76 lines
4.3 KiB
TeX
76 lines
4.3 KiB
TeX
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\begin{wrapfigure}[16]{r}{0.45\linewidth}
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\vspace{-2em}
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\begin{tikzfigure}[Schematic representation of the NASS added below the sample and the control architecture used]
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\label{fig:system_control}
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\centering
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\includegraphics[width=0.9\linewidth]{./figs/system_control.pdf}
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\end{tikzfigure}
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\end{wrapfigure}
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\textbf{6 DoF Metrology System}
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In order to achieve the positioning accuracy and stability requirements (shown on Table \ref{table:specifications}), a direct measurement of the relative position from the sample to the optical element is mandatory.
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\emph{Laser interferometry} is chosen as it offers many advantages such as high resolution, high stability and large measurement range.
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\vspace{1em}
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\textbf{6 DoF Active Stabilization Stage}
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In order to actively compensate the positioning error of the sample in all 6
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DoF, a short stroke Stewart platform is added just below the sample as shown Figure~\ref{fig:system_control}.
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By inverting the dynamics of the Stewart platform, it is possible to control independently the position of the mobile platform in all 6 DoF with respect to the fixed platform \cite{McInroy2000}.
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\vspace{1em}
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\textbf{Control Objective}
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The control objective is to stabilize the position of the sample using the NASS actuators based on the 6DoF measurements provided by the metrology system.
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Using this architecture, all the imperfections that cannot be compensated using the actual system (thermal drifts, guidance flexibilities, etc.) will be measured and compensated using a feedback control loop.
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\vspace{1em}
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\begin{minipage}[t]{0.63\linewidth}
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\textbf{Requirements For The NASS}
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The required stroke for the NASS should correspond to the maximum global
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positioning error of the end station without the NASS (\(\approx \SI{10}{\micro\metre}\) in translations).
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The repetability of the NASS is determined by the global specifications (Table \ref{table:specifications}).
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Other requirements such as stiffness and dynamical properties will be determined using the model presented below.
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\end{minipage}
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\begin{minipage}[t]{0.37\linewidth}
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\vspace{-1em}
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\begin{tikztable}[Rough estimation of the NASS specifications]
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\label{table:nass_specification}
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\centering
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\begin{tabular}{ccc}
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\toprule
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\textbf{Motion} & \textbf{Stroke} & \textbf{Repetability}\\
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\midrule
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\(T_{xyz}\) & \(\SI{\pm 10}{\micro\metre}\) & \(\SI{10}{\nano\metre}\)\\
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\(\theta_{xyz}\) & \(\SI{\pm 10}{\micro\radian}\) & \(\SI{1.7}{\micro\radian}\)\\
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\bottomrule
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\end{tabular}
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\end{tikztable}
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\end{minipage}
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% \textbf{Model Based Design}
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% Such positioning system with multiple stages is highly coupled and presents many physical effects such as wobble that are difficult to model with a simple model based on measurements.
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% Therefore, we have chosen to develop a 3D finite mass model. The software used is Simscape which is a toolbox for modeling multidomain physical systems within the Simulink environment.
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% Each stage is represented as a 3D rigid body connected with the other stages by joints. Springs and dampers are added to take into account the finite stiffness of the mechanical guidance.
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% Actuators and sensors dynamics are also included in the model.
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% Finally, sources of perturbation and noise such as ground motion and sensor noise are also modeled.
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% Thanks to the individual identification of each stage, stiffness and damping representing the flexibilities can be tuned properly.
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% This model has numerous utility.
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% First, it allows to conduct simulations of experiments such as tomography. That will help us to attest the performances of the system and compare various control architecture.
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% Second, it permits to study the effect of the sample mass on the mechanical behavior of the system and verify the robustness properties of the controlled system.
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% Finally, this model will be of great help for designing the NASS. Indeed, many parameters have to be properly chosen such as geometric configuration, leg stiffness, actuator type and rotational joints.
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% In the following, the NASS is modeled as a Stewart platform with a cubic configuration, voice coil linear actuators and ideal rotational joints.
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%%% Local Variables: ***
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%%% mode:latex ***
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%%% TeX-master: "2018 - Student Day.tex" ***
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%%% End: ***
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