Draw line perpendicular to another line without touching

I'm trying to draw the following figure using TikZ:

So far I have everything except the vertical line going to I. I can't figure out how to find the mid point between S and P and then draw the line just above the line to I. I don't want to use absolute coordinates. I've tried various things but can't work out how to do it. For example I found that I can so this: \draw (A) -- (B) coordinate[midway] (M); but it didn't help me. This is the code so far:

\documentclass{article}
\usepackage{tikz}
\begin{tikzpicture}[>=latex',node distance = 2.5cm]

\node (S) {$S$};
\node[above of = S, node distance = 1.5cm] (I) {$I$};

\node [left of = S] (Xo) {};
\draw[->,line width = 1.2pt] (Xo) to node[above] {$v_1$} (S);

\node [right of = S] (P) {$P$};
\draw[->,line width = 1.2pt]  (S) to node[below] $v_2$} (P);

\node [right of = P] (X1) {};
\draw[->,line width = 1.2pt] (P) to node[above] {$v_3$} (X1);

% This is obviously wrong
\draw[|-,line width = 1.2pt] (S) to (I) {};

\end{tikzpicture}
\end{document}


There are several ways for doing this. One option is to create a temp coordinate midway between S and P, then use it to draw the perpendicular line. To calculate the mid-point, the calc library is required.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows,calc} % <-- for arrow tips and calculations
\begin{document}

\begin{tikzpicture}[>=latex',node distance = 2.5cm]

\node (S) {$S$};
\node [left of = S] (Xo) {};
\draw[->,line width = 1.2pt] (Xo) -- node[above] {$v_1$} (S);

\node [right of = S] (P) {$P$};
\draw[->,line width = 1.2pt] (S)  -- node[below] {$v_2$} (P);

\node [right of = P] (X1) {};
\draw[->,line width = 1.2pt] (P)  -- node[above] {$v_3$} (X1);

% This obviously works
\draw[|-,line width=1.2pt] ([yshift=4pt]$(S)!.5!(P)$) --++(0,1.5cm)node[above]{$I$};

\end{tikzpicture}

\end{document}