mar/content/case_experiment_end-effector.tex

With the preparation phase completed, the development cycle is next.
This consists of three steps: Feature selection, Rapid Development and Variable Approach.
The current section explains the first development cycle during the design.
The end-effector is the feature that is selected in the first cycle.
The implementation of the end-effector was not successful, as the design was too complex.
Fortunately, this failure did give valuable insight on the design method.

    \subsection{Feature Selection}
        \label{sec:case_feature_selection_1}
        \begin{table}[]
            \caption{Overview of the different features and their dependencies, number of tests that can be completed and the risk/time factor.
            The risk/time factor is calculate as risk divided by time.}
            \label{tab:firstfeatureselection}
            \begin{tabular}{|l|l|l|l|l|l|}
                \hline
                Feature      & Dependees   & Tests  & Risk   & Time    & Risk/Time  \\ \hline
                SCARA        & -           & 3      & 40\%   & 10 days & 4          \\ \hline
                End-effector & SCARA       & 2      & 60\%   & 8 days  & 7.5        \\ \hline
                Carriage     & -           & 2      & 30\%   & 10 days & 3          \\ \hline
            \end{tabular}
        \end{table}
        The feature is selected according to the feature selection step, explained in \autoref{sec:feature_selection}.
        For each component in the system the dependees, tests and risk/time factor is determined.
        These values are combined into \autoref{tab:firstfeatureselection}.

        The SCARA is dependent on the end-effector, as was explained in the initial design.
        However, for the carriage no dependency was defined even though it has to lift the other two components.
        This is mainly because the behavior of the SCARA changes depending on the end-effector, resulting in a possible design change.
        For the carriage it only changes the mass that has to be lifted.
        Upgrading the motor torque is a minor parametric change and the dependency is therefore insignificant.

        The testing number is directly the number of tests that can be completed by implementing that single component.
        For the risk and time it was an engineering judgement and no specific protocol to determine the values.
        The estimated risk is high for the end-effector due to the collision dynamics of the operation.
        It has to grab something and that is difficult to model. Furthermore, it was not known if that design would work.
        The SCARA has the most moving parts, but no difficult dynamics and has therefore an estimated risk of medium.
        For the carriage the there was no real risks and got therefore a low risk indication.
        
        The SCARA would be implemented first based on number of tests, but is dependent on the end-effector.
        Beginning with the end-effector is an obvious choices. 
        It unlocks the SCARA and has the highest risk/time factor.

        \subsubsection{Evaluation}
            This first step of the detail design phase did go well.
            A more refined method for this step could be very useful.
            But the risk and time assessment will probably always be a engineering judgement from the developer.
            Within a design team a form of planning poker\footnote{\url{https://en.wikipedia.org/wiki/Planning_poker}{Wikipedia entry: Planning Poker}} could be a good option.
        
    \begin{figure}
        \centering
        \includegraphics[width=0.9\linewidth]{graphics/end-effector.pdf}
        \caption{Operation of the end-effector. The marker is clamped in the springloaded end-effector (1).
        To release the marker, the upper part of the clamp is placed against the holder (2). This extra arm length is used as a leverage to open clamp while the end-effector moves downwards (3,4).
        The arm length also allows to move backwards without disturbing the marker (5,6). To grab the marker, the process is repeated in reverse.}
        \label{fig:gripper}
    \end{figure}

    \subsection{Rapid Development of the End-Effector}
        The end-effector will operate as an interface between the SCARA and the different tools.
        For that it has to be able to grab and release the tools.
        The end-effector is mounted at the end of the SCARA.

        The development starts with an initial design of the system.
        The next step is to develop that further into a model and prototype.
        This development did not get past the basic model implementation due to unforeseen difficulties.
        However, the evaluation gave new useful insight on the design plan.

        \subsubsection{Initial design}
        There are multiple options to connect a tool with a moving arm.
        However, there is a trade-off to be made with the SCARA feature, the heavier the end-effector is, the more force the SCARA must deliver.
        And because the goal is to make the SCARA light and quick, this end-effector should be light-weight.
        The best options in this case is to go with a simple spring-loaded clamp.
        It is light-weight, and provides sufficient clamping force and precision for this application.
        To release the tool, the clamp must be forced open.
        Instead of using a servo, the movement of the SCARA can force the clamp open, resulting in a significant simplification of the design.
        The initial design of the clamp and the operation is shown in \autoref{fig:gripper}. 

        \subsubsection{Behavior Modelling}
        The next step is to implement this design with the corresponding behavior in a dynamic model.
        The challenge in this case is the modelling of the contact dynamics.
        Based on some experience in modelling with collisions, I decided to use the 20-sim 3D mechanics editor.
        There is little tooling available and there are no debugging options if the model does not behave as expected.
        The marker kept falling trough the gripper or flew away.
        With the small amount of progress made in two days the implementation was not promising. 
        A system freeze caused the model to corrupt, where the complete configuration of the shapes and their collisions was lost.
        Based on the loss of work and the low feasibility of the implementation, it was decided that the end-effector would no longer be part of the design.

        With the end-effector removed, the SCARA will get a direct connection with the marker.
        The lifting of the marker from the will be included in the SCARA as well.
        Furthermore, this means that the wiping will no be possible via the SCARA.

        \subsubsection{Evaluation}
            The lost progress of the model is unfortunate, but the implementation did not go as expected anyway.
            It was probably for the best as it forced an evaluation of the design and avoided a tunnel vision while trying to get it to work.
            However, it did show the value of the risk/time analysis.
            This early failure resulted in changes for other components.
            But as none of the components were implemented yet, no work was lost.