Developing Grasping Pre-Shaping in Virtual Environment based on Real Object Shape Analysis
The research conducted in neuro-psychology showed that a prehensile movement can be decomposed in three main stages (figure 1.1): (1) visual location of the target, including object recognition, orientation determination and shape and size analysis from visual information, (2) reaching of the target including a ballistic movement, when the hand preshapes according to the shape and size of the object, and fine movements near the target, (3) grasping corresponding to the fingers adjustments against the target , . In virtual environments we deal with the second and the third stage: preshaping and virtual grasping. For virtual grasping to be completed one has to have a precise match of the virtual gripper with the stereoscopic representation of the object.
In real world, in a dextrous manipulation, one of the major problems that remain to be solved, is the determination of an appropriate preshape for grasping given the object, under a given manipulation task. In order to have a successful grasp execution, the system should have a successful grasp preparation at preshaping phase. Hence, the grasp preparation or preshaping is one of the critical issues for manipulation with a dextrous robot hand  The hand preshaping has usually been described by terms such as hand aperture  and hand orientation . The term ‘hand aperture’ describes hand opening during the approaching phase of grasping and is defined as the distance between the thumb and the index finger. The advantage of such a description is its simplicity. Despite the fact that the thumb and index finger probably have an important role in grasping, it should not be forgotten that most of the grasping modes require the cooperation of all five digits and not only the thumb and the index finger. Therefore, such a definition of hand aperture unjustifiably ignores the influence of other fingers but is useful because it can be refined using the concept of virtual fingers . A virtual finger corresponds to a set of real fingers moving together (there are two virtual fingers when grasping a glass, one is the thumb and the other consists of 4 remaining fingers). Using the concept of virtual fingers, the hand aperture is the distance between the thumb and the virtual finger.
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