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MODELLING AND CONTROL OF INDUCTIVE POWER TRANSFER SYSTEM

CHAPTER ONE

INTRODUCTION

1.1 Background of the Study

Inductive power transmission has become a more and more popular method to deliver power to mobile electronic devices and small appliances with a power consumption of up to 100W [1]. In its large meaning, inductive power transfer can take the form of electricity, light, heat or even sound and wind. The inductive power transfer involves a starting point (source) and an ending point (receiver) without intermediate material that would be intentionally added to achieve some purpose. In the scope of this thesis, the starting and ending points are both characterized by electric energy, with in between no additional wires or material.

Light energy transfer can be obtained by photons travel from a source such as the sun or a laser, to a receiver that is generally a photovoltaic cell. The advantage of the laser as a source is that it provides a very directional illumination thanks to its high degree of temporal and spatial coherence. However, the efficiency of light energy transfer is very low, because the losses at each step of the transfer processes are important, namely in the source, in the air and in the receiver. Furthermore, the economical aspects are not attractive [45]. For these reasons, light energy is interesting only for applications that require a transfer over a very large distance, for example in the space.

The microwaves can also transfer energy over a relatively large distance that can be greater than the emitter and receiver sizes. They are electromagnetic waves ranging from 1 GHz to 300 GHz and are used for far-field energy transfer [83]. The most common source for domestic applications is the magnetron thanks to its unexpensive cost, but high frequency waves can be also generated by antennas. The receiver is a rectenna which is actually an antenna with a rectifier used to convert microwaves into DC electricity. The main drawbacks of this technology are the health risks and the relatively low efficiency that can be obtained.

The most used way to transfer contactless energy is the magnetic induction whose principle is shown in Fig. 1.1. It uses near-field electromagnetic waves in the range of frequencies between 10 kHz and a few MHz. The characteristic distance of transfer is the same order as the size of the receiver (secondary coil) and the emitter (primary coil). Such inductive power transfer systems are based on the coreless transformers theory. The working principle consists in applying a high frequency current in a primary coil. In the surrounding air, it generates a magnetic field that induces a voltage to a secondary coil if placed in proximity. It is necessary to operate at high frequency because coreless transformers suffer from weak mutual coupling. To enhance the coupling between primary and secondary, both coils must be well aligned and the air gap must be as small as possible. With this description, a distinction can be made between inductive inductive power transfer and RFID systems.

Indeed, the former involves a transfer of power through a relatively short distance as defined above, while the latter is used to transfer information (with power generally lower than 100mW) through a large distance.

Figure 1.1: Principle of Inductive Power Transfer

1.2 Scope of the thesis

1.2.1 Historical context and current state

Nicola Tesla proposed the first theories of inductive power transfer in 1899-1900. He carried out various wireless transmission and reception experiments through air or matter. At his Colorado Springs laboratory, he experimented for example a remote supply of 200 light bulbs through the ground from a distance of about 40 km [14].

The first reported researches on the so-called energy transport by inductive coupling date from the 1960s [71]. Although the principle of inductive inductive power transfer was known for a long time, this technology has remained immature for a long time as the first industrial applications appear in the 1990s with the electric toothbrushes. Even nowadays the number of electric devices supplied by inductive power transfer is relatively low. This absence is probably due to the lack of standards and regulations for this technology, and also to the uncertainty from the average population concerning the inherent health dangers.

In 1998, a scientific committee has published general guidelines [25] to avoid any kind of health risks concerning the exposure of the population to electromagnetic fields. Basically these recommendations provide the restrictions for the public exposure as a function of the operating frequency, the immersed body proportion and the size of the coils. However, in practice, they are not relevant according to [10]. Therein, a study on the maximum power transfer, based on these restrictions, shows that common applications including coils size of 40mmto 100mmcould transfer no more than about 30mW, which is roughly two orders of magnitude lower than existing products on the market.

1.2.2 Technical and industrial background

More and more electric applications require an energy transfer without wires and contacts.

Especially in the domain of desktop applications such as computer peripherals, wireless technologies (Bluetooth, ZigBee, RF, IR,WiFi) that allow transfer of information are very trendy, while the supplying process uses massively wires or batteries. In a desktop environment, removing the cables between the power source and electronic devices would be convenient. It would allow to gain a certain amount of place and to clean the surface from wires pollution.

In this context, the contactless energy transfer by inductive coupling meets more and more success. Some products are already available for a few applications, such as mice or mobile phones. However, a full platform enabling to supply simultaneously several consumers is still not to be reported.

In the framework of collaboration between Logitech SA and the Laboratory of Integrated Actuators (LAI), inductive power transfer systems are studied for notebook and desktop applications. This collaboration is divided into two different practical applications:

1. The inductive notebook charger is aiming to realize the prototype of a inductive power transfer system from a platform to a static notebook. The said platform is a product already available on the market under the commercial name Alto (Fig. 1.2);

2. The Inductive Power Transfer table is aiming to realize a prototype of a inductive power transfer system embedded in a table in order to supply multiple desktop peripherals.

Logitech [9] is a Swiss company developing and marketing products such as peripheral devices for PCs, including mice, keyboards, loudspeakers, microphones and webcams. They are responsible in the above-mentioned projects for providing the equipment and sharing their know-how on hardware and manufacturing of electronic peripherals.

Figure 1.2: Pictures of the Alto platform. (a) Without notebook [1]. (b) With notebook [2].

1.2 Statement of the Problem

Inductive power transfer systems can be classified into two categories. The first one concerns fixed position systems wherein the devices to be supplied are static. The second one concerns the free position systems involving devices that can be freely moved on the charging surface. With such a definition, it is obvious that the inductive notebook charger belongs to the first one and the Inductive power transfer table to the second one.

1.3.1 Fixed position systems

The fixed position system is the simplest inductive Inductive power transfer method. Nowadays, it involves almost all the existing industrial applications. It usually charges one load and the energy is transferred from a single primary coil to a single secondary coil. Furthermore, both coils have to be approximatively the same size and well aligned to ensure a good mutual coupling, a sufficient amount of transferred energy and a good efficiency.

Induction cookers

Fixed Inductive power transfer is traditionally the method used in induction cookers, except that the secondary coil is replaced by the cooking vessel made of ferromagnetic and conductive metal. The initial researches and patents date from the early 1900s, but the first production of induction cookers was performed in the 1970s by the Westinghouse Electric Corporation[15].

The principle of the induction heating is shown in Fig. 1.3. The magnetic field generated by the primary coil creates Eddy currents in the pot that cause the heating Joule effect. The wires of the primary coils generally exhibit a flat and spread geometry in order to enhance the distribution of the magnetic field that reaches the cooking vessel. The transferred power is in the order of 1 to 2 kW and the operating frequency is situated in a range from 20 kHz to 50 kHz.

Figure 1.3: Principle of an induction cooker

Electric toothbrushes

Inductive power transfer is interesting for applications that require no exposed electrical contacts, such as devices that are used in a moist environment or even immersed in water. Since the early 1990s, rechargeable toothbrushes (for example from the Oral-B brand [7]) use this technology that allows to enclose and therefore fully insulate the wires. It gives the advantage to protect the user against electric shocks due to apparent contacts and to prevent short-circuits that could damage electronics. The system generally includes a ferromagnetic core that increases the coupling between the coils. The operating frequency is around 10 kHz or more, and the transferred power is between 10 and 15W. Similar inductive power transfer systems are also integrated into electric shavers.

Desktop peripherals and mobile phones

The first inductive power transfer studies dedicated to mobile phones were realized in the early 2000s. For example, the prototype of a small platform allowing recharging a mobile phone battery is proposed in [33]. A picture of the prototype is given in Fig. 1.4(a). The coreless transformer is made of printed circuit board (PCB) coils that have to be precisely aligned to start the charging process.

The operating frequency is ranged between 920 and 980 kHz, and the power transferred to the battery is 3.3W, but the transformer has been tested to transfer up to 24W.

More recently, many desktop applications have been marketed. An example of existing inductive power transfer application is the battery-free optical mouse from A4 Tech [8] (c.f.Fig. 1.4(b)). Instead of batteries, the mouse uses inductive CET to provide energy via the included mouse pad, which is connected to a computer USB port. Mice are very low powered devices, generally the amount of transferred power is less than 1W. In a similar way, the HP Touchstone is a small Dock station powered by the USB port and can transfer the power (5W) to recharge a phone or a Palm device [6].

Concerning systems involving inductive power transfer to multiple devices, many products are commercially available. They remain in this category of fixed position charging because they do not offer the possibility to supply the devices freely placed on the whole surface, but only at predefined places. For example, the first inductive power transfer table developed by Fulton Innovation under the denomination of eCoupledallows transferring power to multiple but fixed devices [5]. This application has the ability to communicate with the devices thanks to a process specifically developed by Fulton, which allows transferring the exact amount of power required by each load on the platform.

The common points to these applications are the low power devices that they can supply (generally less than 5W), the predetermined position of the devices on the platform and the integrated intelligence that detects and recognizes the devices.

Figure 1.4: (a) One of the first prototypes involving an inductive power transfer system to charge a mobile phone [34]. (b) Inductively charged mouse from A4 Tech [8]. (c) Portable Powermat station that allows to charge up to three devices simultaneously.

Electric vehicles

A new niche market that may explode in the future for fixed positioning inductive power transfer systems is the electric vehicles charging. Many researches are ongoing in this domain [49], prototypes are being tested by Siemens or BMW [4], and some applications are on the verge to be commercially available [19]. For instance, the typical specifications for a prototype consist of transferring a power of 30 kW to the vehicle battery. The operating frequency is 20 kHz. The main issue here comes from the large airgap of 45mmwhichmakes the coupling low.

1.4 Thesis structure

The thesis presents the design and modeling of inductive power transfer system.

It is divided into six main chapters in addition to the introduction and conclusion. In the present chapter is given a general introduction on inductive power transfer system with a state of the art of the field and the main objectives of the thesis research.

Chapter 2 is dedicated to the modeling of the coreless transformers. First the “magnetic” part of the modeling allows to calculate resistances and inductances of the coils based on the geometry of the coreless transformers. Then the “electric” part allows to determine power magnitudes, current and voltage intensities, based on the resolution of an equivalent electric circuit of coreless transformers. Concepts of resonance and reactive power compensation are introduced then.

Chapter 3 deals with the high frequency effects in the coils. After defining the problem and providing the main inherent hypotheses to resolve it, two methods to compute the AC resistances that vary with the frequency are provided. The first one is based on the resolution Of Maxwell’s equations in a particular case, and the second one is derived from finite-element method (FEM) simulations. The issue of losses in the coils is then addressed and the impact of the high frequency is discussed.

Chapter 4 is probably the most important one because it provides innovative tools to design and optimize different inductive power transfer systems. In the first part, a sensitivity analyzis of the main parameters of coreless transformers is presented. This allows to identify the ones that need to be optimized, as well as their variation range. In the second part, the optimization method itself is described. The main concepts of genetic algorithms and, in particular, multi-objective genetic algorithms are introduced. The implementation of a new algorithm based on a very common one (called NSGA-II) is then presented. It integrates notably several improvements that make it highly efficient. It is then tested with some often-used functions for multi-objective algorithms evaluation, and successfully applied to different inductive power transfer system problems.

Chapter 5 presents the different prototypes built during this thesis work. The design the electronics are discussed in details. Notably, for the inductive power transfer system table, the strategy used to control the detection and the local activation of the table is thoroughly presented.

Chapter 6 ends with a general overview of the results obtained in this thesis. The perspectives and main contributions are also analyzed.