Characterization of Human Pen Grasp With Haptic Displays

Throughout this thesis we have, theoretically and experimentally, analyzed various properties of pen grasp interaction. We showed that to experimentally measure the mechanical impedance of the human hand we need a haptic device with specific characteristics, such as low inertia, almost zero friction and very high stiffness. Part of the research activity was devoted into designing a haptic device that would satisfy these stringent requirements. Using a theoretical model and experimental data we found that pen grasp manipulation is superior to single finger manipulation, both in terms of mechanical impedance and accuracy of motion control. Moreover, because of its parallel structure, the pen grasp stiffness ellipse is not very sensitive to changes in the kinematic configuration. The haptic display can be tuned to a specific user and task introducing an additional term in the device controller. The stiffness and damping ellipses can be enlarged, rotated, and their shape altered, so that better rejection to disturbance while executing precise motion control can be achieved. However we discovered that the potential benefits are limited by the natural performance of the device itself, since simulating dynamic elements, such as springs and dampers, introduces noise into the system. Our analysis was just a preliminary test and much more remains to be done. We analyzed four different types of grasp, with particular attention to four-fingered grasps, and found similar stiffness ellipses for all of them. Some of these grasps were chosen observing different people interacting with a pen, others by trying different configuration on a custom computer animated model.