DEGREE PROJECT IN ELECTRICAL ENGINEERING AND COMPUTER SCIENCE, FIRST LEVEL STOCKHOLM, SWEDEN 2019
Energy harvesting from ambient WiFi energy
A method of harvesting and measuring ambient WiFi energy
ALPHA FOFANA CARL MOSSBERG
KTH ROYAL INSTITUTE OF TECHNOLOGY ELECTRICAL ENGINEERI NG AND COMPUTER SCIE NCE
3 | Introduction
Energy harvesting from ambient WiFi energy
A method of harvesting and measuring ambient energy
Alpha Fofana
Carl Mossberg
2019-07-31
Bachelor’s Thesis Examiner Carl-Mikael Zetterling
Academic adviser Bengt Molin
Industrial adviser Sebastian Kullengren, Cybercom
KTH Royal Institute of Technology School of Electrical Engineering and Computer Science (EECS) Department of Electronics SE-100 44 Stockholm, Sweden
Abstract | i
Abstract
The aim of this thesis was to investigate the question of how to harvest RF energy and if we can harvest enough RF energy for it to be useful in an application. It is aimed towards sensor node applications, commonly used in a typical office environment. The WiFi band was chosen since it is omnipresent in the same environment. With the current development within wireless technology and the IoT domain the demand for low power electronic applications has increased and one of the challenges is to find efficient and sustainable ways of powering these types of devices.
The best possible theoretical power content was initially calculated followed by measurements in an office. A circuit was designed containing an impedance matching network and rectifier. A measurement application was constructed using a microcontroller. Measurements were made in an office environment and the maximum harvested energy over 24 hours was 350 mJ. The energy was stored in a supercapacitor and is estimated to be enough to power a low energy sensor for about 30 seconds.
A large part of the thesis is devoted to impedance matching involving calculating, simulating and experimenting to get a good result.
Keywords Energy harvesting, Rectenna, Impedance matching, Embedded system, IoT, WiFi, Sustainable Energy
Sammanfattning | iii
Sammanfattning
Med den nuvarande utvecklingen inom trådlös teknik och IoT-domänen har efterfrågan på elektroniska applikationer med låg effekt ökat och en av utmaningarna är att hitta effektiva och hållbara sätt att driva dessa typer av enheter. Syftet med detta projekt var att undersöka frågan hur vi skördar radiovågsenergi och kan vi skörda tillräckligt mycket med energi för att den ska vara användbar i en applikation. I ett typiskt kontor finns fler källor till radiovågor, däribland WiFi som antas ha en hög nyttjandegrad. Projektet valde att inrikta sig på WiFi bandet och undersöka om det går att utvinna tillräckligt med energi där.
Projektet strävade efter att leverera en färdig produkt med alla ingående delar, en antenn, en likriktare, en lagringsenhet och ett matchningsnätverk för att anpassa antenn och likriktare till varandra. För att undersöka hur mycket energi som finns att skörda gjordes först beräkningar och sedan mätningar i bland annat ett typiskt kontor. Det konstaterades att det rör sig om väldigt låga nivåer och betonas att de apparater som använder WiFi klarar av att känna av signaler som är långt mycket lägre än de som krävs för att kunna utvinna energi. Detta innebär alltså att apparaterna kan kommunicera felfritt samtidigt som energiinnehållet är så lågt att det inte går att utvinna någon energi.
Projektet ägnar stor del åt att optimera den impedansmatchning som måste ske mellan antenn och likriktare för att största möjliga effektutbyte ska kunna ske. Basen är ett kretskort med ett typiskt impedansnätverk och genom beräkningar, simuleringar och experiment tas en prototyp fram. För att kunna analysera resultaten används en mikrokontroller som tar de analoga värdena, omvandlar dem till digitala och skickar dem till en PC för analys.
Mätningar gjordes i en kontorsmiljö och den maximala mängden energi som gick att utvinna var 350 mJ på 24 timmar. Energin lagrades i en superkondensator och bedöms vara tillräcklig för att driva en lågenergisensor i ca 30 sekunder.
Nyckelord
Utvinna energi, Impedansmatchning, Inbyggda system, IoT, WiFi, Hållbar utveckling
Acknowledgments | v
Acknowledgments
We would like to thank our academic advisor who is also our program’s director Bengt Molin for his endless patience and support. Bengt Molin has taught us much of what we know within this domain and was wise enough point out where the difficult parts would be and where our challenges would lie. Thank you for all the help and resources you have provided us with.
We would also like to thank our examiner Carl-Mikael Zetterling for the encouraging tips along the way and Anne Håkansson for letting us use the picture of “The portal of research methods and methodologies”. Thanks to our fellow students Johan Hansen and Simon Chobot for a thorough opposition of our report and helpful feedback.
We would like to thank Cybercom as a company for generously letting us be a part your fantastic team. A special thanks to Sebastian Kullengren, our industrial advisor and Charlene Sequiera who helped us feel at home at Cybercom.
Stockholm, June 2019 Alpha Fofana and Carl Mossberg
Table of contents | vii
Table of contents
Abstract ...... i Keywords ...... i Sammanfattning ...... iii Nyckelord ...... iii Acknowledgments ...... v Table of contents ...... vii List of Figures ...... ix List of Tables ...... xi List of acronyms and abbreviations ...... xiii 1 Introduction ...... 1 1.1 Background ...... 1 1.1.1 Company background ...... 1 1.2 Problem ...... 1 1.3 Purpose ...... 1 1.4 Goals ...... 1 1.4.1 Deliverables ...... 2 1.4.2 Results ...... 2 1.4.3 Social benefits, Ethics and Sustainability ...... 2 1.5 Research Methodology ...... 2 1.5.1 Main category of research ...... 3 1.5.2 Philosophical assumptions ...... 3 1.5.3 Research Methods ...... 4 1.5.4 Research approach ...... 4 1.5.5 Methodologies ...... 4 1.5.6 Data Collection Methods ...... 4 1.5.7 Data Analysis Methods ...... 4 1.5.8 Quality Assurance ...... 4 1.6 Delimitations ...... 5 1.7 Structure of the thesis/ Disposition ...... 5 2 Theoretical Background ...... 7 2.1 Ambient Radio Wave Environment ...... 7 2.1.1 Overall Spectrum usage in Sweden ...... 7 2.1.2 Radio Waves in office Environment ...... 8 2.2 Harvesting Energy from Radio Waves ...... 8 2.2.1 Fundamental antenna theory ...... 9 2.2.2 Reflection and Impedance theory ...... 9 2.2.3 Schottky diodes equivalent circuit model analysis ...... 12 2.2.4 Transient operation analysis ...... 13 2.2.5 Circuit simulations ...... 15 2.2.6 Rectifier theory ...... 16 2.2.7 Energy storage ...... 17 2.3 Measurement application theory ...... 17 2.4 Full prototype system design ...... 17
viii | Table of contents
2.4.1 Implementation of hardware ...... 18 2.4.2 Implementation of software ...... 18 2.5 Related work ...... 18 3 Methodologies and Methods ...... 19 3.1 Data Collection ...... 19 3.2 Planned Measurements ...... 19 3.2.1 Test environment ...... 19 3.2.2 Hardware/Software to be used ...... 21 3.3 Assessing reliability and validity of the data collected ...... 22 3.3.1 Reliability ...... 22 3.3.2 Validity ...... 22 3.4 Planned Data Analysis ...... 22 3.5 Evaluation framework ...... 22 3.6 Circuit development ...... 23 3.7 Development and project method ...... 23 4 Design Procedure ...... 25 4.1 Ambient Radio Wave Environment ...... 25 4.1.1 Theoretical calculations ...... 25 4.1.2 Measurements ...... 26 4.2 Energy harvester ...... 28 4.2.1 Antenna ...... 28 4.2.2 Rectifying network ...... 28 4.2.3 Impedance matching network ...... 28 4.3 The measuring application ...... 35 5 Results and Analysis ...... 37 5.1 Major results ...... 37 5.2 Reliability Analysis...... 38 5.3 Validity Analysis ...... 38 6 Conclusions and Future work ...... 39 6.1 Conclusions ...... 39 6.2 Limitations ...... 39 6.3 Future work ...... 39 6.4 Reflections ...... 40 References ...... 41 Appendix A: Detailed results, graphs ...... 45
List of Figures | ix
List of Figures
Figure 1 Portal of Research Methods and Methodologies [2] ...... 3 Figure 2 Spectrum allocation in Sweden [5] ...... 7 Figure 3 Typical energy harvesting circuit layout ...... 8 Figure 4 Quarter wave and half wave dipole ...... 9 Figure 5 Circuit to demonstrate impedance matching ...... 10 Figure 6 Simple circuit to explore power transfer ...... 11 Figure 7 Efficiency and maximum power ...... 12 Figure 8 The equivalent circuit for a Schottky diode ...... 12 Figure 9 Cycle operation of the rectifier ...... 14 Figure 10 Half-wave rectifier ...... 16 Figure 11 Full-wave rectifier ...... 16 Figure 12 Voltage doubler and cascade voltage doubler ...... 17 Figure 13 Diagram with types of ambient energy sources ...... 18 Figure 14 Room Ka-305 at KTH, Kista ...... 20 Figure 15 Office space at Cybercom, Kista, AB9 ...... 21 Figure 16 Illustration of free space model ...... 25 Figure 17 Measurements using Spectrum Analyzer at Cybercom AB9 office ...... 27 Figure 18 Measurements using Spectrum Analyzer at KTH Campus Kista ...... 27 Figure 19 Di-pole and meander antenna...... 28 Figure 20 Theoretical values of the input impedance as a function of the output voltage ...... 29 Figure 21 Optimization in ADS finding parameters for matching circuit ...... 30 Figure 22 Optimization cockpit in runtime ...... 31 Figure 23 Matching parameters results from simulation (left) and measured matching parameters (right) ...... 31 Figure 24 Antenna impedance ...... 32 Figure 25 Impedance of the rectifying network ...... 32 Figure 26 Impedance matching using Smith program ...... 33 Figure 27 Impedance matching and reflection ...... 33 Figure 28 Reflection comparison matching circuit (Green) And Antenna (Yellow) .... 34 Figure 29 Impedance matching using capacitors ...... 34 Figure 30 Result, both circuits, for detail, see Appendix A ...... 37
List of Tables | xi
List of Tables
Table 1 SPICE values for schottky diode 1SS351 ...... 29
List of acronyms and abbreviations | xiii
List of acronyms and abbreviations
AC Alternating current ADC Analog to Digital Converter DC Direct current HF High Frequency IDE Integrated Development Environment IoT Internet of Things RF Radio Frequency SMC Surface Mounted Component Supercap Super Capacitor PCB Printed Circuit Board SPICE Simulation Program with Integrated Circuits Emphasis
Introduction | 1
1 Introduction
The use of wireless technology and wireless communication has been increasing extensively and we keep finding new uses and applications for this technology. With the rapid growth of wireless technology, energy in the radio frequency spectrum is increasing. The ability to harvest and utilize this energy would prove very beneficial.
This chapter covers the background of the project with a brief introduction to energy harvesting and the background to how this is done utilizing power from the radio frequency spectrum. The purpose of the report and its objectives are presented together with the projects problem statement, delimitations and the methodology for the workflow of the thesis.
1.1 Background
With the current development within wireless technology and the IoT domain the demand for low- power electronic applications has increased and one of the challenges is to find efficient and sustainable ways of powering these types of devices. The development and harvesting of energy sources such as solar energy, kinetic energy, wind energy, thermal energy and other energy sources is a good alternative for these low-power devices, however they also have some drawbacks depending on application and environment.
These devices are also evolving, they are becoming smaller and require less energy making it feasible to power them using harvested energy from other energy sources. A possible energy source is energy from ambient radio waves. Radio waves can be harvested and converted into current, the harvested energy can then be used to either charge a battery or drive a sensor or other types of low- powered circuits directly [1].
1.1.1 Company background
The thesis project was carried out in collaborations with Cybercom, an IT consulting company with experience within IT and communication technology. Cybercom emphasizes the importance of technology and innovation to develop companies, cities and communities in a sustainable way. As a part of this they participate in the context of the UN Global Compact Private Sector Forum and integrate this in their work to contribute to the UN global goals for sustainable development.
1.2 Problem
How do we harvest RF energy, and can we harvest enough RF energy for it to be useful in an application?
1.3 Purpose
The project aims to provide a method for harvesting energy and showing its feasibility. The purpose of the project is to develop a testbed for harvesting ambient energy from the WiFi band. The testbed should be used to evaluate how radio wave energy could be harvested.
1.4 Goals
The goal of this project is to harvest energy from the WiFi band. This has been divided into the following sub-goals.
1. Measure the ambient energy and calculate theoretical power that could be harvested. 2. Create a circuit to harvest energy from the bandwidth of interest. 3. Construct an application to measure the received power.
2 | Introduction
1.4.1 Deliverables
In the planning stage it was decided that the following two deliverables should be produced from the project: • A working prototype for harvesting energy and an application for delivering measurements of the harvested power levels. • A Bachelor thesis which documents the process and describes the theory used to achieve the project’s goals.
1.4.2 Results
The project delivered a working prototype for harvesting energy and an application for delivering measurements of the harvested power levels. The application harvested 350 mJ of energy over a period of 24 hours in a typical office environment.
1.4.3 Social benefits, Ethics and Sustainability
Energy harvesting technologies present valuable social benefits as they could provide alternative energy sources. The technology could potentially prolong the life time of battery-powered applications or replace them with autonomous energy harvesting wireless applications. As battery- powered devices would not need to be replaced, autonomous devices with an energy harvesting power source would require less maintenance and lower costs. They would also be more environmentally friendly as batteries contain chemicals and metals that are harmful to the environment. Having this type of sustainable energy source becomes even more important in remote or hostile environments where it is complex and/or costly with the use of batteries.
1.5 Research Methodology
The purpose of this chapter is to provide an overview of the research method used in this thesis. In order to sort out and understand our choices regarding methodologies and methods this chapter is partly based on [2]. Following the division found there as sub-chapters, the first presents our chosen methodologies and methods and why these are choosen.
As mentioned above, in [2] the quest to find suitable methodologies and methods for this thesis is illustrated in Figure 1 where you need to decide what way to go in each category.
3 | Introduction
Figure 1 Portal of Research Methods and Methodologies [2] The project’s aim is to answer the research question mentioned in Chapter 1.2. In doing this it also deals with development of a circuit to harvest the energy. The project is in a way both a research project and a development project. It is the research project’s method that is described in the following sub-chapters. How the project measures the reality and evaluates the effectiveness of the developed circuit and the method used for development and method for organizing the project is presented in Methodologies and Methods
1.5.1 Main category of research
The two main categories of research are qualitative and quantitative. Although it is often thought that it is either or, it should rather be viewed as opposite ends of a scale [3]. This research is qualitative, it studies a phenomenon and investigates it in order to develop a kind of solution [2]. The project could be called quantitative since it is all about numbers and the measurements are from the real world with no interpretation involved. The project does not involve large data sets but rather use small portions of measurements to understand the reality. The project iterates towards understanding the problem and work out a solution to answer the research question.
1.5.2 Philosophical assumptions
A project’s philosophical assumption is how the project and/or participants views the world. It is the foundation upon which you view the world you want to study, what methods to use to measure it and what needs to be considered when evaluating the result. There are four major paradigms; positivism, realism, interpretivism and criticalism.
The project studies physical data which is not affected by the one studying it (interpretivism) nor is the data affected by historical or social constructs (criticalism). Positivism or more towards realism is more suited for this project. It views the world as independent of the observer but do not set out to prove a theory, but to help understand the reality in order to develop a solution.
4 | Introduction
1.5.3 Research Methods
A research method is the theoretical framework in which the research is carried out. In Chapter 3 the actual strategy or design is presented. The project uses the applied research method as it aims towards solving a practical problem using existing research and collects and evaluates data in order to develop a practical application.
1.5.4 Research approach
The approach to the research can be either deductive or inductive, sometimes complemented by abductive as an approach between the two. A deductive approach would be to test a theory’s implication on relevant data and find if the theory is supported or not. Reversely, an inductive approach would be to collect data about the topic of interest, take a step back and analyze it in order to develop a theory. The research in this project does not conform to any of these main ideas as it does not strive to test a theory, nor does it strive to develop a new one.
1.5.5 Methodologies
The research methodologies are the research strategies or designs used for carrying out the research. Since the project deals with both research and development, this methodology is in respect to the research part. It can be compared to the process of designing a system where the systems performance needs to be evaluated. Then this is about the evaluation, how to research the systems performance. The project uses an exploratory research method in the way that it varies several variables in order to obtain the best result. On the other hand, it could be called pure experimental since the project control most of the variables in the current reality. That would be too far out on the quantitative side of the research portal and the project does not deal with large amounts of data in this aspect.
1.5.6 Data Collection Methods
The project collects data by recording several variables but for a limited amount time and space, thus resulting in smaller datasets than normally associated with experimental data collection methods. The project collects data by means of a case study, where you study more in depth and in a smaller environment.
1.5.7 Data Analysis Methods
The project intends to measure, evaluate and change according to the result of the evaluation. This is done in an iterative manner. This is called analytical induction and is mostly used in social sciences but fits our purpose [4]. In an engineering project, a statistical data analysis would maybe fit better but that would require actual data to analyze which this project lacks.
1.5.8 Quality Assurance
The quantitative research, with a deductive approach, must apply and discuss validity, reliability, replicability and ethics, see Chapter 3.3.
5 | Introduction
1.6 Delimitations
This bachelor thesis project was conducted by two bachelor students in electronics and computer engineering at The Royal Institute of Technology (KTH) in Stockholm, Sweden. The project was completed within a time frame of 10 weeks with 40 hours of work per week. Due to the short time frame the thesis focuses on creating a physical implementation of an energy harvesting circuit, using existing components and technologies where possible.
Similar work has been done where they look in to realizing and optimizing specific parts of an energy harvesting circuit presented in the related works section. This thesis aims to deliver an end- to-end energy harvesting application to measure how much ambient energy that can be harvested.
1.7 Structure of the thesis/ Disposition
This thesis is divided into several chapters where each chapter covers steps towards answering the research question. In Chapter 2 the theory behind antennas, impedance matching and rectifiers as well as how to measure the result is explained. The methods used during the research and development of the project is covered in Chapter 3. Chapter 4 explains what was done and how, including measurements of the ambient radio frequency power content and impedance matching techniques. The thesis continues with the result and analysis in Chapter 5 and conclusions and future work in Chapter 6.
Theoretical Background | 7
2 Theoretical Background
Ambient radio waves are all around us, the source is often unknown, and both direction and frequency vary. An initial approach would be to harvest all energy from all frequencies, but this is not feasible due to its complexity. An antenna is designed based on what frequency it should operate in. It will receive other frequencies, especially multiplies of the frequency it was originally designed for, but not as efficient. In order to optimize our harvesting and get the most out of the antenna, it needs to be matched with the rest of the circuit. The chosen frequency plays a big role in constructing the harvester.
The amount of power in RF signals is very dependent on the distance from the transmitting source and arguably the sensor nodes located in an indoor office environment would have WiFi as their closest source. As stated earlier, WiFi is both 2.4 and 5 GHz but due to limitations in equipment and narrowing the scope for this thesis it will only cover 2.4 GHz. In this chapter the theory will be presented while the result of measurements of ambient RF signals is presented in Chapter 4.1.2.
2.1 Ambient Radio Wave Environment
There are different environments and means for radio waves to be transmitted and propagate through space. In the sections below, the theory behind the environment in which the application will operate is provided.
2.1.1 Overall Spectrum usage in Sweden
In Sweden it is PTS (Post- och telestyrelsen) that is responsible for ensuring that media and communication means work and is available. They are responsible for the regulation of broadcasting, networks and service of public telecommunications. This includes licensing, pricing, definition of basic conditions for the provision of common and international communications facilities, as well as the planning, coordination, distribution and allocation of radio spectrum. In the case of telecommunications networks and services, the agency ensures that they are accessible to everyone in Sweden on a reasonable commercial basis and through the requirements set out in licenses for operators or suppliers of these services. Figure 2 shows how the different frequency bands have been allocated for the different telecom operators [5].
Figure 2 Spectrum allocation in Sweden [5]
8 | Theoretical Background
Apart from the spectrum presented in the figure the frequency range 80-115 MHz is allocated for FM Radio transmissions, 2.401-2.484 GHz and 5.180-5.240 GHz for 802.11 WiFi transmissions. The full documentation of the requirement for use of radio transmissions published by PTS can be found [6].
2.1.2 Radio Waves in office Environment
This thesis will focus on RF transmission in an indoor environment, more specifically the WiFi band operating at 2.4 GHz. In an indoor office environment where computers are used it is expected that the frequencies used for WiFi communication is used frequently, thus radiating a lot of energy and therefore these will be the ones that are focused on in this thesis. The signal strength and propagation loss may differ a lot especially in an indoor environment. The maximum transmission signal strength from a device in Sweden are regulated by law as too high energy emission can disturb other RF bands.
International Telecom Union, ITU, has propagation data and prediction methods that can help in calculating the spread of a WiFi signal indoors [7]. This method is useful when modeling and doing calculations to find a theoretical max value for power available in ambient frequency spectrum.
It is also worth noting that the required signal strength (power) for communications is much lower compared to the required signal strength for harvesting energy. While a communication device typically can be sensitive enough to receive a signal at -70 dBm (0.1 nW), a harvester would need to deliver approximately 1 µW (-30 dBm), depending on intended use [8].
2.2 Harvesting Energy from Radio Waves
An energy harvester is typically composed of four different parts, an antenna, a rectifier, a storage or load component and a matching circuit to optimize power transfer. The different components are illustrated in Figure 3 below. The following sub-chapters will deal with each part of the construction in detail.
Figure 3 Typical energy harvesting circuit layout
9 | Theoretical Background
2.2.1 Fundamental antenna theory
When choosing an antenna or creating your own, there are many factors to take into consideration. This can be a complex task, but this thesis will not cover more than the basic design parameters necessary which are used in this project. The antenna converts a radio frequency signal to an electromagnetic wave. An isotropic antenna radiates power in all directions equally, this is also known as an ideal antenna. Real antennas do not perform equally well in all directions, they have directivity which means that they will transmit more power in certain directions.
The frequency together with the wavelength is the most important factor when designing an antenna. An antenna both receives and transmits with the same properties, it sends and receives with the same frequency.
A wire antenna is the simplest form of an antenna and is usually the shape that people associate with the word antenna. The length of the antenna needs to be half a wavelength in order to work, a half-wave dipole antenna. If you put a conducting plane below and perpendicular to the wire, a quarter wavelength is best, a quarter-wave dipole antenna. See Figure 4.
Figure 4 Quarter wave and half wave dipole
A meander antenna is using electrical properties caused by the meander pattern to make the antenna electrically longer while keeping the electrical size to a minimum [9]. A meander antenna has a small footprint when mounted on a PCB as it is part of the printed copper and will not need any protruding wire.
An antenna has a certain impedance when you look “into it” from the connection side. Since antennas operate at high frequencies, the conductors themselves act as capacitors or inductors as well as pure resistors. This impedance is usually designed to be 50 Ω without any reactive deviation, capacitive or inductive.
2.2.2 Reflection and Impedance theory
The antenna has a certain impedance as described above. In order to harvest as much energy as possible the antennas impedance needs to be matched to the rest of the circuit. The rectifier network will also have an impedance and this needs to be taken into consideration when matching the antenna.
10 | Theoretical Background
The goal is to transfer as much power as possible from the antenna to the rest of the circuit [10]. Figure 5 shows a purely resistive circuit to explain maximum power transfer theorem. The impedance of the circuit to the left is known and the right circuit needs to be matched.
Figure 5 Circuit to demonstrate impedance matching The relation between power and voltage, current and resistance is as follows
2 푉푖푛 푉푖푛 푃 = 푉푖푛 ∗ 퐼푖푛 = 푉푖푛 ∗ = (2. 1) 푅푖푛 푅푖푛
Introducing a voltage divider, based on Figure 5
푅푖푛 푉푖푛 = 푉 ∗ (2. 2) 푅푖푛 + 푅표푢푡
2 2 2 푉 푅푖푛 푉 푃 = ∗ 2 = 2 푅푖푛 (푅푖푛+푅표푢푡) 푅표푢푡 (2. 3) 푅푖푛∗(1+ ) 푅푖푛
Which gives 푃푚푎푥 when the expression 2 푅표푢푡 푅푖푛 ∗ (1 + ) (2. 4) 푅푖푛
is at its minimum. To find the value for 푅푖푛 for this condition,
2 푑 푅표푢푡 [푅푖푛 ∗ (1 + ) ] = 0 (2. 5) 푑푅푖푛 푅푖푛
and 2 2 푑 푅표푢푡 푅표푢푡 2 2 [2푅표푢푡 + 푅푖푛 + ] = 0 → 1 − 2 = 0 → 푅표푢푡 = 푅푖푛 → 푅표푢푡 = ±푅푖푛 (2. 6) 푑푅푖푛 푅푖푛 푅푖푛
11 | Theoretical Background
Since resistance is always positive, this shows that in order to maximize the power transferred the resistance of the load and the source should be equal. An analogue argument holds when there are capacitive and inductive loads but then 푍푖푛 = −푍표푢푡, they should be each other’s conjugate [10]. Even with a perfect impedance matching the maximum amount of power transferred is only half of what the antenna receives. Figure 6 represents a simplified circuit to show the principle of the power transfer. All circuits can usually be represented in this way.
Figure 6 Simple circuit to explore power transfer
The relation between power, voltage and current.
푃푚푎푥 = 푉푠 ∗ 퐼 (2. 7) Using a voltage divider 2 푅퐿 2 ( ∗ 푉푆) 푅퐿 푉퐿 푅퐿 + 푅푆 푃퐿; 푉퐿 = ∗ 푉푆 → 푃퐿 = = (2. 8) 푅퐿 + 푅푆 푅퐿 푅퐿
To measure the efficiency, introduce 푅 1 푒푓푓푖푐푖푒푛푐푦 = 퐿 = (2. 9) 푅 + 푅 푅 2 퐿 푆 1 + ( 푆) 푅퐿
As the ratio of power dissipated by the load 푅퐿, to power developed by the source 푉푠.
In Figure 7 the 푒푓푓푖푐푖푒푛푐푦 and the ratio between 푃퐿and 푃푚푎푥 is plotted to show that the maximum power transfer occurs when the ratio between 푅퐿 and 푅푆 is equal to 1. They have the same value and that this is not the most efficient when considering power into 푅퐿. What needs to be taken into consideration is that when there is maximum efficiency the power has decreased and the aim is to maximize the power.
12 | Theoretical Background
Figure 7 Efficiency and maximum power
2.2.3 Schottky diodes equivalent circuit model analysis
The main component in the rectifying circuit is the Schottky diodes which are non-linear devices where the input impedance changes as a function of input power and frequency. A Schottky diode analysis method introduced in [11] and verified in [12] and [13] where they use circuit analysis to calculate the input impedance can be used. The method is based on solving Kirchhoff’s relations.
Figure 8 The equivalent circuit for a Schottky diode
13 | Theoretical Background
In Figure 8 the equivalent circuit for a Schottky diode is shown where the numbers represent the diode pins, Cj is the junction capacitance, Cp and Lp the parasitic packaging capacitance and inductance. C23 accounts for the second diode connected between pins 2 and 3. To be able to determine the impedance of the rectifier between pins 1 and 3, the voltage across and the current through the diode d needs to be determined. The electrical behavior of the rectifier is investigated assuming a voltage source with a frequency f0 as input, Vg = |Vg|cos(2πf0t). The generator impedance is denoted by Rg = 50. The behavior of this circuit can be described with the following expressions [11]:
휕퐼 푉 = 퐼 푅 + 퐿 𝑔 + 푉 (2. 10) 𝑔 𝑔 𝑔 푝 휕푡 퐶푝
푉퐶푝 = 푉푑 + 푉푅푠 (2. 11)
( ) 푉푅푆 = 푅푠 (퐼퐶푗 + 퐼푑) 2. 12
휕푉 퐼 = 퐶 푑 (2. 13) 퐶푗 푗 휕푡
훼푉푑 퐼푑 = 퐼푠(푒 − 1) (2. 14)
휕푉 1 휕퐼 푞 푑 𝑔 ( )푉푑 = {휓 ( ) − 푅푠퐼푠 (푒 푛퐾푇 − 1)} (2. 15) 휕푡 푅푠퐶푗 휕푡
휕퐼 휕퐼 Where 휓 ( 푔) = 푉 − 퐼 푅 − 푉 − 퐿 푔. Using these equations and applying the fourth-order 휕푡 𝑔 𝑔 𝑔 푑 푝 휕푡 Runge–Kutta algorithm (RK4) the voltage Vd across the diode d can be calculated and using (2.15) the current Id through the diode is determined [11],[14]. After evaluating Vd and Id, the input impedance of the diode is found using Ohm’s law:
푉푑 푍푑 = (2. 16) 퐼푑
2.2.4 Transient operation analysis
Another way to calculate the input impedance of the rectifier is to perform a transient input impedance analysis as introduced in [15]. For this method they analyze the rectifier from the moment that it is completely discharged and assumes the rectifier as a nonlinear load. Then based on a power method, an analysis is done from the input impedance point of view. The impedance is then computed based on the transient charging time and the input power.
14 | Theoretical Background
Figure 9 Cycle operation of the rectifier
As depicted in Figure 9 the input voltage Vin varies over time and the gray areas represent the charging operations where one of the diodes are forward biased. The rectifying circuit is initially assumed to be discharged and then a voltage with the amplitude of Vm and the angular frequency of ωm is applied to the input (Vin = Vm sin(ωmt)). Each half-cycle one of the diodes are switched on and between turn-on times there is an interval which both diodes are off. During the charging time, the capacitors are charged, and their average voltage increases.
Assuming that the diodes are the same, the amplitude of the input Vm constant and the ripple at the output voltage is neglected, a function for the input impedance can be derived depending on the two operation states that occur in each cycle.
푍푖푛 = 푓(푍푖푛,푂푁, 푍푖푛,푂퐹퐹) (2. 17)
Where 풁푖푛,푂푁 is the average input impedance of the rectifier when only one of the diodes is on and 푍푖푛,푂퐹퐹 is for when both diodes are turned off. Then in each cycle the input impedance of the rectifier significantly changes due to switching of the diodes between on and off states. The separate input impedances for each operational state is calculated as:
푍퐷,푂퐹퐹 1 푍푖푛,푂퐹퐹 = + (2. 18) 2 푗퐶푠휔푚
푍퐷,푂퐹퐹푍퐷,푂푁 1 푍푖푛,푂푁 = + (2. 19) 푍퐷,푂퐹퐹+푍퐷,푂푁 푗퐶푠휔푚
where ZD,OFF and ZD,ON are the average impedances of the diodes in the on and off regions, respectively. As per the appendix in [15] the on/off state of the diodes are calculated as:
푅푗 푧푑 = 푅푠 + (2. 20) 1 + 푗푅푗퐶푗휔
푧푑 푍퐷 = + 푗퐿푑휔 (2. 21) 1 + 푗푧푑퐶푝휔
15 | Theoretical Background
The input impedance of the rectifier is estimated based on a power method. First, the average input complex power of the rectifier is found as follows:
푇 푇 2 1 1 푣푖푛 푆푖푛 = ∫ 푣푖푛푖푖푛푑푡 = ∫ 푑푡 (2. 22) 푇 0 푇 0 푍푖푛
Using (2.20) the final input impedance can be derived as described more in depth in [15] giving the full expression for the input impedance as:
−1
1 1 1 1 1 −1 2푉푡ℎ + 푉표푢푡 −1 2푉푡ℎ + 푉표푢푡 푍푖푛 = [( + ) + ( − ) · (2푠푖푛 ( ) − 푠푖푛 (2푠푖푛 ( )))] (2. 23) 푍푖푛,푂퐹퐹 푍푖푛,푂푁 휋 푍푖푛,푂퐹퐹 푍푖푛,푂푁 2푉푚 2푉푚
2.2.5 Circuit simulations
The modeling and analysis can also be done through simulations to create an approximation of the results that will be obtained with the real PCB implementation. A simulation tool for this is Advanced Design System (ADS) from Keysight. ADS have the same functionality as other SPICE programs like PSPICE, Qucs and LTspice but has an edge when it comes to RF simulations. Like many other commercial SPICE programs, the software comes with a significant number of predefined libraries and components that can be used. There are several different simulations that ADS can perform [16].
Circuit performance and load resistance sweep are simulated by the Harmonic balance ADS module. Harmonic balance is a frequency-domain analysis technique for simulating distortion in nonlinear circuits and systems. It is usually the method of choice for simulating analog RF and microwave circuits. Within the context of high-frequency circuit and system simulation, harmonic balance simulation is ideal and offers several benefits over conventional time-domain transient analysis. Harmonic balance simulation obtains frequency-domain voltages and currents, directly calculating the steady-state spectral content of voltages or currents in the circuit. Many linear models are best represented in the frequency domain at high frequencies [16], [17],[18].
16 | Theoretical Background
2.2.6 Rectifier theory
The function of a rectifier is to convert the RF AC input and convert it into DC to suit our needs. This is usually done with a diode which only allows current through in one direction in combination with capacitor(s) to smoothen the rectified voltage. In its simplest form, only one diode is used and then only half of the sinewave is rectified, the other half is discarded. As seen in Figure 10, the first diagram is the original sinewave, the middle one shows the use of a diode, and the bottom one shows the combination of a diode and a capacitor.
Figure 10 Half-wave rectifier
A half-wave rectifier produces a large alternating component and discards half of the cycle. It is however very simple to implement. In order to use the whole cycle, you need to rectify the other half as well, flipping it to the positive side. This can be done in several ways, one of the commonly used is the Graetz diode bridge rectifier, named after its creator. In Figure 11 a Graetz diode bridge rectifier is implemented. The top diagram shows the original waveform, the middle one shows the rectified waveform and the bottom one shows the use of capacitor in combination with the diode bridge.
Figure 11 Full-wave rectifier
17 | Theoretical Background
The goal of rectifying is to provide a stable voltage capable of supplying electronic equipment. With one or more capacitors the valleys between waves can be bridged. The project includes high frequencies requiring extra care when choosing rectifying diodes. Schottky diodes are most suited for this implementation since they are capable of switching between on and off very fast and additionally have low forward voltage.
An additional advantage of rectifiers is that they, depending on construction, can multiply the voltage they deliver. Figure 12 shows a voltage doubler with two reservoir capacitors which act like two batteries in series and thus doubles the voltage. Figure 12 shows a cascade voltage doubler which can be extended in multiples to deliver the voltage needed.
Figure 12 Voltage doubler and cascade voltage doubler
2.2.7 Energy storage
The harvested energy needs to be stored somewhere. It can be a battery, a capacitor or a supercapacitor. A battery often needs a higher voltage to start receiving charge than a capacitor. A regular capacitor is easily charged but has trouble keeping it due to self-leakage. A supercapacitor can receive small charges, last longer and can keep the charge for a longer period of time. A downside for regular capacitors is that the voltage drops as the charge drops and this affects the amount of power that can be delivered at a specific voltage.
2.3 Measurement application theory
To register measurements from the developed circuit a microcontroller will be used. It needs to have an ADC in order to translate the analog value measured to a digital value that the microcontroller will understand. These ADC values, the voltage over the mounted load, will be calculated to energy. The value of energy will then be sent to a PC for further analysis and storage. There are several ways of sending data from the microcontroller, via cable or via Bluetooth. It will most likely be a cable solution since a RF solution will affect the measurements since it operates the same band as WiFi.
2.4 Full prototype system design
The testbed the project aims to deliver will consist of a working harvesting circuit who’s harvested energy is measured by a microcontroller which allows logging as well as real time overwatch. This will involve circuit design and manufacturing in the beginning, impedance matching and choosing correct values of components and the development of a measurement application to evaluate the result.
18 | Theoretical Background
2.4.1 Implementation of hardware
Because of this project’s limited time, a circuit needs to be ordered early in the project. A circuit needs to be constructed and decisions on hardware and design need to be made, then manufactured and shipped back. The design of a generic impedance network enables a flexible platform for further development.
2.4.2 Implementation of software
The project aims towards delivering a working testbed which will be implemented on a microcontroller. The setup will initially have a basic design and evolve if there is enough time. Two major issues need to be handled, capture data and display data. Depending on how much energy the harvester delivers, the capture will be done in different ways, all involving analogue to digital conversion. The data will initially be sent via cable to a command window on a computer and can then be evolved to be displayed on an LCD screen or streamed via Bluetooth to a mobile device. All coding will be done in C.
2.5 Related work
The last years there has been an increased interest and research effort in energy harvesting technologies due to the increased demand for sustainable and environmentally friendly energy sources. There is a wide variety of ambient energy sources and technologies with different states of technical maturity available. The level of energy that can be delivered from each technology is also an important factor where you can see differences from low power micro-Watt levels to triple digits in the milli-Watt range. The energy harvesting systems can be classified based on the energy sources as shown in Figure 13.
Figure 13 Diagram with types of ambient energy sources
Ambient energy harvesting to power wireless sensors and other applications has been utilized for a long time. Research where some of these energy sources have been utilized are summarized per the references [19] ,[20], [21] and [22]. Despite its low energy density this thesis will focus on RF energy harvesting. As per the feasibility study made in [23] it is concluded that it is possible given an efficient application. Similar energy harvesting system using RF energy has been studied in [17] where they design and construct an antenna and evaluate different rectifying configurations. Similarly, in [1] the focus is to construct a RF energy harvester that is to be compared between two alternative power sources. There has also been studies made on different antenna configurations as presented in [24] and [25], multiple band energy harvesting [26],[27] and the rectifying component of energy harvesting application in [28]. Optimization of harvesting from multiple wireless transmission nodes are presented in [29].
Methodologies and Methods | 19
3 Methodologies and Methods
The purpose of this chapter is to provide an overview of the research method used in this thesis. As described in Chapter 1.5 it is based on [2] to sort out and understand the choices regarding methodologies and methods. In Chapter 1.5 the choices involved in the research process are explained in detail. Below are the choices regarding data collection, testing and development found as well as a discussion regarding validity and reliability.
3.1 Data Collection
The data to be collected is measurements of ambient radio waves energy content using a spectrum analyzer. The measurements will be conducted at KTH, Kista, and at Cybercom, Kista, in order to have access to both laboratory equipment and a typical office environment.
Although the measurements do not contain any personal data or pictures containing people, it does store the use of WiFi at a certain location at a specific time and date. The stored data does not contain any information about what has been sent using WiFi or anything like that, just frequency and the associated value of the power content in decibels with reference to one milliwatt [dBm].
It could be argued that the stored values can reveal something about the use of a location at a given time. In this context it is worth mentioning that the values stored will seldom represent the real use at any given time but the provoked use, induced by uploading and downloading from computers belonging to the project’s participants.
3.2 Planned Measurements
As the project progress, measurements will be repeated using different circuits for energy harvesting. First, measurements were taken in order to establish a baseline together with theoretical calculations. Then measurements will be taken several times over using different antennas and rectifying circuits but under the same circumstances as before in order to evaluate the circuit and answer the research question.
3.2.1 Test environment
Below is the test environment described. This is where the baseline is established, and models tested. The measurement done is a max-hold type which means that the maximum value that is reached during the test period is stored and used.
Methodologies and Methods | 20
3.2.1.1 KTH, Kista
In room Ka-305, the electronics lab, 4 different tests were conducted. Some used for understanding the effect of distance and equipment used and some for the test that are later done with the developed circuits. During tests, one computer is located at point 4, in some cases actively uploading and downloading data using the 2.4 GHz WiFi band in order to “provoke” traffic. See Figure 14.
Figure 14 Room Ka-305 at KTH, Kista
Where
1. Represents a point within centimeters of the access point (AP) 2. Represents a desk in the lab, 6 meters from (AP) 3. Represents a desk in the lab, 7 meters from (AP) 4. Represents a desk in the lab, 4 meters from (AP)
Methodologies and Methods | 21
3.2.1.2 Cybercom, Kista
At Cybercom’s office called AB9 in Kista, tests were conducted in 3 places. During tests, one computer is located at point 1, in some cases actively uploading and downloading data using the 2.4 GHz WiFi band in order to “provoke” traffic. See Figure 15.
Figure 15 Office space at Cybercom, Kista, AB9
Where 1. Represents our desks at the office, 8 meters from (AP) 2. Represents a desk just below the access point 3. Represents a point within centimeters of the access point
3.2.2 Hardware/Software to be used
The spectrum analyzer used is a RIGOL DSA832 which stores measurements and exports it to an USB memory stick [30]. The antenna used for baseline measurements and together with some of the circuits is a dual band antenna from Delock [31]. The network analyzer used to preform tests regarding antenna performance and circuit matching is a Hewlett Packard 8753D.
The report was written in Microsoft Word together with OneDrive, the circuit was made in Dip Trace [32] and the calculations for the impedance matching network was made in the program “Smith” which can be downloaded from [33]. Advanced Design System (ADS) from Keysight was used to simulate our circuit [34]. To log and track the extracted data in real time SerialPlot [35] was used together with Mathematica to capture and analyze data [36]. The hardware used to perform the measurements was a development board and associated software from STMicroelectronics [37], this together with the IDE IAR Workbench [38]. In order to keep track of the software development GitHub was used [39].
22 | Methodologies and Methods
3.3 Assessing reliability and validity of the data collected
It is important to ensure the validity, reliability, replicability and ethics in order to deliver a high- quality result and avoid uncertainties. Chapter 3.1 deals with ethics regarding data collection, the validity, reliability and replicability is described below.
3.3.1 Reliability
The reliability is ensured by applying established methods for measuring the power content in radio waves by using a spectrum analyzer. It records the power content in the radio waves in the environment it is used. The power content is not the same as energy content which means that the measured power content does not give a reliable view of the energy possible to harvest at a certain location. However, the power content measured, and the power content calculated can be compared.
The project focuses on harvesting energy and it needs a circuit to match impedance and rectify as mentioned in the introduction. Using a PCB is beneficial in terms of flexibility but introduce more challenges constructing the impedance network. This needs to be considered if someone wants to reproduce the prototype constructed in this project. The project will be a learning as well as an explorative experience.
3.3.2 Validity
Since this project is of an exploratory nature it is difficult to use the traditional way of defining validity, as a measure of how well the theoretical construct used in the project resembles the world it is describing. By experimenting the project evolves the model and design a testbed. It is understood that there are large discrepancies between the calculated values contra the measured, as well as the measured contra the values one can expect to harvest. By definition, this is not valid, but it is also a way forward in the project where the aim is set to develop a working testbed.
3.4 Planned Data Analysis
The data collected will be in two batches, one with measured power using a spectrum analyzer and the other with measured energy using the developed measuring application. The measured power is spread over a frequency spectrum while the measured energy is spread over time.
The power measurements will be recorded in specified locations under specified conditions, see Chapter 3.2. The data will be collected and then plotted using Mathematica. The power content in the 2.4 GHz band will be shown and it will be possible to compare the plots from different locations and conditions as well as to previous calculations made.
The energy measurements will be recorded in specified locations under specified conditions, see Chapter 3.2. It is the ambition to do these measurements in all the locations and conditions where the power was measured for later comparison. The energy collected over time will be plotted using Mathematica. The energy collected over time will be shown and it will be possible to answer the research question.
3.5 Evaluation framework
There will be a question of whether the amount of energy is enough. This amount differs a lot depending on application, it is up to the reader to decide if it is useful in a specific application. The project aims to deliver an application that can measure the energy level in real time.
In order to evaluate the project, the harvested energy delivered by the circuit developed in the project will be measured. This will be a fundamental way of evaluating the project. The project will develop a harvesting circuit and the delivery of this will also be evaluated.
23 | Methodologies and Methods
3.6 Circuit development
As hardware needs to be designed, produced and delivered the process takes time. The project needs to begin with designing and ordering as soon as possible to have the possibility of further development. This is a risk since this is done in parallel with the pre-study phase and because of this the circuits designed might not be the optimal choice. The knowledge acquired during the pre-study might not emerge in time to be implemented in the design. It is only the basic design of the PCB that is set, the actual components and their values will be possible to change during the development and the knowledge acquired can be used in this aspect.
3.7 Development and project method
To keep track of the project’s development and ensure forward momentum several tools and methods where utilized. In order to drive the project, an agile method was used which included Kanban where a virtual board was used to gather all tasks and check out tasks to work with. Agile workflows are based on iterative development where functionality is added in each iteration until a working system is produced which meets the set requirements. This was used to develop the circuit and the measuring application.
Trello, a Scrum/Kanban inspired project management tool was used to keep track of the project tasks and what to do [40]. The broad plan was documented in a Gantt scheme. Meetings where held with our supervisor from KTH on a weekly basis in the lab, with more formal meetings in the initial and middle stages of the project. Similar kinds of meetings where also held with our supervisor from Cybercom every week to follow-up on the progress, together with team meetings where all thesis workers at Cybercom participated.
Design Procedure | 25
4 Design Procedure
The purpose of the project was to design a circuit that can harvest the ambient WiFi energy and measure and present this. The project was realized in two phases since it was unknown how much energy that could be harvested. First the harvesting circuit was developed and when it was working, a way of measuring the energy was designed and developed.
4.1 Ambient Radio Wave Environment
To get an understanding of what kind of power that could be harvested calculations and measurements of the power level and the ambient radio wave environment was made.
4.1.1 Theoretical calculations
There are several ways of calculating and estimating the theoretical maximum power level. One approach is to calculate the propagation in free space assuming no loss other than the spread. Presuming a spherical shape representing the signal power that originates from the transmitting antenna and letting it grow as the power is propagating through free space. Assume the receiving antenna has a surface area that absorbs part of the sphere, this would represent the total power that can be received. An antenna with a fixed area will receive less power the farther away it is located from the transmitting antenna as the power density decreases [41].
Figure 16 Illustration of free space model
This attenuation is called free space loss and can be express as the ratio of the radiated power Pt and the power received by the antenna Pr. Free space loss for the ideal isotropic antenna is expressed as[41]:
푃 (4휋푑)2 (4휋푓푑)2 푡 = 2 = 2 (4. 1) 푃푟 휆 푐
When solving for Pr in Equation 4.1 the received power can be calculated. To give an intuition and a better understanding of the free space model that is going to be used this will be derived as described before, this is also known as Friis transmission formula. Assuming that the power Pt watts are transmitted in a spherical manner from an isotropic antenna resulting in the power density:
26 | Design Procedure
푆푢푟푓푎푐푒 퐴푟푒푎 표푓 푎 푠푝ℎ푒푟푒 퐴 = 4휋푟2 (4. 2) 푃 푃 푝 = 푡 = 푡 (4. 3) 퐴 4휋푟2
Including the gain from the transmitting antenna Gt and changing the power density to received power by including the receive antennas effective aperture Aer, the following equation is produced:
푃 푃 = 푡 · 퐺 · 퐴 (4. 4) 푟 4휋푟2 푡 푒푟
휆 The effective aperture Aer can be expressed as · G and when substituted in to the equation the final 4휋 result is: 휆 2 푐 2 푃 = ( ) 푃 · 퐺 · 퐺 = ( ) 푃 · 퐺 · 퐺 (4. 5) 푟 4휋푟 푡 푡 푟 4휋푟푓 푡 푡 푟
Where Pt Gt = PEIRP = 100 mW, Gr = 5 dBi, c = speed of light and f = 2.4 GHz.
To get a more accurate model, taking the propagation losses into account, ITUs propagation model can be used. This model takes walls and other obstacles into account using the settings for a typical office building [7].
The propagation loss
푑 퐿푏 = 퐿(푑0) + 푁 ∗ log10 , 퐿(푑0) = 20 ∗ log10(푓) − 28 , 푑0 = 1, 푁 = 30 푓푟표푚 퐼푇푈 푚표푑푒푙 (4. 6) 푑0
푃푡 ∗ 퐺푟 ∗ 퐺푡 푃푟 = 푤ℎ푒푟푒 퐺푡 = 퐺푟 = 5푑퐵푖 푓푟표푚 푎푛푡푒푛푛푎 푠푝푒푐, 푃푡 = 4 푑퐵푚 (푃퐸퐼푅푃 = 100푚푊) (4. 7) 퐿푏
When using the first approach the power at 1 meter is -13 dBm (0.05 mW). Using the second method the power at 1 meter is -17 dBm (0.02 mW). As a comparison, a small sensor or a microcontroller may require several milliwatts for continuous use. It is also important to consider that the calculated power is momentaneous, it does not say anything about actual energy content over time. It is known that the traffic in the WiFi band works in short bursts, which means that the power content over time will be lower.
4.1.2 Measurements
As mentioned in 3.2.1 two test environments were chosen to represent school- and office environment. Measurements were made to measure the power content in several places to get an intuition of how much power could harvest. In some cases, traffic was induced by setting a computer to only work on the 2.4 GHz band and send and receive repeatedly to maximize the power content. A measurement method called max hold was used to collect the data. The downside of this is that it is not representative for the energy content since it stores max values obtained during a set time period, in our case 3 minutes. In Figure 17 is the measurements from Cybercom office AB9 in Kista and in Figure 18 is the measurements from KTH campus in Kista as described in 3.2.1. The related figures are found on the next page to give them justice.
These measurements show the max power level measured for 3 minutes. They do not show the energy content or average power. They represent the equivalent of the calculations made earlier in this chapter but now measured in reality.
Design Procedure | 27
Figure 17 Measurements using Spectrum Analyzer at Cybercom AB9 office
Figure 18 Measurements using Spectrum Analyzer at KTH Campus Kista
Design Procedure | 28
4.2 Energy harvester
4.2.1 Antenna
Initially the project aimed towards constructing the antenna for the harvester, but it was soon realized that this is a complicated process and this idea was discarded as it was not the projects scope.
It was decided that two antennas were to be evaluated. One traditional di-pole antenna bought of- the-shelf and one antenna integrated onto a PBC. A dual-band WiFi antenna was bought, see Figure 19, and a meander antenna was designed and drawn onto a PCB using a blueprint from Texas Instruments, see Figure 19 [42].
Figure 19 Di-pole and meander antenna The di-pole antenna was used to do the pre-study measurements. The work concentrated on one antenna at a time since the impedance matching process proved to be difficult. As a result, the PCB mounted meander antenna was never realized further due to time constraints.
4.2.2 Rectifying network
In order to rectify the harvested energy from alternating current to direct current a rectifying network was implemented. A full wave rectifier was chosen since there are no space restrictions and all energy is needed. The simplest way to do this is by a voltage doubler which will both rectify and boost the voltage at the same time with minimal losses. A voltage doubler called a Delon-circuit was chosen, see Figure 12, [10].
Since the frequency of the alternating current radio wave is very high a Schottky diode was chosen in order to cope with the fast switching speed required. A Schottky diode has the advantage of having a low forward voltage as well. This makes it well suited for energy harvesting.
4.2.3 Impedance matching network
In order to match the impedance of the rectifying network to the antenna’s impedance a separate impedance matching network was designed. The impedance is strongly linked to frequency, if several frequency bands are needed, several matching networks are the most natural way to go. As the project focuses on WiFi frequencies (2.4 and 5 GHz) and the 5 GHz were beyond the project reach in terms of measuring equipment, only 2.4 GHz were chosen.
There are two main ways of constructing an impedance matching network when it comes to topology and how to actually implement it. One is to use “stubs” a way of constructing reactive
29 | Design Procedure
components with use of copper traces with a specified length and width and with or without via holes at the end. The other is to use SMC in order to create the right impedance.
As mentioned earlier the project started with the design of a circuit with the ambition to avoid being caught waiting for the PCB to be manufactured and delivered. A decision was made to use SMC as this was known to the project participants and that the pre-study indicated that using stubs was both difficult and not always with success [43].Below will follow the different approaches to reach component values for the SMC.
4.2.3.1 Calculations
The “Schottky diodes equivalent circuit model” described in chapter 2 turned out to be quite difficult to solve, the mathematical equations in the referred paper does not show the actual solving. As it needed an implementation of RK4 that may be time consuming to create the decision was made to use the “Transient operation analysis method”.
From the datasheet of the Schottky diode 1SS351 the SPICE values in Table 1 are obtained which are the parameters needed to calculate the impedance using the transient operation analysis method [44].
Table 1 SPICE values for schottky diode 1SS351
Using the parameters for the diode and assuming a low input voltage (Vm) of the rectifier during the harvester operation the average applied voltages is obtained for the different regions. Using equations 2.20 together with 2.21 results in ZD,OFF = 78 -j23.8 and ZD,ON = 3.4 +j4.5. Furthermore Zin,OFF and Zin,ON are calculated by using equations 2.18 and 2.19, but as this project uses another rectifying configuration (as shown in Figure 12) the Cs term at the end is dropped. Finally using 2.23 the input impedance can be computed as a function of the output voltage showed in Figure 20.
Figure 20 Theoretical values of the input impedance as a function of the output voltage
30 | Design Procedure
These input impendence values may be valid for the rectifier but as the rest of the PCB construction introduce other factors, like connectors and traces the impedance for the whole circuit may differ a lot.
4.2.3.2 Simulation
In this section simulations of a model for the circuit is presented that can be used for testing and optimizations. The first simulations where carried out in a regular SPICE based program, in this case LTSpice. Due to the limitations of handling HF design ADS from Keysight was used instead.
The circuit was reconstructed and simulated in ADS with the details that could be used from the design made in Dip Trace. The basic component configurations where used and with the large libraries in ADS most components already existed. Harmonic Balance and S-parameter simulations are used to simulate the circuits impedance characteristics and reflection properties.
To find the best matching impedance and performance of the circuit the built-in optimization tools where used. It consists of different tuning options together with trial and error simulations that tries to achieve a predefined set of performances goals. A random optimization type is used as it is one of the most commonly used and optimized for the parameters; imaginary input impedance(imag(Zin)), the real input impedance(real(Zin)) and the forward reflection coefficient (S11). The goal was to find a set of values for component L, C and/or R of the designed circuit, making sure to minimize the value of S11 at 2.4 GHz and keeping real (Zin) close to 50 Ω and imaginary (Zin) close to 0. The optimization setup is displayed in Figure 21.
Figure 21 Optimization in ADS finding parameters for matching circuit
31 | Design Procedure
During the optimization there are possibilities to seed, tune and choose the number of iterations to run. In Figure 22 the optimization cockpit is showed where the simulation is followed in realtime.
Figure 22 Optimization cockpit in runtime
More about ADS and S-parameter simulation and optimization is described in [45]. Figure 23 shows the results from the simulations together with the measured values. The optimization reaches a minimum value of -22 dB for S11 at 2.4 GHz with a minimal deviation from the reference goal of 50 Ω which would mean a maximum transfer of power from the antenna to the rectifying circuit.
Figure 23 Matching parameters results from simulation (left) and measured matching parameters (right)
32 | Design Procedure
When applying the matching circuit from the simulations and measuring the results minor changes had to be made to correct for the differences from the model and the components that where available. With a similar configuration it turned out to be big differences, the measured values for S11 at 2.4 GHz was at -7 dB and an impedance of 27−j25 Ω.
4.2.3.3 Measurement
Since the circuit board needed to be designed in the beginning of the project, space was left for a typical impedance network on the circuit board, at the time not knowing which components should be where. For this to work, it was decided to use surface mounted components. SMCs can differ in value and introduce some uncertainty but allows for a lot of flexibility. The PCB was designed with relatively long copper traces, introducing impedance in the conducting copper traces.
The bought WiFi antenna was connected to a network analyzer using a semi-rigid coaxial cable and SMA connectors. As seen in Figure 24 the antenna held its promise and was a good match having 50 Ω at 2.4 GHz.
Figure 24 Antenna impedance
The rectifying network was then measured in the same way, but one end was soldered onto the PCB. In order to measure the electrical delay introduced by the connection, it was first electrically shorted using a small solder blob. As seen in Figure 25the impedance of the rectifying network was 9.3+j47.8 Ω.
Figure 25 Impedance of the rectifying network
33 | Design Procedure
With the use of the software “Smith v4.1” the networks impedance was inserted, and an impedance matching network designed [33]. As seen in Figure 26 the program calculates the component values needed in order to match the antenna with the rectifier. There are several ways to reach our goal, as seen in the different paths plotted in the right Smith chart.
Figure 26 Impedance matching using Smith program
All the different matching methods resulted in a set of components that could be tested on the prototype. The components for the matching circuit were soldered on to the PCB and the result was measured using the same setup with the network analyzer. Unfortunately, none of the methods yielded a good match and needed additional tweaking and trial and error. Using the network analyzer as a reference and matching using trial and error it resulted in a close 50 Ω matching having no reactive influence at 2.4 GHz displayed in Figure 27.
Figure 27 Impedance matching and reflection
Design Procedure | 34
The impedance was a close match providing a low reflection. Looking at the reflection coefficient a reflection of -17 dB is reached at 2.4 GHz and even lower around 1.8 GHz. Comparing this with the antenna the matching circuit almost accepts as much energy as the antenna as seen in Figure 28.
Figure 28 Reflection comparison matching circuit (Green) And Antenna (Yellow)
This configuration included resistive components which means there may be losses in the harvester. Since the losses should be as low as possible the work continued with trying to realize a matching network that only consisted of reactive components. The same method was used and a solution with only capacitors was chosen based on the results displayed in the network analyzer. As seen in Figure 29 the matching was not as good, the reflection is still quite high.
Figure 29 Impedance matching using capacitors
35 | Design Procedure
4.2.3.4 Result
It is obvious that the impedance matching and design of an impedance matching network is difficult and it took a lot of time to reach the results presented above. The calculations made to calculate the impedance of the diode rectifier in order to be able to match it to the antenna wasn’t valuable. However, that does not mean that the suggested way of calculating is wrong, there are a lot of sources for error. The model varies its result depending on the output voltage, input voltage and other variables that needed to be assumed. Moreover, the measurement included a bit of copper trace that the calculations did not. To conclude, the calculation model did not suit us.
The simulations made to find the impedance of the diode rectifier and calculate a matching network to match the antenna did not return satisfactory results either. ADS is a powerful tool but can only work with what the user can put into the simulation. As we have learned along the way, the circuit is full of impedance in itself and all of the values are not known. This knowledge together with the realization that even very small changes in the value of SMC (and consequently also all other components, copper traces and antennas) have a big impact on the resulting impedance or the matching result. Simply put, it is very sensitive. To conclude, ADS simulation model did not match reality and because of that, could not deliver good matching results.
The experimental design process is the one that gave best result. It was time consuming but delivered the best results compared to calculations and simulations, given the arguments above.
In the beginning of Chapter 4.2.3, stubs were mentioned and discarded. As the project has evolved, the use of stubs has become more and more attractive but because of time constraints we could not explore that option since it would have involved new PCB. See more in Chapter 6.
4.3 The measuring application
The STM32F303 development board from STMicroelectronics was chosen as the microcontroller, it is a familiar platform for the project members. It has several ADC units and is capable of communication via cable to a PC via UART using a FTDI cable for the USB to UART conversion.
The development board has a lot of tools to help set up and develop own code. IAR Workbench was the IDE used for programming and loading the program to the board. The program uses the ADC to read the boards own supply voltage as reference and the harvester’s voltage over the supercap as a measurement. It then calculates the energy charged as it is dependent on the voltage measured and the supercaps capacitance which is known.
The resulting energy value is then sent over UART using the code libraries from STMicroelectronics. A FTDI cable is used to convert the signal from UART to USB as well as handle the supply voltage difference between PC and development board (5 volts v.s. 3.3 volts). The serial data could then be read by either Serial Plot or Mathematica to record and analyze it.
GitHub was used to handle version control of the code. The code can be found at GitHub using the address https://github.com/Alphafofana/Thesis-project .
Results and Analysis | 37
5 Results and Analysis
As mentioned in Chapter 4.2 two matching circuits were constructed, one with a combination of reactive and resistive loads and one with only reactive components. Although it was not in the scope of the project to compare the two circuits, this chapter will present the result from both prototypes.
5.1 Major results
The thesis project had two major deliverables, presented in Chapter 1.4.1, which the project successfully delivered.
✓ A working prototype for harvesting energy and an application for delivering measurements of the harvested power levels.
✓ A Bachelor thesis which documents the process and describes the theory used to achieve the project’s goals.
The main concern and the research question, “How do we harvest RF energy, and can we harvest enough RF energy for it to be useful in an application?” presented in Chapter 1.2 may be a bit ambiguous when it comes to how useful the actual amount of harvested energy is. This is dependent on the application and in this thesis, we assume a low energy sensor node. The results are presented in the graphs below showing the amount of collected energy over a period of time. These results have been produced in a real office environment, Cybercom office location 2, with no computer dedicated to induce extra traffic.
Figure 30 Result, both circuits, for detail, see Appendix A
Suppose 350 mJ stored energy, this is approximately what one of the circuits harvested over 24 consecutive hours. In theory this is enough to power a sensor which needs 6 mW to operate for approximately 60 seconds. Since this energy needs to be delivered at a certain voltage, maybe 30 seconds is more realistic. 30 seconds activity over 24 hours might be enough to measure and send some kind of data for a sensor. 350 mJ would also be enough to run the STM32F303 microcontroller (same as on the Discovery board) for 10 seconds at 8 MHz.
An important consideration is that the measurements were made by the MCU with the ADC peripheral. Using the ADC may introduce different types of conversion errors which has not been taken into account, however initial verification where made with multimeter to ensure that the results could be used in the application. More about ADC conversion errors can be found in the application note from ST Microelectronics [46].
38 | Results and Analysis
As for the big difference in harvested energy between the two circuits, it is difficult to explain in other than general terms. This project shows that small discrepancies affect the impedance and reflection a lot. Since the circuit with only reactive components have a higher reflection and not as god impedance matching it is not a surprise that it would not perform as well. The circuit with mixed components may have a loss over the resistors but that doesn’t seem to affect it too much.
It was the project’s ambition to measure energy harvested from all the locations where power measurements were made but this was not possible due to time constraints. We choose to do the most relevant measurements, in a real office environment.
5.2 Reliability Analysis
As mentioned in Chapter 3.3 the reliability of the measurements made are disputable. The power is measured and can then be compared to calculations made earlier but not to the end result which is energy. It does give an idea about what kind of energy content that can be expected.
The reliability of the result is also questionable since small variations in construction and component values result in large movement in impedance. The process needed some trial and error that may be hard to reproduce.
5.3 Validity Analysis
The method used when measuring the power level was “max hold” which means that it records the maximum levels reached over the frequency spectrum during the measuring time period. Since it is the maximum values, it cannot be said to represents the average content over time but rather a wishful representation of what could have been if there was a lot more computers using 2.4 GHz band of WiFi.
The validity of the result is good. Although the interpretation of what is enough energy differ, the fact that the harvester harvested approximately 350 mJ in 24 hours hold true.
Conclusions and Future work | 39
6 Conclusions and Future work
The goal of the project was to develop an energy harvester and evaluate how much energy could be harvested via a measurement application. This has been realized and measurements delivered. The research question “How do we harvest RF energy, and can we harvest enough RF energy for it to be useful in an application?” has been answered.
From calculations and measurements it became apparent that there was not much ambient RF energy to be harvested. The antenna, rectifier and impedance matching network are a whole, they need to work together as one unit in order to be as effective as possible.
6.1 Conclusions
The decision to design flexibility into the PCB design in the beginning proved to be beneficial. This made it possible to develop the harvesting circuit in an iterative process and improve the functionality. It is possible that using stubs instead of SMD could improve the impedance and reflection even more but then one might need a new antenna as well.
The goals mentioned in Chapter 1 are met with the three subgoals reached. The ambient RF power was measured, and we concluded that there is not much energy to harvest. A circuit to actually harvest energy was constructed and the importance of impedance matching learnt. A microcontroller was used to measure the harvested energy and send the information to a computer.
If we were to do it again, we would try to set up the project in such a way that we could iterate even the PCB design and construction. We would use a modern design and simulation tool to both develop and then simulate our circuits before ordering. We would explore stubs as an option and would probably take a crash course in antenna design.
6.2 Limitations
The shortage of time made it difficult to do iterations of the actual circuit and the lessons learned along the way could not be put into the project.
The amount of energy harvested is 350 mJ per 24 hours. The result is limited to an office area and it can be argued that the measurement locations are not where a typical user of an energy harvesting system would be situated. On the other hand, a typical RF harvester based on WiFi would probably harvest in the 5 GHz band as well and consequently harvest more energy than this project.
6.3 Future work
The project manufactured two circuit, one with a SMA connector for the bought antenna and the other with a surface mounted meander antenna on it. Due to time constraints only one of the circuits were fully developed. To complete and evaluate the other circuit would be a top priority as it is much closer to an actual usable device due to its small size.
Next step would be to design a sensor and communication device and program it to work together with the harvester. A kind of microprocessor would also be required to control when to send measurements and control the charge. Lastly, it would be interesting to evaluate what a DC-DC booster circuit could do to better control charge and load.
40 | Conclusions and Future work
6.4 Reflections
The ability to harvest energy and power sensor nodes without batteries, who has a limited lifespan, is a gamechanger in what is known as IoT. During the pre-study we found a lot of other studies with the aim set to solve the energy issue. This project has shown that it is possible to harvest energy in this way.
As long as ambient, as opposed to induced, RF energy is used in this aspect it is implicated that the energy harvested is something that would be there even if the harvester would not. As such, this technique is environmentally friendly although the extra components in the added energy harvester must be weighed against the environmental footprint from a battery.
The same can be argued for economics, comparing a battery against a harvester, which is cheaper in the long run. If there are one or more battery changes involved in the lifespan of the product and they can be replaced by a harvester, then a lot of money can be saved.
References | 41
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43 | Conclusions and Future work
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Appendix A: Detailed results, graphs | 45
Appendix A: Detailed results, graphs
46 | Appendix A: Detailed results, graphs
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