Human Exposure to Close-Range Resonant Wireless Power Transfer Systems as a Function of Design Parameters

In this study, human exposure to close-range wireless power transfer (WPT) systems operating in the frequency range 0.1-10 MHz with coil diameters up to 150 mm are investigated. Approximation formulae, which include scaling factors derived from numerical simulations that take variations of complex human anatomies into consideration, are proposed to conservatively estimate human exposure with respect to the most authoritative exposure guidelines. The approximation has been verified for two precommercial prototype WPT systems, the first of which, a 5-W system operating at 100 kHz, has been evaluated in this study; the second system been verified was reported in a separate study and operates at 6.78 MHz with a nominal current of 5.4 A rms. Based on the results obtained, the optimal operational frequency range for WPT with respect to compliance with exposure safety guidelines is revealed to be ca. 1-2.5 MHz. In summary, this study provides novel and insightful information for the design of an exposure-compliant close-range magnetic resonant WPT system.

Robust Smith Predictor Design for Time-Delay Systems with H infinity Performance

Vinicius De Oliveira, Alireza Karimi

A new method for robust fixed-order H-infinity controller design for uncertain time-delay systems is presented. It is shown that the H-infinity robust performance condition can be represented by a set of convex constraints with respect to the parameters of a linearly parameterized primary controller in the Smith predictor structure. Systems with uncertain dead-time and with multimodel and frequency-domain uncertainty can be considered straightforwardly in the proposed approach. Furthermore, it is shown that the design method can also be extended to design of robust gain-scheduled dead-time compensators. The performance of the method is illustrated by simulation examples.

2013

Analysis of Spatially Coupled Systems using the Potential Functional with Applications to Coding Theory

Rafah El-Khatib

For the past 70 years or so, coding theorists have been aiming at designing transmission schemes with efficient encoding and decoding algorithms that achieve the capacity of various noisy channels. It was not until the '90s that graph-based codes, such as low-density parity-check (LDPC) codes, and their associated low-complexity iterative decoding algorithms were discovered and studied in depth. Although these schemes are efficient, they are not, in general, capacity-achieving. More specifically, these codes perform well up to some algorithmic threshold on the channel parameter, which is lower than the optimal threshold. The gap between the algorithmic and optimal thresholds was finally closed by spatial coupling. In the context of coding, the belief propagation algorithm on spatially coupled codes yields capacity-achieving low-complexity transmission schemes. The reason behind the optimal performance of spatially coupled codes is

seeding'' perfect information on the replicas at the boundaries of the coupling chain. This extra information makes decoding easier near the boundaries, and this effect is then propagated into the coupling chain upon iterations of the decoding algorithm. Spatial coupling was also applied to various other problems that are governed by low-complexity message-passing algorithms, such as random constraint satisfaction problems, compressive sensing, and statistical physics. Each system has an associated algorithmic threshold and an optimal threshold. As with coding, once the underlying graphs are spatially coupled, the algorithms for these systems exhibit optimal performance. In this thesis, we analyze the performance of iterative low-complexity message-passing algorithms on general spatially coupled systems, and we specialize our results in coding theory applications. To do this, we express the evolution of the state of the system (along iterations of the algorithm) in a variational form, in terms of the so-called potential functional, in the continuum limit approximation. This thesis consists of two parts. In the first part, we consider the dynamic phase of the message-passing algorithm, in which iterations of the algorithm modify the state of the spatially coupled system. Assuming that the boundaries of the coupled chain are appropriately

seeded'', we find a closed-form analytical formula for the velocity with which the extra information propagates into the chain. We apply this result to coupled irregular LDPC code-ensembles with transmission over general BMS channels and to coupled general scalar systems. We perform numerical simulations for several applications and show that our formula gives values that match the empirical, observed velocity. This confirms that the continuum limit is an approximation well-suited to the derivation of the formula. In the second part of this thesis, we consider the static phase of the message-passing algorithm, when it can no longer modify the state of the system. We introduce a novel proof technique that employs displacement convexity, a mathematical tool from optimal transport, to prove that the potential functional is strictly displacement convex under an alternative structure in the space of probability measures. We hence establish the uniqueness of the state to which the spatially coupled system converges, and we characterize it. We apply this result to the (l,r)-regular Gallager ensemble with transmission over the BEC and to coupled general scalar systems.

EPFL2016

An Energy Efficient Power Converter for Zero Power Wearable Devices

Milad Ataei Ashtiani

Early diagnosis of Alzheimer's and epilepsy requires monitoring a subject's development of symptoms through electroencephalography (EEG) signals over long periods. Wearable devices enable convenient monitoring of biosignals, unlike complex and costly hospital equipment. The key to achieving a fit and forgettable wearable device is to increase its operating cycle and decrease its size and weight. Instead of batteries, which limit the life cycle of electronic devices and set their form factor, body heat and environmental light can power wearable devices through energy-scavenging technologies. The harvester transducers should be tailored according to on the application and the sensor placement. This leaves a wide variety of transducers with an extensive range of impedances and voltages. To realize an autonomous wearable device, the power converter energy harvester, has to be very efficient and maintain its efficiency despite potential transducer replacement or variations in environmental conditions. This thesis presents a detailed design of an efficient integrated power converter for use in an autonomous wearable device. The design is based on the examination of both power losses and power transfer in the power converter. The efficiency bound of the converter is derived from the specifications of its transducer. The tuning ranges for the reconfigurable parameters are extracted to keep the converter efficient with variations in the transducer specifications. With the efficient design and the manual tuning of the reconfigurable parameters, the converter can work optimally with different types of transducers, and keeps its efficiency in the conversion of low voltages from the harvesters. Measurements of the designed converter demonstrate an efficiency of higher than 50% and 70% with two different transducers having an open-circuit voltage as low as 20 mV and 100 mV, respectively. The power converter should be able to reconfigure itself without manual tunings to keep its efficiency despite changes in the harvesters' specifications. The second portion of this dissertation addresses this issue with a proposed design methodology to implement a control section. The control section adjusts the converter's reconfigurable parameters by examining the power transfer and loss and through concurrent closed loops. The concurrent loops working together raise a serious concern regarding stability. The system is designed and analyzed in the time domain with the state-space averaging (SSA) model to address the stability issue. The ultra-low-power control section obtained from the SSA model estimates the power and loss with a reasonable accuracy, and adjusts the timings in a stable manner. The entire control section consumes only 30 nW dynamic power at 10 kHz. The control section tunes the converter's speed or its working frequency depending on the available power. The frequency clocks the entire architecture, which is designed asynchronously; therefore, the power consumption of the system depends on the power available from the transducer. The system is implemented using 0.18 µm CMOS technology. For an input as low as 7 mV, the converter is not only functional but also has an efficiency of more than 40%. The efficiency can reach 70% with an input voltage of 50 mV. The system operates in a range of just a few of millivolts to half a volt with ample efficiencies. It can work at an optimal point with different transducers and environmental conditions.

EPFL2018