###### Abstract  
In recent years, consumer-level virtual-reality (VR) devices and content have become widely available. Notably, establishing a sense of presence is a key objective of VR and an immersive interface with haptic feedback for VR applications has long been in development. Despite the state-of-the-art force feedback research being conducted, a study on directional feedback, based on force concentration, has not yet been reported. Therefore, we developed directional force feedback (DFF), a device that generates directional sensations for virtual-reality (VR) applications via mechanical force concentrations. DFF uses the rotation of motors to concentrate force and deliver directional sensations to the user. To achieve this, we developed a novel method of force concentration for directional sensation; by considering both rotational rebound and gravity, the optimum rotational motor speeds and rotation angles were identified. Additionally, we validated the impact of DFF in a virtual environment, showing that the users' presence and immersion within VR were higher with DFF than without. The result of the user studies demonstrated that the device significantly improves immersivity of virtual applications.  
user interface; human-computer interaction; virtual reality; force feedback; force concentration [1  
wearer's movements depending on the VR contents [18]. However, none of these innovations focused on providing directional feedback during interaction with objects possessing inertia (i.e., free-falling objects from various heights or objects travelling laterally with momentum). Despite the state-of-the-art force feedback research being conducted, a study on directional feedback, based on force concentration, has not yet been reported.  
In this study, we focus on developing a method of force concentration in different directions for an immersive experience when interacting with moving objects in VR. We developed a device, directional force feedback (DFF), that uses the rotations of motors to concentrate force and deliver directional sensations to the user. DFF generates directional haptic feedback for VR applications using mechanical force concentrations, and comprises actuators and mechanical structures to provide an immersive virtual experience. The device rotates attached weights at controlled speeds to provide directional sensations. As shown in Figure 1, users hold DFF as they would any other input device before interacting with VR objects. In-depth interviews followed by three user studies were conducted to test the impact of DFF. In the first user study, we validated the capability of force concentrations to generate force feedback with directional sensations. The second user study tested the extent to which the sensation of weight can be delivered via DFF. In the final study, we evaluated the impact of DFF on the immersivity of an experience in a virtual environment. To the best of our knowledge, this is the first study reported that uses force concentrations to generate directional force feedback. The highlights of the proposed research are as follows. A novel method of force concentration was developed for directional sensation we identified the optimum rotational motor speeds and rotation angles, considering both rotational rebound and gravity. We validated the impact of DFF in a virtual environment, showing that the users' presence and immersion within VR were higher with DFF than without.