## 4 Discussion  
### Factors Influencing Operation of Control Elements in VR  
In this study, we identified, for the first time, the factors that influence the positioning accuracy, time on task, presence, user experience, and mental workload when operating joysticks and rotary controls in VR. Mathematical models were used to calculate how these factors must be designed to enable users to operate the control elements efficiently in VR and to experience a high presence and user experience and a low mental workload. Finally, the VR-adapted control elements were validated by user tests. Figures 4 and 6 show the influence of the **angular resolution** and the **positioning speed** of the control elements on the evaluation parameter time on task and positioning accuracy. The higher the angular resolution or the positioning speed, the less time the test participants required to complete the tasks. However, the test participants with sensitive control elements also made more errors. The 8%/s calculated by the optimization calculation is therefore a compromise between time on task and positioning accuracy. The **diameter** of the rotary control had a significant influence on the efficiency dimension of the UEQ and on mental workload. Test participants rated the mental workload higher and the efficiency dimension lower when the rotary control had a large diameter of 110 mm. However, the diameter of the rotary control had no influence on the positioning time or positioning accuracy. Thus, the test participants only had the feeling that the operation of the big rotary control was not as efficient. Objects are often displayed larger in \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Pred. Mean** & **Std Dev** & **95\% PI Low** & **Data Mean** & **95\% PI High** \\ \hline Time on Task & 34.65 & 14,3 & 15.46 & 23.10 & 53.81 \\ Pos. Accuracy & 23 & 10 & 9 & 10 & 37 \\ UEQp & 1.97 & 1.20 & 0.60 & 2.25 & 3.35 \\ UEQe & 2.91 & 0.65 & 1.94 & 2.06 & 3.88 \\ UEQs & 2.29 & 0.76 & 1.32 & 1.13 & 3.27 \\ UEQN & 3.00 & 0.71 & 1.75 & 1.31 & 4.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Results of confirmation runs for the joystick. Predicted parameter results vs. measured results of the validation runs. Figure 7: Optimized shape and height of the VR joystick. VR for better readability or representation. The results of this study show that, at least for control elements, an enlarged display may not be useful despite good operability, because test participants perceived the operation of the control element as less efficient. The factor **input device** had a significant effect on the positioning accuracy. The test participants with the Valve Index controller primarily had problems releasing the control element at the end of the positioning process. The test participants had to remove their fingers from the controller simultaneously to release the control element in VR. If this was unsuccessful and the test participants continued to move their hands, the control element continued to rotate. With the HTC Vive controller, only the trigger button must be released. Problems with the release did not occur with this controller. Test participants who tested rotary controls with **haptic feedback** were able to complete the tasks significantly faster than test participants who tested rotary controls without haptic feedback. We expected that haptic feedback would also influence the positioning accuracy. However, the test participants apparently acted more cautiously and thus slower with the rotary controls without haptic feedback in order to make as few errors as possible. The test participants were not informed that the time for the positioning tasks or the number of errors were measured. The **shape** of the rotary control had no significant influence on the time on task and positioning accuracy, but it influenced the mental workload. The test participants with the knurled rotary control experienced the lowest mental workload. The highest mental workload was reported with the cylindrical rotary control.