ENERGY HARVESTING FOR MICRO-ELECTROMECHANICAL-SYSTEMS (MEMS)

Transcription

1 ENERGY HARVESTING FOR MICRO-ELECTROMECHANICAL-SYSTEMS (MEMS) GURKAN ERDOGAN TABLE OF CONTENTS ABSTRACT (223 words)... 2 INTRODUCTION (758 words)... 3 A CRITICAL LITERATURE REVIEW (1867 words)... 6 SUMMARY AND CONCLUSION (682 words) BIBLIOGRAHY APPENDIX A DETAILED EXAMINATION OF A PAPER (513 words) APPENDIX B SEARCH AND SELECTION PROCEDURE OF THE PAPERS (245 words) APPENDIX C CURRICULUM VITAE APPENDIX D UNIVERSITY TRANSCRIPT APPENDIX E ORIGINAL DESCRIPTION OF THE TOPIC... 21

2 ABSTRACT Sensors that can be used in remote or not easily accessible places are becoming an attractive solution in a wide variety of applications such as habitat or structural monitoring. Advances in low power VLSI (Very Large Scale Integration) design and CMOS (Complementary Metal Oxide Semiconductor) fabrication have considerably reduced power requirements of these sensors. However, one aspect of the approach how to wirelessly generate power tends to cancel out the low power advantage. In addition, the limitations in providing power with batteries and distance constraints in passive wireless strategies have led to a growing interest in energy harvesting. Energy harvesting is a technology that converts the excess energy available in an environment into usable energy for low power electronics. Many ambient energy sources have been considered for this purpose such as incident light, vibration, electromagnetism, radio frequency (RF), human body functions, temperature gradient etc. However, each of these energy sources has its own drawbacks. For example, although the solar cells offer excellent power supply in direct sun light, they are inadequate in dim office lighting. On the other hand, the circuit design for transmitting the power harvested from low level vibrations is another challenging problem. In the critical literature review, different energy sources and harvesting techniques for powering MEMS sensors will be discussed briefly. Vibration-based energy harvesting techniques will be examined in detail. 2

3 INTRODUCTION Energy harvesting, also known as Energy Scavenging, Parasitic Energy, or Micro- Generators in the literature, is a process performed by a conversion mechanism for generating electric power from available ambient energy sources. Incident light, thermal gradients, machine vibrations and human body functions are the well known examples of ambient energy sources receiving the attention of many researchers. Since energy harvesting systems offer maintenance-free, long-lasting, green power supply for many portable, low-powered electronic devices, they are likely to become an essential part of power management systems. Figure (1) Wireless Self-powered Sensor Nodes The growing interest in the field of energy harvesting systems is also due to great developments in related technologies such as micro-electromechanical-system (MEMS) technology, wireless sensor network (WSN) technology, very large scale integration (VLSI) design technology, and complementary metal oxide semiconductor (CMOS) fabrication technology. Sensors emerge as a promising application of energy harvesting techniques where these state of the art technologies work together as shown in figure (1). While advances in micro fabrication techniques have allowed the development of various MEMS sensors that integrate intelligent electronic control systems with a mechanical system on the micro scale, VLSI design and CMOS fabrication techniques have reduced power requirements of the sensors to the range of tens to hundreds microwatts. Such low power dissipation opens up the possibility of powering the sensors by scavenging ambient energy from the environment, eliminating the need for batteries and extending the lifetimes indefinitely [1]. In addition, WSN consisting of large numbers of sensor nodes capable of wireless communication makes it possible to locate sensor nodes in remote and sometimes unrecoverable locations. With the development of WSN, long- 3

4 lasting wireless power supplies have become a must in applications since it is impractical to use and replace batteries, even long-lived types. The labor needed and other costs associated with changing hundreds of batteries not only make systems more expensive, but also inefficient. Furthermore, since the size of the sensor nodes is also critical, nodes not bigger than a dime are required in many applications where batteries might be too bulky. In table (1), some battery types and most applicable energy conversion mechanisms are compared with respect to their long term and short term power densities. Power density, meaning the amount of average energy generated per unit time and volume, is the most convenient and widely used criterion in the literature. Table (1) Comparison of Energy Sources [1] According to this table, batteries are reasonable for one year applications, whereas energy harvesters are required for long lifetime applications. In addition, if we ignore the direct sun light case which is highly ambient dependent, especially piezoelectric and shoe insert mechanisms stand out as the main vibration energy conversion mechanisms. In figure (2), components of a vibration energy harvesting system are depicted. This flow chart can be generalized for all energy harvesting systems in which an energy source, a conversion device, a conditioning circuit and an electric load are the main components of the general energy harvesting system. 4

5 Figure (2) Vibration Energy Harvesting System Components The general system basically aims to accomplish five consecutive tasks: Collecting the maximum energy from the energy source Converting the ambient energy into electric energy efficiently Rectifying and storing the maximum amount of electric energy Regulating the output voltage level depending on the application Transmitting the electric energy to the load when it is required Constructing mathematical models and manufacturing prototypes in order to estimate or ameliorate the efficiency and the performance of an energy conversion mechanism constitute the major areas of research. Many energy harvesting mechanisms are at their very early stage of being prototyped and more efficient systems can be obtained in the future by optimizing the tasks listed above. Availability of the ambient sources, the power densities of the converters, the duty cycles and the power needs of the electric loads, however, are the primary limitations of the energy harvesting systems. For example, solar cells can generate excellent power densities in direct sun light; but they need to be optimized for conditions of dim light or no light at all. Thermoelectric energy converters need large energy gradients to generate substantial power. Power delivery and user comfort are critical while generating power by means of body functions such as breathing, blood pressure, walking etc. Since there are abundant and continuous vibration sources such as machinery, wave, and wind vibrations, vibration energy converters seem to be the most promising mechanism thus far, and will be examined in detail in the next section. 5

6 A CRITICAL LITERATURE REVIEW Energy harvesting from a vibration source for low power electronic devices and sensors is an appealing idea, since there are various kinds of vibration sources around ranging from the wind and sea waves to human body motion and vibrating machinery in the industry. Vibration sources are usually preferable to incident light or thermal gradient energy sources requiring an appropriate operating time and running condition. Therefore, many research programs focusing specifically on vibration to electric energy converters have been conducted for various medical, industrial and military applications for more than a decade. Despite the large variety of prototypes designed for this purpose so far, the technology behind these conversion mechanisms is mainly based on three well-known effects in physics, namely the electrostatic, electromagnetic and piezoelectric effects. In brief, electrostatic, electromagnetic and piezoelectric designs require a variable capacitor, a magnet and a piezoelectric material respectively inducing a voltage on plates, in a coil and between the electrodes as they oscillate. However, the design of an energy converter, especially in microscale, becomes a little more sophisticated and therefore attractive for the researchers, when the system emerges as a vibration energy dissipation problem needing to be examined for various aspects to achieve a maximum power density and efficiency. While some of the reported generators have already been fabricated using MEMS techniques, others have been made on a mesoscale with the intention of later miniaturizing the devices using MEMS [2]. Williams et al. (1995) [1] introduced a generic model for estimating the power that can be generated in a microscale device. In this model, any electric component in which the energy conversion takes place is considered as an energy dissipation element (other than the inherent mechanical dissipation element) of the mechanical system as depicted in figure (3). The vibration source here is assumed to be infinitely large with respect to the system so that it is not affected by the motion of the conversion system. (1) 6

7 An electromagnetic micro-generator with dimensions of around 5x5x1mm and a deflection of 50µm is analyzed and assuming a viscous damping model as given in equation (1), power generations of 1µW and 100µW are estimated from vibration sources at a frequency of 70Hz and 330Hz, respectively. Figure (3) Generic Model for a Vibration Energy Converter In order to generate power, the relative motion of the mass, the voltage, the current and the damping force induce one another in the given sequence. One important result of this process is that the damping force changing with the current depends on the load resistance (R L ) accordingly. (2) (3) The generated power (P) is calculated as the rate of work done by the electrically induced damping force as described in equation (2) and consequently its magnitude is obtained as given in equation (3). The magnitude of the power as a function of the excitation frequency is also depicted in figure (3) for various damping factors. It is obvious that the maximum power generation is possible when the total damping factor (ζ) is minimal and the excitation frequency (ω) matches the undamped frequencies (ω n ) of the system. However, in case of a distributed frequency spectrum of the source, large damping factors are desired since it helps the converter harvest more energy from a broad band. 7

8 Mitcheson et al. (2004) [2], classified the vibration-driven micro-generators reported in the literature so far based on three fundamental architectures, namely the velocitydamped resonant generators (VDRG), coulomb-damped resonant generators (CDRG), and coulomb-force parametric generator, for establishing a unified analytical framework for such devices and providing a methodology for designing optimized generators for particular applications. First of all, they adapted the deflection limit of the proof mass, a key constrained in a MEMS application, to the general formulation. Roundy et al. [2] analyze the design parameters of electrostatic and piezoelectric converters, and then fabricate and test their prototypes shown in figure (4). Figure (4) Piezoelectric (on the left) and Capacitive (on the right) Converter Prototypes The mathematical model introduced by Williams et al. is modified for each mechanism by substituting the system specific design parameters. The estimated powers of the optimized converters are given in table (3) where a vibration source with a fundamental frequency of 100Hz and an acceleration magnitude of 2.25m/s 2 is employed. On the experimental side, the piezoelectric prototype without an optimum design is reported to generate an average power of 60 µw, however no comparable output power is stated for the electrostatic converter prototype. 8

9 Table (2) Estimated Powers with an Optimum Design Three types of electrostatic conversion mechanism topologies, namely (a) in-plane overlap, (b) in-plane gap closing, and (c) out-of-plane gap closing as depicted in figure (5), are also compared. The dark areas are fixed to the substrates, while the light areas are free to move in the arrow directions. Designs allowing the light areas move parallel and vertical to the substrate s surface in order to create a change in capacitance are called inplane and out-of-plane, respectively. In addition, the terms overlap and gap closing are used to indicate area and distance changes in the capacitor, respectively. Since the design (a) had stability problems and the design (c) suffered surface adhesion problems design (b) is chosen as the most convenient topology, despite its extra mechanical stops. Figure (5) (a) in-plane overlap, (b) in-plane gap closing and (c) out-of-plane gap closing Umeda et al. [3] first studied a piezoelectric converter which transforms mechanical impact energy to electric energy. The efficiency of the system with respect to the input energy and the load resistance R L were discussed. A thorough examination of this work can be found in Appendix A. Umeda et al. [4], next study energy storage characteristics of the same system examined in [3]. The load resistance in the first work is substituted by a bridge-rectifier and a 9

10 capacitor for storing electric energy. The efficiency and the amount of energy stored in the capacitor are examined with respect to the capacity and the initial voltage of the capacitor while the initial potential energy of the ball is fixed. In the case where the ball bounced only once, the energy conversion efficiency is defined as the ratio between the stored energy in the capacitor and the initial potential energy of the ball. They observed that a capacitor having an optimum capacitance stored a great portion of the input energy since it continues to be charged until the end of the plate oscillations and, as a result, achieved a maximum efficiency of 7%. However, efficiencies are low and the stored energy amounts are small for the higher capacitances due to having the same electric charge, but the lower voltage levels. That is the case for lower capacitances as well, due to both low electric charges and voltages. It is also observed that the efficiency of the converter increases and the stored charge amount decreases with the increasing initial capacitor voltage and after a certain voltage level, the efficiencies depend little on the capacitance. In the case in which the ball is allowed to bounce repeatedly until it stops, the energy stored by a single impact is replaced with the total energy by multiple impacts in the efficiency expression. The other considerations are exactly the same as the ones stated in the single impact case only if the multiple impacts case is considered as an accumulation of the energy generated by a sequence of single impact cases. A maximum efficiency of 35%, being over three times that of a solar cell, is achieved under the multiple impacts and high initial voltage conditions. However, it is mentioned that in most practical uses the electrical charge is more useful than the high efficiencies reached by means of high initial voltages. Starner [5] notes the possible energy harvesting locations around the human body and hence a great interest in wearable power supplies rapidly began to grow following his survey. The amount of power generated from a number of human activities ranging from body heat and exhalation to walking and typewriting are estimated for an average person by using some fundamental laws of physics and were given in table (3). Although the estimations highly depend on the constitution of a person and the activity being 10

11 performed, they are promising since a small percentage of them can power a microprocessor. Table (3) Power from Body Driven Sources Approximately 67W of power is calculated from the walking of an average man which is relatively a large value among the other body functions. The question, however, is how this power can be recovered without adding a disagreeable load on the user. Piezoelectric materials and rotary generators are proposed as conversion mechanisms. It is estimated that about 5W of power from a piezoelectric shoe insert application and 8.4W of power from a rotary generator application can be achieved. Since the efficiencies are obtained by means of some crude assumptions and comparisons, the estimations seem somewhat optimistic when they are compared with the other results in the literature. Kymissis et al. [6] study three different prototypes as in figure (6) that can be built into a shoe to harvest excess energy and generate electrical power parasitically while walking. Stave multilayer PVDF foil is one way of parasitically tapping energy to harness the bending of the sole, which is attempted in the first device. Uniform strip PZT is another promising mode of harnessing parasitic power in shoes to exploit the high pressure exerted in a heel strike. Rotary magnetic generator is the last system for extracting power from foot pressure by adapting a standard electromagnetic generator. Figure (6) Prototypes for Harvest Energy While Walking 11

12 The piezoelectric generators are terminated with a 250KΩ load resistance which is approximately equal to the equivalent source resistance at the excitation frequencies, hence yielded maximum efficiency. A peak power of 20mW for the PVDF stave and 80mW for the PZT unimorph were achieved. Because of the slow excitation rate, i.e. normal gait rate of an average person, the average powers were considerably lower; the PVDF stave produced about 1mW in average, while the unimorph did about twice that value. In contrast, the shoe mounted magnetic generator terminated with a 10Ω generated 230mW of power in average, but it was far less applicable because of its bulky design. The reported results for piezoelectric materials here are about three orders less than the powers estimated by Starner. This discrepancy is mainly due to the difference between the deflections assumed in theory and those that can be achieved by the systems in practice, aiming not to interfere greatly with one s gait. Figure (7) Power Circuit Design Piezoelectric systems are used together to power an RFID tag which has immediate applications in active environments, enabling the user to transmit their identity. A circuit as in figure (7), which has later found use in the work of several other researchers, was designed for rectifying, accumulating (6.5>V>12.6), and regulating (5V) the generated voltage. 12

13 SUMMARY AND CONCLUSION Energy harvesting from ambient energy sources for MEMS sensors and low-power electronic devices is an active research topic with growing application areas such as wireless sensor networks and wearable devices. As the power consumptions of the electronic devices are decreased by the advancements in micro fabrication techniques, various energy sources including incident light, thermal gradient, human body functions and vibrating industrial machinery which are available in the environment have aroused the interest of many researchers as ambient energy sources convertible into electric energy. However, ambient vibrations stand out as a promising and convenient energy source for many applications among others, since they are usually available continuously and abundantly in the surroundings of the energy harvesting systems. Vibration conversion mechanisms are mainly based on three physical effects, namely electrostatic, electromagnetic, and piezoelectric effects. Inducing voltage from a relative motion, which is a common feature of these effects, appears to be a vital phenomenon in the conversion process and is made use of in the development of a variety of prototypes examined in literature. The conversion mechanism is described by a linear model consisting of a damped spring mass system coupled with an oscillating platform. The proof mass creates a relative motion with respect to the vibrating platform, while the spring stores and discharges potential energy in the vibrating system. Since energy conversion can be considered as a way of dissipating energy, the electrical component where energy conversion phenomenon takes place can be modeled as a damper other than the inherent damping element of the vibrating system. The viscous damping model gives satisfactory results in many applications in which damping force is directly proportional to the relative velocity of the proof mass. Consequently, the amount of generated power can be calculated from the rate of work done by the electrically induced damping force on the proof mass. The power expression obtained from the previous analysis contains the fundamental design parameters including proof mass, electrically induced and total viscous damping factors, amplitude of the source acceleration and the excitation frequency. With the appropriate selection of these parameters maximum energy transformation efficiency 13

14 should be achieved. However, when microscale energy converters are aimed, the size of the system becomes a primary constraint for reaching large amounts of power. This is mainly because a large amount of power means large deflection which is available in resonance condition and limited by the size. If most of the energy is available in the low frequency bands of the source, the system requires a bigger proof mass in order to be in resonance, which is also limited by the size. On the other hand, the efficiency of the system depends highly on the electrically induced damping factor of the system controlled by the load resistance. A very low damping factor increases the output power of the system, but decreases the selectivity of the converter meaning that the converter can only harvest energy from a very narrow band in the vicinity of the resonance frequency. Many prototypes are being fabricated and tested, some of which have already been in microscale, while others have been in mesoscale and are intended to be miniaturized later. Of the three types of vibration to electrical energy conversion mechanisms, electrostatic and piezoelectric converters appear to be more suitable for microscale implementations, because an electromagnetic generator requires a neat design for that purpose despite its relatively high power density. The generated powers are generally satisfactory and sufficient to power a MEMS sensor or a microprocessor. However, power management occurs to be an issue for researchers because of the duty cycles and regulated voltage needs of these low-power devices. Storing the harvested energy in a capacitor or a battery might be an effective solution for some applications in order to accord the availability times of the sources and running periods of the devices. There is a vast amount of work done in literature on this issue, which is mostly studied by the researchers specialized in electronics. However, only an overview of the circuit design could be given in this literature review, since the topic is out of the author s scope. 14

15 BIBLIOGRAPHY 1. C.B. Williams, R.B. Yates, Analysis of a Micro-Electric Generator for Micro- Systems, Proceedings of the Transducers 95/Eurosensors IX (1995) S. Roundy, P.K. Wright, J. Rabaey, A study of low level vibrations as a power source for wireless sensor nodes, Computer Communications, v 26, n 11, Jul 1, 2003, p M. Umeda, K. Nakamura, S. Ueha, Analysis of the Transformation of Mechanical Impact Energy to electric Energy Using Piezoelectric Vibrator, Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes & Review Papers, v 35, n 5B, May, 1996, p M. Umeda, K. Nakamura, S. Ueha, Energy storage characteristics of a piezogenerator using impact induced vibration, Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes & Review Papers, v 36, n 5B, May, 1997, p T. Starner, Human Powered Wearable Computing IBM Systems Journal 35 (3) (1996) J. Kymissis, C. Kendall, J. Paradiso, N. Gershenfeld, Parasitic Power Harvesting in Shoes, 2 nd IEEE International Conference on Wearable Computing pp

16 APPENDIX A A DETAILED EXAMINATION OF [4] [3] Umeda et al. propose a conversion mechanism which transforms mechanical impact energy into electrical energy. In their experimental setup, an impact force acting on the center of a circular plate with a piezoelectric material underneath is created by the free falling of a steel ball from a certain height as depicted in figure (8). Figure (8) Experimental Setup The phenomenon is investigated by separating the action into two sequential time periods; the first one starting when the ball hits the center of the plate and continuing until the moment the ball bounces up and leaves the plate and the second one starting when the ball leaves the plate and continuing until the vibration of the plate is decayed to zero. In addition, it is assumed that there were two distinct mechanical systems: one in the first period in which the ball and the plate are adhered to each other, and so they are oscillating together, and the other one in the second period in which the plate is oscillating by itself. The equivalent circuit model of the systems given in figure (9) is constructed by means of variable circuit elements maintaining the continuity between two time periods. Two critical assumptions are made at this point; one is that the system is operated in the linear region meaning the impact force is not too big to create large deflections causing nonlinearity and the other one is that only the first bending mode is dominating the motion in both cases, so the effects of other modes can be ignored. These assumptions can be violated if we let the ball fall from a relatively high point or strike a point on the plate other than its center. 16

17 Figure (9) Equivalent Circuit Model After the circuit parameters are estimated from the measured admittance characteristics of the systems, the output voltage is solved from the constructed equivalent model for an initial potential energy of the ball. The output voltage is also measured, and then compared with the simulated output voltage in order to verify the equivalent circuit accuracy. This validated model is employed to examine the transformation efficiency. Transformation efficiency is defined as the ratio between the dissipated energy in the load resistance and the input energy, i.e. the initial potential energy of the steel ball. Depending on the efficiencies calculated for various initial potential energies, it is observed that an increase in the potential energy of the ball would decrease the transformation efficiency. The reason for this phenomenon is not analyzed deeply and is guessed as a result of nonlinear effects. From the energy budget of the system it is seen that a significant amount of energy was spent for bouncing of the ball and is also concluded that the generated energy would be larger if the steel ball did not bounce off after an impact but rather vibrated with the plate. A maximum efficiency of 52% was simulated for this case. Finally, effects of the piezoelectric vibrator are examined briefly and it is stated that the efficiency increased as the quality factor, Q, and coupling coefficient, k 2, increased, and the dielectric loss, tan(δ), decreased. 17

18 APPENDIX B SEARCH AND SELECTION PROCEDURE OF THE PAPERS Compendex /Engineering Index program available in the university library website was used to carry out a literature survey. General keywords associated with the topic were searched as a first attempt, including energy, source, harvesting, scavenging, micro-generator, parasitic, electric and conversion. However, a vast amount of published works on various energy sources, conversion mechanisms and power circuits were obtained. Therefore, specific terms such as vibration, piezoelectric, electrostatic, electromagnetic, MEMS, and sensor were employed for reaching distinctive journal papers and conference proceedings that focus on energy conversion mechanisms utilizing ambient vibrations as an energy source. A reasonable number of papers were obtained through this iterative search process and the abstracts of papers were reviewed carefully. The full texts of the most fundamental and interesting works were supplied from library resources or internet. The papers examined in this literature review were selected according to three criteria. First, the sophisticated studies with specific application areas were eliminated and more fundamental works were preferred for maintaining a basic understanding of the topic. Consequently, those papers appeared to be the most frequently cited works in literature at the same time. Secondly, after drawing the outline of this review work, the ones that were highly related to the context were preferred. In case of a serial work of authors such as [3] and [4] of Umeda et al., both were tried to be covered and included in this review work. Lastly, a balanced presentation of experimental and analytical works in literature was sought. 18

19 APPENDIX C CURRICULUM VITAE EDUCATION Ph.D. ITU *, Construction and Manufacturing Program (not finished) M.Sc. ITU, Theory of Machines and Control Program B.S. ITU, System Dynamics and Control Program (*) ITU stands for Istanbul Technical University, Turkey PUBLICATION M. Gurgoze, G. Erdogan, S. Inceoglu, Bending Vibrations of Beams Coupled by a Double Spring Mass System, Journal of Sound and Vibration (2001) 243(2), EMPLOYMENT Full-time Research and Development Engineer in Arcelik ** Part-time Graduate Student Team Member in Arcelik (**) Arcelik AS (Leading Home Appliance Company in Turkey, 7 th in Europe), R&D Department, Vibration and Acoustics Lab. COMPLETED PROJECTS [1] Vibro-Acoustic Design of Exhaust Pipes in Hermetic Refrigerator Compressors [2] Vibration Path Analysis of Dishwasher Motors [3] Measurement of the Mechanical Properties of the Visco-Elastic Damping Materials [4] Subjective Evaluation of Dishwasher Noise with Sound Quality Metrics and Jury Test 19

20 APPENDIX D UNIVERSITY TRANSCRIPT University of Minnesota Unofficial Transcript Name : Erdogan, Gurkan Student ID : Birth date : Print Date : University of Minnesota, Twin Cities MOST RECENT PROGRAMS Institution : University of Minnesota, Twin Cities Program : Graduate School Plan : Mechanical Engr Ph D Major Degree Sought: Doctor of Philosophy Beginning of Graduate Record Spring Semester 2005 University of Minnesota, Twin Cities Graduate School Mechanical Engr Ph D Major Attempted Earned Points AEM 8442 Nav. and Guidance Sys EE 5141 Microsystem Technology 4.00 GRAD 5102 Prep Univ Tchg NN Eng Spkrs 2.00 TERM GPA : TERM TOTALS : University of Minnesota Summary Information Graduate Career Totals Attempted Earned Points UMN GPA : UMN TOTALS : Transferred Courses Engineering Mathematics 3.00 BA Advanced System Dynamics and Control 3.00 AA Mechanics of Multi Body Systems 2.00 AA Industrial Applications of Fuzzy Logic 3.00 AA Intelligent Systems and Software Computing 3.00 BA Applied Numerical Methods 3.00 AA Nonlinear Vibrations 2.00 BA Finite Element Method 3.00 BA Nonlinear Analysis an Control 2.00 BA 20

21 APPENDIX E ORIGINAL DESCRIPTION OF THE TOPIC [1] The development of micro-electromechanical-systems has highlighted a wide range of applications for miniature sensors and actuators. This has made it possible to implant micro sensors and actuators into a host of different structures for applications such as medical implants and embedded sensors in buildings and bridges. In many applications, the microsystem must be completely embedded in the structure, with no physical connection to the outside world. The problem with this is that a remote device has to have its own power supply. The conventional solution is to use batteries, but batteries can be undesirable for many reasons: they tend to be quite bulky, contain a finite amount of energy, have a limited shelf life, and contain chemicals that could cause a hazard. A promising alternative to batteries is miniature self-contained renewable power supplies. Renewable power supplies convert energy from an existing source within their environment into electrical energy. The source of energy available will depend on the application. Some possible energy sources are: Light energy from ambient light source such as sunlight Thermal energy miniature thermoelectric generators that generate electricity when placed across a temperature gradient. Volume flow flow of liquids or gases. Mechanical energy energy from movement and vibration Of these sensors, light and thermal energy have already been exploited for use in micro power supplies. However there are many applications where there is insufficient light or thermal energy, and so other sources of energy should be considered. Therefore, we propose a new power supply that generates electricity from mechanical energy. This is intended for use in vibrating structure. 21