Development of a wireless sensor unit for tunnel monitoring

Development of a wireless sensor unit for tunnel

monitoring

by

Sivaram M.S.L. Cheekiralla

Bachelor of Technology, Civil Engineering (2001)

Indian Institute of Technology, Bombay

Submitted to the Department of Civil and Environmental Engineering

in partial fulfillment of the requirements for the degree of

Master of Science in Civil and Environmental Engineering

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

February 2004

©

Massachusetts Institute of Technology 2004. All rights reserved.

A u th o r ...

; .. ...

Department of Civil and Environmental Engineering

January 16, 2004

Certified by...

... . . .~~ . ... . . . . . j. . . . . .

Ruaidhri M. O'Connor

Assistant Professor

it

The is Supervisor

(I

1/1

Accepted by ...

_,...

_...

_....

_...

Heidi M. Aepf

Chairman, Department Committee on Graduate Students

MASSACHUSETTS INSTITE

OF TECHNOLOGY

Development of a wireless sensor unit for tunnel monitoring

by

Sivaram M.S.L. Cheekiralla

Submitted to the Department of Civil and Environmental Engineering on January 16, 2004, in partial fulfillment of the

requirements for the degree of

Master of Science in Civil and Environmental Engineering

Abstract

In this thesis we describe the development of a wireless sensor module for tunnel monitoring. The tunnel in question is a part of the London Underground system. Construction of a new tunnel beneath the existing tunnel is anticipated to cause quantifiable vertical displacement. To ensure safe operation of the tunnel during the construction activity, a real-time monitoring system has been created to measure vertical displacements along the critical zone near Highbury & Islington station. A geomechanical analysis, provided by a third party, is used to establish the allowable maximum displacement.

A custom wireless sensor module was developed from off-the-shelf components.

This module consists of a sensor device, microcontroller, ADC and RF transmitter. The integration of these components is described in detail. Deployment details and some preliminary results are presented.

Thesis Supervisor: Ruaidhri M. O'Connor Title: Assistant Professor

Acknowledgments

First of all, I would like to thank Prof. Rory O'Connor for his support. His words of encouragement during tough times were really motivating. I am grateful to him for being my advisor.

James Brooks has been a great person to work with. Most of this work has been done with him. His keen insights for solving problems are really inspiring. I appreciate his patience during the tough times we had during this project. I also thank him for listening to some of my fancy suggestions and actually implementing some of them!

I would also like to thank all the members of the CMI team. I am thankful to

Prof. Whittle and Dr. Germaine for their constructive criticism and useful suggestions during the course of this project. Special thanks to Prof. Amaratunga and George Kokossalakis for their help and support for my qualifying exams.

I am thankful to my friends Hari, Phani, Deepak, Dhanush, Sreekar, Bhanu,

Dinesh and Srini for all their support and encouragement. Special thanks to Kunal Kunde and Murthy for all the help over the years.

I am thankful to my relatives for providing me a homely environment whenever I

felt home-sick. Special thanks to Manasa and Jyotsna.

I would like to thank my parents, brother and grandmother for all their love,

support and encouragement.

I would like to dedicate this thesis to my late friend Hari Narayan Sugavanam.

This research was sponsored by the The Cambridge MIT Institute's New Tech-nologies for Condition Assessment and Monitoring of Ageing Infrastructure project.

Contents

1 Introduction

1.1 M otivation . . . .

1.2 Wireless monitoring . . . . 1.2.1 Great Duck Island project . 1.2.2 Wireless Modular Monitoring

1.3 LUL tunnel monitoring project . .

1.3.1 Other Methods . . . . 1.3.2 Proposed Method . . . . 1.4 Present work . . . .

. . . .

. . . .

. . . .

Systems (WiMMS) . . . . . . . . . . . . . . . . 2.2 Accelerometers 2.3 2.4 2.5 2.2.1 Sensing Method . . . . 2.2.2 Capacitive accelerometers . . . . 2.2.3 Piezoelectric accelerometers . . . 2.2.4 Potentiometric accelerometers . . Strain gages . . . .

Linear Variable Differential Transformer Sensors used in the tunnel monitoring pro

2.5.1 Pressure transducers . . . . 2.5.2 Ultrasonic transducers . . . .

j ect

2.6 MEMS 15 15 16 18 18 21 23 23 25 2 Sensors 2.1 Sensors.. ... 27 . . . . 27 . . . . 2 8 28 29 29 29 30 30 31 31 32 36

2.6.1 Fiber optic sensors . . . . 3 Microcontroller 3.1 A rchitecture . . . . 3.1.1 Clocking scheme . . . . 3.1.2 Memory Organization . . . . 3.1.3 Pin Description . . . . 3.2 Features . . . .

3.2.1 Universal Synchronous Asynchronous Receiver Transmitter

(US-ART) module . . . .

3.2.2 Analog to Digital Converter . . . .

3.2.3 Master Synchronous Serial Port (MSSP) module . . . . 3.2.4 Timer and interrupt features . . . .

3.3 Instruction Set . . . . 3.3.1 Programming in assembly language . . . .

3.3.2 Programming in C . . . .

3.4 Operation of the microcontroller . . . .

4 Communications

4.1 Wire-based systems . . . . 4.1.1 Synchronous serial communications 4.1.2 Asynchronous serial communications

4.1.3 Bit-banging . . . .

4.2 Wireless Technologies . . . .

4.2.1 MICA motes . . . .

4.2.2 i-Beans . . . .

4.2.3 Linx Technologies RF ICs . . . .

5 Integration of components

5.1 Interfacing the ADC . . . .

5.2 Interfacing the i-Bean . . . .

8 38 41 41 43 44 46 47 48 48 49 49 50 50 51 51 53 53 53 55 58 59 61 62 64 67 67 74

5.3 Interfacing the ultrasonics . . . . 5.4 Others . . . ..

5.5 Sensor Board . . . .. . . . .

5.6 Protocols . . . . 5.7 Protocol for i-Bean . . . . 5.8 Protocol for the Ultrasonic transducer

6 Summary

6.1 Sum m ary ...

6.2 Directions for future research . . . . . 6.2.1 Wireless technology . . . .

6.2.2 Hardware . . . . 6.2.3 Operating system . . . .

6.3 Conclusions . . . .

A Configuration details for ultrasonic transducer using HE860

B Sample microcontroller program in assembly language and in C B.1 Program in assembly language . . . .

B.2 Program in C ... ... ... .... ... ....

C Latest version of the microcontroller code

C.1 Code to interface the ultrasonics . . . .

D ASCII version of CMI Communication Protocol

78 82 84 86 87 88 91 91 93 93 95 96 96 97 99 99 103 105 121 125

List of Figures

1-1 Architecture diagram for the wireless sensor network deployed on the Great D uck Island [53]. . . . . 1-2 Figure showing the MICA mote in an enclosure [53]. . . . . 1-3 Figure showing the prototype sensing unit for WiMMS [37]. . . . . 1-4 Figure showing a tunnel boring machine used in the CTRL project [13].

1-5 Planned sensor installation in the tunnel. . . . .

1-6 Figure showing the layout of the wireless sensors to measure pressure . . . 2-1 2-2 2-3 2-4 2-5 2-6 2-7

Simplified model of an accelerometer [23]. . . . . Figure showing the parts of an LVDT [36]. . . . . The MPX2200AP pressure transducer [49]. . . . . HE860 positioning device and HE 240 transducer head [25]. E-152, ultrasonic transducer from Massa corporation [40]. .

Figure showing a MEMS accelerometer [3]. . . . .

Figure showing a fiber optic strain gage [59].. . . . .

3-1 Block diagram of Harvard and Neumann architectures. . . . .

3-2 Pin diagram of PIC16F877 [45]. . . . .

4-1 Figure showing the data pulses for sending the character 'U' at a baud rate of 9600... ...

4-2 _{a) figure showing the MICA processor board and b) MICA sensor board [12].} 4-3 Figure showing i-Beans and the base station. . . . .

4-4 Linx RM series a)transmitter [34] b) receiver [33]. . . . .

19 20 21 22 25 26 . . . . 28 . . . . 31 . . . . 33 . . . . 34 . . . . 35 . . . . 37 . . . . 40 42 46 60 61 63 64

5-1 Pin diagram of AD7713 [4]. . . . . 68

5-2 Interfacing the ADC with the microcontroller. . . . . 70

5-3 Timing diagram for writing data to the ADC [4]. . . . . 72

5-4 Timing diagram for reading data from the ADC [4]. . . . . 74

5-5 Interfacing the i-Bean with the microcontroller. . . . . 76

5-6 Timing diagram for the i-Bean provided by Millennial Net, Inc. . . . . 77

5-7 Figure showing the interface between microcontroller and the ultrasonic transducers. . . . . 80

5-8 Figure showing the ultrasonic transducers in boxes. . . . . 82

5-9 Graph of the calibration experiment. . . . . 83

5-10 Schematic of the sensor board. . . . . 85

5-11 Figure showing the sensor board and the wireless sensor unit. . . . . 86

6-1 Figure showing the installed sensors in the tunnel. . . . . 92

6-2 Figure showing the receiving unit and the laptop placed in the access shaft of the tunnel. . . . . 93

6-3 Summary of the wireless sensor operation. . . . . 94

List of Tables

5.1 Table showing the timing constraints provided by Millennial Net, Inc. . . 75

A. 1 Table showing the Configuration String . . . . 97 A.2 Table showing the bits of the PCB for a Beacon . . . . 98 A.3 Table showing the bits of the PCB for a Pilot . . . . 98

Chapter 1

Introduction

1.1

Motivation

Infrastructure 1 monitoring can be defined as the use of sensor technologies to char-acterize a system's condition, performance or response.

Civil infrastructure undergo a lot of changes in their structural and serviceabil-ity characteristics over time. Some of the common reasons for these changes are degradation of material properties, adverse loading and climatic conditions, improper maintenance, etc. These changes may be gradual over time or can be sudden due to events like earthquakes. In order to ensure safety to the public; rehabilitation, retrofit and repair of the infrastructure becomes necessary. To establish the need for maintenance, the existing state of the structure needs to be known for applying the techniques mentioned above.

Apart from the safety issue, economic factors are also important. "Civil infras-tructure systems are generally the most expensive investments/assets in any country (an estimated $ 20 trillion in the United States) and these systems are deteriorating at an alarming rate" [11]. For example, in the United States itself, nearly 50% of the bridges were built before the 1940's. A survey in 1996 showed that 42% of the bridges are functionally deficient or obsolete and the cost of correcting all of these

'The word infrastructure here refers to Civil infrastructure such as buildings, bridges, tunnels, etc.

deficient bridges exceeds 90 billion dollars [41]. The nationwide maintenance cost for

civil infrastructure is estimated at 1.4 trillion dollars [24]. These figures show that

there is a strong necessity to evaluate the state of the existing infrastructure.

The state of the infrastructure can be predicted or estimated by monitoring and

other Non Destructive Evaluation (NDE) methods. A review of different methods

for structural health monitoring can be found here [14]. Monitoring also helps in

understanding the structure's performance and in studying the structural response

during hazardous or other events which might affect the performance of the structure.

Another scenario for infrastructure monitoring is the concept of I-city. "The

concept of I-city, envisages an entire metropolis linked to a web-based monitoring

system" [58]. With the availability of cheap computing power and advances in sensor

technology, the concept of I-city is much more feasible. Again, the chief task in

envisaging such a concept is monitoring.

These are just some of the reasons for structural monitoring. The goal of this

thesis is to explain the different components of a wireless sensor unit, which can be

used for wireless infrastructure monitoring. A specific example of a wireless sensor

unit is described in this thesis. This sensor unit is currently being used to monitor

the behavior of an aging underground train tunnel system in London, UK.

1.2

Wireless monitoring

Wireless infrastructure monitoring makes use of wireless sensor technologies. Wireless

sensors use wireless communications as a means to transmit data to and from a base

station or for inter-sensor communication. Most (if not all) of the monitoring systems

existing in Civil Engineering are wire-based systems. The primary purpose of the

wires is for data communications, and may also be used for powering the sensors.

Some of the important limitations of wire-based systems are:

1. It can be difficult to instrument an existing structure with a wire-based

moni-toring system. This is because of the practical difficulties associated with laying

out wires in an existing structure. Accessibility for laying out wires, and

ing for the wires are major issues. For outdoor installations, the wires need additional shielding from the harsh weather conditions.

2. Wire-based systems are expensive because of the initial cost associated with laying out the wires and the maintenance costs associated with the system. To install a wire-based system, installing the wires alone be approximately 25% of the total system cost and 75 % of the installation time focussed solely on the

installation of system wires [37].

3. Wire-based systems require additional architectural and engineering design to

incorporate wires unobtrusively.

Wireless systems, to a great extent, are devoid of the above mentioned problems. Wireless monitoring systems also have the added advantage of being modular. They can be moved to new locations as needed. The availability of low-powered and cheap computing power (microcontrollers, Digital Signal Processing (DSP) chips, etc.), Ra-dio Frequency (RF) Integrated Circuits and the development of new wireless stan-dards has fueled a lot of interest in wireless sensor systems. Wireless sensor technology is listed as one of the promising ten technologies of the future [60].

A wide variety of research areas related to wireless sensors have emanated. These

include development of new energy-efficient wireless networking algorithms, real-time distributed sensing and control, routing protocols for sensor networks, etc. On the application side, a lot of research is being done in the use of wireless sensor systems for structural and environment monitoring. In structural monitoring, wireless sensors are used for sensing structural integrity during and after an earthquake [20], devel-oping modular systems for monitoring bridges and buildings [38]. In environmental monitoring, wireless sensors are used for monitoring rare and endangered species of plants [54], wild life habitats [22], vineyards [63], etc. In the next couple of sections, a few of these applications are described.

1.2.1

Great Duck Island project

The Great Duck Island habitat monitoring project is a testbed for wireless sensor

network technologies. The wireless sensors used are a custom version of the MICA

motes2 (which are discussed in chapter 4 of this thesis). Great Duck Island lies near

Desert Island, Maine. It approximately contains 5000 pairs of petrels '. The goal of

this project is to study the petrel usage pattern of nesting burrows and to monitor the

environmental changes during the breeding season. One of the primary requirement

of the sensor system was that it should be non-invasive [53]. Presently, around 50

weather motes (for monitoring weather) and around 70 burrow motes (to monitor

temperature, humidity, etc. of the burrows) are deployed.

A specially manufactured weather board was used for this project. This sensor

board contains photo-resistor (for light), temperature, barometric pressure, humidity

and passive infrared sensors (for occupancy). Figure 1-2 shows the MICA mote with

its enclosure. The enclosure is designed for robustness and minimum disruption in

the functionality of the mote. Figure 1-1 shows the architecture diagram for the

monitoring project. More information about this project and live sensor data can be

found at the following site [22].

1.2.2

Wireless Modular Monitoring Systems (WiMMS)

The WiMMS group at Stanford University is concentrating on the development of

low-cost modular structure monitoring systems. The focus is also on using a

de-centralized approach for monitoring. In a de-centralized approach, all the sensors report

to a base station 4. The base station is the unit which is responsible solely for taking

decisions, collecting data, etc. Sensors in this modality usually don't communicate

among themselves. In a de-centralized approach, the sensors have some intelligence

and they can communicate with other sensors and make autonomous decisions if

2

MICA motes are a compact wireless sensor module developed by University of California, Berke-ley and now sold commercially by Crossbow Technology, Inc.

3

A petrel is a kind of seabird.

4

The base station need not be a sensor. It is usually a receiving unit connected to a PC or a laptop.

Figure 1-1: Architecture diagram for the wireless sensor network deployed on the Great Duck Island [53].

necessary [37].

The WiMMS group have developed a wireless sensing unit which has a MEMS5

-based accelerometer, a wireless modem, a 16-bit Analog to Digital Converter (ADC) and a computational core consisting of a 8-bit microcontroller. Figure 1-3 shows the prototype sensing units. The group also has developed another sensing unit that has the additional computational power of a 32-bit microcontroller. This increased power of the microcontroller allows computation to calculate estimates for damage detec-tion using statistical time-series algorithms. Damage detecdetec-tion is predicted from the frequency response of the structure. The validation of the sensing unit was performed on the Almosa Canyon Bridge [39]

Figure 1-2: Figure showing the MICA mote in an enclosure [53].

The positive thing about these monitoring applications is that the research

com-munity is actively interested in using the latest technology to solve some of the

prob-lems in structural and environmental monitoring. Also, these applications serve as

good educational tools. Students can learn how complex systems behave in the real

world. A good example of such a project is the flagpole project at MIT. A virtual

laboratory was developed as a part of this project. The virtual laboratory was created

to " enhance the comprehension of concepts in structural dynamics, sensor

technol-ogy and signal processing using a real-world structural system as a laboratory model"

[58].

The complex nature of civil engineering systems makes it difficult to develop

mon-itoring systems which are more practical and useful. Also, the heterogeneous nature

of the infrastructure means each application is different from the other. Therefore,

Figure 1-3: Figure showing the prototype sensing unit for WiMMS [37].

wireless systems should be designed based on the application rather than designing for a generic application. Thus, for the tunnel monitoring project, a new wireless sen-sor system had to be developed. Some of the reasons for this design are discussed in the subsequent chapters of this thesis. In the next section, the London Underground Limited tunnel monitoring project is described.

1.3

LUL tunnel monitoring project

The core of this thesis describes the design and development of a wireless sensor unit, to be used in a tunnel monitoring application. The wireless sensor unit is intended for deployment in a section of the London underground tunnel system during a disruptive construction activity. The construction activity being the construction of a new tunnel below the existing and operational London underground tunnel. The new tunnel is being bored using a Tunnel Boring Machine (TBM, see figure 1-4). Due to the removal of material to accommodate the TBM movement, some displacement in the existing tunnel is expected. These displacements should not exceed the critical value that forces the tunnel to be shutdown. The goal of this project is to use wireless

technology to measure these displacements in real-time. An additional objective is to

evaluate the potential of wireless technology for monitoring applications.

The new tunnel that is being constructed is a part of the Channel Tunnel Rail

Link (CTRL) Project. The CTRL is a high speed line running for 109 km between St.

Pancras station in London and the Channel Tunnel. This project is being constructed

in two sections named Section I and Section II. Section I was opened and became

operational in September 2003, while Section II is expected to be completed by the

end of 2006. The new tunnel is a part of the work comprising of construction of twin

7.5 km bored tunnels from Stratford station to King's Cross station [13]. The existing

tunnel's diameter is around 12 ft and is a section of the Victoria Line (Northbound).

The nearest station to the tunnel section of interest is Highbury & Islington. The new

tunnel will be perpendicular to the existing tunnel. See figure 1-5

1.3.1

Other Methods

A recently established method to monitor displacements due to excavation makes use

of electrolevels. This method is also used for monitoring the section of the tunnel

where the wireless sensor system will be deployed [28]. An electrolevel is a " robust, stable, gravity referenced tilt transducer" [15]. Tilt can be thought of as being

synony-mous with slope. Electrolevels are used for measuring the distortion and alignment of

various parts of a structure [15]. Using electrolevels is one of the ways of to measure

displacements vertically in the direction of the tunnel. The tilts are integrated over

length to get vertical displacements in the direction of the tunnel. This method has

the following disadvantages.

" Errors get accumulated over the distance and are also exacerbated through the

integration process.

" System is not redundant in the sense that if one of the electrolevels fail, all

subsequent electrolevels become disconnected from the absolute reference frame.

Another potential way of measuring these displacements is to use fiber optics. The research group at the Department of Civil Engineering, The University of Birming-ham, UK is studying the use of fiber optic sensors for tunnel monitoring. The goal is to develop a remote system for measuring tunnel settlement, distortion and rotation of tunnels. Their current research is focused on developing a test rig for simulating tunnel movements and measuring them using fiber optic sensors and strain gages [17].

Some of the limitations aforementioned are reduced by the wireless tunnel moni-toring scheme proposed below.

1.3.2

Proposed Method

The two critical values that need to be measured continuously in this project are the vertical displacements along the length of the tunnel and the transverse deformation at the section where the tunnel is directly undercut by the TBM. This section is the critical section. These displacements are calculated in the following manner.

1. Vertical displacements along the length of the tunnel: Measurements of the

relative change in the absolute pressure in a fixed hydraulic line are used for calculating these displacements. The hydraulic line is laid out along the length of the section of a tunnel (around 50 m of length) and is connected to a fixed reservoir at a known reference location (in this case, this is the access shaft of the tunnel). The relative (with respect to a fixed point) pressure variation in the tube is then proportional to the vertical displacement. Absolute pressure at a few points along the length of the tunnel is measured using pressure transduc-ers. The absolute pressure is measured with respect to the fixed reservoir. This is depicted in figure 1-6. At these points, the boxes containing the sensor boards are fixed. These points are approximately placed every 5 meters. The sensor farthest from the construction activity is assumed 6 to be the fixed point and is used as a reference for calculating the displacements. After taking the pressure measurements, these data are sent wirelessly over to the base station. Temper-ature is also measured on the sensor board to correct the pressure transducer measurements for temperature. These adjustments are based on calibration data previously determined in a controlled laboratory setting.

2. Transverse deformation at the critical section: Two ultrasonic transducers fac-ing each other are to be installed in the tunnel for calculatfac-ing the transverse displacements at the critical section. The transducers measure the time of flight for an acoustic signal between the pair and use this measurement to calculate the distance between them. Any change in the transducer measurements indi-cate movement of the transducers or deformation at the critical section '. After the distance between the two transducers is calculated, the distance readings are sent wirelessly to the base station. Figure 1-5 shows how the sensors are to be installed in the tunnel.

6

This point is assumed to be fixed. If this point moves, the displacements calculated will be relative to the displacement of this fixed point.

7

This change in transducer measurements could be also due to a change in temperature, pressure relative humidity etc. Calibration for these factors is also necessary. Only preliminary calibration has been performed to date, and additional benchmarking remains necessary for accurate measurement.

50 m (monitoring distance)

12 ft

Highbury &Islington Nearest Station (Circle Line)

D

)

Critical Section

Hydraulic Line & Wireless Sensor Node Ultrasonic Transducer

Figure 1-5: Planned sensor installation in the tunnel.

The proposed scheme is considered better than that of the electrolevels for the following reasons:

" The system is redundant in the sense if one of the pressure transducers fail, it is

still possible to calculate displacements. Although, if the hydraulic line breaks, the system also fails.

" It is easier to install this system.

* It is relatively cheaper and reusable for other similar applications.

1.4

Present work

In this thesis, a detailed discussion of the different components of a wireless sensor unit for infrastructure monitoring and integration of these components to construct the wireless sensor unit are presented.

Chapter 2 discusses the various sensors that are commonly used in civil infras-tructure monitoring and then talks about the sensors used in this project.

loin

10

Channel Tunnel Rail Link (CTRL)

Reservoir

()

Hydraulic

Line

Pressure sensor

with

RF module

Figure 1-6: Figure showing the layout of the wireless sensors to measure pressure

Chapter 3 discusses microcontroller architectures and specifically PIC16F877, which has been used in this project. The architecture, memory details and other relevant features of this microcontroller are explained.

Chapter 4 discusses the various forms of communication arising in this project. In particular this chapter addresses synchronous and asynchronous (wire-based com-munications) communication standards and some relevant wireless technologies that have been tested during the course of this project.

Chapter 5 describes how the various components were integrated so as to construct the wireless sensor unit.

Chapter 6 summarizes the thesis and states the directions for future research.

Chapter 2

Sensors

This chapter gives a brief overview of some sensor types used for various monitor-ing applications. These include the accelerometer, strain gage and Linear Variable Differential Transformer (LVDT). Following this, a brief overview of the sensors used for the tunnel monitoring project is given. In the later sections a brief discussion on

MEMS and fiber optic sensors follows.

2.1

Sensors

A sensor is a device which measures a physical quantity. Sensors are typically used

for measuring the parameters which affect a system's state. Common examples in-clude temperature sensors, pressure sensors, etc. For monitoring applications, sensors provide the data by measuring various physical parameters over time. A transducer is a subset of a sensor; a transducer converts the physical quantity measured into an electrical signal proportional to the value of the measured physical parameter [50]. The electrical output of the transducer (which is an analog signal) can be digitized and can be processed using a microcontroller or a Digital Signal Processing (DSP) chip. To convert an analog signal to a digital signal, an Analog to Digital Converter

(ADC) is typically used.

In the next couple of sections, a brief description of some of the more commonly used sensors in monitoring applications for Civil Engineering systems is given. These

are accelerometers, strain gages and LVDT.

2.2

Accelerometers

As the name suggests, accelerometers or acceleration transducers measure

accelera-tion. Measures of acceleration can also be interpreted to detect tilt, vibration and

shock [50]. Linear acceleration is measured in m/s2, or often expressed in terms of

g's, where g is the gravitational constant. The standard value of g is 9.81m/s 2. There

are many types of accelerometers and a few of which are discussed below.

2.2.1

Sensing Method

Accelerometers use a "sensing method in which the acceleration acts upon a seismic

mass (proof mass) that is restrained by a spring and whose motion is usually damped

in a spring-mass system" (figure 2-1). Acceleration is proportional to the distance

moved by the proof mass from its reference position [23].

Figure 2-1: Simplified model of an accelerometer [23].

28

Spring

Mass

2.2.2

Capacitive accelerometers

In this type of accelerometers, the proof mass is a freely suspended electrode, which moves with respect to a fixed electrode or stator plate. When acceleration is applied, the distance between the electrodes change and this changes the capacitance. This change in capacitance measured is proportional to the value of the acceleration [50].

2.2.3

Piezoelectric accelerometers

The basic principle of these accelerometers is piezoelectric transduction. The proof mass is connected to a piezoelectric crystal. The piezoelectric crystal is pre-loaded and this loading changes whenever acceleration is experienced. This changes the electrical output of the piezo-material. This gives a relative measure of the acceleration. Typical piezoelectric materials include quartz crystals, Barium Titanate, etc. Piezoelectric transducers usually have a low output signal and very high output impedance. This requires the use of amplifiers which not only magnify the signal, but also act as impedance converters [50].

2.2.4

Potentiometric accelerometers

These accelerometers can be produced at a relatively low cost, have moderate, but acceptable accuracy and a limited frequency range. The displacement of the spring-mass system causes a displacement of a potentiometric element '. This change in resistance is measured to get the value of the acceleration.

Details about other types of accelerometers can be found at [50] and [23]. In the next section, strain gages are discussed

1

Potentiometric element has a potentiometer and the resistance of this element changes because of the displacement.

2.3

Strain

gages

Strain is defined as "the ratio of dimensional change to the total value of the dimension

in which the change occurs" [50]. Since strain is dimensionless (as it is a ratio of two

physical quantities having the same units), there is no specific unit for strain. It is

usually measured in percent or microstrain.

Strain is usually measured using piezoresistive materials. Piezoresistive substances

undergo a change in their resistance when a stress is applied. Stress is related to

strain by the modulus of elasticity, and is defined as the ratio of stress to strain. The

sensitivity of a strain gage is measured by the gage factor which is defined as the ratio

of unit change in resistance to the ratio of unit change in length.

GageFactor, GF =AR/R = AR/R

AL/L C

The resistance change of a strain gage is converted into voltage using a Wheatstone

bridge [50]. Details about different kinds of strain gages can be found at [50] and [23].

2.4

Linear Variable Differential Transformer

Linear Variable Differential Transformer (LVDT) is used for measuring displacement.

The absence of physical contact of the sensing element gives the LVDT a high degree

of robustness [35]. The LVDT consists of one primary and two secondary magnetic

coils and a magnetic core. In its null position, the magnetic field in the two secondary

coils gets canceled. Hence, at its null position, the output of a LVDT is zero volts.

When the magnetic core is displaced, there is be an imbalance in the electromagnetic

field in the two secondary coils. This imbalance in electromagnetic field causes a

differential output voltage that is proportional to the displacement and the direction

of displacement. Apart from being a very rugged instrument, an LVDT has a high

resolution [36]. Figure 2-2 shows the different parts of an LVDT.

In the next section the sensors used in the London underground tunnel project

are discussed.

COIL WINDING ASSEMBLY

MAGNETIC CORE

ELECTROMAGNETIC SHIELDING

CYLINDRICAL CASE

Figure 2-2: Figure showing the parts of an LVDT [36].

2.5

Sensors used in the tunnel monitoring project

The sensors used in the tunnel monitoring project are the pressure transducer and the ultrasonic transducers. Though ultrasonic transducers haven't yet been installed in the tunnel, they are described for completeness. It is anticipated that the ultrasonics will be deployed in future monitoring projects.

2.5.1

Pressure transducers

Pressure transducer measurements could be absolute (i.e. with reference to a vac-uum or zero pressure) or relative (with respect to atmosphere). Pressure transducers are widely used in pressure control systems, flow measurement systems, vehicle air bag deployment, etc [50]. There are different types of pressure transducers. Com-mon types are piezoelectric pressure transducers, strain-gage pressure transducers,

capacitive pressure transducer, etc. Information about different types of pressure transducers can be found here [50]. The pressure transducer used in the project is the MPX2200AP from Motorola Corporation (shown in figure 2-3). This pressure transducer is a silicon piezoresistive sensor.

A piezoresistive substance changes its resistance when it is stressed. In a

piezore-sistive transducer, when there is a change in pressure at the input of the transducer, the stress on the piezoresistive material also changes and hence its resistance. This change in resistance is usually detected as a differential voltage across a bridge circuit. Some of the advantages of using a mono-crystalline semiconductor as the piezoresis-tive element are:

* "High sensitivity > 10 mV/V" [52].

0 "Good linearity at a constant temperature" and the "ability to track pressure

changes without hysteresis" [52].

Some of the disadvantages are large initial offset, strong non-linear dependence on temperature and strong drift with temperature. These can be compensated with proper electronic circuitry. More details can be found here [52].

The MPX2200AP measures absolute pressure (i.e. with reference to a vacuum or zero pressure). It has a good linearity (±0.25 % linearity) and is temperature compensated over the range of 00C to 85 C. Other details of this transducer could

be found here [49]. The differential outputs of the transducer are connected to an

ADC. The ADC converts the analog output of the transducer to digital format. The

digital output from the ADC is collected by the microcontroller. These details are discussed in Chapter 5.

In the next section, the ultrasonic transducers are discussed.

2.5.2

Ultrasonic transducers

Ultrasonic waves are those sound waves whose frequency is greater than 20 kHz. Ultrasonic transducers can be classified into two kinds. They are:

Figure 2-3: The MPX2200AP pressure transducer [49].

" Magnetostrictive transducers, which are mainly used in ultrasonic cleaning baths (used by watchmakers and in the electronics industry) [57].

" Piezoelectric transducers, they have low power consumption than the magne-tostrictive transducers and are used in security devices etc. [57].

There are two important parameters related to ultrasonic transducers. They are beam spread and target angle. Beam spread defines the maximum spreading of the ultrasonic sound beam from the transducer. The target angle measures the maximum amount by which the target can be tilted and still be detected by the ultrasonic transducer [57].

Ultrasonic transducers are used to calculate distances by using the time of flight principle. The precision of ultrasonic transducer system allows it to be used to mea-sure small movements (< 1 mm) [25]. The idea of using ultrasonic transducers is to measure deformations at the critical section. That is, the cross-section of the tunnel below which the tunnel boring machine will pass. This is where the largest vertical

displacement and related deformation of the tunnel liner occur. The deformations

might be in the order of 2 mm or less [56]. The original idea was to place ultrasonic

transducers along the circumference of the tunnel and calculate the distance between

any two pair of transducers and observe any changes in these distance measurements

over time. The change in distance is an indication of relative deformation at the

critical cross-section 2

The transducers that were chosen in order to achieve the goal were the HE240

series of transducers from Hexamite Devices (shown in figure 2-4). In addition to

the transducers, a positioning device is needed (for each transducer) which has the

necessary electronic and control circuitry for operating the ultrasonic transducers.

The positioning device measures the time of flight and calculates the distance from

this measurement and sends this measurement over a RS-485 network to a controlling

device. The positioning device that was chosen was HE860 (from Hexamite Devices)

[25].

Figure 2-4: HE860 positioning device and HE 240 transducer head [25].

One of the main obstacles in using the transducers was the problem of unstable

readings. The transducers gave very unstable readings when they were not facing each

other and moderately stable readings when they were directly facing each other. The

2

This is not strictly true as temperature and other factors might affect the distance measurements made by the ultrasonic transducers

reason for this might be a narrow beam angle of the Hexamite ultrasonic transducer.

This problem was partially solved when a transducer head from another company

(Massa Corporation, figure 2-5) was used [40]. The probable reason for this could

be a wider beam angle of the Massa transducer head than the Hexamite transducer

head. Because of the narrow beam angle, it was decided that only two transducers

will be used,and these transducers will be installed such that they are facing each

other directly (see figure 1-5). Some of the results of the tests that were done using

the transducers from these two different companies are discussed in Chapter 5.

Figure 2-5: E-152, ultrasonic transducer from Massa corporation [40].

Each ultrasonic transducer is configured either as a pilot or a beacon. One of the

pilot nodes (called the master node), initiates a timing distance acquisition signal.

The master node transmits a synchronization signal (electrical) and measures the

time to receive an ultrasonic wave from the beacons. This measured time gives the

distance between the pilot and the beacon nodes [25]. Chapter 5 gives the details

of the interface between the microcontroller and the ultrasonic transducer. Details

of configuring the ultrasonic transducers using the positioning device are given in

Appendix A.

In the next two sections, two (relatively) new sensor technologies are briefly

sen-sors. These new technologies will play an important role in the future monitoring applications.

2.6

MEMS

Micro Electro-Mechanical Systems or MEMS "is the integration of mechanical ele-ments, actuators, and electronics on a common silicon substrate through the use of microfabrication technology" [23] . As evident from the name, the size of MEMS de-vices are on the order of the microns (10-6). Since bulk fabrication techniques from the micro-electronic devices can be applied (lithography, etching, micro-machining, etc. ), mass fabrication of these devices make them potentially cheap and more reli-able than their macro-mechanical counterparts. Three of the main application areas

of MEMS are:

* Biotechnology, applications of MEMS in biotechnology include developing biochips for detecting hazardous chemicals and biological agents and developing mi-crosystems for drug delivery [42].

" Communications, the focus in this area is to develop MEMS-based electronic

components that can be used for making low-power and low-cost, high-frequency RF circuits. An important use for this is in making light-weight cellular phones [42]. Other important application is developing micro-mirrors for telecommuni-cations.

" Accelerometers, low-cost MEMS accelerometers are being used for crash air-bag

systems in automobiles [42]. MEMS (figure 2-6) accelerometers are also being used in wireless sensors. A particular example of this is the MICA mote devel-oped by University of California, Berkeley and Crossbow Corporation. Details of the MICA mote are provided in Chapter 4.

MEMS have great potential in monitoring applications providing low-cost and

low-power sensors. Unfortunately, there are some challenges in using this technology

widely. They are:

Figure 2-6: Figure showing a MEMS accelerometer [3].

1. For manufacturing or prototyping MEMS devices, micro-fabrication technology is required and this fabrication technology is not easily accessible. Building such a fabrication unit is very expensive [42].

2. Packaging MEMS is more difficult than packaging ICs. One reason is protection against the harsh environments in which MEMS devices may be used. This is because of the harsh environments in which MEMS devices may be used. Also, the packaging has to be customized for different applications. Packaging MEMS devices is one of the major obstacle the MEMS community is facing [42].

3. Numerical and simulating tools for MEMS are very limited [42].

4. In order to design a MEMS device, the designer would need an extensive knowl-edge of fabrication techniques. The process of fabricating even a simple MEMS

device requires a lot of effort and research [42].

More details on MEMS can be found here [42].

2.6.1

Fiber optic sensors

This section focuses on fiber optic sensors. Fiber optic sensors have existed for quite

some time (past 10 years approximately) and hence is not a new technology. "Fiber

optics (optical fibers) are long, thin strands of very pure glass about the diameter of a

human hair. They are arranged in bundles called optical cables and used to transmit

light signals over long distances" [27]. Fiber optic sensors can be broadly classified as

extrinsic and intrinsic sensors. Extrinsic sensors use an optical fiber to convey light to

a sensing element and use the same or another optical fiber to convey the processed

light to a photo detector. On the other hand, intrinsic sensors are those sensors, which

use a sensing mechanism that is a part of the optical fiber [41]. Another classification

of fiber optics sensors is to classify them on the property of light that is affected by

the sensing mechanism. They are:

" Intensiometric sensors: these sensors use the variation of light power transmitted

through optical fiber for sensing mechanism [41].

* Interferometric sensors: these sensors use the changes in phase of the light as

the sensing mechanism [41].

" Polarimetric sensors: these sensors measure temperature or strain by detecting

the changes in polarization eigenmodes of an optical fiber [41].

" Modalmetric sensors: these sensors use the changes in "transverse spatial mode

distribution of light" as the sensing mechanism [41]

" Spectrometric sensors: these sensors use Raman and Brillouin scattering

mech-anisms for determining temperature profiles [41].

Details of these mechanisms is beyond the scope of the thesis and more information

could be found here [41]. Some of the important applications (related to monitoring) of fiber optic sensors are:

1. For bridge monitoring fiber optic sensors are potentially useful in performing

the following tasks [41].

(a) Fiber optic sensors could be used in evaluating the long term

perfor-mance of new construction materials like Carbon Fiber Reinforced Poly-mers (CFRP).

(b) For studying the wind and seismic response of bridges by integrating Fiber

Reinforced Polymer (FRP) strengthening elements with fiber optic sensing technology. This can also be used for studying bridge vibration due to traffic loading and other environmental conditions.

2. Fiber optic sensors can be used to measure other mechanical and thermal infor-mation. For example, a fiber optic strain gage is shown in figure 2-7. They can also be used as both the sensor and the data channel which has such uses such as measuring strain and temperature information from any point of its length [41].

(a) They could be used for monitoring the degree of cure and temperature of concrete structures and hence indicate when it is appropriate to post-tension or moving the concrete structure [41].

Important considerations that should be kept in mind while using this technology is the integrity of fiber optic sensors such that installation should not affect the light properties adversely. More details on fiber optic sensors can be found here [41]

Smart sensors are those sensors which integrate some kind of intelligence from a microcontroller, DSP chip, or an application-specific integrated circuit (ASIC) [18]. The wireless sensor units developed for the tunnel monitoring project are smart sen-sors as they have integrated microcontrollers. In the next chapter, the brain of the wireless sensor unit, the microcontroller is discussed.

Chapter 3

Microcontroller

A Microcontroller is a miniature computer. It is an Integrated Chip (IC) that has a

Central Processing Unit (CPU), Random Access Memory (RAM), Read Only Memory

(ROM) and other components that are also present in a computer. The present

chap-ter describes the PIC 'family of microcontroller in detail. Specifically, the PIC16F877

microcontroller is examined, which is used in this project. The PIC microcontroller

is manufactured by Microchip Technology, Inc.

In this chapter, the architecture and instruction set for the PIC mid-range devices are described. In the architecture section, the clocking scheme, the memory orga-nization and pin description of PIC16F877 microcontroller are discussed. Specific features of the PIC16F877 microcontroller are described and finally the operation of the microcontroller on the sensor board is described.

3.1

Architecture

This section explains the architecture of PIC mid-range microcontrollers 2. The clock-ing scheme, memory organization and pin description of the PIC16F877 microcon-troller are described.

iPIC stands for Peripheral Interface Controller. PICgis a registered trademark of Microchip Technology, Inc.

2PIC microcontrollers which have 14 bit length instructions are classified as mid-range microcon-trollers

Many of the present day microcontrollers (including the PIC16F877) have a

Har-vard Architecture. In HarHar-vard Architecture, the program memory and data memory

reside separately in two different blocks of memory. While in a Neumann

Architec-ture, the program and data memory reside in the same block of memory. This is

depicted in the figure 3-1, which shows the block diagrams of both the architectures.

As seen in the figure below, in a Neumann Architecture a single or a shared bus is

used to access data and program memory, as they reside in the same block of the

memory. In Harvard Architecture separate/different buses are used for accessing the

program and data memory. This allows execution of an instruction using data from

the data memory and fetching of the next instruction from the program memory[31]

in the same instruction cycle.

PROGRAM MEMORY

DATA PROGRAM MMR

MEMORY CPU M MEMORY CPU

DATA MEMORY

a) Harvard Architecture b) Neumann Architecture

Figure 3-1: Block diagram of Harvard and Neumann architectures.

Apart from having a Harvard Architecture, the PIC16F877 has some common

fea-tures that are found in other RISC (Reduced Instruction Set Computers)

microcon-trollers

3

_[47].

_{They are discussed below:}

" Long word instructions, each instruction is 14 bits long for a PIC16F877

micro-controller.

" Single word instructions: the microcontroller has a 14 bit program memory bus

which can access the program memory. Since the instructions are single word

3

RISC microcontrollers have an instruction set comprising of very few instructions. The PIC16F877 microcontroller has only 35 instructions.

instructions, all the locations in the program memory are all valid instructions. This leads to a better utilization of the program memory. This also implies that the microcontroller has to read just a single memory location in the program memory for executing an instruction.

3.1.1

Clocking scheme

In this section, the clocking scheme and instruction cycle of the PIC microcontroller are described. A microcontroller needs a clock input to synchronize and execute the instructions in its program memory. The external clock input to the microcontroller is through pins OSC1 and OSC2. The PIC16F877 microcontroller communicates with the external world using the pins. The pin description of the PIC16F877 is given later. The four valid clock modes are4 described below:

1. RC mode. In this mode, an external resistor and a capacitor circuit provides

a clock input to the microcontroller on the OSCI pin. The frequency of the clock input is a function of the supply voltage, and the values of the resistor and capacitor. The device data sheet has more details regarding the range of resistors and capacitors that can be used [45]. In this mode, the microcontroller outputs a clock pulse on the OSC2 pin, which can be used for clocking other devices. The frequency of this clock pulse is one-fourth of the frequency of the

RC clock signal on the OSC1 pin.

2. LP mode. In this mode, the microcontroller uses an external crystal of low speed (usually less than 2 MHz, for exact values, see [45]). Of all the 4 modes, this mode consumes the least power.

3. XT mode. In this mode, the microcontroller uses an external crystal. Typical clock speeds include 2-4 MHz [45].

4. HS mode. In this mode, the microcontroller uses an external crystal. The clock

4

All the modes that are discussed here are the external clocking modes. Some microcontrollers have internal oscillators/clocks. The PIC16F877 does not have an internal oscillator.

speeds are usually greater than 4MHz. Of all the 4 modes, this mode consumes the highest power.

The clock input from the OSCI pin of the microcontroller is formed into four non-overlapping quadrature clock cycles namely Q1, Q2,

Q3

and Q4. These four clock cycles together form an instruction cycle for the microcontroller. During an instruction cycle, an instruction is executed and the next instruction is fetched si-multaneously. Hence, each instruction of the microcontroller takes a time of 4/F..c seconds, where F,,, is the frequency of the clock input to the microcontroller. Com-plex instructions like GOTO, which branch the program take more than one instruction cycle to execute [45]. Understanding clocking scheme and the instruction cycle helps in writing better programs to program the microcontroller.

3.1.2

Memory Organization

In this section, the memory organization of the PIC16F877 is discussed. The mem-ory of a PIC microcontroller is divided into two main components. They are the program memory and the data memory. The PIC16F877 is an Electrically Erasable Programmable Read Only Memory (EEPROM) device (also known as a flash micro-controller) [45].

Program Memory

The program memory of the PIC16F877 microcontroller is an EEPROM. The size of this memory is 8K X14 bits (as each instruction is 14 bits long). This program memory is divided into four equally sized blocks called pages. The two important locations in the program memory are:

* Reset vector. This is the address at which the microcontroller looks for instruc-tions whenever there is a device reset or whenever the microcontroller starts. The address of the reset vector is Oh 5.

5

0h is 0 in hexadecimal notation.

* Interrupt vector. This is the address to which the program branches

when-ever there is an enabled interrupt. Interrupts are special events which might

have to be serviced depending upon the application. Common examples are

timer interrupts, serial port interrupts, etc. In order to service an interrupt, the interrupt enable flag for that particular interrupt and the global interrupt

flag should be set. If the aforementioned flags are set, whenever the interrupt

occurs, the microcontroller services the interrupt by making the program jump

to the interrupt vector. Enabling and disabling of the interrupts is done in the

program.

Data Memory

Data Memory can be divided into three categories. They are:

1. General Purpose Registers. General Purpose Registers (GPR) and the Special

Function registers (SFR) form the RAM (Random Access Memory)6of the mi-crocontroller. The size of this RAM is 368 bytes for PIC16F877 [45]. GPR are

used for storing the variables and other temporary values that are used in the

program.

2. Special Function Registers. The SFR controls the various core and

periph-eral features of the microcontroller. The core features are the features

neces-sary for the microcontroller to operate. These include the STATUS, PCL, etc.

The peripheral features include features like the Analog to Digital Converter

(ADC), USART (Universal Synchronous Asynchronous Receiver Transmitter),

etc. These features are explained later in this chapter.

3. Data EEPROM. The Data EEPROM is an EEPROM. This can be used for

storing permanent data or some other valuable information. The

microcon-troller uses a protocol for writing and reading data from the data EEPROM

6

Similar to the computer's RAM, the microcontroller's RAM is volatile. After a device reset all the values in the RAM registers are reset to their reset values.

so that the existing data is not written over accidentally. PIC16F877's data EEPROM has 256 locations and the size of each location is 1 byte [45].

3.1.3

Pin Description

As stated earlier in this chapter, the microcontroller communicates with the exter-nal world using its pins. In this section, the pin layout of PIC16F877 is described. Figure 3-2 shows the pin diagram of PIC16F877. These can broadly be classified as Input/Output (I/O) pins and power pins. Each class is now described.

TIONPP/THw - R D RAW/ANO _m( RA1/AN1 rs-a[ RA24AN2NREF-..- ₍ R A3ANNRIF + -RAC/TOCKI _e[ RAFWAN4/b"7I r--[ REBM"EAN5 S-RE E1/WRAN6 m[ RE2CIAN7 L VDD -B -[ OSC1UCLKIN -[ OSC2/CLKOUrT r---[ RCT10SO/T1CKI ,m-E [R RC1ITISVCCP2 "--[ RC2VCCP1 _L-[ RC3SCKISCL [ RDCVPSP0 e--[ RD1/PSP1 mC 1

K740

2 39 3 38 4 37 6 36 6 35 7 34 8 33 9 _ 32 10 r 31B 11 CD 30 12 CD 29 13 28 14 27 is 26 16 25V 17 24 18 23 19 22 20 21 R87NPGD RBGJPGC RBS RB4 RB3jPGM~ RB2 RB:1 RB0.1NT VD N/SS RD7,rPSP7 RDGPSP6 RDWPS RD4VPSP4 RC7/RXQDT RC&TX(CK RMsSDO RC4/SDIX-RDA RD30PSP3 RD)+PSP2

Figure 3-2: Pin diagram of PIC16F877 [45].

I/O pins

I/O ports are the ports by which the microcontroller communicates with external

devices for I/O operations. The PIC16F877 has 40 pins, of which 33 are I/O pins.

Most of the I/O pins are multiplexed to perform other functions. The 33 I/O pins are divided into 5 ports named A,B,C,D and E. The registers corresponding to these ports

46 J --] '4--b S-j ] ---) --. ] 4--] -3 ) 3-a :]1 --- ,

3 s-a

]3 -a 3]--.

are PORTA, PORTB, PORTC, PORTD and PORTE respectively [45]. An I/O pin

can be either configured as input or output by setting or clearing7 the corresponding bit in a corresponding register. These registers are referred by the prefix TRIS.

(TRISA corresponding to PORTA and so on). TRIS stands for tri-state. As the name

suggests, tri-state devices have three states. These are logical 1, logical 0 (when the

device is in its output mode) and off state (when the device is in its input mode). For

example, to configure PIN6 of PORTB as an input pin, bit 6 of the TRISB register

has to be set high or logical 1.

Power pins and other pins

These pins correspond to the pins which are either connected to the power or ground

lines. The other pins apart from the power pins are the pins for the clock input, which are OSCI and OSC2.

3.2

Features

In this section, some of the important features of the microcontroller are described.

These features are not the complete set of features. Details of other features can be found here [45]. The features that are now described are:

1. Universal Synchronous Asynchronous Receiver Transmitter (USART) module

2. Analog to Digital Converter (ADC)

3. Master Synchronous Serial Port (MSSP) module

4. Timer and interrupt features

7

By set we mean writing a "1" and by clearing we mean writing a "0" at the appropriate memory location.