Energy harvesting for low-power sensing has drawn great attention, but still faces challenges in harnessing broadband random motions. Inspired by the parasitic relationship in plants, a host-parasite vibration harvester is designed to scavenge random low-frequency vibrations by incorporating bi-stability and frequency up-conversion within such a design. A hosting beam is formed in a buckled condition by clamping it at both ends and applying an axial compression load. Two parasitic piezoelectric beams are fixed at the center of the hosting beam and plucked at the free ends by two plectra on the hosting beam, while it oscillates in an inter-well mode. The low-frequency hosting beam oscillation is converted to high-frequency parasitic beam's vibration at resonance due to the plucking effect, allowing the harvester to convert the broadband low-frequency motion into electricity effectively. The electromechanical dynamics are modeled and the design is validated experimentally. The harvester is capable of harnessing low-frequency random vibration (0.0018 g2/Hz @ 5–400 Hz) over a wide bandwidth. More than 1 mJ energy was collected over 100 s under this pseudorandom vibration.
Energy harvesting has been recognized as one of the key enablers for self-powered sensing applications in the era of Internet of things.1–4 However, enhancing the energy harvesting effectiveness requires significant efforts, especially for different energy sources under various conditions, such as low-frequency human motion,5,6 random aircraft vibrations7 or ocean waves.8 Harnessing a random, broadband and low-frequency kinetic energy is one of the key challenges, and different mechanisms have been developed to enhance the conversion performance.
Nonlinear dynamics are one major consideration to enhance the operation bandwidth.9–11 Different harvesters have been developed with monostable,12–14 bistable15–17 and multistable behaviors.18–20 The aim is to alter harvesters' potential shape by applying preloads using magnetic forces21–23 or displacement constraints.24,25 A good example of a bistable harvester using displacement constraints is a device designed for harvesting energy from the passing traffic and pedestrians.26 A scissor-like structure was adopted to transfer the vertical passing weight to a horizontal axial force for buckling and excitation.
In addition to nonlinearity, frequency up-conversion is another mechanism that is often employed in harvesting low-frequency motions, especially human motion.27–29 This mechanism uses a beam plucking effect to convert the low-frequency plucking motion into high-frequency transducers' vibration, in which the transducers normally operate at resonance after each plucking. Direct impact30 and magnetic plucking31 are typical methods to achieve the plucking motion and activate the transducers. In a recent work, Halim and Park developed an impact-driven harvester for harvesting the human-limb motion.32 A metal ball was designed to impact a flexible side-wall where a piezoelectric beam is fixed. The impact motion excited the beam, and the low-frequency limb motion was up-converted to a high-frequency beam vibration.
The parasitic relationship is a well-known phenomenon in nature. Figure 1(a) provides a good example in plants, where a dodder is reliant on a host plant. In terms of motion and dynamics, the parasitic plant intertwines with the hosting plant and moves along with the host; however, due to the freedom of its own structure, the parasitic plant exhibits more complex dynamics even under simple hosting plant's motions, such as airflow-induced low-frequency vibration.