06-05-2013, 04:12 PM
TECHINICAL SEMINAR MICRO POWER GENERATOR
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ABSTRACT:-
Over the past few years, there has been a huge reduction in size and power consumption of MEMS devices like transducers and sensors. These devices are usually designed to run on batteries the replacement of batteries is not practical. The limited lifespan of batteries may induce costly maintenance, in the case of contaminated areas for instance. Moreover, batteries dying without warning cause serious problems in safety monitoring applications. That led to a surge of research in the area of energy harvesting. Sustainable power generation may be achieved in converting ambient energy into electrical energy. Some possible ambient energy sources are, for instance, thermal energy, light energy and mechanical energy.
The design of the mechanical system that transmits the surrounding vibratory energy to the electromagnetic generator is a critical importance. This thesis presents an optimization of an electromagnetic micro generator. It describes the theory, design and simulation of an energy converter based on electromagnetic induction. The objectives of this research are designing, improving the performance and operational reliability of electromagnetic micro generator. This paper deals with the design and simulation of a number of flat springs to be used for supporting the moving magnet of an electromagnetic micro generator. The flat spring and moving magnet are equivalent to a basic spring-mass system, in which the moving magnet is attached to a platform suspended by four beams
Introduction:
The advancements made in fabrication and micromachining techniques have enhanced the functionality of MEMS devices, which is one of the key reasons for the emergence of sustainable power sources. Power MEMS is one of the newest categories of MEMS, which encompasses micro devices and micro systems for power generation and energy conversion. The research and development of power MEMS have been promoted by the need for compact power sources with high energy and power density. We need for compact power sources with high energy and power density. Power MEMS has expanded to include not only various MEMS based power generators but also small energy machines and micro devices for macro power generators. As various devices and systems, such as energy harvesting micro devices. Their power levels vary from ten nano watts to ten of microwatts.
Overview of the existing power sources for MEMS:-
Most MEMS devices generally utilize macroscopic power sources and they greatly limit the main advantage of size reduction of MEMS devices. Batteries are commonly used for powering MEMS, however, they exhibit short lifetime, limited power storage and large weight and size in comparison with the device they power. Also chemicals contained in the batteries may be toxic. Moreover, in some applications the battery replacement is an expensive and difficult process or may be inconvenient for devices that are implanted in human patients like medical sensors. Therefore, spectacular efforts have been made by researchers to find alternative power supplies to cover the drawbacks of using batteries in MEMS applications. Some of these supplies can be specially designed to convert the ambient energy in the environment into electrical power.
Solar energy conversion:-
An abundant source of ambient energy is solar energy. A solar cell is a device that produces electrical energy directly from solar radiation. The power density of the solar radiation is about 1.4 kw/m2 [Roundy et.al., 2003] and it changes slightly during the year by no more than 0.1%. The solar cells are fabricated from single crystal, polycrystalline and amorphous silicon.
When illuminated, photo generated electron-hole pairs are generated throughout the solar cell. If the cell is connected to a load, current will flow from one region of the cell, through the load, and back to the other region of the cell.
Solar self powered devices such as calculator and watches are
Common place. Lee et al. [Lee, 2001] developed a thin solar cell that is specifically designed to
produce the open circuit voltages which is required to supply MEMS electrostatic actuators .
Thermal energy conversions:-
Temperature differences can also provide an ambient energy exploited to generate power. Thermoelectric generators are devices used to convert thermal energy directly to electrical energy. Such devices are based on thermoelectric effects involving interactions between the flow of heat and of electricity through solid bodies. When a load is properly connected, electrical current flows. Starner [Starner, 1996] calculates the amount of energy that could be extracted from the skin temperature of a human being. The temperature difference between the skin and the surrounding atmosphere drives a flow of heat energy that could be captured.
Stordeur and Stark [Stordeur and Stark, 1997] have presented a thermoelectric micro generator based on thermocouples and it manly developed to MEMS powering. The output power of the thermoelectric micro generator has been developed by the improvement of the thermo electronics [Castano et.al, 1997; Jacquot et.al. , 2002; Li et.al., 2003].
Vibrational energy conversions:-
Mechanical energy associated with the low amplitude vibration in the environment is another source of ambient energy that can be converted to electrical power. Low amplitude vibration is present in structures such us tall buildings, bridges, vehicles and industrial mechanics. The main design concept of the vibration based generator utilizes the free vibration available in the environment to produce movement and convert this movement of a suspended mass into electrical power by an electromagnetic induction .
Human body energy:-
Human body energy like breath movement, blood flow, walking, finger movement on a keyboard and body heat can be feasibly extracted and converted to electric power. Starner, and Shenck et.al. [Starner, 1996 and Shenck and Paradiso., 2001]. The amount of power was about 5-8W. The power density available from the shoe inserts can be useful for computing and communication devices.
Gravitational fields:-
Seiko Kinetics wrist watch invented by Hayakawa [Hayakawa, 1991] is an example of converting the gravitational energy to electric energy. The ac generator works by connecting a weight with an eccentric centre of rotation to a speed increasing gear train. The gear train supplies rotation to a dynamo at
an increased rate of rotation. Power of 200 μW was estimated based on a weight of 2 grams falling through 1cm once a second.
Electrostatic generators:-
Extracting electric power from vibration by electromagnetic induction has the interest of a group from Sheffield University [Williams et.al, 1996; Shear wood et.al., 1997; Williams et.al, 2001]. They have published an analysis of the vibration-based electromagnetic micro generator. An electromagnetic micro generator is an inertial device that is anchored at one end and free at the other end. The micro generator model consisting of a moving spring (k), rare earth permanent magnet, the mass of the spring (m) and an electrical coil represented by the dashpot (d) [Williams et.al, 1996]. The magnet is attached to the spring to form the spring mass system. When the generator is vibrating the magnet will move towards the electric coil. As a result, electric power will be induced across the coil according to Faraday’s law of induction.
General model and design concept of an electromagnetic micro generator:-
It has been proposed by Williams and Yates [Williams and Yates, 1995]. It is a general model for the conversion of the kinetic energy of the vibrating mass to electrical power based on linear system theory. It does not specify the mechanism by which the conversion takes place. The main design concept utilizes the free vibration available in the environment to produce movement and convert this movement of a suspended mass into electrical power by electromagnetic induction.The conversion of energy from the oscillating mass to electricity looks like a linear damper to the mass spring system.
General Principal of operation electromagnetic micro generator:-
The operating principle of the device is as follows: when the housing is vibrated, a mechanical input force feeds into a second order mechanical system, the mass moves relative to the housing and energy is stored in the mass-spring system. This relative displacement, which is sinusoidal in amplitude, causes the magnetic flux to cut the coil. This in turn induces a voltage on the coil due to the varying flux linkage within the motion between the magnet and the coil as stated by Faraday’s law of induction. The electrical system involved is simply a first-order LR circuit with the inductance of the coil (L) in series with the load resistance and parasitic resistance of the coil ®. In the former case, the amount of electricity generated depends upon the strength of the magnetic field, the velocity of the relative motion and the number of turns of the coil. The voltage on the coil is determined by Faraday’s Law.
Conclusion:
This work presents the analysis, fabrication process, and preliminary experimental results of a set of variable geometry MEMS-fabricated electromagnetic generators using vibration source energy for the purpose of an integrated MEMS power harvesting device. The experimental results indicate a sub-optimal power output, 12.5μW per Cu spiral layer, than of that which was predicted by theory, 53.3μW. An approach to increase the performance of the electromagnetic generator is to oscillate the magnet through the center, as depicted in Figure (1). A way in which to extrude the hole through the inner diameter of the coil is currently being tested and utilizes a diamond abrasion technique. This technique appears to work well with slow, low pressure abrasion of the micro-devices inner diameter. As for conclusions of the geometrical empirical analysis, maximizing the number of turns, while keeping the gap width wide enough to ensure no shorting of the coils, and providing for approximately a 6:5:1 ratio of spiral coil width to gap width will maximize the performance output of the micro-electromagnetic power harvester.