\documentclass{standalone}
\usepackage{tikz}
\usepackage{amsmath}
\usetikzlibrary{shapes,positioning,calc}
\usetikzlibrary{narrow}
\usetikzlibrary{arrows,decorations.markings}

% The double arrow comes from
% http://www.texample.net/tikz/examples/double-arrows/
% with modification in 
% http://tex.stackexchange.com/questions/169061/tikz-increase-arrow-head-size

\tikzset{blockMinus/.style={draw,minimum width=2em, 
    minimum height=2em,line width=1.5pt,
    label=center:{$-1$}}}

\tikzset{every picture/.style={auto, line width=1pt,
     >=narrow,font=\small}}

\tikzset{block/.style={draw, rectangle, line width=1.5pt,
     minimum height=3em, minimum width=3em,
     outer sep=0pt}}

\tikzset{sumcircle/.style={draw, circle, outer sep=0pt,
      label=center:{{$\sum$}}, minimum width=2em}}

\tikzset{vecArrow/.style={
  line width=1pt,
  decoration={
    markings,mark=at position 1 with {
       \arrow[scale=2.5,line width=0.4pt] {open triangle 60}
    }
  },
  double distance=4pt, shorten >= 13pt,
  preaction = {decorate},
  postaction = {draw,line width=4pt, white,shorten >= 10pt}
}}


\begin{document}
\begin{tikzpicture}
\node[sumcircle](sum){};
\node[block, right=25mm of sum](g_p){$G_P$};
\node[block, below=13mm of sum.center, xshift=15mm](l){$-L$};
\node[block, left=30mm of sum](l_r){$l_r$};
\node[blockMinus, below=26mm of sum](minus_e){};
\node[block, left=32mm of l](l_i){$-l_i$};
\node[block, left=10mm of l_i](integrator){$\dfrac{1}{s}$};
\node[sumcircle, left=10mm of integrator](sum_e){};
\coordinate[above=15mm of sum_e](input);
\draw[->](l_r.west)++(-3.3,0)--node[xshift=-10mm]{$R$}(l_r);
\draw[->](l_r)--(sum);
\draw[->](sum)--node{$U$}(g_p);
\draw[->](g_p.east)--node(output){$Y$}+(2,0);
\draw[vecArrow](g_p)|-node[xshift=4mm, yshift=8mm]{$X$}(l);
\draw[->](l)-|(sum);
\draw[->](input)--(sum_e);
\draw[->](output)|-(minus_e);
\draw[->](minus_e)-|(sum_e);
\coordinate[right=5mm of l_i](l_i1);
\draw[-](l_i)--(l_i1);
\draw[->](l_i1)--(sum);
\draw[->](sum_e)--node{$E$}(integrator);
\draw[->](integrator)--node{$x_i$}(l_i);
\end{tikzpicture}
\end{document}