Introduction

Recently, the development of VR technology has enabled users to have a highly immersive experience. The development of force-feedback devices that present the sense of force applied to the body in a VR environment has been advanced. Many of these devices have been studied for upper-limbs using motors and reduction gears [1]. In contrast, the use of MR fluid brakes can safely present a sense of force. In addition, force-feedback not only to the upper-limbs but also to the lower-limbs is expected to be applied to contents such as effective VR training. Therefore, the objective of this research is to develop an exoskeletal lower-limb force-feedback device that enables a wide range of mobility. In this paper, the presentation of underwater motion sensation by the proposed device was evaluated mainly from the sense of presence.

Added Mass Model

When an object in a fluid is accelerated, and the fluid around the object is also accelerated, and the equivalent amount of extra energy is consumed. At this time, the inertial force of the object increases as if its mass had increased. This apparent increase in mass is called added mass. The added mass depends on the shape of the object. As a force-feedback model of the added mass force, we construct a model in which a load torque proportional to the joint angular acceleration is applied to the lower-limb.

Experiment

The purpose of this experiment is to verify the effect of the proposed added mass force model on the sense of presence of knee motion in water in a VR space by incorporating it into the developed force-feedback device. For Comparison, a “Drag” condition proportional to velocity, a “Base” MR torque condition and a “None” condition without the device were prepared. The experimental environment and the results of the VAS questionnaire for sense of presence are shown in Fig. 1 and Fig. 2. The “Added mass” condition using the added mass model was significantly higher sense of presence than the “None” condition, providing approximately 70 % more sense of presence.

Fig. 1 Experimental environement.


Fig. 2 VAS score of sense of presence in the comparison experiment.

Force feedback,Exoskeleton,MR brake