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4 anos atrás · 3d23e026d3
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 %&tex
    As the previous development cycle was aborted prematurely, that cycle did not finish.
    The second cycle is picks up at the feature selection step in the Development Cycle.
    As the previous development cycle was aborted prematurely, the development cycle is repeated for the next feature.

    \subsection{Feature Selection}
        The implementation of the end-effector proofed to be impractical.
        This means that only two features are left.
        The updated table in \autoref{tab:featurestab2} shows the updated feature comparison.
        \autoref{tab:featurestab2} shows an updated feature comparison.
        Compared with the previous feature selection in \autoref{tab:firstfeatureselection}, the number of tests for the SCARA decreased and the Risk/Time increased.
        This is because System Test \ref{test_tool_change} relied on both the SCARA and the End-effector and is no longer applicable.
        This is because \autoref{test_tool_change} relied on both the SCARA and the End-effector which is no longer applicable.
        Based on the feature comparison, the next component to implement is the SCARA.

        \begin{table}[]
            \caption{}
            \caption{Comparison of the two remaining features in the design process. This table is an updated version of \autoref{tab:firstfeatureselection}.}
            \label{tab:featurestab2}
            \begin{tabular}{|l|l|l|l|l|l|}
                \hline
@@ -27,12 +26,14 @@

    \subsection{Rapid Development for SCARA}
        The goal is to present a functional model of the SCARA.
        Based on the tests and requirements, it must be able to write three characters within 2 seconds.
        The basic design principle is based on the initial design and shown in \autoref{fig:combined}.
        The lowest level of detail is a kinematics model of the design.
        This does not involve any physics simulation yet, but gives insight in the operation range, arm length and joint behavior.
        In the following steps, the level of detail is gradually increased until it is a competent model.
        However, planning all the different steps in advance is difficult as design decisions still need to be made.
        The specifications state that it must be able to write three characters within 2 seconds.
        And to pass \autoref{test1} it must draw a \SI{50}{\milli\meter} by \SI{70}{\milli\meter} rectangle within 1 second.
        The basic design principle is based on the initial design as shown in \autoref{fig:combined}.
        For the lowest detail level of the design, I decided on a kinematics model.
        The model is stays very simple as it does not implement any physics.
        However, the model enables me to tinker with the design parameters, such as the lengths of the linkages and joint angles.
        In the following steps, the level of detail is gradually increased to arrive at a competent model.
        Planning all the different steps in advance is difficult as design decisions still need to be made.
        Nonetheless, I can describe at least the following levels of detail for the model:
        \begin{enumerate}
            \item Basic kinematics model, no physics.
@@ -52,12 +53,18 @@
        \subsubsection{Evaluation}
            The current steps in the rapid development are difficult to perform.
            There is, unsurprisingly, lack of a clear vision of the end-product.
            Making describing all the different levels of detail explicitly farfetched.
            However, it was still possible to describe some levels of detail and a couple of expected elements that are added later.
            Which makes an explicit description of all the different levels of detail unfeasible.
            However, it was still possible to describe the initial steps in the level of detail of the design.
            The remaining elements, that are essential to the design, will take shape in a later stage of the development.
            Apart from this small deviation, the deliverables of this step are a good start of this development cycle.

    \subsection{Variable Detail Approach}
        The following steps is to increase the detail of the model.
        This is done according to the steps in the previous section.
        The following steps is to increase the level of detail of the model.
        The initial model together with the set of steps in the detail level is inherited from the previous design step.
        To start, I will implement the basic model and implement the different levels of detail.
        Based on the model after those steps, it is possible to make more detailed design decisions.
        The decisions make it possible to plan the subsequent levels of detail.
        Implementing these details as well, results in a competent model.

        \subsubsection{Basic Kinematics Model}
            \begin{marginfigure}
@@ -103,8 +110,7 @@
            In \autoref{fig:scaradesign}, this setup is shown as configuration 1.
            The disadvantage is that a motorized joint is heavy and has to be accelerated with the rest of the arm.
            Other configurations in \autoref{fig:scaradesign} move the motor to a static position.
            Configuration 2 is a double arm setup, but has quite limited operating range.
            Due to a singularity in the system when both arms at the top are in line with each other.
            Configuration 2 is a double arm setup, but has quite limited operating range, caused by a singularity region in the system when both arms at the top are in line with each other.
            Configuration 3 also has such a singularity, but due to the extended top arm this point of singularity is outside of the operating range.
            However, this configuration requires one axis with two motorized joints on it.
            Even though this is possible, it does increase the complexity of the construction.
@@ -116,12 +122,12 @@
                \label{fig:scaradesign}
            \end{figure}

            The actuation of the arm is done with stepper motors.
            The advantage of stepper motors over simple DC-motors is that they hold a specific position.
            There is no extra feedback loop required to compensate for external forces.
            They are heavier and more expensive as well.
            The additional mass is probably beneficial as adds momentum to the base, reducing the counter movement of the base when the arm is actuated.
            The extra costs are easily compensated as it save development time due to the simplified control law.
            The actuation of the arm is done with stepper motors, which have the advantage over DC-motors with their holding torque.
            The holding torque removes the need of a feedback controller to compensate for external forces.
            Instead, the stepper motors can be fully operated with a feedforward controller.
            However, they are heavier and more expensive.
            The additional mass is beneficial as increased inertia of the base, reducing the displacement due to the reaction force of the SCARA acceleration.
            The extra costs are easily compensated as it saves development time due to the simplified control law, and the removed need for extra angle sensors used in feedback control.

            Due to the aborted implementation of the end-effector, the SCARA must also lift the marker of the board.
            The chosen configuration of the SCARA makes it possible to add an extra joint in the linkage.
@@ -129,33 +135,35 @@

        \subsubsection{Implementing details}
            The new concrete design decisions, make it possible to plan the next steps of adding detail.
            The following steps are an addition of steps in as described in the previous section:
            The following steps are an addition to the steps as described in the previous section:
            \begin{enumerate}
                \setcounter{enumi}{4}
                \item Stepper motor behavior.
                \item Updating physics model to 3D physics.
                \item Marker lifting behavior, servo lifts marker of the board.
            \end{enumerate}
            The first step was to replace the DC-motor with a stepper motor model.
            This is based on a model by \textcite{karadeniz_modelling_2018}.
            Starting with replacing the DC-motor with a stepper motor model, which is based on a model by \textcite{karadeniz_modelling_2018}.
            The controller is updated as well, to accommodate for the behavior of the steppers.
            The next step is to implement a dynamic model of the configuration (4) as shown in \autoref{fig:scaradesign}.
            The next step is to implement a dynamic model of configuration 4 in \autoref{fig:scaradesign}.
            The dynamics of the SCARA are based on a serial link structure \autocite{dresscher_modeling_2010}.
            This serial link structure was makes it easy to add or extend joints and bodies to the system.
            This serial link structure makes it easy to add and extend joints, bodies and mass points to the system.
            Therefore, the last detail, the marker lifting, was added without any difficulty.
            The servo is connected via a linkage with the marker such that it rotates away from the board.

        \subsubsection{Component Design}
            At this point the development has produced a design with a competent dynamic model.
            The developed design does, however, not incorporate the physical component design.
            Nevertheless, must these components be designed to validate that the SCARA and its components can be constructed.
            At this point the development has reached a detailed design together with a dynamic model representing that design.
            The dynamic model is a useful tool to test and evaluate the system behavior.
            However, it does not include the shapes of the components and can therefore not be used to evaluate clearance or collision between components.
            By implementing the design using CAD software, it is possible to search for collisions.
            Furthermore, this model can than also be used to print the custom parts.
            For the mechanical part I used OpenSCAD as CAD software, based on prior experience with the software.
            With this it was possible to implement all the components that have to be made, as well as the \ac{ots}-components.
            With this it was possible to implement all the custom components as well as the \ac{ots}-components.
            Using the inverse kinematics model from the basic design of the SCARA, the angles were directly applied on the components in system.
            Allowing me to change the configuration of the SCARA and inspect the clearance between each component.
            Following the rectangular path as defined in \autoref{test1}, it revealed that collision occurred between some parts.
            These collisions were resolved by adding an indentation and moving linkage and are shown in \autoref{fig:scad_clearance}
            The configuration with the stepper motors, servo and marker is shown in \autoref{fig:scad_carriage}.
            Following the rectangular path as defined in \autoref{test1} revealed that collisions occurred between parts.
            These collisions were resolved by adding an indentation in one linkage and moving another linkage.
            These changes are shown in \autoref{fig:scad_clearance}
            The complete setup with the custom parts and the \ac{ots}-components, such as stepper motors, servo and marker, is shown in \autoref{fig:scad_carriage}.
            \begin{figure}
                \centering
                \includegraphics[width=0.8\linewidth]{graphics/scad_scara_circles.png}
@@ -188,3 +196,12 @@
            For example, changing the arm lengths and evaluate the new behavior.
            Did it improve? Is this as expected?
            Implicitly, the system was very often tested and changed based on test results.

    \subsection{Conclusion}
        With the development of the SCARA completed.
        Following the design plan, the development has to be repeated for the design of the Cable bot.
        However, the evaluation of the development until this point resulted in enough information to draw conclusions about the design plan.
        I expect that executing this development a third time is not beneficial to the case study, given the additional effort.
        Time is better spent on the realization of a prototype and improving the current design method.
        Therefore, the next section will go into the construction of the prototype instead of the development of the Cable bot.