This thesis reports five novel techniques for harvesting vibration energy based on piezoelectric and converting them to electrical energy. These harvesters have multiple modes to absorb destructive vibrations generated by machines and then convert them to useful electricity. The nature and characteristics of such multimodal energy harvester were investigated. The theoretical studies, modelling simulation and analysis, and the experimental validations were also presented. A single degree and multiple degrees of freedoms mechanical structures were constructed and used in the experimental tests. These structures were then improved to find the maximum power output that would occur at their resonance frequencies. A wide frequency bandwidth was also allowed to be generated by the harvester to cover multimodal resonance vibration modes so that vast electrical power was produced. The contributions of the above research work are summarised as follows.

The frequency converter embedded with a simple cantilevered beam structure that belonged to a single degree of freedom in such mechanical structures was first investigated. The longer the beam converter is, the smaller the first resonance frequency the simple harvester has. An external RLC circuit was designed to maximise the output values of voltage and power. The impedance matching was achieved by adding an external inductor. In summary, the power output harvested could be enhanced by laminating a piezoelectric to a cantilever beam so that the harvester could be vibrating at its resonance frequency.

Inspirited by the previous results, four different types of cantilevered multimodal structures were designed and compared. From the experimental results, the harvester, which had a structure of E-folded with two tip masses, exhibited that the first three modal resonance frequencies were under 50 Hz. The three resonance modes were vibrating at 18.18 Hz, 23.6 Hz, and 26.8Hz. At a frequency of 18.18 Hz, it had produced a voltage of 800.1 mV. By comparing with the other two modes, the maximum power output was produced at the resonance frequency of 18.18 Hz. In summary, the harvester’s structure that is E-folded with two tip masses has the best performance among the four piezoelectric energy harvester designs in producing maximum power.

The E-folded with two tip masses structure was then further investigated to seek the feasibility of producing higher power. The wing beam length and the tip mass were found to be two significant parameters that could play an essential role in deciding the resonance frequency and the power output. Through experimental results, the performance of the harvester that had a left wing of 110 mm outperformed the other designs. The length ratio of the left and the right wing was found to be best at 11/15. At this ratio, the harvester provided a strong asymmetric stiffness for the structure. It yielded more substantial deformation when it was vibrating close to the resonance frequency peak.

The material that was used to make the tip mass was replaced by a smart material called magnetorheological elastomer (MRE). The application of the MRE could enable the device to become a frequency-tunable vibration energy harvester because the MRE could change its stiffness when it is exerted on an external magnetic field. Then the resonant frequency could be changed flexibly by controlling the magnetic field. Therefore, the application of MRE material provides a smart and flexible way to control the stiffness of the whole energy device. The short MRE structure which was defined by the smart material in replacement of the shorter wing beam tip mass produced a maximum voltage of 11.85 V when it was vibrating at the first resonance frequency of 21.2 Hz. The usage of smart material offers an alternative to enhance the dynamic behaviour of the energy harvester.

The above research work focused on the improvement of the mechanical structure design which could produce close multiple resonance vibration modes. However, the nonlinearity can also enhance the performance of the harvester with multiple resonance peaks. Therefore, a nonlinear electro-magneto-mechanical (EMM) vibration energy harvester with piezoelectric was proposed. It consisted of a substrate plate A, a substrate plate D and a host beam with the piezoelectric material. It could collect destructive vibrations generated from rotational machines vibrating at low frequencies. This harvester device not only reduced the amplitude of vibration transit from the machine to other machines but also converted the vibrations into useful electricity. It was developed from multimodal vibrations that have different degrees of freedom by having different cantilevered beam structures. The application of nonlinearity induced by the EMM system could change the vibration mode from a single resonance mode into two resonance modes at a frequency range from 20 Hz to 50 Hz. Moreover, the substrate plate D contributed on average a higher voltage output at the first vibration mode, while substrate plate A contributed voltage output when the nonlinear EMM system is vibrating at the second vibration mode. Hence, such nonlinear characteristics make this harvester more flexible in operation and can absorb destructive vibrations generated by machines and then convert them to useful electricity.

Finally, a piezoelectric-based energy harvester that has a horizontal and asymmetric structure and in U-shape (U-VPEH) was built. It consisted of the geometric and magnetic nonlinearity coupling with a multimodally designed structure. This nonlinear U-VPEH model yielded a closer two resonance frequencies located closely to each other. The first and the second eigenfrequencies of the nonlinear U-VPEH model was recorded as 17.167 Hz and 22.951 Hz. It had produced a higher voltage output than that of the linear U-VPEH model. The nonlinear U-VPEH model yielded a bandwidth at a low-frequency range that could cover the maximum power output frequency range. The maximum voltage response was found to be 8.743 V at 16.5 Hz under the up-sweeping signals, while the maximum voltage response was 14.18 V at 15.41 Hz under the down-sweeping signals. In conclusion, the nonlinear U-VPEH harvester exhibits a promising performance in energy harvesting. Its natural properties that produce higher energy output, lower resonance frequency and closer location of resonance peaks have broadened its flexibility and practical usage.

With the help of these five new energy harvesters, the waste vibration energy can be converted into usable electricity. This electricity can continuously power-up the wireless sensors used for monitoring the health of building-related rotary machines. Hence, frequent change of batteries can be minimised, and the use of line power can be reduced. Most importantly, the monitored machines can extend their lives due to being less subject to destructive vibration. Hence, the machines can have a longer operating life and use green and sustainable energy.