%&tex
\chapter{Starting Point}
\label{chap:background}
The goal of the design plan is to develop a \ac{cps}.
Due to the nature of \ac{cps}, it involves a multi-domain design approach.
Therefore, the subject of \ac{se} is relevant to this approach.
Furthermore, the \ac{ridm} is discussed in more detail in this chapter.

The \ac{ridm} does not initiate from the problem description step.
As this step is required a design from scratch, the \ac{ridm} is combined with the approach from \ac{se} establish the required design steps.

\section{Systems Engineering}
\label{sec:SE}
\textcite{blanchard_systems_2014} describe \ac{se} in their book as: "an interdisciplinary approach and means to enable the realization of successful systems."
Their book extensively covers multiple design methods and design steps in detail.
For this thesis, their approach on \emph{Bringing a Systems into Being} and \emph{Preliminary System Design} are especially relevant.
\ac{se} is a complete field of engineering on its own and only the top of the iceberg is used in this thesis.

\section{Rapid Iterative Design Method}
\label{sec:RIDM}
The \ac{ridm} by \textcite{broenink_rapid_2019} describes a methodology using two core components for the implementation: the rapid development cycle and the variable-detail approach.
The design method also describes the preparation steps that are required prior to this implementation.
In short, the preparation prepares a list of features.
These features are implemented one by one with in the rapid development cycle using the variable-detail approach.
The following sections discus each of these three design steps.

\subsection{Rapid Development Cycle} 
\label{sec:background_rdc}
The goal of the rapid development cycle is sequential implementation of the features in a system.
Each iteration of the rapid development incorporates the following steps:
\begin{enumerate}
    \item Create an initial design and corresponding tests for the next feature.
    \item Implement and test that feature.
\end{enumerate}
The first step is to create an initial design and tests that are used to verify the requirements of the current feature.
During the second step, the initial design is developed into a detailed design of the feature.
This detailed design of the feature is develop with the variable-detail approach, in which the level of detail is stepwise incremented.
When the second step is completed, the implemented feature contains all the required details and passes all the tests as defined in the first step.
From this point the rapid development cycle is repeated for the next feature, or, when no features are left, finish the development.

\subsection{Variable-Detail Approach}
The variable-detail approach starts with a low-detailed model and increases the detail discretely over multiple iterations.
The low-detailed model is for example a single transfer function of the system.
In the following iteration, the detail of the model is increased by adding, for example, non-linearity, non-continuity or parasitic elements.

The tests, as specified in the first step of the rapid development cycle, are performed after each addition of detail.
If the tests show that the added detail is not conform the requirements, the added detail is reviewed or redesigned.
When the added detail passes the tests, the process is repeated to add more detail.
The variable-detail approach is finished when all the tests are passed and all the detail is added.

\subsection{Preparation steps}
Although the \ac{ridm} does not specify the complete steps for the preparation, it does state some requirements.
The rapid development cycle requires a list of features that can be implemented and tested individually.
These features are gained by partitioning the functionality of the system.

For each feature it is required to specify the feature requirements and the corresponding test protocol.
The feature requirements are based on the system requirements and the tests are used to validate that the feature meets its requirements.
About the order of implementation, the \ac{ridm} states that critical features must be implemented first, as these features have an increased chance of invalidating the complete design.
Would such a feature fail, the investment loss is limited, because the development is still in an early stage.

Following the feature separation step is the system test protocol.
The goal of this step is to describe how the requirements of the system are tested.
These tests can cover a single feature or multiple features.

\begin{marginfigure}
    \centering
    \includegraphics[width=55mm]{graphics/design_flow.pdf}
    \caption{Combined design plan, where the first three steps are based on the \ac{se}-approach and the other four steps are taken from the \ac{ridm} \autocite{broenink_rapid_2019}}
    \label{fig:design_flow}
\end{marginfigure}
\section{Combination}
To create a complete design plan, design approaches from both \ac{se} and the \ac{ridm} are combined.
The \ac{ridm} requires an initial design that is then split into features.
To meet this requirement, a linear set of steps from problem description to initial design is used from \ac{se}.
These three steps are shown at the top of \autoref{fig:design_flow} in the \acl{se} group.
The steps show some similarity with the first steps of a V or Waterfall model.

The requirements and initial design are used in the four steps of \ac{ridm} continue the design process.
These four steps are grouped at the bottom of \autoref{fig:design_flow}.
