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We present an energy harvester for environments that rotate through the earth's gravitational field. Example applications include shafts connected to motors, axles, propellers, fans, and wheels or tires. Our approach makes use of the unique dynamics of an offset pendulum along with a nonlinear bi-stable restoring spring to improve the operational bandwidth of the system. We apply our approach to a tire pressure monitoring system (TPMS) that is mounted on a car rim. Simulation and experimental test results show that the prototype generator is capable of directly powering an RF transmission every 60 seconds or less over a speed range of 6 to 95 mph.

Wireless sensor networks are poised to become a very significant enabling technology in many sectors. Already a few very low power wireless sensor platforms have entered the marketplace. Almost all of these platforms are designed to run on batteries that have a very limited lifetime. In order for wireless sensor networks to become a ubiquitous part of our environment, alternative power sources must be employed. This paper reviews many potential power sources for wireless sensor nodes. Well established power sources, such as batteries, are reviewed along with emerging technologies and currently untapped sources. Power sources are classified as energy reservoirs, power distribution methods, or power scavenging methods, which enable wireless nodes to be completely self-sustaining. Several sources capable of providing power on the order of 100 μW/cm3 for very long lifetimes are feasible. It is the authors’ opinion that no single power source will suffice for all applications, and that the choice of a power source needs to be considered on an application-by-application basis.

Magnetic plucking applies the strategy of frequency up-conversion in inertial energy harvesting when the energy source, such as human motion, only provides excitations with very low and irregular frequencies. This paper presents an analysis of three different magnet configurations to achieve magnetic plucking based on a three-dimensional analytical cube permanent magnet model: direct repulsive configuration, orthogonal configuration and indirect repulsive configuration. Simulation and experimental results indicate that the indirect repulsive configuration generates the largest tip displacement given the pratical constraints in designing a wearable energy harvester. We have implemented this configuration in a wrist-worn rotational energy harvester to pluck multiple piezoelectric beams. Other configurations, however, can potentially be advantageous in applications with alternative constraints.

Ambient intelligence (AmI) depends on the existence of vast quantities of wireless sensors distributed throughout the environment. While advances in IC fabrica- tion technologies, circuit designs, and networking techniques have greatly reduced the cost, size, and power consumption of potential wireless sensor platforms, the development of suitable power sources for many applications lags. The purpose of this chapter is both to review technologies for scavenging energy from the environment to power wireless sensors, and to discuss challenges and future research directions for vibration-based energy scaven- ging. Many potential energy scavenging technologies are presented along with current state of research and theoretical maximum power densities. Special focus is given to scavenging energy from mechanical vibrations extant in many environments. Results from vibration- based energy scavengers using piezoelectric structures developed by the authors are pre- sented demonstrating power production of approximately 375 mW=cm 3 . While wireless sensor nodes have successfully been powered by vibration-based energy scavengers, several improvements are possible, and indeed necessary, for widespread deployment. Three areas

Vibration energy scavenging with piezoelectric material can currently generate up to 300 microwatts per cubic centimeter, making it a viable method of powering low-power electronics. Given the growing interest in small-scale devices, particularly wireless sensor networks, concerns over how to indefinitely power them have become extremely relevant. Current limiting factors in the field of piezoelectric vibration energy scavenging include: coupling coefficients, strain distribution, and frequency matching. This paper addresses each of these three factors with a novel design and a corresponding analysis of its performance. For example, the power output of a cantilevered rectangular piezoelectric beam is limited by its uneven strain distribution under load. A prototype scavenger using a harmonically matched trapezoidal geometry solves this problem by evening the strain distribution throughout the beam, increasing by 30% the output power per unit volume. Another design is created which softens the frequency response of the generator, relaxing the constraint of frequency matching. The paper concludes that each of the three challenges to vibration energy scavenging can be met through creativity in mechanism design, making higher power densities possible and broader applications more feasible.

The rapidly decreasing size, cost, and power consumption of wireless sensors has opened up the relatively new research field of energy harvesting. Recent years have seen an increasing amount of research on using ambient vibrations as a power source. An important feature of all of these generators is that they depend on the resonance frequency of the generator device being matched with the frequency of the input vibrations. The goal of this paper, therefore, is to explore solutions to the problem of self-tuning vibration based energy harvesters. A distinction is made between “active” tuning actuators that must continuously supply power to achieve the resonance frequency change, and “passive” tuning actuators that supply power initially to tune the frequency, and then are able to “turn off” while maintaining the new resonance frequency. This paper analyzes the feasibility of tuning the resonance frequency of vibration based generators with “active” tuning actuators. Actuators that can tune the effective stiffness, mass, and damping are analyzed theoretically. Numerical results based for each type of actuator are presented. It is shown that only actuators that tune the effective damping will result in a net increase in power output, and only under the circumstance that no actuation power is needed to add damping. The net increase in power occurs when the mismatch between driving vibrations the mismatch between driving vibrations the resonance frequency of the device is more than 5%. Finally, the theory and numerical results are validated by experiments done on a piezoelectric generator with a smart material “active” tuning actuator.

Chapters 4 and 5 have dealt with piezoelectric converters. The direction will now change to the consideration of electrostatic converters. This chapter will deal with the modeling, design, and optimization of electrostatic converters. The following chapter will discuss the processing and fabrication of electrostatic converters. And finally, chapter 8 will cover test results.

In order to determine how much power can be converted from vibrations, the details of the particular vibration source must be considered. This chapter presents the results of a study undertaken to characterize many commonly occurring, low-level, vibrations. A general vibration to electricity model, provided by Williams and Yates (Williams and Yates 1995), is presented and discussed. The model is non-device specific, and therefore the conversion mechanism (i.e. electromagnetic, electrostatic, or piezoelectric) need not be established for the Williams and Yates model to be used. Power output can be roughly estimated given only the magnitude and frequency of input vibrations, and the overall size (and therefore mass) of the device.

There are three methods typically used to convert mechanical motion to an electrical signal. They are: electromagnetic (inductive), electrostatic (capacitive), and piezoelectric. These three methods are all commonly used for inertial sensors as well as for actuators. Conversion of energy intended as a power source rather than a sensor signal will use the same methods, however, the design criteria are significantly different, and therefore the suitability of each method should be re-evaluated in terms of its potential for energy conversion on the meso and micro scale. This chapter will provide an initial, primarily qualitative, comparison of these three methods. The comparison will be used as a basis to identify the areas that merit further detailed analysis.

A qualitative comparison of electrostatic, electromagnetic, and piezoelectric converters was presented in chapter 3. Chapter 4 will consider the modeling, design, and optimization of piezoelectric converters. Basic design configurations will first be discussed and evaluated. Models are then developed and validated. These models are then used as a basis for optimization.

A sequence of prototype devices has been designed and fabricated. The previous chapter explained the essential elements of the fabrication processes used. This chapter is dedicated to reporting fabrication and testing results.

The design of electrostatic converters has been discussed in the previous chapter. Perhaps more than with other manufacturing technologies, the design of MEMS devices is closely tied to the target processing technology. Therefore, a very short discussion of the manufacturing process to be used has been given in chapter 6 in order to effectively produce a design. The processing that has been used will be covered in more depth in this chapter. A number of variants on the basic SOI process have been used to fabricate a string of prototypes. Each of these variants and the purpose for which the variant was used will be discussed.

A model to predict the output of piezoelectric generators was developed and discussed in the previous chapter. This model was validated with a prototype to ensure its accuracy and suitability of for use as a basis for design optimization. Optimal designs were generated and discussed. Actual converters were designed and built based on these optimizations. This chapter will discuss.the implementation of these converters and present test results showing the improvement of the optimal designs. The converters have been used to power small wireless sensor devices, and results from such tests will also be presented.

Advances in low power VLSI design, along with the potentially low duty cycle of wireless sensor nodes open up the possibility of powering small wireless computing devices from scavenged ambient power. Low level vibrations occurring in typical household, office, and manufacturing environments are considered as a possible power source for wireless sensor nodes. This work focuses on the design of electrostatic vibration-to-electricity converters using MEMS fabrications technology. Detailed models of three different design concepts are developed. The three design concepts are evaluated and compared based on simulations and practical considerations. A formal optimization of the preferred design concept is performed, and a final design is produced using the optimal design parameters. Simulations of the optimized design show that an output power density of 116 μW/cm3 is possible from input vibrations of 2.25 m/s2 at 120 Hz. Test devices have been designed for a Deep Reactive Ion Etching (DRIE) process that etches MEMS structures into the top layer of a Silicon On Insulator (SOI) wafer. The devices are currently being fabricated.

Energy harvesting from human motion addresses the growing need for battery-free health and wellness sensors in wearable applications. The major obstacles to harvesting energy in such applications are low and random frequencies due to the nature of human motion. This paper presents a generalized rotational harvester model in 3 dimensions to determine the upper bound of power output from real world measured data. Simulation results indicate much space for improvement on power generation comparing to existing devices. We have developed a rotational energy harvester for human motion that attempts to close the gap between theoretical possibility and demonstrated devices. Like previous work, it makes use of magnetically plucked piezoelectric beams. However, it more fully utilizes the space available and has many degrees of freedom available for optimization. Finally we present a prototype harvester based on the coupled harvester model with preliminary experimental validation.

This paper outlines a mathematical framework to determine the upper bound on extractable power as a function of the forcing vibrations. In addition to determining the upper bound on power output, the method described provides insight into the dynamic transducer forces required to attain the upper bound. This relationship, between input vibration parameters and transducer force gives a critical first step in determining the optimal transducer architecture for a given vibration input. The method developed is applied to two specific vibration inputs; a single sinusoid, and the sum of two sinusoids. For the single sinusoidal case, the optimal transducer force is found to be that produced by a linear spring, resonant with the input frequency, and a linear viscous damper, with matched impedance to the mechanical damper. The solution to this first case was previously known, but has been used here to validate the methodology. The resulting transducer force for the input described by a sum of two sinusoids is found to be inherently time dependent. This time dependency shows that an active system can outperform a passive system. Furthermore, the upper bound on power output is shown to be twice that obtainable from a linear harvester centred at the lower of the two frequencies.

We present an energy harvester for environments that rotate through the Earth’s gravitational field. Example applications include shafts connected to motors, axles, propellers, fans, and wheels or tires. Our approach uses the unique dynamics of an offset pendulum along with a nonlinear bistable restoring spring to improve the operational bandwidth of the system. Depending on the speed of the rotating environment, the system can act as a bistable oscillator, monostable stiffening oscillator, or linear oscillator. We apply our approach to a tire pressure monitoring system mounted on a car rim. Simulation and experimental test results show that the prototype generator is capable of directly powering an RF transmission every 60 s or less over a speed range of 10 to 155 kph.

In today’s globally distributed business environment, resolution of the communication problems between team members is a critical issue. A project named the Design Consultant has been launched to improve inter/intra-departmental communications. Leveraging available Internet technology, the Design Consultant enables designers to share design information with other team members including colleague designers, CAE analysts, manufacturing engineers, and marketing engineers. The network accessible DFX modules assist designers to simulate manufacturability and to estimate cost and time for fabrication resulting in faster product development. Building blocks of the Design Consultant were established based on a 3-tier client/server architecture, and feasibility tests were successful. As part of the feasibility tests, the Design Consultant for Part Reuse and the Design Consultant for Rubber Molding were implemented to prove the concept of the network-based Concurrent Engineering.

Subcentimeter wireless computers capable of interfacing physically with their environment and communicating with each other have progressed from concept to commercial reality in the past decade. Wireless sensor nodes are an exciting technology, as they provide a backbone to measure almost any quantity in a spatially disperse way, allowing time-synchronized correlations over meters or miles. Before these devices can be deployed to monitor and protect environments (such as grid power distribution systems, buildings, factories, or even the human body) for long periods of time, they need a power source. Environmental generation looks to be a promising method.

This paper is a comparative study of the design spaces of two bio-implantable acoustically excited generator architectures: the thickness-stretch-mode circular piezoelectric plate and the bending-mode unimorph piezoelectric diaphragm. The generators are part of an acoustic power transfer system for implanted sensors and medical devices such as glucose monitors, metabolic monitors, drug delivery systems, etc. Our studies indicate that at small sizes the diaphragm architecture outperforms the plate architecture. This paper will present the results of simulation studies and initial experiments that explore the characteristics of the two architectures and compare their performance.