Design method for piezoelectric bending generators in energy harvesting systems

Abstract

The increased demand for mobile systems using low-power electronics leads to a need for new power sources. Using batteries as power source may be inapplicable in distributed systems like wireless sensor networks because the batteries have to be exchanged frequently. Energy Harvesting systems are one possible energy source for such systems exploiting environmental energy like mechanical vibrations. One good solution to convert vibration energy is the use of piezoelectric generators usually realised as piezoelectric bending beams. The generators convert mechanical energy to electrical energy due to resulting strain of the element. However, the power output of piezoelectric generators is a challenging task even if low-power applications have to be driven. Due to the low electric power output of piezoelectric generators, it is an important task to obtain a suitable geometric design of the transducer element. Beside the element dimensions the electric power output depends on the input excitation as well as on the electric load to be powered. To analyse the system behaviour, input variables and the generator itself have to be described in a mathematical model. This enables the calculation of optimal elements in principle. A modal electro-mechanical model of the piezoelectric element assuming to be base-excited is used in this paper. Although the modal model is very helpful to analyse the system, it cannot be easy used to determine a proper design of the piezoelectric elements. The problem is that the parameters of the model do not show any apparent relations to geometric dimensions or material data. Therefore, a mathematical method to obtain the parameters from the physical properties of a piezoelectric bending element is briefly described. The knowledge of the link between physical and modal parameters allows the usage of the mathematical model as a qualified design method. The input parameters of the linked model are the material data which can be found on data sheets. Additionally, boundary conditions of the environment like the impedance of the driven load and the vibration excitation has to be specified. The linked model shows the influences on power output to connected electric loads. The given power demands of applications which have to be satisfied yields in a design space of suitable elements. The design method enables the development engineer to select piezoelectric generator elements.