Multi-input Circuits of Vibration Piezoelectric Energy Harvesters Based on the Synchronized Switch Technique


Student thesis: Doctoral Thesis

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Award date9 Nov 2023


Piezoelectric energy harvesters (PEHs), due to their ability to convert widely dispersed mechanical energy in the environment into electrical energy, along with advantages such as high energy density, ease of integration, and maintenance-free operation, are extensively utilized in fields including the Internet of Things (IoT), wearable devices, wireless sensor networks, and aerospace. To effectively convert and utilize the AC (alternating current) output of PEHs for practical applications, a rational energy control circuit design is necessary. Piezoelectric energy control circuits based on synchronized switch technique can effectively enhance the conversion of mechanical energy to electrical energy in PEHs. However, existing piezoelectric energy control circuits based on synchronized switch technique mainly serve a single PEH during electrical harvesting, and cannot effectively extract and superimpose multiple piezoelectric AC input energies; during electrical dissipation, they cannot dissipate the electrical energy stably and efficiently under the excitation of external vibrations of varying intensity.

Hence, this thesis focuses on how to design energy control circuits based on synchronized switch technique, to realize multi-input electrical harvesting and electrical dissipation of PEHs, in order to achieve reliable enhanced output power and stable strong structural damping, respectively. Specifically, the first circuit is based on the P-SSHI (parallel synchronized switch harvesting on inductor) technology, which avoids the inductor access conflict problem caused by the piezoelectric voltage phase difference near π by splitting the inductor. At the same time, the energy storage capacitor is divided to reduce the number of rectifier diodes and decrease conduction losses. Experimental results show that the proposed split-inductor-capacitor topology realizes multi-input energy extraction and merging of PEH arrays under arbitrary phase differences.

To reduce the number of inductors to one in the first multi-input electrical harvesting circuit, this thesis extends the single-channel SSDCI (synchronized switching and discharging to a capacitor through an inductor) topology to a multi-input SSDCI topology and designs the second piezoelectric multi-input electrical harvesting circuit. Full-bridge rectification at the front end of SSDCI ensures that the resonant current direction of each channel is the same during inductor accessing (piezoelectric voltage inversion), thereby ensuring that electrical harvesting of PEH arrays based on synchronized switch technique can be achieved with a shared inductor under any phase difference. The SSDCI topology, combined with precise active peak detection, enables the overall circuit to achieve high-efficiency energy conversion.

As the output power of the second multi-input SSDCI electrical harvesting circuit fluctuates with load variations and the output power needs further enhancement, the third multi-input electrical harvesting circuit with MPPT (maximum power point tracking) functionality has been developed. To further enhance power output and use a shared inductor, the ReL-SSHI (rectifier-less synchronized switch harvesting on inductor) topology is chosen for multi-input expansion, sacrificing the energy addition performance of the circuit when the piezoelectric voltage phase difference is close to π. The theoretical analysis of ReL-SSHI in this thesis reveals that the optimal rectified voltage is proportional to the amplitude of the piezoelectric voltage. Therefore, each channel can independently track the maximum power point using a resistive voltage divider to adjust the rectified voltage according to the amplitude of the piezoelectric voltage obtained by the envelope detector. Experimental results show that the multi-input electrical harvesting circuit achieves a fast-tracking response speed for each channel and boosts output power by 485% compared to the full-bridge rectifier.

With respect to the control of the electrical dissipation, the SSDV (synchronized switch damping on voltage) technique, which can significantly enhance structural damping, is unstable under weak excitation. Inspired by the MPPT principle of the third piezoelectric multi-input electrical harvesting circuit, this thesis proposes a direct adaptive SSDV (DA-SSDV) method, which does not require additional excitation level monitoring modules and can adaptively adjust the voltages of the series voltage sources based on the external vibration excitation intensity. The closed-loop control of the series voltage sources is achieved only through a passive envelope detector and a resistive voltage divider. Experimental results show that the DA-SSDV circuit is stable and realizes a good displacement transmissibility. Finally, this thesis further improves the phase lag of the peak detection and optimize the power consumption of the DA-SSDV circuit.