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- 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.
- For this first cycle of the design process, I design the end-effector.
- However, not long after the start of the development process, the implementation proved to be too complex.
- This led to the decision to abort the implemention of the end-effector.
- Eventhough no progress was made, this attempted implementation did provide valuable insight in the desing process.
-
- \subsection{Feature Selection}
- \label{sec:case_feature_selection_1}
- \begin{table}[]
- \caption{Overview of the different features and their dependencies, number of tests that are covered 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
- \ac{scara} & - & 3 & 40\% & 10 days & 4 \\ \hline
- End-effector & \ac{scara} & 2 & 60\% & 8 days & 7.5 \\ \hline
- \ac{cdc} & - & 2 & 30\% & 10 days & 3 \\ \hline
- \end{tabular}
- \end{table}
- For each feature in the system the dependees, tests and risk/time factor is determined, as explained in \autoref{sec:feature_selection}.
- These values are combined into \autoref{tab:firstfeatureselection}.
-
- The \ac{scara} depends on the end-effector, as explained in the initial design.
- However, for the \ac{cdc} no dependency was defined even though it has to lift the other two components.
- This is mainly because the torque and range requirements of the \ac{scara} depending on the implementation of the end-effector.
- Especially the required range depends on the method of grabbing and releasing tools.
- For the \ac{cdc} 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 apply to that feature.
- The risk and time values are not determined with a specific protocol, but with simple engineering judgement.
- 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 \ac{scara} has the most moving parts, but no difficult dynamics and has therefore an estimated risk of medium.
- For the \ac{cdc} there was no real risks and got therefore a low risk indication.
-
- Based on \autoref{tab:firstfeatureselection}, the end-effector is implemented first.
- The end-effector has the most dependees, and is therefore chosen above the other two.
-
- \subsubsection{Evaluation}
- This first step of the detail design phase did go well.
- Although risk and time assessment is always depend on some engineering judgment, this human factor introduces uncertainty in the assessment.
- However, an improved approach for the risk assesment can drastically reduce this human factor.
- Within a design team a form of planning poker \autocite{grenning_planning_2002} could be a good option.
-
- \subsection{Rapid Development of the End-Effector}
- \label{sec:case_development_cycle_1}
- This section explains the process of the development of the end-effector.
- The first step is to create an initial design of the model.
- In subsequent steps, detail is added to this model.
-
- The previous section explained the relative high risk assessment for the end-effector.
- Which was not exaggerated as the implementation proved to be troublesome.
- Eventually, the implementation was unfeasible and was therefore cut short.
- Nonetheless did it result in useful evaluation points on the design method.
- The process of this step is explained in the following sections.
-
-
- 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}
- The end-effector is mounted on the \ac{scara} and acts as an interface.
- With the end-effector, the \ac{scara} is able to grab and release tools.
- There are multiple approaches to handle these tools.
- However, there is a trade-off to be made with the \ac{scara} feature, the heavier the end-effector is, the more force the \ac{scara} must deliver.
- And because the goal is to make the \ac{scara} light and quick, this end-effector must be light-weight as well.
- The best options in this case is a simple spring-loaded clamp.
- To release the tool, the clamp is forced open, pushing it against the holder.
- As the end-effector is connected to the \ac{scara}, the \ac{scara} is responsible for the pushing force.
- Because the actuation force of the \ac{scara} is used, it removes the need for an additional servo in the end-effector.
- Resulting in a simpler and lighter design.
- The initial design of the clamp and the operation is shown in \autoref{fig:gripper}.
- Although this design requires the \ac{scara} to deliver more force.
- The relative low mass of the end-effector also keeps the moment of inertia small.
- Therefore, the current design reduces the impact on the acceleration of the \ac{scara} to a minimum.
- \begin{figure*}
- \centering
- \includegraphics[width=151mm]{graphics/end-effector.pdf}
- \caption{Operation of the end-effector. The clamp is forced open against the holder to release the marker.
- Instead of releasing, the marker is grabbed by reversing the order of executing for these steps.}
- \label{fig:gripper}
- \end{figure*}
-
- \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.
- Unfortunately, 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, the decision was made to remove the end-effector from the design.
-
- With the end-effector removed, the \ac{scara} gets a direct connection with the marker.
- The lifting of the marker from the is included in the \ac{scara} as well.
- Furthermore, this means that the erasing is no longer possible as a feature.
-
- \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.
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