\section{Introduction}
-Cantilevers are used inside atomic force microscope which provides high
+Cantilevers are used inside atomic force microscope (AFM) which provides high
resolution images of surfaces. Several technics have been used to measure the
displacement of cantilevers in litterature. For example, it is possible to
-determine accurately the deflection with optic
-interferometer~\cite{CantiOptic89}, pizeoresistor~\cite{CantiPiezzo01} or
-capacitive sensing~\cite{CantiCapacitive03}. In this paper our attention is
-focused on a method based on interferometry to measure cantilevers'
-displacements. In this method cantilevers are illiminated by an optic
-source. The interferometry produces fringes on each cantilevers which enables to
-compute the cantilever displacement. In order to analyze the fringes a high
-speed camera is used. Images need to be processed quickly and then a estimation
-method is required to determine the displacement of each cantilever.
-In~\cite{AFMCSEM11} {\bf verifier ref}, the authors have used an algorithm based
-on spline to estimate the cantilevers' positions. The overall process gives
+determine accurately the deflection with different mechanisms.
+In~\cite{CantiPiezzo01}, authors used piezoresistor integrated into the
+cantilever. Nevertheless this approach suffers from the complexity of the
+microfabrication process needed to implement the sensor in the cantilever.
+In~\cite{CantiCapacitive03}, authors have presented an cantilever mechanism
+based on capacitive sensing. This kind of technic also involves to instrument
+the cantiliver which result in a complex fabrication process.
+
+In this paper our attention is focused on a method based on interferometry to
+measure cantilevers' displacements. In this method cantilevers are illuminated
+by an optic source. The interferometry produces fringes on each cantilevers
+which enables to compute the cantilever displacement. In order to analyze the
+fringes a high speed camera is used. Images need to be processed quickly and
+then a estimation method is required to determine the displacement of each
+cantilever. In~\cite{AFMCSEM11}, the authors have used an algorithm based on
+spline to estimate the cantilevers' positions.
+
+ The overall process gives
accurate results but all the computation are performed on a standard computer
using labview. Consequently, the main drawback of this implementation is that
the computer is a bootleneck in the overall process. In this paper we propose to
\section{Measurement principles}
\label{sec:measure}
+
+
+
+
+
+
+
\subsection{Architecture}
\label{sec:archi}
%% description de l'architecture générale de l'acquisition d'images
%% avec au milieu une unité de traitement dont on ne précise pas ce
%% qu'elle est.
+In order to develop simple, cost effective and user-friendly cantilever arrays,
+authors of ~\cite{AFMCSEM11} have developped a system based of
+interferometry. In opposition to other optical based systems, using a laser beam
+deflection scheme and sentitive to the angular displacement of the cantilever,
+interferometry is sensitive to the optical path difference induced by the
+vertical displacement of the cantilever.
+
+The system build by authors of~\cite{AFMCSEM11} has been developped based on a
+Linnick interferomter~\cite{Sinclair:05}. It is illustrated in
+Figure~\ref{fig:AFM}. A laser diode is first split (by the splitter) into a
+reference beam and a sample beam that reachs the cantilever array. In order to
+be able to move the cantilever array, it is mounted on a translation and
+rotational hexapod stage with five degrees of freedom. The optical system is
+also fixed to the stage. Thus, the cantilever array is centered in the optical
+system which can be adjusted accurately. The beam illuminates the array by a
+microscope objective and the light reflects on the cantilevers. Likewise the
+reference beam reflects on a movable mirror. A CMOS camera chip records the
+reference and sample beams which are recombined in the beam splitter and the
+interferogram. At the beginning of each experiment, the movable mirror is
+fitted manually in order to align the interferometric fringes approximately
+parallel to the cantilevers. When cantilevers move due to the surface, the
+bending of cantilevers produce movements in the fringes that can be detected
+with the CMOS camera. Finally the fringes need to be
+analyzed. In~\cite{AFMCSEM11}, the authors used a LabView program to compute the
+cantilevers' movements from the fringes.
+
+\begin{figure}
+\begin{center}
+\includegraphics[width=\columnwidth]{AFM}
+\end{center}
+\caption{schema of the AFM}
+\label{fig:AFM}
+\end{figure}
+
+
%% image tirée des expériences.
\subsection{Cantilever deflection estimation}
\subsection{FPGA constraints}
+A field-programmable gate array (FPGA) is an integrated circuit designed to be
+configured by the customer. A hardware description language (HDL) is used to
+configure a FPGA. FGPAs are composed of programmable logic components, called
+logic blocks. These blocks can be configured to perform simple (AND, XOR, ...)
+or complex combinational functions. Logic blocks are interconnected by
+reconfigurable links. Modern FPGAs contains memory elements and multipliers
+which enables to simplify the design and increase the speed. As the most complex
+operation operation on FGPAs is the multiplier, design of FGPAs should not used
+complex operations. For example, a divider is not an available operation and it
+should be programmed using simple components.
+
+FGPAs programming is very different from classic processors programming. When
+logic block are programmed and linked to performed an operation, they cannot be
+reused anymore. FPGA are cadenced slowly than classic processors but they can
+performed pipelined as well as pipelined operations. A pipeline provides a way
+manipulate data quickly since at each clock top to handle a new data. However,
+using a pipeline consomes more logics and components since they are not
+reusable, nevertheless it is probably the most efficient technique on FPGA.
+Parallel operations can be used in order to manipulate several data
+simultaneously. When it is possible, using a pipeline is a good solution to
+manipulate new data at each clock top and using parallelism to handle
+simultaneously several data streams.
+
%% contraintes imposées par le FPGA : algo pipeline/parallele, pas d'op math complexe, ...
