\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{relsize}
\usetikzlibrary{shapes,arrows}

\begin{document}

\tikzstyle{block} = [draw, fill=blue!20, rectangle,
    minimum height=3em, minimum width=6em]
\tikzstyle{sum} = [draw, fill=blue!20, circle, node distance=1cm]
\tikzstyle{input} = [coordinate]
\tikzstyle{output} = [coordinate]
\tikzstyle{pinstyle} = [pin edge={to-,thin,black}]

\begin{tikzpicture}[auto, node distance=2cm,>=latex']
    % We start by placing the blocks
    \node [input, name=input] {};
    %\node [block, right of=input, node distance=3cm] (controller) {Tracking Controller};
    \node [sum, right of=input,node distance=3cm] (sum) {};

    \node [block, right of=sum,
            node distance=3cm] (system) {$\frac{\omega_n^2}{s^2+2\zeta\omega_ns+\omega_n^2}$};
    % We draw an edge between the controller and system block to
    % calculate the coordinate u. We need it to place the measurement block.
    %\draw [->] (controller) -- node[name=u] {$u$} (system);
    \node [output, right of=system,node distance=5cm] (output) {};
    \node [block, below of=system] (measurements) {$\frac{\Gamma_1s+\Gamma_2}{s^2+2\Psi \omega_c s+\omega_c^2}$};

    %add some texts below block
    \node [below of = system ,node distance=0.75cm] {\tiny Undamped Resonant System};
     \node [below of = measurements ,node distance=0.75cm] {\tiny PVPF Controller};

    % Once the nodes are placed, connecting them is easy.
    \draw [draw,->] (input) -- node {$r$} (sum);
    %\draw [->] (controller) -- node {$r$} (sum);
    \draw [->] (sum) -- node {$e$} (system);
    \draw [->] (system) -- node [name=y] {$y$}(output);
    \draw [->] (y) |- (measurements) ;
    \draw [->] (measurements) -| node[pos=0.99] {$+$}
        node [near end] {$y_m$} (sum);
\end{tikzpicture}
\end{document}