Object motion-decoupled internal force control for a compliant multifingered hand
Compliance in multifingered hand improves grasp stability and effectiveness of the manipulation tasks. Compliance of robotic hands depends mainly on the joint control parameters, on the mechanical design of the hand, as joint passive springs, and on the contact properties. In object grasping the primary task of the robotic hand is the control of internal forces which allows to satisfy the contact constraints and consequently to guarantee a stable grasp of the object. When compliance is an essential element of the multifingered hand, and the control of the internal forces is not designed to be decoupled from the object motion, it happens that a change in the internal forces causes the object trajectory to deviate from the planned path with consequent performance degradation. This paper studies the structural conditions to design an internal force controller decoupled from object motions. The analysis is constructive and a controller of internal forces is proposed. We will refer to this controller as object motion- decoupled control of internal forces. The force controller has been successfully tested on a realistic model of the DLR Hand II. This controller provides a trajectory interface allowing to vary the internal forces (and to specify object motions) of an underactuated hand, which can be used by higher-level modules, e.g. planning tools.
Hand-Tool-Tissue Interaction Forces in Neurosurgery for Haptic Rendering
Haptics provides sensory stimuli displaying the interaction with virtual or tele-manipulated objects. The haptic feedback can be provided to the user via tactile information and via kinesthetic feedback. Here we focused on measuring the interaction forces during neurosurgical tasks performed on a brain phantom, with the aim of understanding which could be the best haptic feedback in a real tele-operation scenario. We instrumented three neurosurgical tools using Force Sensitive Resistors, for measuring the contact forces exerted by surgeons to tools. A load cell placed under a brain phantom measured the tool-tissue forces. Three neurosurgeons were asked to perform typical actions on the phantom. The measured surgeon-tool contact forces ranges, i.e., 0.01 - 3.49 N for the thumb and 0.01 - 6.6 N for index and middle fingers, fit the range of the cutaneous sensitivity of the human finger pad. The measured tool-tissue interaction forces were from six to eleven times lower with respect to the contact forces, i.e., 0.01 - 0.59 N. Eventually, we believe that convey only the cutaneous component of the haptic feedback would transmit a sensation comparable to that present when both cutaneous and kinesthetic feedback are given. Additionally, this approach does not compromise the stability of the haptic feedback loop.