Eric M. Yeatman

Energy Harvesting From Human and Machine Motion for Wireless Electronic Devices

Energy harvesting generators are attractive as inexhaustible replacements for batteries in low-power wireless electronic devices and have received increasing research interest in recent years. Ambient motion is one of the main sources of energy for harvesting, and a wide range of motion-powered energy harvesters have been proposed or demonstrated, particularly at the microscale. This paper reviews the principles and state-of-art in motion-driven miniature energy harvesters and discusses trends, suitable applications, and possible future developments.

Architectures for vibration-driven micropower generators

Several forms of vibration-driven MEMS microgenerator are possible and are reported in the literature, with potential application areas including distributed sensing and ubiquitous computing. This paper sets out an analytical basis for their design and comparison, verified against full time-domain simulations. Most reported microgenerators are classified as either velocity-damped resonant generators (VDRGs) or Coulomb-damped resonant generators (CDRGs) and a unified analytical structure is provided for these generator types. Reported generators are shown to have operated at well below achievable power densities and design guides are given for optimising future devices. The paper also describes a new class-the Coulomb-force parametric generator (CFPG)-which does not operate in a resonant manner. For all three generators, expressions and graphs are provided showing the dependence of output power on key operating parameters. The optimization also considers physical generator constraints such as voltage limitation or maximum or minimum damping ratios. The sensitivity of each generator architecture to the source vibration frequency is analyzed and this shows that the CFPG can be better suited than the resonant generators to applications where the source frequency is likely to vary. It is demonstrated that mechanical resonance is particularly useful when the vibration source amplitude is small compared to the allowable mass-to-frame displacement. The CDRG and the VDRG generate the same power at resonance but give better performance below and above resonance respectively. Both resonant generator types are unable to operate when the allowable mass frame displacement is small compared to the vibration source amplitude, as is likely to be the case in some MEMS applications. The CFPG is, therefore, required for such applications.

Surface tension-powered self-assembly of microstructures - the state-of-the-art

Because of the low dimensional power of its force scaling law, surface tension is appropriate for carrying out reshaping and assembly in the microstructure size domain. This paper reviews work on surface tension powered self-assembly of microstructures. The existing theoretical approaches for rotational assembly are unified. The demonstrated fabrication processes are compared. Mechanisms for accurately determining the assembled shape are discussed, and the limits on accuracy and structural distortion are considered. Applications in optics, electronics and mechanics are described. More complex operations (including the combination of self-assembly and self-organization) are also reviewed.

Optimization of inertial micropower Generators for human walking motion

Micropower generators, which have applications in distributed sensing, have previously been classified into architectures and analyzed for sinusoidal driving motions. However, under many practical operating conditions, the driving motion will not be sinusoidal. In this paper, we present a comparison of the simulated performance of optimized configurations of the different architectures using measured acceleration data from walking motion gathered from human subjects. The sensitivity of generator performance to variations in generator parameters is investigated, with a 20% change in generator parameters causing between a 3% and 80% drop in generator power output, depending upon generator architecture and operating condition. Based on the results of this investigation, microgenerator design guidelines are provided. The Coulomb-force parametric generator is the recommended architecture for generators with internal displacement amplitude limits of less than /spl sim/0.5 mm and the velocity-damped resonant generator is the recommended architecture when the internal displacement amplitude can exceed /spl sim/0.5 mm, depending upon the exact operating conditions. Maximum power densities for human powered motion vary between 8.7 and 2100 /spl mu/W/cm/sup 3/, depending upon generator size and the location of the body on which it is mounted.

Surface plasmon microscopy

The use of surface plasmon resonance measurements for the imaging of surfaces has been investigated. Images of dielectric patterns deposited on silver films are presented, with thickness sensitivity of about 3 A and a lateral resolution of about 25μm.

Resolution and sensitivity in surface plasmon microscopy and sensing

The use of surface plasmons on planar surfaces for the characterisation of those surfaces is analysed. In particular, analytic expressions are derived for the sensitivity of such characterisation to the variation of bulk refractive indices, and to the addition of thin dielectric layers on the plasmon supporting surface. Comparison between surface plasmon and dielectric waveguide techniques is made. Surface plasmon microscopy is then considered, and relationships between sensitivity and resolution are derived. The interaction of plasmons with surface features is considered, and the effects in imaging of step and periodic features are specifically analysed. Other aspects relating to implementation and possible improvements to the technique are discussed.

Power-Extraction Circuits for Piezoelectric Energy Harvesters in Miniature and Low-Power Applications

When a piezoelectric energy harvester is connected to a simple load circuit, the damping force which the piezoelectric element is able to generate is often below the optimal value to maximize electrical power generation. Circuits that aim to increase the power output of a piezoelectric energy harvester do so by modifying the voltage onto which the piezoelectric current source drives its charge. This paper presents a systematic analysis and comparison of all the principal types of power extraction circuit that allow this damping force to be increased, under both ideal and realistic constraints. Particular emphasis is placed on low-amplitude operation. A circuit called single-supply prebiasing is shown to harvest more power than previous approaches. Most of the analyzed circuits able to increase the power output do so by synchronously inverting or charging the piezoelectric capacitance through an inductor. For inductor Q factors greater than around only 2, the single-supply prebiasing circuit has the highest power density of the analyzed circuits. The absence of diodes in conduction paths, achievable with a minimum number of synchronous rectifiers, means that the input excitation amplitude is not required to overcome a minimum value before power can be extracted, making it particularly suitable for microscale applications or those with a wide variation in amplitude.

Demonstration of three-dimensional microstructure self-assembly

Self-assembly of three-dimensional microstructures using the surface tension force of molten solder to produce out-of-plane rotation is demonstrated. The generic nature of the technique is illustrated by reconfiguring structures formed in both Ni metal and single crystal Si. The structures do not have a hinge to constrain the rotation. This considerably simplifies fabrication and eliminates problems associated with the compatibility of a suitable hinge material. Details of the fabrication processes are given and results are presented for rotated structures.

Ultrasonic vs. Inductive Power Delivery for Miniature Biomedical Implants

In this paper we compare two methods of wireless power delivery to implanted microdevices: ultrasonically and via inductive coupling. We build models for both methods and compare them in terms of power transmission efficiency, for different separations and receiver sizes. The simulation results show that at small distances between source and receiver (1 cm) the inductive system outperforms the ultrasonic one (efficiency of 81% vs. 39% for a receiver of 10 mm diameter). At larger distances (10 cm) the efficiencies of both systems reduce significantly, but the ultrasonic system demonstrates much better performance (0.2% vs. 0.013% for a 10 mm receiver). As the receiver gets smaller this gap increases drastically (0.02% vs. 0.02•10^-3% for a 2 mm receiver) while the distance after which the ultrasonic system outperforms the inductive one reduces (from 2.9 cm for a 10 mm receiver to 1.5 cm for a 5 mm receiver).

Performance limits of the three MEMS inertial energy generator transduction types

In this paper, trends from the last 10 years of inertial micro-generator literature are investigated and it is shown that, although current generator designs are still operating well below their maximum power, there has been a significant improvement with time. Whilst no clear conclusions could be drawn from reported fabricated devices with respect to preferred transducer technology, this paper presents operating charts for inertial micro-generators which identify optimal operating modes for different frequencies and normalized generator sizes, and allows comparison of the different transduction mechanisms as these parameters vary. It is shown that piezoelectric generators have a wider operating range at low frequency than electromagnetic generators, but as generator dimensions increase, the frequency to which piezoelectric transducers outperform electromagnetic transducers decreases.