In this study, a haptic rendering method named indirect haptic feedback is proposed. In traditional haptic interfaces for the index finger, the contact point of the finger and the point of application of force are the same when touching a virtual object. As a result, in the augmented reality environment, a user is unable to touch a real object with his/her bare hand through traditional haptic interfaces. In this study, separate contact and reaction force points are selected for haptic rendering. To realize this, a prototype system, which consists of a motion tracker, a 2-DOF haptic interface that is equipped with a force sensor, and a visual display, is developed. The system measures the position of the fingertip of a user and produces an appropriate horizontal force on the thenar eminence (the group of muscles on the palm of the human hand at the base of the thumb) of the user. Through the visio-haptic interface, a user can perceive the surfaces of 3D virtual objects. In addition, the user can perceive the frictional force on the surface of the objects. Through two evaluation tests, it was verified that the subjects could effectively perceive the 3D virtual objects using this system.
The system consists of a 24-inch LCD display, a 2-DOF haptic interface equipped with a motion tracker and a force sensor, and a PC for controlling them. The 2-DOF interface is equipped with two linear actuators (KR33, produced by THK). Each actuator has an AC servomotor (MSMD011P1S, produced by Panasonic) and an optical shaft encoder. Their maximum speed is 833 mm/s and maximum force is 200 N. One actuator is placed on top of another actuator, horizontally perpendicular to each other. The size of the operating region of the end effector is 300 mm x 300 mm. On top of the end effector of the interface, a motion tracker (Leap Motion controller, produced by Leap Motion) is placed under the fingertip to measure the relative position of the fingertip from the end effector. In addition, a force sensor (CFS034CA301, produced by Leptrino) is placed on top of the end effector. The PC (Windows7 Professional OS, 64-bit CPU, Intel Core i5 650) communicates with the interface through a master control board (HCRTEXsd, produced by Vanguard Systems). The position of the finger is measured by the encoders and the tracker. Force applied to the thenar eminence is measured by the force sensor.