[Tex/LaTex] Difficulty in using multicolumn and multirow table commands

Question

I am trying to implement the following table (picture seen) in LaTeX. I am facing difficulty in writing the code since it consists of both multicolumn and multirow table commands. I truly appreciate if you guys can help me with it:

Here is the initial part of code using multicolumn and multirow, I can't understand where I am getting it wrong:

\begin{table}[tp]
\toprule
\renewcommand{\arraystretch}{1.3}
\setlength{\arrayrulewidth}{0.15mm}
\setlength{\doublerulesep}{0.12mm}
\caption{VSI Parameters}  
\label{tab:2}
\centering
\begin{tabular}{t|t|t|t}
\hline
\hline
\multicolumn{2}{*}{\textbf{VSI Controller}  \\
                       \textbf{Parameters} \\
&  \textbf{Minimum}  &  \textbf{Maximum}   \\
&    \textbf{Value}  &   \textbf{Value}    \\\otoprule
\hline
\hline
\multirow{4}{*}{Outer Control Loops} & \math{k_{p1}} & \SI{0.002}{} \\
& \math{T_{i1}} & \SI{0.0001}{} \\
& \math{k_{p2}} & \SI{0.01}{} \\
& \math{T_{i2}} & \SI{0.0001}{} \\ \hline  



\end{tabular}
\end{table}

EDITED: @Bernard: I really like your format; if I opt for it in making all my tables, what is the modified code for the below table in my script:

\begin{table}[tp]
\toprule
\renewcommand{\arraystretch}{1.3}
\setlength{\arrayrulewidth}{0.15mm}
\setlength{\doublerulesep}{0.12mm}
\caption{Step Changes}
\label{tab:1}
\centering
\begin{tabular}{t|t|t}
\hline
\hline
Step Change   &  Initial &  Final \\
Number        &    $\math{(W)}$  &   $\math{(W)}$ \\\otoprule
\hline
\hline
\SI{1}{}   &   \SI{0}{}  &   \SI{500}{}\\
\hline
\SI{2}{}    &   \SI{500}{}    &   \SI{250}{}\\
\hline
\SI{3}{}    &   \SI{250}{}    &   \SI{750}{}\\
\hline
\hline
\end{tabular}
\end{table} 

Answer 1

Here is a solution, using makecell, siunitx, caption and booktabs. I removed all vertical rules for a more professional look:

\documentclass{article}

\usepackage{makecell, caption, booktabs, multirow, siunitx}
\renewcommand\theadfont{\bfseries}
\renewcommand\cellalign{lc}
\newcommand{\otoprule}{\midrule[\heavyrulewidth]}
\usepackage{lipsum, fancyvrb}

\begin{document}

\lipsum[3]

\begin{table}[tp]

  \renewcommand{\arraystretch}{1.3}
  \caption{VSI Parameters}
  \label{tab:2}
  \centering
  \begin{tabular}{@{}p{25mm}>{$}c<{$}@{\hskip1.5em}S[table-format=1.4]S[table-format=1.1]@{}}
    \toprule
    \multicolumn{2}{@{}l}{\bfseries\makecell{VSI Controller\\ Parameters}} & {\thead{Minimum\\Value}} & {\thead{Maximum\\Value}} \\
    \otoprule
    \multirowcell{4}{Outer Control \\ Loops} & k_{p_1} & 0.002 & 5 \\
                                   & T_{i_1} & 0.0001 & 0.5 \\
                                   & k_{p_2} & 0.01 & 3 \\
                                   & T_{i_2} & 0.0001 & 0.5 \\
    \midrule
    \multirowcell{4}{Inner Control \\ Loops} & k_{p_3} & 0.1 & {\Verb+15+} \\
                                   & T_{i_3} & 0.001 & 0.5 \\
                                   & k_{p_4} & 0.1 & {\Verb+10+} \\
                                   & T_{i_4} & 0.001 & 0.5 \\
    \bottomrule
  \end{tabular}
\end{table}

\end{document}