# Natural Virtual Reality User Interface to Define Assembly Sequences for Digital Human Models
## 4 Discussion
These results are also supported by the work of [15], as a user interface with direct manipulation metaphors provides interaction techniques that users know from the real world and can thus need lees trials to learn the interaction process. To sum up, the VATS is easier to learn, and we can accept hypothesis two as well. The subjective workload was measured with the NASA-TLX questionnaire. The results show that the workload has no significant differences between the items, except in the dimension time. The participants experienced a higher pressure of time using the WIMP interface. This, in our opinion, comes from the number of different tasks the users have to perform to define the assembly sequence in the IPS IMMA (WIMP) interface. Thus, the users needed significantly more time to conduct the task of defining the assembly sequence with the WIMP interface than with the VATS. This leads to the point that the VATS creates less temporal workload, and we have to decline hypothesis three, as we expected no difference between the VATS and the IPS IMMA interface. The self-developed questionnaire shows significant positive results, except in the item \"system accuracy\" and that the \"tracking system is accurate enough.\" In our system, we use the leap motion as the tracking device. Whereas a previous study shows [34] that the system accuracy was significantly negative, we implemented several features to improve the accuracy: we rotated the leap motion at an angle of 45\({}^{\circ}\) so that the optimal tracking volume is in front of the users' torso. Furthermore, we implemented a warning for the users, which tells them if the hand has a higher distance than 45 cm between the leap motion and the wrist. Thus, the users always had their hands in the optimal tracking volume. To sum up, the VATS is a system that enables engineers to define an assembly sequence. The time to conduct the definition of the assembly sequence is lower than with the existing WIMP interface. Still, it is not as precise as the WIMP interface, but users can correct the errors from the VATS in the WIMP interface. In general, we expect, after reducing the error rate of the VATS, that more companies and engineers are going to use digital human models, as the engineers won't need the same amount of training, due to the more natural way to define an assembly sequence. This can improve the ergonomics of many assembly stations. In the end, this can have a positive influence on the workers who have to conduct the assembly tasks in the real factory lines daily. We don't see any impediments of using a head-mounted display, as our participants mentioned they would use head-mounted displays daily at their workplace. Future work will focus on improving the interaction method. We are going to work on using haptic gloves to interact in the virtual environment and use the reaction forces of each finger, as well as the interactions as simulation inputs, for the assembly sequence. In addition to this, we will extend the system to be able to interact with flexible parts like cables and flat flexible parts (e.g., car door panels). Furthermore, we are going to investigate different hand and finger tracking devices to improve hand tracking accuracy. We expect to be able to use the VATS also for other DHMs like JACK, Human Builder, Ramsis, and EMA, and will investigate the possible necessary adjustments to the VATS. Furthermore, we see several different use cases for the system:  
* Creation and validation of training for assembly and MRO (maintenance, repair, and overhaul);
* Creation and validation of assembly and MRO assistance systems, e.g., Augmented Reality, digital workables, adaptive instructions;
* Creation and validation of fitting and assembly processes itself, e.g., collision-free assembly planning. These use cases will be addressed in further research projects. Conceptualization, A.G.; Methodology, A.G. and E.B. ; Software, A.G.; Validation, A.G., E.B. ; Resources, R.S. ; Writing--original draft preparation, A.G. and E.B.