Make a TikZ picture automatic

Question

I'm trying to make this picture in TikZ:

.

I wrote

\documentclass{standalone}
\usepackage{tikz}

\newcommand*{\rinn}{2}
\newcommand*{\rout}{3.8}
\newcommand*{\tangle}{37}
\newcommand*{\sangle}{-30}

\begin{document}
\begin{tikzpicture}
\coordinate [label=above:$T_1$]        (T1) at (\tangle:\rinn);
\coordinate [label=above right:$T_2$]  (T2) at (\tangle:\rout);
\coordinate [label=below right:$S_1$]  (S1) at (\sangle:\rinn);
\coordinate [label=below right:$S_2$]  (S2) at (\sangle:\rout);

\draw (0,0) circle [radius=\rinn];
\draw (0,0) circle [radius=\rout];

\draw (0,0) -- (T2);
\draw (0,0) -- (S2);

\end{tikzpicture}
\end{document}

that looks like

.

The image is automatic but the label position needs adjustment every time something is changed, and even then they're not well placed.

• Is \newcommand the best way to define a variable for TikZ?
• Is there a way to adjust the label position so that it always stays clear of the other lines?

Answer 1

Here is a possible solution:

\documentclass[tikz]{standalone}
\begin{document}
\foreach {\tangle} in {80,79,...,10}{
  \pgfmathsetmacro{\sangle}{-\tangle}
  \begin{tikzpicture}[auto]
    % constants
    \def\rinn{2}
    \def\rout{3.8}

    % just to fix the global bounding box
    \path circle[radius=\rout cm+5mm+2em];

    \draw (0,0) circle [radius=\rinn];
    \draw (0,0) circle [radius=\rout];

    \coordinate (T1) at (\tangle:\rinn);
    \coordinate (T2) at (\tangle:\rout);
    \coordinate (S1) at (\sangle:\rinn);
    \coordinate (S2) at (\sangle:\rout);

    \draw (0,0) -- (T1) -- (T2);
    \draw (0,0) -- (S1) -- (S2);

    \path (T1) ++(\tangle+45:5mm) node{T1};
    \path (T2) ++(\tangle+45:5mm) node{T2};
    \path (S1) ++(\sangle-45:5mm) node{S1};
    \path (S2) ++(\sangle-45:5mm) node{S2};

  \end{tikzpicture}
}
\end{document}

Answer 2

Not perfect solution ...

\documentclass[tikz,border=3mm]{standalone}
\usepackage{}

\newcommand*{\rinn}{2}
\newcommand*{\rout}{3.8}
\newcommand*{\tangle}{37}
\newcommand*{\sangle}{-30}

\begin{document}
\begin{tikzpicture}[auto]
\coordinate (T1) at (\tangle:\rinn);
\coordinate [label=\tangle:$T_2$]  (T2) at (\tangle:\rout);
\coordinate (S1) at (\sangle:\rinn);
\coordinate [label=\sangle:$S_2$]  (S2) at (\sangle:\rout);

\draw (0,0) circle [radius=\rinn];
\draw (0,0) circle [radius=\rout];

\draw (0,0) -- (T1) -- node[near start] {T1}  (T2);
\draw (0,0) -- (S1) -- node[near start] {S1}   (S2);

\end{tikzpicture}
\end{document}