the input shaft at a frequency of approximately 1.5 Hz and a displacement of 40 mm. After around 40 s, two temperature and humidity monitors and two digital calculators connected to the power management circuit can all be turned on and remain functional. Once the temperature and humidity monitors have started up, they can be detected and connected by the mobile phone via Bluetooth 5.0. Once the connection is established, continuous updates of the environmental temperature and humidity data can be transmitted and monitored on the mobile phone.
6. Further Comparison
To further demonstrate the performance of the proposed energy harvester, a simple comparison is conducted against other state-of-the- art MMR-based electromagnetic (EM) energy harvesters that have Fig. 20. Comparison of open-circuit voltage waveforms of the designs with
consistent and alternating magnet polarities by finite element simulation.
Fig. 21.Comparison of open-circuit voltage waveforms from the designs with different magnet dimensions by finite element simulation: (a) magnet height of 6.5 mm and 9 mm with diameter of 10 mm and (b) magnet diameter of 10 mm and 14 mm with height of 6.5 mm.
Fig. 22. Power management circuit: (a) prototype and (b) circuit diagram.
similar mass, volume, and are specifically designed for low frequency applications (Table 2). Since the electrical power generated by electro- magnetic energy harvesters is generally proportional to the square of excitation frequency, a figure of merit commonly used (FoM 1 [27]) is introduced to evaluate the performance of the energy harvesters:
FoM 1=P
f2 (1)
where P and f are the maximum generated power and excitation fre- quency, respectively. Furthermore, as the power output of the energy harvester is generally proportional to its mass or volume, the following figures of merit are also introduced to evaluate the normalised perfor- mance of the energy harvesters [26]:
FoM 2=FoM1
Mass (2)
FoM 3= FoM1
Volume (3)
The first comparison presented in Table 2 highlights the superior performance of the designed energy harvester when compared against its counterpart [28] that similarly employed one-way bearing to achieve MMR. The designed energy harvester demonstrates superior out- performance across both FoM 1 and FoM 2, with the increases of 33 % and 143 %, respectively. This disparity highlights the proposed energy harvester’s exceptional performance in low excitation frequency circumstance, which is even more favourable by taking into account the normalized mass.
In comparison with other energy harvesters incorporating ratchets for MMR realization [27,29], the designed energy harvester exhibits superiority by surpassing the counterparts by a factor of at least 10 across all three FoMs. This further demonstrates the effectiveness of the novel design of the one-way bearing and the innovative structural
arrangements in the proposed harvester.
7. Conclusions
In this work, to address the problem of energy harvesting from ul- tralow frequency motions, a contra-rotating free-spinning electromag- netic energy harvester is designed, prototyped, characterized and demonstrated for powering electronic devices. The novelties of this design include: a customised one-way bearing without springs that en- ables free-spinning with minimal friction; a novel arrangement of the customised one-way bearings in the magnet and coil layers for motion rectification that provides a simple design without cumbersome output shaft and gearing systems; the high-speed contra-rotating motion ach- ieved for high-performance energy harvesting. The designed energy harvester can provide the highest RMS open-circuit voltage of 5.56 V and a maximum power of 8 mW at an optimal load resistance of 800 Ω under the excitation displacement of 90 mm and frequency of 1 Hz, which significantly outperforms the device working in the modes without or with only a single one-way bearing. Finite element simula- tion further confirms the suitability of the arrangement and dimensions of the selected magnets. The energy harvesting capability of the devel- oped harvester has been demonstrated by simultaneously powering and sustaining the continuous operation of multiple electronic devices, including wireless temperature and humidity sensors. The novel design proposed in this work sheds light on developing high-performance electromagnetic energy harvesters used in the scenarios of ultralow frequency, intermittent and reciprocating motions.
CRediT authorship contribution statement
Haopeng Ma: Formal analysis, Writing – original draft. Lihua Tang:
Conceptualization, Methodology, Validation, Supervision, Writing – review & editing, Funding acquisition. Haipeng Liu: Conceptualization, Fig. 23.Demonstration of the energy harvester powering electronic devices. (a) demonstration with shaker; (b) demonstration with hand pulling.
Table 2
Comparison with reported MMR-based electromagnetic energy harvesters.
Energy
Harvester Mechanism Frequency
(Hz) Power
(mW) Volume (cm3) Mass
(g) Mass Power Density (mW/
g)
Volume Power Density (mW/
cm3)
FoM 1 (mW/
Hz2)
FoM 2 (mW/Hz2/ g)
FoM 3 (mW/
Hz2/cm3)
This work EM with One-way
Bearing MMR 1 8.0 232 127 0.0630 0.0345 8.00 0.0630 0.0345
Hao et al.
[28] EM with One-way
Bearing MMR 1 ~ 6 / 231.5 0.0259 / 6.00 0.0259 /
Fan et al.
[29] EM with Ratchet
MMR 3.5 2 65.8 67.6 0.0296 0.0304 0.163 0.00242 0.00248
Zhang et al.
[27] EM with Ratchet
MMR 1 ~ 0.2 97 / / 0.00206 0.200 / 0.00206
5 7 97 / / 0.0722 0.280 / 0.00289
Investigation, Formal analysis, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgement
This work is financially supported by Early and Mid Career COVID Research Restart Fund from The University of Auckland (No. 3726669) that provided the scholarship to the first author.
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