How to get a consistent zigzag line?

Question

I want to get a consistent zigzag line that will be used to mimic a physical spring.

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pst-coil,multido}
\usepackage[nomessages]{fp}

\FPset\CoilArm{.2}
\FPset\Windings{7}

\psset
{
    coilarm=\CoilArm,
    linejoin=1,
}

\def\System#1#2{% #1: total length includes the arms, #2: coil width
    \FPeval\CoilWidth{trunc(#2,9)}%
    \FPeval\CoilHeight{trunc((#1-2*CoilArm)/(CoilWidth*Windings),9)}%
    \pszigzag[coilwidth=\CoilWidth,coilheight=\CoilHeight](1,0)(1,-#1)%
    \ignorespaces
}

\begin{document}

\begin{pspicture}(13,-7)
\multido{\nx=0+1.25,\nl=2.50+.50,\nw=1.00+-0.10}{10}{\rput(\nx,0){\System{\nl}{\nw}}}
\end{pspicture}
\end{document}

In the figure below, the red cross represents inconsistent zigzag lines as its top end begins with a different orientation of segment.

I have increased the precision of trunc by choosing a large number of digit. But it does not seem to work. How to solve this issue?

Facts

One method to get rid of this inconsistency is by choosing a constant coilwidth. However, with constant coilwidth the animation will not look realistic anymore. That is the problem.

Answer 1

The last resort: don't rely on pst-coil!

\documentclass[pstricks,border=12pt]{standalone}
\usepackage{pst-node,pst-plot}
\usepackage[nomessages]{fp}

\FPset\CoilArm{.6}
\FPset\Windings{7}

\def\System#1#2{% #1: total length includes the arms, #2: coil width
    \FPeval\CoilWidth{#2}%
    \FPeval\Lambda{(#1-2*CoilArm)/Windings}%
    \FPeval\PlotPoints{trunc(4*Windings+1,0)}%
    \curvepnodes[plotpoints=\PlotPoints,algebraic]{0}{\Lambda\space \Windings\space mul}{\CoilWidth*sin(2*Pi*t/\Lambda)|-t-\CoilArm}{P}%
    \pscustom[linejoin=1]
    {
        \psline(0,0)
        \psnline(0,\Pnodecount){P}
        \psline(0,-#1)
    }%
    \ignorespaces
}

\begin{document}

\begin{pspicture}(13,-7)
\multido{\nx=1.00+1.25,\nl=2.50+.50,\nw=.50+-.05}{10}{\rput(\nx,0){\System{\nl}{\nw}}}
\end{pspicture}
\end{document}