tikz extending spring with constant number of segments

tikz extending spring with constant number of segments

Is there as simple way to create a pdf animation with tikz of an oscillating mass attached to a spring which extends and compresses as the mass oscillates. The number of "spring elements" should be constant during the oscillation.
I came up with this pretty solution so far. However, I do not quite understand how to chose the right units for the segment length parameter:
\documentclass[tikz]{standalone}
\usepackage{tikz}
\usetikzlibrary{shadings, calc, decorations.pathmorphing}

\begin{document}

\def\frames{50}
\def\amplitude{1.5}
\def\z0{3.0}
\def\nloops{5.0}

\foreach \n in {0, 1, ..., \frames}
{
    \begin{tikzpicture}[x=1cm, y=1cm]

    % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    % calculations
    % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \pgfmathsetmacro{\height}{\amplitude * cos(2.0 * pi * \n / \frames r) - \z0};
    \pgfmathsetmacro{\seglength}{(-\amplitude * cos(2.0 * pi * \n / \frames r) + 3.0) / \nloops * 1cm};


    % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    % help lines
    % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \draw[color=white,fill=white] (-1.70, 0.4) rectangle (2.00, -6.0);


    % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    % the scene
    % %%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    % the ceiling  
    \draw[color=black!70, line width=2.0pt, line cap=round] (-1.70, 0) -- (1.70, 0);
    \foreach \x in {-1.60, -1.20, ..., 2.0}
    {
        \draw[color=black!70, line width=1.00pt, line cap=round] (\x, 0) -- ++ (0.35, 0.4);
    }
    \draw[color=black!70, line width=2.00pt, line cap=round] (-0.5, 0) -- +(0.0, -0.2);
    \draw[color=black!70, line width=2.00pt, line cap=round] (+0.5, 0) -- +(0.0, -0.2);


    % the mass
    \draw[ball color=orange!90!black!70, rounded corners=1ex] (-1.0, \height) rectangle +(2.0, -0.75);
    \draw[color=black!70, line width=2.00pt, line cap=round] (-0.5, \height) -- +(0.0, 0.2);
    \draw[color=black!70, line width=2.00pt, line cap=round] (+0.5, \height) -- +(0.0, 0.2);

    % the spring
    \draw[decorate, color=red!70, line width=2.00pt, line cap=round, decoration={coil, amplitude=0.25cm, segment length=\seglength}] ($(-0.5, \height) + (0, 0.2)$) -- (-0.5, -0.2);

    \end{tikzpicture}
}
\end{document}
To the spring should have a minimum extension of 1.5 cm and a maximum extension of 4.5 cm. Here is an image of the spring at minimum extension:

Can someone point me into the right direction?