Terence O'Donnell

A micro electromagnetic generator for vibration energy harvesting

Vibration energy harvesting is receiving a considerable amount of interest as a means for powering wireless sensor nodes. This paper presents a small (component volume 0.1 cm3, practical volume 0.15 cm3) electromagnetic generator utilizing discrete components and optimized for a low ambient vibration level based upon real application data. The generator uses four magnets arranged on an etched cantilever with a wound coil located within the moving magnetic field. Magnet size and coil properties were optimized, with the final device producing 46 µW in a resistive load of 4 k? from just 0.59 m s-2 acceleration levels at its resonant frequency of 52 Hz. A voltage of 428 mVrms was obtained from the generator with a 2300 turn coil which has proved sufficient for subsequent rectification and voltage step-up circuitry. The generator delivers 30% of the power supplied from the environment to useful electrical power in the load. This generator compares very favourably with other demonstrated examples in the literature, both in terms of normalized power density and efficiency.

Self-powered autonomous wireless sensor node using vibration energy harvesting

This paper reports the development and implementation of an energy aware autonomous wireless condition monitoring sensor system (ACMS) powered by ambient vibrations. An electromagnetic (EM) generator has been designed to harvest sufficient energy to power a radio-frequency (RF) linked accelerometer-based sensor system. The ACMS is energy aware and will adjust the measurement/transmit duty cycle according to the available energy; this is typically every 3 s at 0.6 m s?2 rms acceleration and can be as low as 0.2 m s?2 rms with a duty cycle around 12 min. The EM generator has a volume of only 150 mm3 producing an average power of 58 ?W at 0.6m s?2 rms acceleration at a frequency of 52 Hz. In addition, a voltage multiplier circuit is shown to increase the electrical damping compared to a purely resistive load; this allows for an average power of 120 ?W to be generated at 1.7 m s?2 rms acceleration. The ACMS has been successfully demonstrated on an industrial air compressor and an office air conditioning unit, continuously monitoring vibration levels and thereby simulating a typical condition monitoring application

Magnetics on silicon: an enabling technology for power supply on chip

Data from the ITRS2003 roadmap for 2010 predicts voltages for microprocessors in hand-held electronics will decrease to 0.8V with current and power increasing to 4A and 3W, respectively. Consequently, low power converters will move to multimegahertz frequencies with a resulting reduction in capacitor and inductor values by factors of 5 and 20, respectively. Values required at 10 MHz, for a low power buck converter, are estimated at 130 nH and 0.6 uF, compatible with the integration of magnetics onto silicon and the concept of power supply-on-chip (PSOC). A review of magnetics-on-silicon shows that inductance values of 20 to 40nH/mm/sup 2/ can be achieved for winding resistances less than 1/spl Omega/. A 1-/spl mu/H inductance can be achieved at 5 MHz with dc resistance of 1/spl Omega/ and a Q of four. Thin film magnetic materials, compatible with semiconductor processing, offer power loss density that is lower than ferrite by a factor of 5 at 10 MHz. Other data reported includes, lowest dc resistance values of 120 m/spl Omega/ for an inductance of 120 nH; highest Q of 15 for an inductance of 350 nH and a current of 1 A for a 1- /spl mu/H inductor. Future technology challenges include reducing losses using high resistivity, laminated magnetic materials, and increasing current carrying capability using high aspect-ratio, electroplated copper conductors. Compatible technologies are available in the power switch, control, and packaging space. Integrated capacitor technology is still a long-term challenge with maximum reported values of 400 nF/cm/sup 2/.

Optimization of an Electromagnetic Energy Harvesting Device

This paper presents the modeling and optimization of an electromagnetic-based generator for generating power from ambient vibrations. Basic equations describing such generators are presented and the conditions for maximum power generation are described. Two-centimeter scale prototype generators, which consist of magnets suspended on a beam vibrating relative to a coil, have been built and tested. The measured power and modeled results are compared. It is shown that the experimental results confirm the optimization theory

PCB integrated inductors for low power DC/DC converter

This paper discusses the use of printed circuit board (PCB) integrated inductors for low power DC/DC buck converters. Coreless, magnetic plates and closed core structures are compared in terms of achievable inductance, power handling and efficiency in a footprint of 10 /spl times/ 10 mm/sup 2/. The magnetic layers consist of electroplated NiFe, so that the process is fully compatible with standard PCB process. Analytic and finite element method (FEM) methods are applied to predict inductor performance for typical current waveforms encountered in a buck converter. Conventional magnetic design procedures are applied to define optimum winding and core structures for typical inductor specifications. A 4.7 /spl mu/H PCB integrated inductor with dc current handling of up to 500 mA is presented. This inductor is employed in a 1.5 W buck converter using a commercial control integrated circuit (IC). The footprint of the entire converter measures 10 /spl times/ 10 mm/sup 2/ and is built on top of the integrated inductor to demonstrate the concept of integrated passives in power electronic circuits to achieve ultra flat and compact converter solutions.

A review of planar magnetic techniques and technologies

This paper presents an extensive survey of techniques and technologies used to implement planar magnetic structures in modern DC to DC converters. The survey emphasises the practical applications of these devices. The trends are analyzed in the context of the marketplace and some predictions of future direction are attempted.

Microfabricated inductors for 20 MHz Dc-Dc converters

This paper presents the design and measured results for micro-fabricated inductors suitable for use in high frequency (> 10 MHz), low power (1 -2 W) dc-dc converters. The design has focused on maximizing inductor efficiency for a given converter specification. Inductors in the range of 100 nH to 300 nH have been fabricated and tested. The small signal measurements show a relatively flat inductance profile, with a 10% drop in inductance at 30 MHz. Inductance vs. dc bias current measurements show less than 15% decrease in inductance at 500 mA current. The performance of the micro-inductors have also been compared to a conventional wire-wound inductor in a 20 MHz dc-dc converter. The converter efficiency is shown to be approximately 4% lower when the micro-inductor is used compared to the when the wire- wound inductor is used. The peak efficiency of the micro-inductor in the converter is estimated to be approximately 93%.

Electrical performance of microtransformers for DC-DC converter applications

This paper presents the electrical performances of the fabricated microtransformers. Measurements for open-circuit and short-circuit impedance up to a frequency of 100 MHz are presented. The measured results are compared with predictions obtained from models based on an analytical approach. The use of a microtransformer in a 2-MHz dc-dc converter is reported. An efficiency of 40% for a power density of 1.2 W/cm/sup 2/ was measured.

Thick photoresist development for the fabrication of high aspect ratio magnetic coils

This paper reports the fabrication of coils for micro-magnetic devices on silicon using thick photoresists commonly used in the manufacture of microelectromechanical systems. A comparison of three photoresists, EPON SU-8, AZ 4562 and AZ 9260, is presented for the fabrication of high aspect ratio conductors. With a thickness of 81 μm, aspect ratios of 6:1 are obtained using the AZ 9260 photoresist. RF inductors and micro-transformers for power conversion applications are fabricated using this technology. The quality factor of the RF inductors shows maximum values of 23 at 0.4 GHz. Very good measurement is also obtained with the micro-transformers: the resistance of the electroplated copper windings is 0.3 Ω up to 2 MHz.

Design considerations of sub-mW indoor light energy harvesting for wireless sensor systems

For most wireless sensor networks, one common and major bottleneck is the limited battery lifetime. The frequent maintenance efforts associated with battery replacement significantly increase the system operational and logistics cost. Unnoticed power failures on nodes will degrade the system reliability and may lead to system failure. In building management applications, to solve this problem, small energy sources such as indoor light energy are promising to provide long-term power to these distributed wireless sensor nodes. This article provides comprehensive design considerations for an indoor light energy harvesting system for building management applications. Photovoltaic cells characteristics, energy storage units, power management circuit design, and power consumption pattern of the target mote are presented. Maximum power point tracking circuits are proposed which significantly increase the power obtained from the solar cells. The novel fast charge circuit reduces the charging time. A prototype was then successfully built and tested in various indoor light conditions to discover the practical issues of the design. The evaluation results show that the proposed prototype increases the power harvested from the PV cells by 30% and also accelerates the charging rate by 34% in a typical indoor lighting condition. By entirely eliminating the rechargeable battery as energy storage, the proposed system would expect an operational lifetime 10--20 years instead of the current less than 6 months battery lifetime.