• Connection Configurations to Increase Operational Range and Output Power of Piezoelectric MEMS Vibration Energy Harvesters

      Du, Sijun; Chen, Shao-Tuan; Jia, Yu; Arroyo, Emmanuelle; Seshia, Ashwin A.; University of Cambridge; University of Chester (IOP Publishing, 2016-09-06)
      Among the various methods of extracting energy harvested by a piezoelectric vibration energy harvester, full-bridge rectifiers (FBR) are widely employed due to its simplicity and stability. However, its efficiency and operational range are limited due to a threshold voltage that the open-circuit voltage generated from the piezoelectric transducer (PT) must attain prior to any energy extraction. This voltage linearly depends on the output voltage of the FBR and the forward voltage drop of diodes and the nature of the interface can significantly limit the amount of extracted energy under low excitation levels. In this paper, a passive scheme is proposed to split the electrode of a micromachined PT into multiple (n) equal regions, which are electrically connected in series. The power output from such a series connected MEMS PT allows for the generated voltage to readily overcome the threshold set by the FBR. Theoretical calculations have been performed in this paper to assess the performance for different series stages (n values) and the theory has been experimentally validated. The results show that a PT with more series stages (high n values) improves the efficiency of energy extraction relative to the case with fewer series-connected stages under weak excitation levels.
    • Experimental and theoretical study of a piezoelectric vibration energy harvester under high temperature

      Arroyo, Emmanuelle; Jia, Yu; Du, Sijun; Chen, Shao-Tuan; Seshia, Ashwin A.; University of Cambridge; University of Chester (IEEE, 2017-08-01)
      This paper focuses on studying the effect of increasing the ambient temperature up to 160 °C on the power harvested by an MEMS piezoelectric micro-cantilever manufactured using an aluminum nitride-on-silicon fabrication process. An experimental study shows that the peak output power decreases by 60% to 70% depending on the input acceleration. A theoretical study establishes the relationship of all important parameters with temperature and includes them into a temperature-dependent model. This model shows that around 50% of the power drop can be explained by a decreasing quality factor, and that thermal stresses account for around 30% of this decrease.
    • High temperature performance of a piezoelectric micro cantilever for vibration energy harvesting

      Arroyo, Emmanuelle; Jia, Yu; Du, Sijun; Chen, Shao-Tuan; Seshia, Ashwin A. (IOP Publishing, 2016-12-06)
      Energy harvesters withstanding high temperatures could provide potentially unlimited energy to sensor nodes placed in harsh environments, where manual maintenance is difficult and costly. Experimental results on a classical microcantilever show a 67% drop of the maximum power when the temperature is increased up to 160 °C. This decrease is investigated using a lumped-parameters model which takes into account variations in material parameters with temperature, damping increase and thermal stresses induced by mismatched thermal coefficients in a composite cantilever. The model allows a description of the maximum power evolution as a function of temperature and input acceleration. Simulation results further show that an increase in damping and the apparition of thermal stresses are contributing to the power drop at 59% and 13% respectively.
    • A New Electrode Design Method in Piezoelectric Vibration Energy Harvesters to Maximize Output Power

      Du, Sijun; Jia, Yu; Chen, Shao-Tuan; Zhao, Chun; Sun, Boqian; Arroyo, Emmanuelle; Seshia, Ashwin A.; University of Cambridge; University of Chester (Elsevier, 2017-07-19)
      A resonant vibration energy harvester typically comprises of a clamped anchor and a vibrating shuttle with a proof mass. Piezoelectric materials are embedded in locations of high strain in order to transduce mechanical deformation into electrical charge. Conventional design for piezoelectric vibration energy harvesters (PVEH) usually utilizes piezoelectric materials and metal electrode layers covering the entire surface area of the cantilever with no consideration provided to examine the trade-off involved with respect to maximize output power. This paper reports on the theory and experimental verification underpinning optimization of the active electrode area in order to maximize output power. The calculations show that, in order to maximize the output power of a PVEH, the electrode should cover the piezoelectric layer from the peak strain area to a position, where the strain is a half of the average strain in all the previously covered area. With the proposed electrode design, the output power can be improved by 145% and 126% for a cantilever and a clamped-clamped beam, respectively. MEMS piezoelectric harvesters are fabricated to experimentally validate the theory.
    • Real-world evaluation of a self-startup SSHI rectifier for piezoelectric vibration energy harvesting

      Du, Sijun; Jia, Yu; Zhao, Chun; Chen, Shao-Tuan; Seshia, Ashwin A.; University of Cambridge; University of Chester (Elsevier, 2017-08-02)
      This paper presents an enhanced SSHI (synchronized switch harvesting on inductor) rectifier with startup circuit and representative environment validation using real world vibration data collected from a tram. Compared to a conventional SSHI rectifier, the proposed rectifier dynamically monitors the working status of the circuit and restarts it when necessary. The proposed rectifier is designed in a 0.35 μm HV CMOS process and its performance is experimentally evaluated. With a 500-second real-world collected vibration data, the conventional and the proposed SSHI rectifiers record average power performance improvements by 9.2× and 22× respectively, compared to a passive full-bridge rectifier. As the startup circuit helps restart the SSHI rectifier several times, it is able to extract energy in an increased excitation range and its average power output performance is 2.4× higher than a conventional SSHI rectifier.
    • Shock Reliability Enhancement for MEMS Vibration Energy Harvesters with Nonlinear Air Damping as Soft Stopper

      Chen, Shao-Tuan; Du, Sijun; Arroyo, Emmanuelle; Jia, Yu; Seshia, Ashwin A.; University of Cambridge; University of Chester (IOP Publishing, 2017-09-20)
      This paper presents a novel application of utilising nonlinear air damping as soft mechanical stopper to increase the shock reliability for MEMS vibration energy harvesters. Theoretical framework for nonlinear air damping is constructed for MEMS vibration energy harvesters operating in different air pressure levels, and characterisation experiments are conducted to establish the relationship between air pressure and nonlinear air damping coefficient for rectangular cantilever MEMS micro cantilevers with different proof masses. Design guidelines on choosing the optimal air pressure level for different MEMS vibration energy harvesters based on the trade-off between harvestable energy and the device robustness is presented, and random excitation experiments are performed to verify the robustness of MEMS vibration energy harvesters with nonlinear air damping as soft stoppers to limit the maximum deflection distance and increase the shock reliability of the device.
    • Utilising Nonlinear Air Damping as a Soft Mechanical Stopper for MEMS Vibration Energy Harvesting

      Chen, Shao-Tuan; Du, Sijun; Arroyo, Emmanuelle; Jia, Yu; Seshia, Ashwin A.; University of Cambridge; University of Chester (IOP Publishing, 2016-12-06)
      This paper reports on the theory and experimental verification of utilising air damping as a soft stopper mechanism for piezoelectric vibration energy harvesting to enhance shock resistance. Experiments to characterise device responsiveness under various vibration conditions were performed at different air pressure levels, and a dimensionless model was constructed with nonlinear damping terms included to model PVEH response. The relationship between the quadratic damping coefficient ζ n and air pressure is empirically established, and an optimal pressure level is calculated to trade off harvestable energy and device robustness for specific environmental conditions.