A drum-like piezoelectric rotational energy harvester via magnetic beating

The on-line wireless condition monitoring of rotating machines and infrastructures, such as wind turbines, tires, and bridges, are receiving considerable attention since it can increase the safety as well as decrease unscheduled downtime and maintenance cost. Rotational energy harvesting technique has been rapidly developing to offer an autonomous power system without regularly replaced low-durability batteries or inconvenient wired power over the past decades.^1–3 The commonly used transduction methods for rotational energy scavenging are mainly based on electromagnetic,⁴ piezoelectric,⁵ and triboelectric⁶ mechanisms. Among them, piezoelectric rotational energy harvesters (PREHs) have attracted significant research interest due to their unique merits of high energy density, architectural simplicity, and easy fabrication.^7–9 Consequently, rotational energy harvesting using piezoelectric transduction is widely applied in the real-time condition monitoring and wireless structural health monitoring systems.

To date, a variety of energy harvesting schemes and methods of exciting piezoelectric vibrators have been developed to effectively convert the ambient rotational vibration into usable electricity in recent years. In the meantime, some intriguing progress has been made in the performance improvement of PREHs. Rui et al.¹⁰ reported a scheme of using the centrifugal force to construct a low-frequency PREH, which was accomplished by altering the distance between the mass and the rotation center. Fang et al.² proposed a music-box-like PREH, where an extremely high output power and a broad operating frequency range were achieved by using the array of piezoelectric vibrators. Janphuang et al.¹¹ demonstrated the energy harvesting from low frequency vibration using contact-type plucking mechanism where a rotating gear was adopted to realize the frequency up-converting harvester. Fu et al.¹² presented a frequency upconversion approach to construct a low-speed broadband PREH, where the low frequency rotation was transformed into high frequency vibration of piezoelectric beam by magnetic plucking. Yang et al.¹³ developed a rotational energy harvester using the ball-impact-induced resonance to enable effective excitation of piezoelectric cantilever beams in order to improve the output power. Fang et al.¹⁴ proposed a comprehensive model to explore an impact-based PREH where a centrifugal-softening driving beam was used to actuate two piezoelectric beams under the gravity excitation. Machado et al.¹⁵ incorporated a flexible stop into a PREH, where the stop could limit the maximum displacement of the piezoelectric cantilever and increase the output power in a low frequency range by means of introducing an impact force. Kan et al.¹⁶ developed a piezo-disk energy harvester excited by rotary magnets to harvest energy from rotating structures, and the effect of exciting magnets on the optimal rotary speeds and effective speed bandwidth was obtained experimentally. Liu et al.¹⁷ reported a low frequency non-contact PREH harvester excited by magnetic coupling to improve the PREH performance at low frequency by optimizing the magnetic field conditions.

As can be demonstrated from the above-mentioned literature reviews, most existing PREHs may be roughly divided to two categories according to the triggering mode of piezoelectric vibrators, namely, contact-type excitation such as plectrums and impact-based balls and contactless excitation like centrifugal force and magnetic plucking. Because piezoelectric cantilever structures were frequently adopted in most previous PREHs, the breakable PZT (lead zirconate titanate) vibrators attached on piezoelectric cantilevers were easily damaged in the long-lasting unforeseeable contact-type triggering force. Thus, it was quite hard to guarantee the long-term reliability of the PREHs using the contact-type mode. Regarding the non-contact triggering mode, since most piezoelectric cantilevers have endured varying degrees of stress concentration, it was very risky to cause the fatigue damage of piezoelectric beams experiencing the long-term large alternating deformation and stress concentration. Apparently, the reduced reliability of PREHs could weaken long-lasting superiority in sustainable power source for wireless sensor systems. Therefore, it is necessary to find a viable solution to ensure the high reliability of PREHs once the piezoelectric vibrators experience an unforeseen large triggering force.

In this Letter, a drum-like PREH (DL-PREH) via magnetic beating is proposed to improve the reliability of the harvesters under an unpredictable large triggering force and stress concentration. Herein, an indirectly excited approach of introducing an air-filled separation chamber is presented to construct the DL-PREH. The external excitation was transferred to the piezoelectric transducer via the intermediate air medium; then, the electric output of the transducer could be induced by the varying air pressure. Meanwhile, because the piezoelectric diaphragm was subjected to the uniform load of air pressure, the stress concentration could be effectively avoided. Hence, the piezoelectric transducer could tolerate the unexpected high-intensity stress. In addition, because the air-filled separation zone could be regarded as an air spring, the resonance frequency of the DL-PREH could be tuned by changing the gap distance (namely the stiffness of the air spring) to improve the rotational speed adaptability. Thus, the DL-PREH was promising to offer a long-term sustainable power source for an energy-autonomous system in unknown harsh environments.

Figure 1 depicts the structural schematic of the presented DL-PREH, which is composed of a disk-type edge-fixed piezoelectric transducer, a round flexible diaphragm, a circular hollow cylinder, exciting magnets distributed evenly on the rotary disk, excited magnet attached on the center of the diaphragm, and aligned with the opposite-pole exciting magnets. An air-filled separation chamber is formed by the transducer, the diaphragm, and the cylindrical shell. When the rotary disk is rotated with the main shaft, a periodically altered magnetic attractive force is induced and imposed on the diaphragm center. Then, the air pressure inside the separation chamber will vary cyclically with the attractive force. Accordingly, the piezoelectric transducer yields continually a bending deformation and thus the corresponding electric output is produced under the alternating air pressure. It could be seen from the above working mechanism of the DL-PREH that the energy conversion from rotational energy to magnetic energy, air pressure energy, vibration energy, and electric energy was mainly involved. In addition, the DL-PREH dealt with the motion transformation from the rotational motion to the bending vibration via the magnetic coupling and fluid-solid coupling, namely the interaction between rotary speed, magnetic force, air pressure, transducer deformation, and electricity.

To prove the feasibility of the presented indirectly excited technique and examine the performance of the DL-PREH, a prototype was fabricated and tested by experimental rotational speed responses. Figure 1 shows the fabricated DL-PREH prototype and experimental setup. The unimorph PZT-4 disk with the size of Ф30 × 0.3 mm² was used as the piezoelectric transducer, and the flexible diaphragm was made of the polyethylene terephthalate (PET) film with the size of Ф56 × 0.2 mm². The inside chamber of the drum-like structure was enclosed by assembling a polymethyl methacrylate (PMMA) hollow open cylinder, top end of the PET film, and bottom end of the unimorph disk. Because of a constant area of the adopted commercial PZT-4 disk, the chamber volume was determined by the cylinder height, i.e., the drum height or the gap distance between the PZT-4 disk and PET film. Moreover, the stiffness of drum chamber similar to a gas spring was closely related to the initial chamber volume (or drum height). Meanwhile, it also dominated the resonance frequency of the drum-like structure. Therefore, this work concentrated on demonstrating the impact of the drum height on the electric output behaviors of the DL-PREH device in the experiments.

Figure 2 depicts the variation of peak-to-peak open-circuit voltage as a function of rotational speed for the DL-PREH with different drum heights. In experiments, a significant electric output from the DL-PREH was obtained under the magnetic force as well as an obvious peak voltage was observed with the varying rotational speed. The output voltage waveforms of different drum heights with respect to the peak are recorded by the oscilloscope, as demonstrated in Figs. 2(b)–2(f). Basically, the waveform of each drum height presented regularly an approximate sine wave. It indicated that the periodical magnetic force has been changed into a sinusoidal air pressure applied on the PZT-4 disk via the separation chamber. In another word, the external excitation could be delivered to the piezoelectric transducer via the air-filled separation zone and meanwhile a desirable electric output could be acquired from the DL-PREH.

Additionally, as predicted, Fig. 2 illustrates that the drum height brings a strong influence on the output voltage of the DL-PREH. Figure 3 directly presents the effect of drum height by extracting the relevant data from Fig. 2(a). As the drum height was changed from 2 to 10 mm, the peak voltage was enhanced from 15.15 to 21.1 V and then dropped to 15.27 V. By comparing with the peak voltage of 15.2 V at the drum height of 2 mm, the peak voltages generated by the DL-PREH with the drum height of 4, 6, 8, and 10 mm are increased by 7.9%, 39.2%, 22%, and 0.8%, respectively. It showed there was an optimal drum height of 6 mm where the peak voltage could reach a maximum of 21.1 V. In addition, the speed corresponding to the peak voltage presented a decreasing trend as the drum height increased. The peak speed gradually decreased from 2464 to 1677 rpm with the rising drum height from 2 to 10 mm. By comparing with the peak speed of 2464 rpm at the drum height of 2 mm, the peak speed with the drum height of 4, 6, 8, and 10 mm is reduced by 13.7%, 22.8%, 27.4%, and 31.9%, respectively. It was in good accordance with the nature of gas spring where the stiffness was reduced with the enhancing initial chamber volume. Therefore, the DL-PREH also exhibited a large tunability of the resonance frequency, which could be readily tuned by adjusting the drum height to meet power generation requirements of various application environments.

To demonstrate the charging capability of the DL-PREH for powering electronic devices, three different capacitors with capacitances of 220, 470, and 1000 μF were charged by the prototype. The alternating voltage from the DL-PREH was first transformed into DC output through a full-wave bridge rectifier, and then, DC voltage was utilized to charge these three capacitors. Figure 4 displays the charging performance of three commercial capacitors and resistance-dependent power of the DL-PREH with a drum height of 6 mm at 1902 rpm. The results showed the device charged the 220, 470, and 1000 μF capacitors to the saturation voltages of 7.8, 7.78, and 7.38 V in about 40, 80, and 160 s, respectively. Accordingly, the electric energy stored into the capacitor could be further estimated and the calculation result indicated that the electric energy of about 0.17 mJ was delivered on the capacitors per second. Regarding the output power test, the commonly used resistive impedance matching with the external resistance was examined to maximize power extraction. It was observed that the output voltage reached the saturation state at about 50 kΩ, and the output current was kept dropping from 1.34 to 0.3 mA with the ascending load resistance from 1 to 70 kΩ. The device could achieve a maximum output power of 10.63 mW at the matching resistance of 24 kΩ. To further prove the practical application capability of the DL-PREH as a power supply, the device is exploited to light up 100 commercial blue light-emitting diodes (LEDs) in parallel with the aid of the full-wave bridge rectifier, as shown in Fig. 5. The above demonstration has fully exhibited a promising capability of the DL-PREH to supply a power source for low-powered electronic devices.

In summary, we have reported an indirectly excited technique of introducing an air-filled chamber to separate the piezoelectric transducer from the trigger to guarantee high reliability of harvesters under an unexpected excessive impact and demonstrated a drum-like piezoelectric rotational energy harvester. The fabricated device presented its good power generation capability and resonance frequency tunability. The drum height played an important role to maximize the electric output. The resonance frequency of the DL-PREH decreased with the increasing drum height. The maximum open-circuit peak voltage of 21.1 V and output power of 10.63 mW were achieved at an optimal drum height of 6 mm. Three capacitors of 220, 470, and 1000 μF capacitance reached a saturation voltage of 7.8, 7.78, and 7.38 V at 40, 80, and 160 s, respectively. The harvester could light up at least 100 blue commercial LEDs in parallel.

This work was supported by the National Natural Science Foundation of China (Nos. 51877199 and 52077201), Zhejiang Provincial Key Research and Development Project of China (No. 2021C01181), and Zhejiang Provincial Natural Science Foundation of China (No. LY20F010006).