A novel piezoelectric wafer-stack vibration energy harvester

Abstract

Piezoelectric vibration-based energy harvesters are attractive as inexhaustible replacements for batteries in low-power requirement wireless electronic devices and thus have received increasing research interest in the last few years. This paper presents a novel piezoelectric harvester based on the wafer-stack configuration to convert the external vibration into usable electrical energy for this purpose. Both analytical and experimental investigations are undertaken at University of Technology Sydney. Firstly, an electromechanical model with a rectified circuit, considering both the mechanical and electrical factors of the harvester, is built to characterise the harvested electrical power across the external loadings. Exact closed-form expressions of the electromechanical model have been given to analyse conditions for maximum harvested power. Finally, a shake table experimental testing was conducted to evaluate the feasibility of the presented PZT wafer stack harvester under standard sinusoidal loadings. Test results show that the harvester can generate a maximum 16mW electrical power for sinusoidal loading with 40mm amplitude and 2Hz frequency. harvester is constructed in wafer-stack configuration that is robust and fit for large force vibration. Platt et al (Platt et al. 2005) have shown that it’s feasible to use stack configuration to increase the amount of harvested power, and both the voltage output and the matching resistive load are much more manageable in a PZT stack than in a monolithic configuration. The development of models for cantilevered PZT harvester has attracted great attentions from researchers. And there are already many models can be used to evaluate the harvested power of cantilevered harvester. However, not much research has been undertaken on the model development of the wafer configuration harvester operating in 33-mode for the large force vibration. Moreover, a PZT harvester system should contain both the mechanical part, which generates electrical energy, and an electrical circuit, which converts and rectifies the harvested energy in a form of an alternating voltage, into a constant voltage. The efficiency of the energy harvester design depends not only on the PZT harvester itself but also on its integration with the electrical circuit. Therefore, an electromechanical model is of great importance to optimise the design as well as for understanding the behavior of the PZT energy harvester. In this paper, we firstly built an electromechanical model with the rectified circuit of this novel PZT wafer-stack harvester in order to investigate the ability of harvesting electrical power and find the optimal condition for maximising electrical power output. Then, a series of tests were conducted to verify the theoretical findings. Test results showed that the electromechanical model for the PZT wafer configuration harvester is accurate and the harvester can generate up to 16mW DC electrical power during the standard sinusoidal motion. 2 ELECTOMECHANICAL MODEL 2.1 Electrical characteristics under applied force Figure 1. Electrical reaction of PZT wafer-stack under force The PZT vibration energy harvester proposed in this paper works in large force vibration condition. The wafer-stack configuration is chosen due to its durability for large exciting force and its ability of generate higher electrical power than single PZT wafer under same vibration condition. PZT material is a inter-medium of mechanical energy to electrical energy and can generate electrical energy on the pressure of a force. In order to build an accurate model for PZT harvester, it’s necessary to investigate the electrical reaction of the PZT disc under external excitation. Figure 1 shows the electrical characteristics of the PZT wafer-stack. According to the IEEE Standard on Piezoelectricity: under the external force, giving by the strain S, stress T, electric field E, and electric displacement D, the constitutive relations of the PZT energy harvesting device are typically defined by: