## 5 Conclusions and Discussion  
In this paper, we presented DFF, a device that generates directional sensations in VR applications through mechanical force concentrations. Our approach was based on inducing force concentrations via controlled motor rotations. A method was developed to optimize and control the rotational speeds and rotation angle intervals to maximize the directional sensations. Three user studies were conducted to investigate the potential impact of the proposed device as a VR force feedback system. Study 1 was conducted to test how effectively DFF can provide directional sensations to the users. The result of this study indicated that the participants can correctly guess the target direction (i.e., up, down, forward, backward) along with the dragged direction. The dragged direction is a direction of force concentration in the target direction. We measured the dragged direction because the \"target direction\" is difficult to estimate, especially when decomposing 360 degrees to 4 directions. During the interview, the participants commented that it was difficult to distinguish directions if the target direction was not exactly the forward, upward, downward, or backward direction. For example, if the force is concentrated at between forward and downward, it is cognitively difficult to classify the target direction. Also, the current DFF sets the target direction with an absolute value. Thus, a user can feel a discrepancy if his/her hand is not placed at the right coordinate. In this case, the inertial sensor can help set the target direction relative to hand position. Future work on optimizing directions in the cognitive level, and installation of the accelerometer can significantly improve the performance of DFF. In study 2, the participants responded that they could accurately feel the variations in weight. We only tested three weight variations (light, medium, and heavy) in the current study; future work on developing more weight variations can widen the scope of applications of DFF. The result of study 3 showed that DFF increased presence and immersion within the VR environment compared with an experiment without DFF. However, users' enjoyment within the VR did not show significant differences between the two experiments. This may have been caused by the simplicity of the VR game. During the  
Figure 12: Survey responses of the third study (error bar indicates standard deviation). interview session, the participants responded that the VR game in study 3 was not entertaining. This is because we developed a simple VR game to test the impact of DFF on VR immersivity. As the users indicated that both presence and immersion increased with DFF, we believe that the enjoyment can be significantly improved if more features were added to the game. We identified several research areas for future work to expand the impact of DFF. As the DFF can generate plausible directional sensations of various degrees via force concentrations, the next step will to investigate the miniaturization of the device. The current version comprises two actuators attached to a cylindrical handle. However, the actuators can be attached to other input devices, including VR joysticks and haptic gloves. Furthermore, safety is an important requirement in VR. The DFF was tested in a lab setting with supervision for safe experimentation. The force concentration requires rotation of weights, therefore protection gear for the DFF needs to be developed. Finally, potential applications for DFF need to be considered in various fields. DFF can be used in educational and professional environments in addition to consumer use. For example, DFF can be used to guide visually impaired people towards the target direction. Additionally, it can be used in VR sport games, and as the interaction interface in social VR. Conceptualization, K.H.H. and S.B. ; methodology, K.H.H. and S.B. ; software, K.H.H. and S.B. ; validation, S.B. ; imsugi, K.H.H. and S.B. ; resources, K.H.H. ; data curation, S.B., writing--original draft preparation, K.H.H. and S.B.