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\begin{document}

\title{{\color{RED}
Sensor Networks for Structural Monitoring: Status, Plans, Problems}
\vspace{2cm}}
\author{
{\bf Ananth Grama}
}

\date{}

%is, if you were going to be putting this together next week, what
%would you do and what challenges you would face and how you would
%solve them.. if you send me a draft at some point, we can iterate
%a couple of times.

\maketitle
\thispagestyle{empty}

\foilhead{{\color{RED} Goal}}
\noindent
{\color{blue}Designing sensing infrastructure for real-time, high fidelity physical measurement retrieval} \\ \\
Test infrastructure: three story $30'\times 50'$ structure \\ \\

\centerline{\psfig{file=str.ps, width=11cm}}
\begin{itemize}
\item Physical phenomenon to be measured:
	\begin{itemize}
	\item Linear displacement
	\item Multi-axial acceleration
	\item Structural strain
	\end{itemize}
\end{itemize}

\foilhead{{\color{RED} Linear Displacement Sensing}}
\begin{itemize}
\item Four commonly-used options:
	\begin{itemize}
	\item Optical tracking
	\item Time of flight and triangulation sensors
	\item Magnetic, inductive and capacitive sensors
	\item Strain gauges and resistive sensors
	\end{itemize}
\end{itemize}

\foilhead{{\color{RED} Linear Displacement Sensing}}
\begin{itemize}
\item {\color{blue} Optical tracking}
	\begin{itemize}
	\item {\color{blue} Mechanism}
		\begin{itemize}
		\item Markers on the surface
		\item Cameras capture mark displacement
		\item Image processing calibrated to measure distance
		\end{itemize}
	\item {\color{GREEN} Pros}
		\begin{itemize}
		\item Non-contact sensing
		\item Simple and safe installation
		\end{itemize}
	\item {\color{red} Cons}
		\begin{itemize}
		\item Expensive video capture equipment
		\item Difficult calibration
		\item High processing requirements
		\item High data transfer rates
		\end{itemize}
	\end{itemize}
\end{itemize}

\foilhead{{\color{RED} Linear Displacement Sensing}}
\begin{itemize}
\item {\color{blue} Time of flight and triangulation sensors}
	\begin{itemize}
	\item {\color{blue} Mechanism}
		\begin{itemize}
		\item Sound or light (laser or IR) waves incident on the surface
		\item Measures round trip time or phase shift of the wave to measure distance
		\end{itemize}
	\item {\color{GREEN} Pros}
		\begin{itemize}
		\item Laser sensors have high accuracy (up to few microns) and
			wide measurement ranges (up to few feet)
		\item Ultrasonic sensors are safe, most laser sensors in the market are eye-safe
		\item Most products usually ship with RS-232 I/O interface
		\end{itemize}
	\item {\color{red} Cons}
		\begin{itemize}
		\item Ultrasonic sensors do not have high accuracy and depend on ambient conditions
		\item Laser sensors are very expensive ($\approx \$ 2,000$) %$
		\item Requires 12V DC power supply
		\end{itemize}
	\end{itemize}
\end{itemize}

\foilhead{{\color{RED} Linear Displacement Sensing}}
\begin{itemize}
\item {\color{blue} Magnetic, inductive and capacitive sensors}
	\begin{itemize}
	\item {\color{blue} Mechanism}
		\begin{itemize}
		\item Magnetic and inductive sensors use the relative movement of a soft iron core
			and a magnetic field to measure displacements
		\item Capacitive sensors measure capacitance changes as distance or overlapping area
			between conductive plates change
		\end{itemize}
	\item {\color{GREEN} Pros}
		\begin{itemize}
		\item These sensors are very cheap
		\end{itemize}
	\item {\color{red} Cons}
		\begin{itemize}
		\item Capacitive sensors and some magnetic sensors have a very short range and span of distances
		\item Most magnetic and inductive sensors with higher ranges involve the movement of
			concentric rods (one attached to the moving surface and other to the fixed support)
		\item Sensors are affected by ambient conditions like temperature and humidity
		\end{itemize}
	\end{itemize}
\end{itemize}

\foilhead{{\color{RED} Linear Displacement Sensing}}
\begin{itemize}
\item {\color{blue}Strain gauges and resistive (potentiometer) sensors }
	\begin{itemize}
	\item {\color{blue} Mechanism}
		\begin{small}
		\begin{itemize}
		\item A shaft in contact with the surface moves, resulting in electrical resistance
			change which is translated to distance
		\item Another design has an extensible chord and a mountable base, whereby pulling
			the chord results in change of resistance of a potentiometer in the base
		\end{itemize}
		\end{small}
	\item {\color{GREEN} Pros}
		\begin{itemize}
		\item These sensors are very cheap
		\item The chord version can extend up to thousands of inches
		\item Essentially the resolution is infinite, however resistance variability may cause
			inaccuracies
		\item Chorded version also allows velocity measurement
		\end{itemize}
	\item {\color{red} Cons}
		\begin{itemize}
		\item Involves moving parts implying low life due to wear
		\item Need external circuitry and calibration
			to translate voltage differences to distance
		\item Requires 25V supply; variation in input voltage may also cause inaccuracies
		\end{itemize}
	\end{itemize}
\end{itemize}

\foilhead{{\color{RED} Linear Displacement Sensing}}
\vspace{0.1\textheight}
\begin{itemize}
\item 
{\color{BLUE} Use of a few laser sensors for high precision measurements and several
chorded potentiometers appear to be an attractive option.}
\item
Battery packs can be used to supply power, however the life expectancy of such
power packs needs to be evaluated.
\item Chorded potentiometers can also be used to evaluate structural strain
\end{itemize}

\foilhead{{\color{RED} Multi-axial acceleration}}
\begin{itemize}
\item 2-{\em axis} and 3-{\em axis} accelerometers are available
\item Most options have are MEMs devices with small size and low power requirements
\item Most common sensors have +/- 2{\em g} range and with +/- 2 m{\em g} sensitivity, and are
available for only tens of dollars
\item Higher end accelerometers are available for a couple of hundred dollars
\item Example: MTS420CA is a data acquisition board with 2-{\em axis} accelerometer, developed by
UC Berkeley and Intel and is compatible with MICA2
\end{itemize}

%\foilhead{{\color{RED} Structural Strain}}
%\begin{itemize}
%\item
%\end{itemize}

\foilhead{{\color{RED} Network Plan}}
\begin{itemize}
\item For our current test infrastructure both wired and wireless options are possible
\item Wireless option allows easier deployment but lower data rates
\item For wireless 433 MHz radio communication equipment is available, this allows a maximum of
	42 packets per second (12.364 Kbps)
\item Using a high number of sensing devices over a large geographical area favors an ad-hoc network of nodes
\item Fewer devices in a small area can all directly report to a single master gateway (forming a star topology)
\end{itemize}

\foilhead{{\color{RED} Network Plan: continued..}}
\begin{itemize}
\item MICA2 mote is a good possibility because it also allows programmability and on board processing capability
\item For a high density deployment an overlay of gateways with one interface to the mote wireless network and
the other to RS-232, Ethernet, or 802.11b can be deployed
\item Data streams from the gateways can be redirected via commodity networks to the data sinks possibly over
campus networks,   WANs,   or even    the  Internet  \item    Bluetooth
communication    devices   are     also   an  option
\end{itemize}

\foilhead{{\color{RED}    Implementation Plan}}
\begin{itemize}
\item In the next few weeks, we must have complete deployment
in preparation for tests.
\item Set up displacement sensors first -- displacement at periphery
is adequate at this point.
\item Perhaps, start from optical sensing and see if we can use
laser rangefinders? (Eyesafe lasers are difficult to scale to larger
distances).
\item Accelerometers are easy to deploy. These can be done at short
notice.
\end{itemize}




\foilhead{{\color{RED}    Research   and    Implementation    Issues}}
\begin{itemize}
\item Data  rate
\item Errors
\item   Calibrations
\end{itemize}




%%%%%%%%%%%%%%%%%%%%%%%
%Affords     high      fidelity monitoring of physical  phenomena.
%\item {\color{blue} Semiconductor technology growth:}
% \begin{itemize}
% \item low cost, small size and higher accuracy instrumentation
% \item wireless communication between nodes  allowing
% ``in vivo'' measurements  with low infrastructure \& deployment costs
%  \end{itemize} %\item {\color{blue} Constraints:}
% \begin{itemize}
% \item  power consumption and life  of nodes
%  \item processing   and memory  capabilities
% \item   node   failures and malfunctioning
% \item data transmission rates
% \item management of thousands of nodes
% \end{itemize}


\end{document}
