Thesis: Jiewen Deng

M.Sc Thesis, St. Anne's College, Hilary Term 2013

Pervasive networks of wireless sensors and communication nodes have been developed during past decades and most of them are powered by fixed-energy sources, e.g., wiring power and batteries. However, these traditional energy sources are impractical for powering wireless devices due to their inherent limitations, e.g., the high setup cost of wiring power and the finite life span of batteries. In view of these facts, more attentions have been drawn on vibration energy sources existing in ambient environment where sensors operate. Dozens of different types of vibration energy scavenging devices have been developed, which are mainly consisted of mechanical systems coupled with transduction mechanisms. The linear mechanical system has been used in most of existing vibration generators. The main drawback of such system is that it has a rather narrow bandwidth, meaning that the device can only effectively harvest vibration energy when the resonance frequency of the system coincides with the excitation frequency. Various methods have been proposed recently to overcome the drawback including utilising mechanical systems with nonlinear mechanisms in order to increase the bandwidth of vibration energy scavenging devices.

The dissertation is intended to find a practically effective non-linear mechanical system with desirable dynamic behaviours for vibration energy scavenging devices. To do so, we first examined three non-linear mechanisms numerically to find the most desirable one based on the corresponding typical mechanical systems. The Numerical Simulation (NS) method built in Matlab has been used to explore the static and dynamic characteristics of these systems with hardening and softening mechanisms firstly. It has been found that these systems with hardening and softening mechanisms whose frequency response curves lean sideways can expand the operation bandwidths.

However, the former has comparatively low output displacements whereas unstable dynamic responses are often associated with the latter after the forcing amplitude exceeds the critical value. To overcome the new hurdles, we have examined a typical mechanical system with a snap-through mechanism using the NS method. It exhibits the characteristics of a system with a softening mechanism when the forcing amplitude is comparatively small, and those of a system with a hardening mechanism when the forcing amplitude increases. Despite the advantages of the typical system, it has been found to be impossible to attach the transduction mechanism to the typical system in order to harvest power.

This has led to the development of a new mechanical system which consists of a central shim with four identical beams bonded by two blocks and magnetic buttons. By numerical simulation and experiments, we are able to show that the new design can be a system with either hardening mechanism or snap-through mechanism with a suitable selection of the column height. Due to the magnetic force from the magnetic buttons, the new design with a snap-through mechanism shows significant advantages in expanding the bandwidth and raising the dynamic displacement. However, the design leads to distortion of the two blocks in vibration, which makes the piezoelectric transduction mechanism unable to be coupled with it efficiently. As a result an improved design has been proposed in which a cross shim is used to replace the rectangular shim in the first design, which can be coupled with the piezoelectric transduction mechanism effectively to form a complete piezoelectric non-linear vibration energy scavenging device. Based on the results of numerical simulation and experiments, it is shown that the new device with a snap-through mechanism can generate enough voltage and power to power an existing wireless senor and has a wide off-resonance frequency bandwidth. Therefore, the new device with a snapthrough mechanism could be an alternative design to overcome the drawbacks of existing linear and non-linear vibration generators in certain conditions.