Vibrational Energy Scavenging Via Thin Film Piezoelectric Ceramics

This work focuses on constructing a vibrational energy scavenging device with a specific application to MEMS wireless sensor networks. The device utilizes vibrations produced by HVAC ducts, traffic in a room, and even wind hitting a window. The advantages of using thin film (~1 micron) PZT (Pb1.15(Zr0.47, Ti0.53)O3) over a larger scale bimorph will be addressed. The thin films are grown using pulsed laser deposition (PLD) to deposit the film epitaxially on MgO. The PZT is then removed from the MgO and attached to a metallic shim, thus creating a usable bimorph. Discussion of the system will reflect that of an external force applied perpendicular to the beam at its tip. Characterization and material analysis will illustrate the effectiveness of this technique in creating an energy scavenging device. Vibrational Energy Scavenging Wireless sensor platforms are currently being used in preliminary experiments to monitor energy use in buildings, deflections of the Golden Gate Bridge, and the health of redwood trees in California forests. These self-contained sensor platforms, or motes, range from approximately 2 to 6 cm 3 in size and include the desired sensor(s), a small-integrated computer, a transmitter, and a battery (Figures 1a and 1b). The size and cost will need to be reduced as the research naturally evolves towards miniaturization. Complete sensor platforms utilizing complementary metal oxide (CMOS) fabrication and microelctromechanical (MEMS) devices have the potential of occupying volumes less than 1 mm 3 . As “smart dust” they may eventually be spray-painted onto the walls of air-conditioning ducts, or integrated into the upholstery of furniture [1-3]. Figures (1a) and (1b): Wireless sensor “nodes” or “motes,” (b) practical size of 2 cm 3 The larger platforms depicted in Figure 1a can carry a supplementary sensor board for a range of sensors, while the meso-scale device shown in Figure 1b requires sensors permanently attached to the circuit board. A broad range of sensors are currently available including sensors for detecting and measuring temperature, humidity, light, motion, and particles. The large platforms have also been connected to conventional strain gauges and accelerometers for stress and vibration measurement. However, substantial obstacles exist for the practical deployment of such sensor platforms. Despite the small size of the electronics, the large battery volume dictates the final size of the wireless platform. Battery volume can be reduced by integrating a renewable power supply, thus reducing the total platform size. The potential and excitement of wireless sensor platforms has initiated research into discovering new energy sources to aid in miniaturization. The focus of this research is to construct both mesoand MEMS-scale energy scavenging (or harvesting) systems driven by ambient vibrations. Batteries are a reasonable solution for short life sensor platforms (less than 1 year), but for industrial and commercial implementation an estimated 10-year life is required. Although research efforts focused on microbattery production for short term applications can be found in the literature [4-6], alternate solutions using photovoltaics and vibrational energy scavenging are more attractive for long term applications. Figure 2 charts the power density versus lifetime of various batteries and energy sources showing the lifetime limitations of batteries. Figure 2. Power density vs. lifetime for various energy sources and batteries. Photovoltaics offer excellent power density in direct sunlight, but in dim, or non-lit areas, they are inadequate [7]. Vibrational energy scavenging is one of the more promising alternative solutions due to the multitude of ambient vibrational sources and the relative ease of electromechanical conversion. The most common methods currently employed for intrinsic vibrational energy scavenging utilize electrostatic, electromagnetic, or piezoelectric devices [810]. Roundy et. al. [11] has analyzed frequency and acceleration output for various vibrational sources as tabulated in Table I. Effective vibrational energy scavenging devices must be designed around these readily available sources to be commercially viable. Table I. Vibrational frequency and peak acceleration of various ambient vibration sources. Vibrational Source Peak Acc. (m/s 2 ) Freq. (Hz) Base of a 3 axis machine tool 10 70 Kitchen blender casing 6.4 121 Clothes dryer 3.5 121 Door frame just as door closes 3 125 Small microwave oven 2.25 121 HVAC vents in office building 0.2 1.5 60 Wooden deck with foot traffic 1.3 385