Development of a Finger Pad Force Display for a Hand Haptic Interface

Most human interfaces in virtual environments use information of visual sensation and auditory sensation. By adding tactile sensation to these human interfaces, the human operator can handle objects with a realistic tactile sense and can perform complex tasks in the virtual reality environment. Haptic interfaces that present force and tactile feeling to the fingertips of a human operator have been investigated extensively over the last two decades [1]-[12]. Although some of these interfaces [9]-[12] provide three-dimensional force feeling to the human fingertip, few haptic interfaces cover the workspace of the human arm. These interfaces are classified into two types: wearable type interfaces [9], which are mounted firmly to the human hand, and opposed type interfaces [12], which are mounted in a position opposed to the human hand. These interfaces apply three-dimensional (3D) forces only to the human fingertips. Medical doctors must use palpation in examining patients, and the force and tactile sensation on both the fingertips and the finger pads are important in such palpation. However, training doctors to perform palpation is difficult, because this requires the cooperation of the patient. Thus a breast palpation training system in a virtual reality environment [13] would be useful as a training tool for palpation. However, due to the limitation of human finger length, developing a haptic interface that displays 3D force feeling to both the fingertips and the finger pads is not easy. The density of tactile sense organs in the human finger [14] is high in the fingertip and low in the finger pad. Hence, the human finger has a high sensitivity to 3D force at the fingertip but a low sensitivity at the finger pad. This suggests that a haptic interface that consists of 1D finger pad force display devices and a 3D fingertip force device would be effective for use in a virtual environment such as a virtual breast palpation training system. The present paper describes a hand haptic interface for use in a virtual training system in which not only fingertip force display but also finger pad force display is required. The hand haptic interface consists of novel finger pad force display devices and a 3D fingertip haptic interface, known as HIRO II [12], which was developed by our group. The developed finger pad force display device is driven by a flat-type brushless DC motor and is easy attachable to the finger pad. The applied force is controlled by a time interval control, which is an open-loop control. Here we present the design concept of the hand haptic interface, the control method and specifications of the finger pad force display device, and the results of an experimental evaluation of manipulating a virtual object. We also provide a comparative