Energy autonomous systems : future trends in devices, technology, and systems

The rapid evolution of electronic devices since the beginning of the nanoelectronics era has brought about exceptional computational power in an ever shrinking system footprint. This has enabled among others the wealth of nomadic battery powered wireless systems (smart phones, mp3 players, GPS, …) that society currently enjoys. Emerging integration technologies enabling even smaller volumes and the associated increased functional density may bring about a new revolution in systems targeting wearable healthcare, wellness, lifestyle and industrial monitoring applications

A 32 mV/69 mV input voltage booster based on a piezoelectric transformer for energy harvesting applications

This paper presents a novel method for battery-less circuit start-up from ultra-low voltage energy harvesting sources. The approach proposes for the first time the use of a Piezoelectric Transformer (PT) as the key component of a step-up oscillator. The proposed oscillator circuit is first modelled from a theoretical point of view and then validated experimentally with a commercial PT. The minimum achieved start-up voltage is about 69 mV, with no need for any external magnetic component. Hence, the presented system is compatible with the typical output voltages of thermoelectric generators (TEGs). Oscillation is achieved through a positive feedback coupling the PT with an inverter stage made up of JFETs. All the used components are in perspective compatible with microelectronic and MEMS technologies. In addition, in case the use of a ∼40 μH inductor is acceptable, the minimum start-up voltage becomes as low as about 32 mV

Development Of Low Frequency Electromagnetic Vibration Energy Harvester

This thesis presents the development of an energy harvester based on electromagnetic system to harvest energy from the low frequency vibration in particular structural vibration of the bridge. The eventual intended application is to power up a wireless sensor node that can be used to monitor structural integrity of a bridge. The electromagnetic vibration energy harvester is developed with four units of voice coil actuators using Neodymium magnets and copper coils. An open circuit voltage of 2.6 V and maximum output power of 25 mW were generated at 10 Hz under constant acceleration of 0.35g respectively. A model of the harvester is made by combining the mechanical of mass-spring-damper system with the electromagnetic resistance and inductance where the velocity of the moving coil from the mechanical system will produce current in the system. The simulation model showed a good agreement with the experimental results at 4 - 10 Hz for one cell harvester with 4.3% difference. The main contribution of this research is the prototype construction of low vibration energy harvester system and development of the energy harvester model using Simulink. The power density of the harvester system is 602 μW/g.cm3 and 1308.2 μW/g.cm3 at 4 Hz and 10 Hz respectively and enough to power up the wireless sensor network device

Ferroelectrets: from material science to energy harvesting and sensor applications

The purpose of this thesis is to develop innovative ferroelectrets that can be used in energy harvesting devices as well as mechanical sensors. In the first stage, the focus lies on the application of ferroelectrets as energy harvesters. The inability to control the environment where the energy harvesters will be applied, requires the use of materials that can be utilized in harsh environment such as high temperature or humidity. Therefore, new ferroelectrets based on polymers with excellent electret properties, such as fluoroethylene propylene (FEP) are developed. Two types of ferroelectrets are considered, one optimized for the longitidunal piezoelectric effect and the other one optimized for the transverse piezoelectric effect in these materials. Hereby, new void structures are achieved through thermally fusing such films so that parallel tunnels (parallel-tunnel ferroelectrets) are formed between them, or by fusing round-section FEP tubes together so that they form a band or membrane. The FEP tube configuration is optimized based on a finite element model showing that implementing a single tube structure (25 mm × 1.5 mm) as the energy harvester exhibits the largest output power. By building the energy harvester and modeling it analytically, it is demonstrated that the generated power is highly dependent on parameters such as wall thickness, load resistance, and seismic mass. Utilizing a seismic mass of 80 g at resonance frequencies around 80 Hz and an input acceleration of 1 g (9.81 m s−2), output powers up to 300 μW are reached for a transducer with 25 μm thick walls. The parallel-tunnel ferroelectrets (40 mm × 10 mm) are characterized and used in an energy harvester device based on the transverse piezoelectric effect. The energy harvesting device is an air-spaced cantilever arrangement produced by additive manufacturing technique (3D-printing). The device is tested by exposing it to sinusoidal vibrations with an acceleration a, generated by a shaker. By placing the ferroelectret at a defined distance from the neutral axis of the cantilever beam and using a proper pre-stress of the ferroelectret, an output power exceeding 1000 μW at the resonance frequency of approximately 35 Hz is reached. This demonstrates a significant improvement of air-spaced vibrational energy harvesting with ferroelectrets and greatly exceeds previous performance data for ferroelectret energy harvester of maximal 230 μW. In the second stage of the dissertation, the focus is shifted to develop ferroelectrets for chosen applications such as force myography, ultrasonic transducer and smart insole. Hereby, new arrangements and manufacturing methods are investigated to build the ferroelectret sensors. Furthermore, and following the recent requirements of eco-friendlier sensors, ferroelectrets based on polylactic acid (PLA) are investigated. PLA is a biodegradable and bioabsorbable material derived from renewable plant sources, such as corn or potato starch, tapioca roots, and sugar canes. This work relays a promising new technique in the fabrication of ferroelectrets. The novel structure is achieved through sandwiching a 3D-printed grid of periodically spaced thermoplastic polyurethane (TPU) spacers and air channels between two 12.5 μm-thick FEP films. Due to the ultra-soft TPU sections, very high quasistatic (22.000 pC N−1) and dynamic (7500 pC N−1) d33-coefficients are achieved. The isothermal stability of the d33-coefficients showed a strong dependence on poling temperature. Furthermore, the thermally stimulated discharge currents revealed well-known instability of positive charge carriers in FEP, thereby offering the possibility of stabilization by high-temperature poling. A similar approach is taken by replacing the environmentally harmful FEP by PLA. Large piezoelectric d33-coefficients of up to 2850 pC N−1 are recorded directly after charging and stabilized at about 1500 pC N−1 after approximately 50 days under ambient environmental conditions. These ferroelectrets when used for force myography to detect the slightest muscle movement when moving a finger, resulted in signal shapes and magnitudes that can be clearly distinguished from each other using simple machine learning algorithms known as Support Vector Machine (SVM) with a classification accuracy of 89.5%. Following the new manufacturing route using 3D-printing, an insole is printed using pure polypropylene filament and consists of eight independent sensors, each with a piezoelectric d33 coefficient of approximately 2000 pC N−1. The active part of the insole is protected using a 3D-printed PLA cover that features eight defined embossments on the bottom part, which focus the force on the sensors and act as overload protection against excessive stress. In addition to determining the gait pattern, an accelerometer is implemented to measure kinematic parameters and validate the sensor output signals. The combination of the high sensitivity of the sensors and the kinematic movement of the foot, opens new perspectives regarding diagnosis possibilities through gait analysis. By 3D-printing a PLA backplate and using it in combination with a bulk PLA film, a new possibility to build ultrasonic transducers is presented. The ultrasonic transducer consists of three main components all made from PLA: the film presenting the vibrating plate, the printed backplate with well-defined groves, and the printed holder. The PLA film and the printed backplate build together the ferroelectret with artificial air voids. The printed holder clamps the film on the backplate and fixes the ferroelectret together. The resulting sound pressure is measured with a calibrated microphone (Type 4138, Bruel & Kjaer) at a distance of 30 cm. The biodegradable ultrasonic transducer exhibits a large bandwidth of approximately 45 kHz and fractional bandwidth of 70%. The resulting sound pressure at the resonance frequency can be increased from 98 dB up to 106 dB for driving voltages from 30 to 70 V. respectively. The obtained theoretical and experimental results are an excellent base for further optimizing ferroelectrets to be accepted in the field of energy harvesting and mechanical sensors, where flexibility and high sensitivity are mandatory for the applications

Energy harvesting system design and optimization for wireless sensor networks

Wireless sensor networks (WSN) are becoming widely adopted for many applications including complicated tasks like building energy management. However, one major concern for WSN technologies is the short lifetime and high maintenance cost due to the limited battery energy. One of the solutions is to scavenge ambient energy, which is then rectified to power the WSN. The objective of this thesis was to investigate the feasibility of an ultra-low energy consumption power management system suitable for harvesting sub-mW photovoltaic and thermoelectric energy to power WSNs. To achieve this goal, energy harvesting system architectures have been analyzed. Detailed analysis of energy storage units (ESU) have led to an innovative ESU solution for the target applications. Battery-less, long-lifetime ESU and its associated power management circuitry, including fast-charge circuit, self-start circuit, output voltage regulation circuit and hybrid ESU, using a combination of super-capacitor and thin film battery, were developed to achieve continuous operation of energy harvester. Low start-up voltage DC/DC converters have been developed for 1mW level thermoelectric energy harvesting. The novel method of altering thermoelectric generator (TEG) configuration in order to match impedance has been verified in this work. Novel maximum power point tracking (MPPT) circuits, exploring the fractional open circuit voltage method, were particularly developed to suit the sub-1mW photovoltaic energy harvesting applications. The MPPT energy model has been developed and verified against both SPICE simulation and implemented prototypes. Both indoor light and thermoelectric energy harvesting methods proposed in this thesis have been implemented into prototype devices. The improved indoor light energy harvester prototype demonstrates 81% MPPT conversion efficiency with 0.5mW input power. This important improvement makes light energy harvesting from small energy sources (i.e. credit card size solar panel in 500lux indoor lighting conditions) a feasible approach. The 50mm × 54mm thermoelectric energy harvester prototype generates 0.95mW when placed on a 60oC heat source with 28% conversion efficiency. Both prototypes can be used to continuously power WSN for building energy management applications in typical office building environment. In addition to the hardware development, a comprehensive system energy model has been developed. This system energy model not only can be used to predict the available and consumed energy based on real-world ambient conditions, but also can be employed to optimize the system design and configuration. This energy model has been verified by indoor photovoltaic energy harvesting system prototypes in long-term deployed experiments

Energy harvesting from body motion using rotational micro-generation

Autonomous system applications are typically limited by the power supply operational lifetime when battery replacement is difficult or costly. A trade-off between battery size and battery life is usually calculated to determine the device capability and lifespan. As a result, energy harvesting research has gained importance as society searches for alternative energy sources for power generation. For instance, energy harvesting has been a proven alternative for powering solar-based calculators and self-winding wristwatches. Thus, the use of energy harvesting technology can make it possible to assist or replace batteries for portable, wearable, or surgically-implantable autonomous systems. Applications such as cardiac pacemakers or electrical stimulation applications can benefit from this approach since the number of surgeries for battery replacement can be reduced or eliminated. Research on energy scavenging from body motion has been investigated to evaluate the feasibility of powering wearable or implantable systems. Energy from walking has been previously extracted using generators placed on shoes, backpacks, and knee braces while producing power levels ranging from milliwatts to watts. The research presented in this paper examines the available power from walking and running at several body locations. The ankle, knee, hip, chest, wrist, elbow, upper arm, side of the head, and back of the head were the chosen target localizations. Joints were preferred since they experience the most drastic acceleration changes. For this, a motor-driven treadmill test was performed on 11 healthy individuals at several walking (1-4 mph) and running (2-5 mph) speeds. The treadmill test provided the acceleration magnitudes from the listed body locations. Power can be estimated from the treadmill evaluation since it is proportional to the acceleration and frequency of occurrence. Available power output from walking was determined to be greater than 1mW/cm³ for most body locations while being over 10mW/cm³ at the foot and ankle locations. Available power from running was found to be almost 10 times higher than that from walking. Most energy harvester topologies use linear generator approaches that are well suited to fixed-frequency vibrations with sub-millimeter amplitude oscillations. In contrast, body motion is characterized with a wide frequency spectrum and larger amplitudes. A generator prototype based on self-winding wristwatches is deemed to be appropriate for harvesting body motion since it is not limited to operate at fixed-frequencies or restricted displacements. Electromagnetic generation is typically favored because of its slightly higher power output per unit volume. Then, a nonharmonic oscillating rotational energy scavenger prototype is proposed to harness body motion. The electromagnetic generator follows the approach from small wind turbine designs that overcome the lack of a gearbox by using a larger number of coil and magnets arrangements. The device presented here is composed of a rotor with multiple-pole permanent magnets having an eccentric weight and a stator composed of stacked planar coils. The rotor oscillations induce a voltage on the planar coil due to the eccentric mass unbalance produced by body motion. A meso-scale prototype device was then built and evaluated for energy generation. The meso-scale casing and rotor were constructed on PMMA with the help of a CNC mill machine. Commercially available discrete magnets were encased in a 25mm rotor. Commercial copper-coated polyimide film was employed to manufacture the planar coils using MEMS fabrication processes. Jewel bearings were used to finalize the arrangement. The prototypes were also tested at the listed body locations. A meso-scale generator with a 2-layer coil was capable to extract up to 234 µW of power at the ankle while walking at 3mph with a 2cm³ prototype for a power density of 117 µW/cm³. This dissertation presents the analysis of available power from walking and running at different speeds and the development of an unobtrusive miniature energy harvesting generator for body motion. Power generation indicates the possibility of powering devices by extracting energy from body motion

Microelectronic Design with Integrated Magnetic and Piezoelectric Structures

This thesis investigates the possibility of integrating the standard CMOS design process with additional microstructures enhancing circuit functionalities. More specifically, the thesis faces the problem of miniaturization of magnetic and piezoelectric devices mostly focused on the application field of EH (Energy Harvesting) systems and ultra-low power and ultra-low voltage systems. It shows all the most critical aspects which have to be taken into account during the design process of miniaturized inductors for PwrSoC (Power System on Chip) or transformers. Furthermore it shows that it is possible to optimize the inductance value and also performances by means of a proper choice of the size of the planar core or choosing a different layout shape such as a serpentine shape in place of the classic toroidal one. A new formula for the correct evaluation of the MPL (Magnetic Path Length) was also introduced. Concerning the piezoelectric counterpart, it is focused on the design and simulation of various MEMS PTs based on a SOI (Silicon on Insulator) structure with AlN (Alluminum Nitride) as active piezoelectric element, in perspective of having a SoC with embedded MEMS devices and circuitry. Furthermore it demonstrates for the first time the use of a PT (Piezoelectric Transformer) for ultra-low voltage EH applications. A new boost oscillator based on a discrete PZT (Lead Zirconate Titanate) PT instead of a MT (Magnetic Transformer) has been modelled and tested on a circuit made up by discrete devices, showing performances comparable to commercial solutions like the LTC3108 from Linear. Furthermore this novel boost oscillator has been designed in a 0.35μm technology by ST Microelectronics, showing better performances as intuitively expected by the developed mathematical model of the entire system

Contribution to modeling and realization of ultralow voltage oscillators with application to energy harvesting

Orientadores: José Antenor Pomilio, Saulo FincoTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: Sistemas autônomos como dispositivos implantáveis, redes de sensores sem fio e sistemas embarcados requerem uma fonte de energia usualmente na forma de bateria ou supercapacitor. A miniaturização e a redução do consumo de potência em dispositivos eletrônicos modernos permite o uso de fontes alternativas como forma de estender a vida útil destes sistemas. A energia pode ser fornecida pelo ambiente, na forma de luz solar, vibração, calor ou ondas eletromagnéticas. O processo de captação e adequação desta energia é chamado de extração ou coleta de energia. O desenvolvimento de sistemas de extração de energia envolve desafios. Algumas fontes fornecem apenas dezenas de milivolts ou nanoamperes. Uma abordagem para extrair energia de fontes de baixa tensão é o projeto de osciladores que possam operar nestas condições. Esta área do conhecimento vem sendo objeto de intensa pesquisa. Várias estratégias são utilizadas, e sistemas operando com até 3,5 mV são descritos. Há, contudo um compromisso entre mínima tensão de operação, capacidade de potência e volume/complexidade. O comportamento de osciladores em tensão ultrabaixa é altamente não-linear e dependente dos parâmetros dos dispositivos e condições de operação. Técnicas convencionais são inadequadas para a análise destes circuitos em situação tão extrema. A impossibilidade de prever com precisão aceitável parâmetros como excursão de tensão e frequência de oscilação e a falta de compreensão mais profunda do mecanismo de funcionamento tornam difícil a especificação dos blocos seguintes ao oscilador no sistema de extração de energia. Este trabalho propõe a aplicação do oscilador com acoplamento nas portas como um módulo de extração de energia de tensão ultrabaixa. O comportamento do circuito é tipicamente como multivibrador astável e uma modelagem utilizando a teoria de oscilações não lineares é apresentada, tanto para circuitos com transistores MOS (MOSFETs) como com transistores bipolares (BJTs). A validade do modelo é verificada através de experimentos com protótipos discretos e uma boa concordância é obtida entre a teoria e a prática. A topologia necessita de uma tensão nas bases dos BJTs ou portas de MOSFETs convencionais de forma que a oscilação possa se iniciar. Um novo módulo, chamado de bloco de partida, é proposto que deriva esta tensão de polarização da fonte de alimentação, tornando o circuito independente de uma tensão preexistente. Um modelo linear para este bloco é apresentado e verificado através da caracterização de um protótipo. Experimentos com circuitos discretos utilizando o bloco de partida mostram que os osciladores podem iniciar sua operação com uma tensão única tão baixa quanto 50 mV. Os protótipos com BJTs e MOSFETs foram capazes de fornecer 173 µW e 560 µW para uma alimentação de 100 mV, respectivamente, demonstrando que a topologia pode ser uma alternativa competitiva em termos de desempenho, tensão de operação e complexidade quando comparada a outras já apresentadas na literaturaAbstract: Autonomous systems, such as implanted devices, wireless sensor networks, and embedded systems, require an energy source which is usually in the form of a battery or a supercapacitor. The miniaturization and reduction of power consumption in modern electronic devices enables the use of alternative energy sources as a way of extending the life-span of these systems. The energy can be supplied by the environment, such as sunlight, vibration, heat, or RF waves. The process of extracting and fitting this energy is usually called energy harvesting. The development of energy harvesting systems presents challenges. Some sources can only supply dozens of millivolts or nanoamperes. One approach to harvest the energy of low-voltage sources is by designing oscillators that can operate in these conditions. This knowledge area is subject of intensive research. Many approaches are proposed, and systems operating with voltages as low as 3.5 mV are described. However, there is a tradeoff between minimum operating voltage, power capacity and volume/complexity. The behavior of oscillators at ultralow voltage is very nonlinear and dependent of the device parameters and operational conditions. Conventional techniques are not able to give reasonable results in the analysis of circuits in such extreme levels. The lack of a prediction of parameters like voltage excursion and oscillation frequency with acceptable precision and a deeper understanding of the working mechanism turn difficult the specification of the blocks that follow the oscillator in the energy harvesting system. This work proposes the application of the oscillator with coupling at the gates as an ultralow voltage energy harvesting module. The behavior of the circuit is typically as astable multivibrator and a modeling using the theory of nonlinear oscillations is presented, when applying MOS transistors (MOSFETs) or bipolar junction transistors (BJTs). The validity of the models is verified through experiments with discrete prototypes and good agreement is found between theory and practice. The topology needs a voltage biasing at the bases of the BJTs or gates of the conventional MOSFETs in order it starts oscillating. A new module, called starting block, is proposed that derives this biasing voltage from the voltage source, turning the circuit independent of a preexistent voltage. A linear model for this block is presented and checked by the characterization of a prototype. Experiments with discrete prototypes with the starting block show that the oscillators can start operating with a unique source as low as 50 mV. Prototypes with BJTs and MOSFETs were able to provide 173 µW and 560 µW from a supply of 100 mV, respectively, demonstrating that the proposed topology can be a competitive option regarding performance, operating voltage and complexity when compared with those previously presentedDoutoradoEletrônica, Microeletrônica e OptoeletrônicaDoutor em Engenharia Elétric