\documentclass{article}

\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows}
\begin{document}
\pagestyle{empty}

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% Define block styles
% TiKz setup stuff -- leave stuff in here alone!
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\tikzstyle{decision} = [diamond, draw, fill=blue!20, 
    text width=4.5em, text badly centered, node distance=3cm, inner sep=0pt]
\tikzstyle{block} = [rectangle, draw, fill=blue!20, 
    text width=5em, text centered, rounded corners, minimum height=4em]
\tikzstyle{line} = [draw, -latex']
\tikzstyle{cloud} = [draw, ellipse,fill=red!20, node distance=3cm,
    minimum height=2em]
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% Some body of text
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This text appears above the image. 
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% The schematic itself
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\begin{tikzpicture} % this command tells LaTeX to begin a tikzpicture, everything between this command and \end{tikzpicture} is considered part of the picture
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%integrator I (the left integrator)
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\huge																% change fontsize to huge
\draw (.7071,.7071) -- (2, .7071); 									% top of box
\draw (2,.7071) -- (2, -.7071); 									% right edge of box
\draw (.7071,-.7071) -- (2,-.7071); 								% bottom of box
\draw (0,0) circle (1cm); 											% wheel
\node at (0,0) {I};													% the text "I" inside integrator I
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%integrator II (the right integrator)
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\draw (10.7071,.7071) -- (12, .7071); 								% top of  box
\draw (12,.7071) -- (12, -.7071); 									% right edge of box
\draw (10.7071,-.7071) -- (12,-.7071); 								% bottom of box
\draw (10,0) circle (1cm); 											% wheel
\node at (10,0) {II}; 												% the text "II" inside integrator II
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% lines representing the rods on the machine
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\normalsize 														% change fontsize back to normal
\draw (-1,-3) -- (12,-3);											% t rod
\node at (12.5,-3) {$t$};											% the letter "t" at the end of the rod
																	%
\draw (-1,-4) -- (12,-4);											% y'' rod
\node at (12.5,-4) {$\frac{d^2y}{dt^2}$};							% the 2nd derivative symbols at the end of the rod
																	%
\draw (-1,-5) -- (12,-5);											% y' rod
\node at (12.5,-5) {$\frac{dy}{dt}$};								% the 1st derivative symbols at the end of the rod
																	%
\draw (-1,-6) -- (12,-6);											% y rod
\node at (12.5,-6) {$y$};											% the letter "y" at the end of the rod
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% arrows from rods to integrators
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\draw[->] (1.35355,-3) -- (1.35355,-.7071); 						% time input to integrator I
\draw[->] (11.35355,-3) -- (11.35355,-.7071); 						% time input to integrator II
																	%
\draw[->] (.5,-5) -- (.5,-.86602); 									% y' input to integrator I
\draw (-.5,-6) -- (-.5,-.86602); 									% y output from integrator I
\filldraw[fill=black!100, draw=black!100] (-.5,-6) circle (.07cm); 	% vertex at output of integrator I
																	%
\draw[->] (10.5,-4) -- (10.5,-.86602); 								% y'' input to integrator II
\draw (9.5,-5) -- (9.5,-.86602); 									% y' output from integrator II
\filldraw[fill=black!100, draw=black!100] (9.5,-5) circle (.07cm); 	% vertex at output of integrator II
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% the adder
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\huge																% change fontsize to huge
\node at (5,-5) [rectangle,draw] {$\Sigma$};						% the text "\Sigma" inside the adder and the box around it
\draw (5,-4.525) -- (5,-4); 										% line from top of adder box to above rod
\filldraw[fill=black!100, draw=black!100] (5,-4) circle (.07cm); 	% vertex at the rod the previous line goes to
\draw (5,-5.47) -- (5,-6); 											% line from bottom of adder box to below rod
\filldraw[fill=black!100, draw=black!100] (5,-6) circle (.07cm); 	% vertex at the rod the previous line goes to
\end{tikzpicture} % this ends the entire picture environment and goes back to the document
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% Back to the body of the paper
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This text appears below the image. Pictured above is the two-integrator schematic for the differential equation $y''=-ky'-y$.
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\end{document}