Designing a smart home environment using a …...Designing a smart home environment using a wireless...

Designing a smart home

environment using a wireless

sensor networking of everyday

objects

LAGUIONIE Olivier

November 27, 2008

Master’s Thesis in Computing Science, 30 ECTS creditsSupervisor at CS-UmU: SURIE Dipak

Examiner: Per Lindstrom

Umea University

Department of Computing Science

SE-901 87 UMEA

SWEDEN

Abstract

This thesis report describes the development of a smart home environment based on anaccurate wireless sensors network using ZigBee[26] communication protocol. In orderto recognize the states/state changes of everyday objects, we used 81 sensors to sensethe 42 everyday objects we selected in a realistic home environment. Knowing thestates of the everyday objects is essential to perform further advanced computation forinstance activity recognition. The promising results we obtained consequently providea good base for such developments. The system has been evaluated in a realistic homeenvironment with background noise (ambient light variations, WI-FI network with samefrequency, etc).The sensing nodes have shown some interesting results with a precision value of 91.2%and a recall value of 98.8% concerning the recognition of the everyday objects’ statechanges. The experimentation has been accomplished by four individual users duringone week. They were all wearing the same equipment for collecting data: a wearablecamera and digital video recorder in order to obtain the ground truth, and an ultraportable laptop (the personal server) connected to the receiving node. The groundtruth obtained from the video recording is compared with the state changes informationrecorded in the personal server to evaluate the performance of the system.

Contents

1 Introduction 1

1.1 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Presentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Problem Description 5

2.1 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Related works 9

3.1 Communication protocols . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.1 Infrared . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.2 WI-FI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1.3 Bluetooth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.4 ZigBee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.2 Existing wireless sensor networking platforms . . . . . . . . . . . . . . . 11

3.2.1 MITes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2.2 Berkeley motes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2.3 The BTnodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2.4 Intel mote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2.5 The U3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2.6 Smart-its . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4 Conceptual design 17

4.1 Everyday objects in a home environment . . . . . . . . . . . . . . . . . . 17

4.2 Sensor nodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.2.1 Everyday object: fridge . . . . . . . . . . . . . . . . . . . . . . . 19

4.2.2 Everyday object: freezer . . . . . . . . . . . . . . . . . . . . . . . 19

4.2.3 Everyday object: microwave oven . . . . . . . . . . . . . . . . . . 20

4.2.4 Everyday objects: ovens and stove . . . . . . . . . . . . . . . . . 20

4.2.5 Everyday object: cupboards and milk box . . . . . . . . . . . . . 22

4.2.6 Everyday object: coffee machine . . . . . . . . . . . . . . . . . . 22

iii

iv CONTENTS

4.2.7 Everyday object: dustbin . . . . . . . . . . . . . . . . . . . . . . 22

4.2.8 Everyday object: drawer . . . . . . . . . . . . . . . . . . . . . . . 23

4.2.9 Everyday object: dish washer . . . . . . . . . . . . . . . . . . . . 24

4.2.10 Everyday object: lamps . . . . . . . . . . . . . . . . . . . . . . . 24

4.2.11 Everyday object: soap provider and flush . . . . . . . . . . . . . 24

4.3 Possible future scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

5 System implementation 27

5.1 Requirements for the wireless sensor network . . . . . . . . . . . . . . . 28

5.2 Hardware implementation . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.2.1 Description of the board . . . . . . . . . . . . . . . . . . . . . . . 28

5.2.2 The main components . . . . . . . . . . . . . . . . . . . . . . . . 28

5.2.3 Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5.2.4 Operating mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.3 Software implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.3.1 The class board . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

5.3.2 Classes board 1 to board 10 . . . . . . . . . . . . . . . . . . . . . 36

5.3.3 Global functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6 Evaluation 39

6.1 The sensor network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6.2 Scenario of evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6.3 The experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6.4 Energy consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

6.5 Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

6.6 Accuracy of the system . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

7 Discussion and limitations 45

8 Future works 47

9 Acknowledgments 49

References 51

A Appendix 55

A.1 Maxstream XBee’s data sheet . . . . . . . . . . . . . . . . . . . . . . . . 55

A.2 Atmal ATMEGA88 data sheet’s front page . . . . . . . . . . . . . . . . 55

A.3 TAOS light to voltage converter data sheet’s front page . . . . . . . . . 55

A.4 Dallas digital temperature sensor TO92 data sheet’s front page . . . . . 55

A.5 Omron microswitch data sheet’s front page . . . . . . . . . . . . . . . . 55

A.6 Microchip low drop voltage regulator data sheet’s front page . . . . . . . 55

List of Figures

2.1 Integration of the sensing board . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1 The first version of the MITes . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2 An example of a berkeley mote(the sensing part) . . . . . . . . . . . . . . . . . 12

3.3 The BTnode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.4 The intel mote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.5 The U3 node . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.6 The 2 modules of the Smart-its . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.1 Light and temperature sensors in the fridge . . . . . . . . . . . . . . . . . . . . 19

4.2 Light and temperature sensors in the freezer . . . . . . . . . . . . . . . . . . . 20

4.3 The sensors of the microwave . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.4 Switches around the knob . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.5 Sensors on the stove/oven panel . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.6 A light sensor in a cupboard (a) and in the milk box (b) . . . . . . . . . . . . . . 22

4.7 Press buttons on the coffee machine (a) and light sensor in the coffee container (b) . 23

4.8 Light sensor in the dustbin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.9 Light sensor in a drawer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

4.10 Light sensors on the dish washer panel . . . . . . . . . . . . . . . . . . . . . . 24

4.11 Light sensor on a lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.12 Press button on a soap provider (a) and on the toilet’s flush (b) . . . . . . . . . . 25

5.1 Photo of the sensing board . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.2 Description of the sensing board . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.3 The XBee transceiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.4 Antenna connector (a) and antenna (b) . . . . . . . . . . . . . . . . . . . . . . 30

5.5 Microcontroller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.6 The voltage regulator (a) and on the battery (b) . . . . . . . . . . . . . . . . . 31

5.7 The light sensor (a) and the temperature sensor (b) . . . . . . . . . . . . . . . . 31

5.8 The switch (a) and the press button (b) . . . . . . . . . . . . . . . . . . . . . 32

5.9 Example of a data frame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

v

vi LIST OF FIGURES

5.10 The classes diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

6.1 A live-in laboratory home environment with nodes placement and signal strength measures 40

6.2 A user wearing our system during the experimentation session . . . . . . . . . . . 42

A.1 XBee datasheet page 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

A.2 XBee datasheet page 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

A.3 Microcontroller datasheet page 1 . . . . . . . . . . . . . . . . . . . . . . . . . 58

A.4 Light sensor datasheet page 1 . . . . . . . . . . . . . . . . . . . . . . . . . . 59

A.5 Temperature sensor datasheet page 1 . . . . . . . . . . . . . . . . . . . . . . . 60

A.6 Switch datasheet page 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

A.7 Voltage regulator datasheet page 1 . . . . . . . . . . . . . . . . . . . . . . . . 62

List of Tables

3.1 Comparative statement of different communication protocols . . . . . . . . . . . . 11

4.1 List of selected everyday objects . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.1 Evaluation of signal strength at 8 different locations . . . . . . . . . . . . . . . . 41

vii

viii LIST OF TABLES

Chapter 1

Introduction

1.1 Outline

This report is composed of the following parts:

– Chapter 1 is a short introduction to this report, describing the goal of this project.

– Chapter 2 explains in detail the problems addressed in this thesis. It also summa-rizes all the important reflections of the undertaken work and draws the conclusionsconcerning the development of the system.

– Chapter 3 references several related works which have been studied during thisthesis.

– Chapter 4 describes in detail the conceptual design of the system.

– Chapter 5 explains the implementation of both the hardware and the software

application.

– Chapter 6 concerns the evaluation of the system.

– Chapter 7 is a discussion about the results of the evaluation of the system anddetails its limitations.

– Chapter 8 makes a list of some possible future works on the system.

– Chapter 9 is the acknowledgments part.

– Appendix: some extracts of the important data sheets.

1

2 Chapter 1. Introduction

1.2 Presentation

This report describes the development of a wireless sensor network[4] of everyday ob-jects in a smart home environment[30] . It will present all the different aspects of thisproject, from the related works to the final evaluation. This thesis is related to the fieldof ubiquitous computing. Ubiquitous computing (computing everywhere) is a vision inusing technology (wireless networking, sensing, actuation, computation, etc) in every-day environments like home, airports, etc. Ubiquitous computing technologies can beused for situation awareness[35], context awareness [2], activity recognition[34], etc. Thesystem to be described in this thesis could be regarded as a human-computer interface(implicit interface) between a user and his/her home environment based on wireless sen-sor networking of everyday objects[28].

There are many related works that exist concerning the design of smart environmentsusing wireless sensor networks[29][31][3]. However, considering the special requirementsfor this research work in obtaining the states/state changes of everyday objects, theoptimal sensors required, the modularity and upgradeability, we have chosen to developour own sensing and communication module.We have used ZigBee communication considering its low power consumption require-ments. The choice of the ZigBee communication protocol can appear to be pretty obviousnowadays for the development of smart objects especially due to the low power consump-tion of these transceivers. When we started to discuss the goals of this project two yearsago, the ZigBee technology was not that widespread compared to other wireless com-munication protocol like for instance Bluetooth[27]. However, considering its efficiencyand the promise shown by this technology, we have decided to use ZigBeeZigBee[5]. Asit can be seen from this thesis, ZigBee communication can be used efficiently for smarthome environments.

The use of simple and cheap sensors has also played an important role concerningour decision in developing our own sensor module. Many wireless sensor projects usecomplex sensors like camera, microphone which need higher level computation for fea-ture extraction. We preferred to use simple sensors like switches or light sensor, whichare cheap, easy to implement, available off-the-shelf, resistant to hostile environmentalconditions and can be used in many different situations.

Furthermore, we also decided to make our sensing board as much generalized aspossible. As mentioned earlier, the wireless sensor network described in this thesis ispart of a larger system[8]. Other modules of the larger system will be added in thenear future using the data collected by the wireless sensor network to enable activityrecognition[6][9], situation awareness, interaction management, etc. Consequently, thechoice of all the components of the electronic board representing the sensing node havebeen very important. This board had to be modular and easily upgradeable to make itpossible to use other kind of sensors (RFID reader[32], accelerometer, etc). That is whyall the components from the microcontroller to the ZigBee transceiver and the way thesensors are connected to the sensing node have been throughly investigated.

Such a generalized board can be also be used for other applications beyond the scopeof this thesis. The size of this board can be minimized by removing some sensor connec-tors depending on the application requirements. One such application requirement is toattach accelerometers to the sensor module presented in this thesis in a wearable form

1.2. Presentation 3

(for the left-hand and right-hand) for gesture recognition[1] and gesture based interac-tion. However the sensor board presented in this thesis is a first version of the prototypeused mainly for activity recognition.Wireless sensor networking could be used for elderly care in a home environment. Forinstance in such a home environment, an efficient activity recognition system could as-sist the elderly person in performing his/her daily activities in an appropriate mannerwithout forgetting important actions.

4 Chapter 1. Introduction

Chapter 2

Problem Description

Many related works can be found concerning the development of a smart environmentor a wireless sensor networking. A literature survey is presented to compare the differ-ent technologies used, the components used, etc. , to understand the advantages andlimitations of the existing works and to see the improvements that are required for ourcurrent design.

As mentioned earlier, this thesis aims at developing an efficient hardware prototypefor sensing the state changes of everyday objects in a home environment as it is shownon this sketch 2.1. The sensing board should resist certain hostile environmental con-ditions in order to be efficient, robust and scalable. The prototype to be developedshould be easy for future developers to be able to modify the board and to extend it.The sensing modules should allow for background noise, real environment constraints,etc, while providing state/state changes information with reliable accuracy. Cost isan important factor to consider and the sensors to be used should be cheap, durableand easy to use. Some sensors have to be set up inside the fridge, inside the freezer, ona microwave oven, on a stove, on a dish washer, etc. , which produce some perturbation.

Figure 2.1: Integration of the sensing board

5

6 Chapter 2. Problem Description

It is also important to investigate the environment in which the system is supposedto be deployed. This includes investigating on the objects present in the environmentand their states/state changes based on the user’s interaction with them while perform-ing everyday activities and to analyse the states/state changes that might be useful forsensing. All the objects do not have the same possible states/state changes and hencecannot be sensed in the same way. This investigation is important to decide the objectsto be sensed, their states, and the sensors that are required for sensing thse states. Amarket survey is also required to investigate on the available sensors, their cost, tech-nology used, compatibility, etc. .

The data that is received from the sensor network is collected in a personal servercurrently under development. The personal server runs an activity centric middlewarefor facilitating personal activity support in smart environments[8]. The operating systemand programming language were chosen according to the projects standards (MicrosoftWindows XP and C++). The middleware module developed as part of this thesis shouldbe modular so that other developers can modify and extend it easily. It also has to bestable and efficient.

2.1 Goals

1. To develop an efficient sensing module that provides an accurate state/state changeinformation concerning everyday objects. Such information is important to de-velop an efficient activity recognizer.

2. To select the appropriate objects in a home environment based on studying theusers activity patterns in a home environment.

3. To select the states that are interesting and important for knowing the activityperformed by the user.

4. To select the appropriate sensors available in the market for sensing the objectsstates.

5. To build a complex sensing mechanism for states that are not directly senseablewith the existing sensors.

6. To basically build the sensing module such that it is easy to modify or upgrade it.

7. To design the system in a power efficient manner since the system is supposed tobe deployed for everyday use in a home environment.

8. Comparison of different communication protocols and to choose an appropriateone.

2.1. Goals 7

9. To design the sensing module such that it is not confined to any specific applica-tion and has a broader usage.

10. To deploy the system built as part of the thesis in a living laboratory home environ-ment for collecting data, evaluating the system and to analyse the results obtained.

8 Chapter 2. Problem Description

Chapter 3

Related works

3.1 Communication protocols

Concerning the communication protocols, several possibilities are available. Each ofthem have some advantages and disadvantages. To choose the most appropriate one,the pros and cons have to be weighted for each one. The different protocols are comparedconsidering the application requirements including data transmission rate, range, batterylife and cost, an it is presented in this comparative table 3.1.

3.1.1 Infrared

The infrared light has a wide bandwidth and a high rate transmission (10 Mb/s), butthe range is short (30 meters). Finally, the infrared technology is relatively old, con-sequently the infrared transceivers are cheap compared to the other technologies. Theinfrared transmissions should also be intense in order to not be confused with other lightsources like television, window, light bulbs, etc. Finally the infrared technology is a wellestablished technology resulting in infrared transceivers which are cheap compared toother technologies. Considering the risk of light confusion, this communication protocolhas not been retained.

– Advantages:

• High rate transmission

• Cheap components

– Disadvantages:

• Short range

• Light confusion

3.1.2 WI-FI

WI-FI is certainly the technology which has the highest data transmission rate with arange of 300 meters with the 802.11b norm. The data transmission rate differs accordingto the norms, but concerning the 802.11b which is the most standard one today, it canreach 11 Mb/s. However the battery life of a WI-FI transceiver is very short rangingfrom half a day to five days and does not suit our requirements.

9

10 Chapter 3. Related works

– Advantages:

• High rate transmission

• Very important range

– Disadvantages:

• Short battery life

• Interaction risk

3.1.3 Bluetooth

Bluetooth is a short range communication technology (< 10 meters), but this range caneasily be extended with a power booster. This communication protocol is consideredas low power consumption, but the battery life of a Bluetooth transceiver is not muchlonger than the battery life of WI-FI transceiver (1 day to 7 days), and is not accordingto our requirements. The data transmission rate is lesser that of WI-FI, but it is stillvery high for our application (720 Kb/s).

– Advantages:

• Sufficient range

• High data rate

– Disadvantages:

• Short life battery

3.1.4 ZigBee

ZigBee is a new standard uniquely designed for low rate wireless personal area networks.It targets low data rate, low power consumption and low cost wireless networking. Therange of the transceivers reaches 100 meters and can be risen until more than 1000meters with a new kind of transceivers. The data rate is certainly the smallest of thiscomparative statement, as it reaches only 250 Kb/s. However, the battery life is muchbigger than the other technologies. A ZigBee transceiver can work from 100 to 1 000days, which is the highest battery life of this comparative statement.

– Advantages:

• High range

• Very low power consumption

• Sufficient data rate?

• Low cost

– Disadvantages:

• Sufficient data rate?

3.2. Existing wireless sensor networking platforms 11

Communication protocols Advantages Disadvantages

InfraredHigh rate transmission Short rangeCheap components Light confusion

WI-FIHigh rate transmission Short battery lifeVery important range Interaction risk

BluetoothSufficient range Short life batteryHigh data rate

ZigBee

High range Sufficient data rate?Very low power consumptionSufficient data rate?Low cost

Table 3.1: Comparative statement of different communication protocols

3.2 Existing wireless sensor networking platforms

3.2.1 MITes

MITes: MIT Environmental Sensors, is a portable wireless sensor platform that can beused to collect data for activity recognition in non-laboratory settings such as homes[33].A photography of the first version of these sensor modules can be found on this figure3.1. The MITes platform includes six environmental sensor types, five of them beingamong the most typically needed in ubiquitous and pervasive computing applications:

– movement using ball, mercury, and reed switches

– movement tuned for object-usage detection (using acceleration)

– light

– temperature

– proximity

– current consumption.

The MITes platform also includes five wearable sensors:

– accelerometers to acquire body motion information

– heart rate

– ultra violet radiation exposure

– an RFID reader in a wristband form factor

– location beacons.

All of these sensors can be used simultaneously, and a single receiver acquires the data,which is sent to a PC or mobile computing device for real-time processing.


Figure 3.1: The first version of the MITes

3.2.2 Berkeley motes

The Berkeley motes are small sensors devices developed at UC Berkeley, which use aradio frequency transmission[13]. The motes contain a microcontroller, a radio frequencytransceiver and many other components. All the sensors are placed on a separate board.The following figure 3.2 is a photography of the sensing board. The sensor boards canbe composed of different sensors considering the need and the function of the wirelesssensing device. Some examples of sensors include light sensor, temperature sensor,accelerometer, magnetometer and humidity sensor. The UC Berkeley also developedTinyOS, an operating system which is especially suited to run on the sensor devices.

Figure 3.2: An example of a berkeley mote(the sensing part)

3.2. Existing wireless sensor networking platforms 13

3.2.3 The BTnodes

The BTnode is an autonomous wireless communication and computing platform basedon a Bluetooth radio and a microcontroller. The BTnode has been jointly developedat ETH Zurich by the Computer Engineering and Networks Laboratory (TIK) andthe Research Group for Distributed Systems[14]. The figure 3.3 is a photography of aBTnode.

Figure 3.3: The BTnode

3.2.4 Intel mote

The Intel mote is built around an integrated microcontroller which is composed ofan ARM 7 core, a Bluetooth radio transceiver, RAM, Flash memory and several I/Ooptions[31]. The Intel motes use a tree topology in which the root node is connected toa pc for the data collection. The motes are connected to an accelerometer board as youcan notice in figure 3.4.

Figure 3.4: The intel mote


3.2.5 The U3

The U3 is a sensor network that has been designed by the university of Tokyo, Japan[29].The sensor node is represented by a 50mm cube that contains 4 separate modules. Eachmodule consists in an independent functional board. The 4 different board of each nodeare placed on top of each other and interconnected inside a cube. A power module, aCPU module, a RF communication module and a sensor module are composing eachmodule. The power module consists in 3 AAA rechargeable batteries. The CPU moduleis based on a PIC18F452. The RF communication module is a 300 MHz RF transceiver,and the range transmission reaches 30m. The sensor module is composed of a temper-ature sensor, a brightness sensor a,d a motion sensor. The concept of this sensor nodeis very interesting especially concerning the design. The fact of having all the differentmodules totally independent makes it easily modular. However, this concept makes thesensor node pretty big, and each module should not exceed a done sized to not increasethe size of the whole module. Besides, the fact of having all the sensors plugged on thesensor module was not suitable for our application. A photography of a U3 node can befound on the figure 3.5.

Figure 3.5: The U3 node

3.3. Conclusion 15

3.2.6 Smart-its

The Smart-its are small devices which are composed of two hardware modules[3]. Thefirst one is the communication module which contains a microcontroller PIC 16F87x anda RFM transceiver TR 1001 at 868.35MHz. The second module is the sensing board.Both modules are interconnected by an I2C data bus and a power bus. These modulesare shown on the figure 3.6. One or several sensing boards can be connected to thecommunication board. The sensing module is composed of a microphone, a light sensor,an accelerometer, a pressure sensor and a temperature sensor.

Figure 3.6: The 2 modules of the Smart-its

3.3 Conclusion

There have been many more related works including IRIS[25], MICAz[23], Imote2[24],etc. Even though many of such systems have their own advantages, they do not meet ourrequirements in sensing the state changes to everyday objects including home appliances,furniture, simple objects, walls, etc, in a home environment.

Chapter 4

Conceptual design

4.1 Everyday objects in a home environment

A typical home environment is a complex one with many everyday objects that an usercan interact with while performing some activities. To recognize the activity of a per-son in such an environment, the system needs to know what object is currently usedbut more precisely its state/state change information[6]. The states of objects are oftwo types. We make a difference between the internal states and the external states.Internal states for instance is a fridge that is open or closed while external states repre-sent the relationship of an object with reference to other objects. For instance a fridgemight contain other objects like milk packet, vegetables packet, etc. The work to bepresented in this thesis will deal with the internal state/state changes alone. Externalstate changes are left for future work. However the wireless sensor networking moduledeveloped as part of this thesis could be used for sensing the external state changes aswell. For instance a RFID reader could be easily integrated to the system presented inthis thesis to sense the external states/ state changes of everyday objects.

Many internal states are common to several objects, but some are specific for individ-ual objects. To know if an object is open or closed; switched on or off, are common states.However to sense these states can be different depending on the object. For instance, toknow if a coffee-maker is switched on can be sensed using a light sensor which detectsthe feedback light signals from the LED integrated onto the coffee-maker. Instead avibration sensor can be used to detect that the blender is switched on. Furthermoresome state like for instance the temperature, the timer or the power specifications arespecific to individual devices. Therefore each individual object that needs to be sensedhas to be studied in detail in order to choose the appropriate states and the best way tosense them. Refer to table 4.1 for a list of selected objects with their state informationimplemented as part of this thesis. Note that V represents the state that are sensed andX the state that are not sensed.

4.2 Sensor nodes

The overall goal for this thesis is to develop sensor and communication nodes for in-dividual objects such that they communicate wirelessly with the user’s personal server

17

18

Chapte

r4.C

onceptu

aldesig

n

Object Open/closed On/off Temperature Full/not full Pressed/released

Fridge V X V X XFreezer V X V X X

Microwave oven V V X X VUp oven X V X X V

Down oven X V X X VStove X V X X V

Cupboards V X X X XCoffee machine V X X V V

Lamps X V X X XDustbins X X X V XDrawers V X X X X

Dish washer X V X X VExhaust fan X V X X XMilk box V X X X X

Toilet’s flush X X X X VSoap provider X X X X V

Table

4.1

:List

ofselected

every

day

objects

4.2. Sensor nodes 19

and provide information related to objects states/state changes. However to reduce theamount of boards which are required we have chosen to use common sensor boards forseveral objects. Our system is composed of 10 boards. Mobility is an important require-ment since the user normally is mobile when interacting with everyday objects hencewe have decided to go with wireless approach instead of a wired approach. The sensingboard is the same for all the objects that includes a microcontroller, a transceiver anda few other electronic components. However the sensors that are embedded in differentobjects are different. A detailed account of the sensors used for sensing the states/statechanges to everyday objects is provided below.

4.2.1 Everyday object: fridge

– State type - open/closed: light sensor

Several sensors can be used to detect if the fridge is open or closed. The simplestsensors which can be used are certainly the switch and the light sensor. The lightsensor has the advantage that there is no mechanical interaction, so it has a longeraverage lifetime. By placing the light sensor inside the fridge it is easy to detectif the door is open or closed as all the fridges have a light inside. Of course, therecan be a problem if the light breaks down. The photography 4.1 is an example ofthe integration of the sensor in the fridge.

– State type - temperature: temperature sensor

To sense the temperature, there is no other choice than to use a temperaturesensor. However this sensor has to be used in a low temperature environment(around 0◦C).

Figure 4.1: Light and temperature sensors in the fridge

4.2.2 Everyday object: freezer


This is similar to the fridge but in this case the sensor has to resist very lowtemperature (from -6◦C to -30◦C following the quality of the freezer). Anotherproblem is that in some freezer there is no light inside when the door is open, but

20 Chapter 4. Conceptual design

this can be probably solved by placing the sensor in such a position that it candetect the outside light (for instance on the door).

– State type - temperature: temperature sensor

Similar to the fridge but the sensor has to be able to work in a very low temperatureenvironment. The photography 4.2 shows an example of the integration of thesensor in the freezer.

Figure 4.2: Light and temperature sensors in the freezer

4.2.3 Everyday object: microwave oven

– State type - open/closed and on/off: light sensor

We placed a light sensor on the glass door of the microwave. This light sensor caneither detect if this door is open by receiving the ambient light or it can also detectif the microwave is switched on by receiving the inside light of the microwave. Anexample of the integration of the sensors can be found in figure 4.3.

– State type - start,stop and defrost: press buttons

A press button has been placed over the start button of the microwave. Thisallows our system to detect when the user is pressing the start button. Stop anddefrost buttons are sensed in a similar way.

4.2.4 Everyday objects: ovens and stove

– State type - on/off: light sensor

Some light sensors have been placed over the LEDs to detect when the devices areswitched on.

– State type - knobs: switches

Some switches have been placed all around the knobs, at each of their differentpositions in order to detect the state of each of them. The sketch 4.4 presents thisway of sensing. It has been used for the experimentation as you can see on thefigure 4.5.


(a) (b)

Figure 4.3: The sensors of the microwave

Figure 4.4: Switches around the knob

(a) (b)

Figure 4.5: Sensors on the stove/oven panel


4.2.5 Everyday object: cupboards and milk box


The open/closed state of the cupboard is quite easy to sense with a switch or alight sensor. The light sensor maybe the most efficient as the door of the cupboardis not obliged to be totally closed to be sensed as closed. A light sensor has alsobeen used for the milk box as you can notice on the figure 4.6.

(a) (b)

Figure 4.6: A light sensor in a cupboard (a) and in the milk box (b)

4.2.6 Everyday object: coffee machine


The state open/closed of the coffee machine is sensed by a light sensor placedinside the coffee machine to detect if this one is opened or closed.

– State type - full/not full: light sensor

A light sensor has been placed in the coffee container of the coffee machine todetect if this one needs more coffee.

– State type - selection buttons: press buttons

Some press buttons have been placed on the coffee machine to detect which selec-tion the user is doing as you can see on the figure 4.7.

4.2.7 Everyday object: dustbin

– State type - full/not full: light sensor

A light sensor has been placed in the dustbin to detect if it is full or not as it canbe seen on the photography 4.8. Indeed, if the dustbin is full then the light sensoris hidden from the ambient light.


(a) (b)

Figure 4.7: Press buttons on the coffee machine (a) and light sensor in the coffee container (b)

Figure 4.8: Light sensor in the dustbin

4.2.8 Everyday object: drawer


The situation of the drawer is very similar to the cupboard. A switch can also beused to detect its state. However the use of a light sensor appeared to be simpler,and has been used as it can be seen in figure 4.9.


Figure 4.9: Light sensor in a drawer

4.2.9 Everyday object: dish washer

– State type - on/off: light sensor A light sensor has been placed on the LED corre-sponding to the on/off state of the dish washer. Another way could be to place anaccelerometer on the door of the dish washer to detect the vibrations generatedduring its functioning.

– State type - program selection: light sensors

Some light sensors have been place on the LEDs corresponding to the differentprograms of the dish washer that the user can select, as it can be seen on thephotography 4.10.

Figure 4.10: Light sensors on the dish washer panel

4.2.10 Everyday object: lamps

– State type - on/off: light sensor A light sensor seems to be the easiest way to sensethe state on/off of a lamp by placing it very close to the bulb, as it can be seenon the photography 4.11. However it has to be isolated from the ambient light.

4.2.11 Everyday object: soap provider and flush

– State type - pressed/released: press button Several possibilities can be found tosense this state. A touch sensor, a press button or other sensors can suit for that.


Figure 4.11: Light sensor on a lamp

The press button seems to be the simplest and cheapest way to sense it. A pressbutton has also been used for the toilet’s flush as it is shown on the photography4.12.

(a) (b)

Figure 4.12: Press button on a soap provider (a) and on the toilet’s flush (b)


4.3 Possible future scenarios

Several future scenarios can be imagined concerning the application of our system.Knowing the state of the objects provide some very useful information for higher levelapplications. First, these data can be used for an activity recognition based systemwhich could analyses all the habits of a user, the time schedule, etc. In case of an el-derly person, such a system could be used as a permanent assistant which could detectif something abnormal is happening. Some automates could also be added in order toeither help the user in case he forgets to perform something, or to simply improve thecomfort of a normal user (switch on the TV and select the channel following the taste,etc). The states of the objects could also be used by a tracking location system to detectwhere the user is situated in the environment.

Chapter 5

System implementation

The device that has been developed during this thesis consists in an electronic board(refer to figure 5.1) on which sensors are connected. This board is powered with abattery and uses ZigBee wireless technology to send the state of the sensors. The datais sent to a receiving board connected to the personal server. We will explain in detailsin the following parts which components are implanted on the sensing board and howthe sensing data is collected.

Figure 5.1: Photo of the sensing board

27

28 Chapter 5. System implementation

5.1 Requirements for the wireless sensor network

A workshop has been conducted with several users and members of the projects in orderto constitute some requirements for our wireless sensor network[7]:

– Usability/Availability/Installation: The users prefer cheap products, available off-the-shelf and easy to set up. They do not want to wear to many devices, and donot want to change their habits in the environment.Consequently, the wireless sensor network has to use a simple network topologywhich does not involve any complex installation. The size of the sensor nodes hasto be as minimized as possible, as the personal server which should remain theonly wearable device carried by the user.

– Performance/Reliability: The reliability of the system has been emphasized by theusers compared to its performance. The transmission range also has to be highenough to obtain a stable system. However, as the user is constantly wearing themobile receiver, the transmission of the state changes should not be a problemwith the everyday objects the user is interacting with. Finally, the battery life isan important criteria to avoid any repetitive recharge cycles of the batteries.

5.2 Hardware implementation

5.2.1 Description of the board

The two main components of the board are the ZigBee transceiver and the microcon-troller. Both communicate through the USART serial communication device, whichconcretely consists in linking the Rx port of the transceiver to the Tx port of the micro-controller and vice versa. A programming interface which consists in a six pins headerhas been had to the board to allow to reprogram the microcontroller without removingit. One I/O microcontroller pin, port D4, is connected to the pin of the transceivercalled sleep control line and is used either to wake up the transceiver or to put it insleep mode. All the other I/O pins are connected to a three pins header which is alsolinked to the ground and +3V. This allows to connect the sensors to the microcontrollerand also to power them. Three of the I/O pins of the transceiver can be used as indi-cators and consequently have been connected to LEDs. All the other I/O pins of thetransceiver are connected to a pin header so that they can be used in a future evolution.The sketch 5.2 presents the different parts of the board.

5.2.2 The main components

– The ZigBee transceiver is a XBee module from Maxstream (figure 5.3), series 1and standard version with a U.FL antenna connector[15]

This transceiver operates at ISM 2.4 GHz frequency and allows data rates up to 250Kbps. This rate is more than enough for our application.The standard version of this module can be replaced by the professional one whichhas a higher transmitting power and consequently a bigger range. We chose to use anexternal antenna instead of an antenna integrated to the transceiver. This allows abetter transmission and reception and also a larger range. A new series (series 2) ofthese XBee modules has been developed whereas this thesis was going to its end. This

5.2. Hardware implementation 29

Figure 5.2: Description of the sensing board

Figure 5.3: The XBee transceiver

new series uses Mesh networking instead of the point to multi point network architectureof the series 1. It could be interesting to use this new series for a more expanded wirelesssensors network.

– The U.FL antenna connector of the XBee is connected to a MMCX (U.FL) toSMA reverse converter[20]:


– This converter is used to connect a 2.4 GHz antenna for SMA reverse connector[21].This antenna (figure 5.4) is a 2.4 GHz omnidirectional antenna with half a wave-length and a gain of 2.90 dBI.

(a) (b)

Figure 5.4: Antenna connector (a) and antenna (b)

– The microcontroller (figure 5.5) is an Atmel ATMEGA88[10]:

Figure 5.5: Microcontroller

This microcontroller has been chosen for several reasons. First, this microcontrolleris pretty cheap and available off-the-shelf. It also provides a very interesting connectionset with for instance five analog-to-digital converters (here used for light sensors, butwhich can also be used for accelerometers or others), USART serial communicationdevice (used for the communication with the XBee transceiver) and SPI serial interface(used as a programming interface). Finally, the firmware can be coded in C languagewith the free environment AVR studio. This microcontroller is running at 8 MHz.

– The power is regulated through a low drop voltage regulator (figure 5.6) whichdelivers 3.0V[22]. The whole board is powered with 3.0V. When operating underthis voltage, the XBee module consumes much less power when it is in sleep mode.The microcontroller can also work with this voltage as the sensors used so far.

– The power is provided by 3 NIMH batteries of 1.2V each and 2600mA[17]:


(a) (b)

Figure 5.6: The voltage regulator (a) and on the battery (b)

5.2.3 Sensors

The sensors used are basic ones. It was important for this thesis to provide a system easyto install, to use, and affordable. Some more suitable sensors for proper state changescertainly exist and we probably be used in a future evolution of our system. However wetried to select some basic sensors for this first version that could be used for the mostcommon state changes. The board has been developed in such a way that it is easy touse other sensors. Consequently, some new sensors can easily be added, and some newstate changes can be sensed.

– Light sensor(figure 5.7): TAOS light to voltage converter[18]

– Temperature sensor(figure 5.7): Dallas digital temperature sensor TO92[19]

(a) (b)

Figure 5.7: The light sensor (a) and the temperature sensor (b)

– Switch(figure 5.8): Omron microswitch[16]

– Press button(figure 5.8): Push button sw. NO


(a) (b)

Figure 5.8: The switch (a) and the press button (b)

5.2.4 Operating mode

Global description

The sensors are plugged into the 3 pins headers corresponding to the I/O ports ofthe microcontroller. Only the light sensors need a special port as it is explained in thefollowing part. The microcontroller is periodically checking the values of the sensors andin case a new value differs from a previous one more than a certain threshold, it wakesup the transceiver, sends it the information, and then put the transceiver back in sleepmode to save some power. The sampling rate is low, around 100 Hz or less, dependingon the amount of sensors. When the transceiver receives some information from themicrocontroller, it automatically sends it to the destination address corresponding tothe receiving node.

The sensors

The temperature sensor The temperature sensor is the most particular sensor usedhere. This is a digital temperature sensor which uses the one wire protocol from Dallas.That means this sensor only uses one wire for data communication with the microcon-troller. This sensor needs to be initialized at the beginning of the processing part of themicrocontroller. Consequently, the temperature sensors need to be plugged to the boardbefore the power is switched on. This sensor is the only one among the other sensorsthat we are using that can be called passive. In fact, even if the temperature is changing,it will not be seen as an event. Consequently, the temperature will be updated in thepersonal server when another state change will occur. An application note with sourcecode examples is provided by Atmel explaining how to communicate with sensors usingthis protocol[11]

The light sensor The light sensor used is a light to voltage converter. Consequently,it needs to be connected to an Analog-to-Digital converter. The Atmel ATmega88provides 6 Analog-to-Digital converters represented by the ports C.


The microcontroller

The microcontroller is constantly checking the values of all the sensors connected to itthrough an endless loop. At each iteration of the loop, the values of all the sensors arecompared to their previous value stored in a variable. If at least one of these valueshas changed, then the microcontroller sends the new values of all the sensors to thetransceiver. First, it wakes up the transceiver using the port D2 which is reserved forthis use. This only consists in clearing the corresponding bit followed by a certain delayto let enough time to the transceiver to wake up. Then, a data frame is sent throughthe UART. This data frame depends on each sensing board and on the sensors used.Indeed, this data frame first contains an identification number unique for each board.Then it contains the values of all the sensors. The number of bytes used depends on thesensors. Finally, a character is added at the end of the frame as you can notice on thesketch 5.9.

Figure 5.9: Example of a data frame

The transceiver

The transceiver works in transparent mode (See the data sheet for more information).When working in this mode, the transceiver immediately sends the data it receives onits RX port of the UART serial communication device. These data are sent to thetransceiver whose address is the destination address of the previous transceiver. In thisthesis, all the transceivers of the sensing boards send the data to the transceiver of thereceiving board plugged to the computer. Consequently, all these transceivers have thesame destination address.

Processing part

The processing part of the microcontroller is more or less the same for all the differ-ent boards. First, it is composed of a delay of a few milliseconds to let all the sensorsand transceivers to be ready, an initialization part which includes the initialization ofthe uart, initialization of the A2D converter, configuration of the ports (input, output,pull up), configuration of the port dedicated to wake up the transceiver before sendingdata, and then put the transceiver in sleep mode. If the microcontroller also deals with atemperature sensor, it also has to initialize the one wire protocol and search for the tem-perature sensors on the bus. For each sensor, the processing part attributes 2 values for


each sensor. One represents the previous value of the sensor and another one representsthe new value. This is used to determine if the value of the sensor has changed or not.After the previous initialization part, all the variables representing the previous valuesof the sensors are initialized with the current values of the sensors. The microcontrollerwakes up the transceiver, sends the states of the sensors (4 times) and then puts thetransceiver back in sleep mode. When the microcontroller is sending the data to thetransceiver, it always sends it 4 times with a delay of a few milliseconds in between.We did that to avoid the loss of data. Indeed, we noticed when the signal strength ispretty low that the data frame sometimes reaches the personal server with some datamissing and consequently cannot be used. (no crc or other error detection algorithmhas been implemented, but has been recognized as a good future improvement). Thenthe microcontroller enters in an endless loop. This endless loop consists in reading allthe new values of the sensors (the reading process depending on the sensors), comparingthese new values with the previous ones, and then, with some specific thresholds andfiltering functions, deciding to send or not these new values. If the new values are sent,then the previous values are replaced with these new values.

The light sensors The light sensors have been the most complicated sensors to cali-brate. In fact, the ambient light plays an important role concerning the accuracy of thesesensors. To obtain the best results we could, we introduced several filtering functionswith different thresholds either in the processing part of the microcontroller and in theapplication of the personal server itself. First of all, to read the value of a light sensor,we read 10 sample values with a delay of 1 millisecond in between, and then deduce thevalue by taking an average of these 10 sample values. In fact, these light sensors are verysensitive and their value can change a lot even if the state of the object did not changeat all. This can be due to the shadow of the user, shadow of an object, light reflectedby an object, etc..., or other kind of perturbation on the sensor. When this new value iscalculated, we compare it with the previous value of this sensor. The difference betweenthe new value and the previous one has to be equal or bigger to a certain threshold tobe interpreted as a possible state change, and sent to the transceiver. This threshold isspecific to each sensor. Indeed, depending on the place where they are situated, a lightsensor is not exposed to the same light conditions. For example, a light sensor placedin a cupboard will detect a pretty low light level when the cupboard will be closed. Inthe contrary, a light sensor placed near the bulb of a lamp will always detect a prettyhigh level of light because the ambient light will always be stronger than the one in aclosed cupboard. Consequently, the range of the light values detected by the light sensorplaced in the cupboard will be larger than the one detected by the light sensor placednear the bulb. That is why they need different thresholds. If we had applied the samethresholds for these 2 sensors, the microcontroller would have constantly detected thenew value of the light sensor in the cupboard as a possible new state change becauseof its too high sensitivity. If a light sensor is not well calibrated, and is too sensitivedue to a too low threshold, it can happen that the corresponding node will constantlysend the new values of this sensor and consequently monopolize the receptor preventingall the other nodes from sending data. Furthermore, another threshold is used in theapplication of the server to interpret the new value of a sensor as a new state or not.This threshold is specific for each sensor. Indeed, this threshold depends on the objectthe sensor is sensing, the place of this sensor in the object, the place of this object inthe environment and the ambient light of the environment. Consequently, to obtain thebest results, this threshold has to be evaluated in different environment light conditions.

5.3. Software implementation 35

Switches ad press buttons The switches and press buttons are the most simplesensors used in this system. There is no threshold or filtering done concerning thosesensors. Their state is directly interpreted in the microcontroller as the state pressed orreleased, and is represented as a simple character in the data frame.

5.3 Software implementation

The application developed for this thesis is responsible for the interaction between thehardware prototype and the laptop (middleware application). It has been written inC++ to respect the standards of coding of the project (object oriented). It has severaltasks. First, it has to read the data received by the reception board. Then, the appli-cation treats these data and updates the state of the objects in consequence. Finally,these states are displayed in order to inform the developer of a possible problem, andare also saved in a .log file to keep track of all the state changes. This application isa multi-threaded application developed with Microsoft visual studio 6.0. One thread isresponsible for the reception of the data from the reception board. Another one treatsthese data and update the states of the objects. A last one displays the information. Theapplication is composed of several classes as you can notice on the class diagram 5.10. 3default classes deal with the application and the dialog boxes. One class is dedicated tothe communication with the serial port: CCom[12]. Indeed, even if the receiving boardused is plugged to the personal server through a usb connection, the driver provided isa usb to serial converter driver. Consequently, this board is recognized as a com port.

Then the application is composed of one class per board: board 1, board 2, ..., board 9.All these classes inherit from a common class board.

5.3.1 The class board

Attributes

This class is composed of 2 attributes. First a string id, unique for each board, used toidentify it, and then an integer state change which notifies if a state change occurred inone of the objects sensed by the board. Both of these attributes are private ones.

Public methods

string get id() Access function which just returns the id of the board.

string identify(string data) This function reads the id (first 3 bytes) of a data framegiven as a parameter and returns it.

string read id(string data) This functions reads the id of a data frame given as aparameter, store this id in the id attributes of the board, removes the id from the dataframe and returns this string.

int has changed() Access function returns the attribute state change of the board.

void event occurred() This function gives the value 1 to the state change attribute.

void event treated() This function gives the value 0 to the state change attribute.


Figure 5.10: The classes diagram

5.3.2 Classes board 1 to board 10

The classes board 1, ..., board 10 all inherit from the class board.

5.3. Software implementation 37

Attributes

Each board class has as attributes all the possible states of the objects they are sensing.For each state, the board has 3 attributes: one for the current value of a sensor, one forthe current corresponding state, and one for the previous state. For example, the boardclass responsible for the fridge will have the following attributes corresponding to thelight sensor of the fridge:

– string fridge light value

– string fridge light state

– string fridge light previous state

All these attributes are private.

Public methods

Attributes’ accessors First, all the board classes have functions in order to get thevalues of all attributes.

int data(string data) This function consists in reading the data frame given as aparameter. It reads in the appropriate order first the id of the board, and then all thevalues of all the sensors. This function depends on the structure of the data frame.

CString display() This function consists in displaying all the values of the sensorsof the board in the debugging window.

string write file() This function consists in writing the states of all the states of thesensed objects in the .log file.

Private methods

These functions are the ones which are composing the previous data method. Theyconsist in reading one of the sensor value included in the data frame, storing the previousvalue in the appropriate attribute, and interpret this value as a new state or not.

5.3.3 Global functions

The global functions consist in all the calibration functions. These functions interpret asensor value as a state, following a certain threshold value. These functions are globalones in order to be used by different classes.

Chapter 6

Evaluation

6.1 The sensor network

For this experimentation, we used 10 boards, one board containing one transceiver. Eachboard represents a node in the wireless sensors network. The 10 nodes represented bythese 10 boards are sending the data to an 11th node which represents the center of thestar topology of this wireless sensor network. The number of the nodes can easily beincreased as the XBee transceivers can take one of the 65,000 addresses available.

6.2 Scenario of evaluation

Our system has to be evaluated in a real living home environment to precisely knowits accuracy and stability. A real environment always offers lots of perturbations thatcannot be thought in advance during the developing state of a project. This step isessential for the evaluation, but also in a first time to test the system and correct thepossible problems.

This system has to be able to face several perturbations. First, a user that does notknow the system and how it works, does not behave in the same way than the person whodesigned it. For instance, if the user knows how the sensor of a specific object works oris fragile, he will differently interact with this object than a user who absolutely does notknow anything about it. Besides, the sensors themselves have to face a lot of constraints.For example, the sensors placed in a fridge or a freezer have to face some humidity andtemperature conditions very specific which cannot be simulated. The shadow of theuser or the ambient light conditions can also represent some constraints that only existin a real environment. Finally, it is important to see how the wireless sensor networkbehaves when the user is interacting with several objects at the same time, and if thepersonal server is not missing any event.

6.3 The experimental setup

The experiments were performed in the fika room at the 4th floor of MIT huset. Theconfiguration of this environment is described in this sketch 6.1.

39

40 Chapter 6. Evaluation

Figure 6.1: A live-in laboratory home environment with nodes placement and signal strength measures

Four users were recruited for the experimentation. The experiments were performedindividually by the users during a week. In addition to the system components (wear-able personal server + receiving node), the users were also carrying a wearable cameraconnected to a mobile digital video recorder (6.2) to obtain the ground truth.

6.4 Energy consumption

We conducted the experimentation of our wireless sensors network in real conditionsduring 4 days. The batteries of the sensing boards were fully charged before we startedand they were switched on during 27 hours. The sensing boards do not function in thesame way following the sensors they are using and the devices they are sensing. Conse-quently, the batteries do not need to provide the same amount of energy following theboard they are powering. However, during these 4 consecutive days, we never encoun-tered any problem due to a low battery level, and the batteries could have certainlybeen used during a larger amount of time.

To reduce the power consumption of the sensing boards, we put the XBee transceiverin sleep mode when the board is not sending any data. But first of all, the whole board ispowered in 3V which consequently reduces the power consumption when the transceiveris in sleep mode. Then the XBee transceiver consumes less than 3µA when it is in sleepmode. If it had been powered in 3.3V, it would have consumed 101µA when it is insleep mode.

6.5 Range

The signal strength measure above the acceptable limit of >10 dB to obtain reliable datacommunication was found to be 97.5% checked at 8 different locations in a home envi-ronment. The results have been summarized in this table 6.1. Finally the transmission-reception range was evaluated to be 33 m with a single wall obstruction and 19 m withmultiple wall obstruction.

6.5

.R

ange

41

Signal Living Dining Kitchen Bedroom Bedroom Toilet Toilet Officestrength space hall (door open) corridor (door closed) (door closed) corridor (door open)

(%) (%) (%) (%) (%) (%) (%) (%)

Best (>30 dB) 100 100 80 60 0 0 60 0Good (>20 dB) 0 0 20 40 40 0 40 60

Medium (>10 dB) 0 0 0 0 60 80 0 40Low (<10 dB) 0 0 0 0 0 20 0 0

Table

6.1

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valu

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at

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loca

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Figure 6.2: A user wearing our system during the experimentation session

6.6 Accuracy of the system

The accuracy of the system is essential for such an application. Indeed, if the sensingdata is not well received by the personal server or if some data is missing, then anyapplication built on top of this wireless sensor network cannot be reliable. To performthe evaluation, the users were performing a set of activities of daily living (Preparinglunch, baking, etc).The characteristic values concerning the evaluation of the system, precision and recallvalues, are defined as follows:

6.6. Accuracy of the system 43

Precision = TruePositives(TruePositives+FalsePositives

Recall = TruePositives(TruePositives+FalseNegatives

There is a True Positive when a state change happens and the system recognizes it.There is a False Positive when the system recognizes a state change whereas it did nothappen in reality.There is a False Negative when the system does not recognize a state change whereas ithappened in reality.

The system got a global precision value of 91.2% and a global recall value of 98.8%.Formore information, please refer to the related paper[7].

These values are promising taking into consideration that some perturbations due tothe environment itself (interferences, light conditions, etc) influence the performance ofthe network. Besides, some state changes were really difficult to sense, and consequentlysome more appropriate sensors would significally improve the results.

Chapter 7

Discussion and limitations

Even though the results obtained after this evaluation are very encouraging, some limi-tations of the system have been revealed and should be reduced in a next version. Firstof all, some important problems have been encountered concerning the use of the lightsensors. Those sensors are very sensitive, and their accuracy depend a lot about theambient light conditions. The experimentation has been performed at different momentsof the day, with different daylight conditions, which caused some troubles for the lightsensors. To solve this problem, we recalibrated the sensors, more precisely, so that theycould work in all cases. However, this represents a lot of work, and could be even moredifficult to realize with a larger amount of sensors. That is why, the use of the lightsensors should be improved. A possibility could be to use several of these sensors tosense a same state (2 or 3) instead of one, and also to sense the ambient light intensitywhich could allow some kind of dynamic sensor calibration.

Another limitation concerns the battery life of our sensor modules. In fact, we didnot really tested for how long a sensor node can work without any power supply problem.However, as the state of the sensors are periodically checked by the microcontroller, anon negligeable amount of energy is wasted when a state change does not occur. Thisshould be improved to increase the battery life of our system.

Finally, one of the constraint of the system is the wearable equipment that the useris carrying. Even if its size and weight are not that important, it still represents a limi-tation for the user. An easy way of avoiding this constraint could be to use a device theuser is wearing by himself. Actually, nowadays almost everybody has a mobile phoneand carries it in any situation. Besides, the mobile phones becoming more and morepowerful (smartphone, PDAphone, etc), it could be possible (or maybe in a near future)to embed our application in such a device.

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46 Chapter 7. Discussion and limitations

Chapter 8

Future works

Several future works can be planned concerning this thesis. First, some solutions can befound in reference to the limitations mentioned above. The hard-coding of the objectsstates should be removed. It would certainly be more efficient to create a database withall the everyday objects and their state changes. Like this, it would be much faster andeasier to treat a larger amount of devices. This would also allow other applications toaccess the sensor data which can then be used for other purposes.

Another improvement concerns the communication protocol. In fact, even if nowa-days ZigBee seems to be the most efficient technology for this kind of work, a receptionboard is needed to receive the data from the sensor nodes to the computer. A technologylike Bluetooth is already embedded in the laptops which avoids to use any additionalboard. It would not be surprising to see in a near future a laptop with an embeddedZigBee transceiver which would then reduce the wearable equipment.

Besides, as it has been explained before, lots of the objects which are sensed alsohave a containment property (fridge, freezer, cupboard, etc) of which the state wouldbe interesting to know. This future work had already been planned since the beginningof this thesis. Some work about the use of an RFID reader and some id tags has beenperformed, and should start to be tested very soon.

The electronic board of the sensor nodes has been designed to be as modular and up-gradeable as possible. However, if this board has to take part of a wearable equipment,for instance connected to several accelerometers on the user’s body, the design shouldbe modified. Indeed, for such an application, the size of the pcb could be significallyreduced.

Finally, a CRC or other kind of error detection algorithm should be implemented toavoid the loss of data during the transmission.

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48 Chapter 8. Future works

Chapter 9

Acknowledgments

I would like to thank all the following persons who helpt me a lot to write this master’sthesis:

– Thomas Pederson, manager of the easyADL project in computer science depart-ment at Umea university, for accepting me in his team and for advising me duringthis long period.

– Dipak Surie, my supervisor, Phd student in computer science and team memberof the easyADL project, for supervising me, orientating me and supporting me.

– Nils-Erik Eriksson, electronic teacher at the TFE department, for bringing me lotof help concerning the electronic part of this thesis.

– All the other members of the project, master thesis students and research assis-tants, for integrating me in the team and helping me in my work.

I also want to thank all the university personal, teachers, international office, and othersfor integrating me so well and supporting me during my stay.

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50 Chapter 9. Acknowledgments

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Appendix A

Appendix

A.1 Maxstream XBee’s data sheet

A.2 Atmal ATMEGA88 data sheet’s front page

A.3 TAOS light to voltage converter data sheet’s front

page

A.4 Dallas digital temperature sensor TO92 data sheet’s

front page

A.5 Omron microswitch data sheet’s front page

A.6 Microchip low drop voltage regulator data sheet’s

front page

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Figure A.1: XBee datasheet page 1

Figure A.2: XBee datasheet page 2

Figure A.3: Microcontroller datasheet page 1

Figure A.4: Light sensor datasheet page 1

Figure A.5: Temperature sensor datasheet page 1

Figure A.6: Switch datasheet page 1

Figure A.7: Voltage regulator datasheet page 1

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