Self-powered remote microsystems and sensor networks are needed in many emerging applications such as environmental and structural health monitoring, industrial sensing applications, security and military uses, and others. The required power for these systems can be provided mainly in three ways: 1) wiring the devices to a power supply, 2) by using electrochemical batteries and micro fuel cells, and 3) by energy scavenging from environmental sources such as ambient heat, light, and vibration. Physical wiring is not very practical and can increase the cost of a sensor up to ten times. Although electrochemical batteries and microfuel cells can provide enough power, they are not desirable for some applications due to their limited lifetime, and size. A battery large enough to last the lifetime of the sensor would dominate the overall system size, and hence is not very attractive. As the sensor network increases in number and the device size decreases, the replacement of depleted batteries and fuel cells is not practical. Energy scavenging has become popular recently, because of the need for clean power generation process and long lifetime. By scavenging energy, power can be generated from various environmental sources such as ambient heat, light, acoustic noise, vibration, and ambient RF signals. The goal of this project is to develop an efficient energy scavenger for converting ambient low-frequency vibrations into electrical power. Low-frequency vibrations present a couple of significant challenges to energy scavenging: 1) as the frequency drops so does the expected power density, and 2) most of the vibrations in the applications enumerated above typically vary in frequency and thus require wide bandwidth generators or non-resonant generators in order to scavenge them effectively.
The goal of this project is to design an efficient energy scavenger for converting ambient low-frequency, and in many cases non-periodic, vibrations into electrical power. The importance of the low-end of the frequency spectrum cannot be overstated. Humans, animals, the environment, bustling urban settings all give of waste kinetic energy, but these are slowly moving systems, providing peaks in the <30Hz frequency range. Further, these environments do not give off steady and periodic vibrations. Instead, their frequency response constantly changes, meaning that generators operating in this region require a high bandwidth. The contributions of this work are: 1) The state-of-the-art in scavenging low frequency, high-displacement vibrations will be improved. A generator architecture will be developed so that scavenging can be performed in these ambient environments. 2) As a means to develop a standalone power generator, effective strategies for power management electronics will be developed and implemented. 3) Low frequency vibrations by their nature have high displacement amplitudes, which is a great impediment to miniaturization. This project will develop “dense” architectures suitable for miniaturization into the micro scale. In vibration scavengers, appropriate mechanical frequency up-converters can be used to up-convert the frequency of mechanical vibrations to higher frequencies where a better efficiency and more power can be extracted.
To address the technical challenges outlined above, a novel non-resonant Parametric Frequency Increased Generator (PFIG) architecture has been developed. Three generations of the PFIG generator have already been fabricated and tested. The first one was a bench-top proof-of-concept version and the second one was a self-contained miniature electromagnetic generator. The latest version of the device uses the piezoelectric effect. This is the first micro-scale piezoelectric generator reported capable of ≤10Hz operation. The generator incorporates a bulk piezoelectric ceramic machined using ultrafast laser ablation. By incorporating a PZT bimorph in this device, the volume and displacement range are reduced by half over our second-generation electromagnetic version. Additional benefits of piezoelectric transduction include: reduced volume (halved), large rectifiable voltage, and the possibility of combining piezoelectric and electromagnetic transduction mechanisms. While work on this device continues, so far the fabricated device is capable of generating peak power of 100mW and an average power of 3.25mW from an input acceleration of 9.8m/s2 at 10Hz. The device operates over a frequency range of 24Hz. The internal volume of the generator is 1.2cm3.