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  A consistent number of preliminary experiments has been performed on single finger modules and sensorial/actuation subsystems in order to test the efficiency of each finger structure and of the control system. The validation has also included verification of the procedures for the object approach, based on the integration of the distance and the position sensor information.
At the moment, a first prototype of the gripper has been completed and installed on a 6 dof antropomorphic robot, a COMAU SMART 3S with a open-control architecture (i.e. a PC connected to the standard robot controller C3G9000) and equipped with a force/torque sensor on the wrist.. The open control architecture allows in particular to sinchronize the tasks of both the gripper and the arm for micro-motion during task execution.

Moreover, a set-up has been prepared in order to perform different experiments using the prototype, where absence of gravity is partially simulated by suspending the objects with a wire. The first experiments include demonstrative tasks of the following procedures:

  • use of proximity sensors for coordinating the approach phase of the fingers;
  • control of the approach/contact phase of floating objects;
  • scanning of the object surface for shape recognition by means of the proximity sensors;
  • choice of optimal grasp configuration according to a criterion of maximum area of friction cones convex;
  • simultaneous application of contacts and test of grasp accuracy and stability.

Concerning the approach and contact phases, it must be observed that the possibility of independently moving the fingers has noticeably increased the capability of grasping moving objects. As a matter of fact, the object may be tracked (if moving) with a coordinated movement of both the arm and the fingers. Once the motion is tracked (i.e. the fingers move syncronously with the object), the grasp may be firmly applied without loosing contact.

As an example of this procedure the positions of the three fingers are shown while approaching a moving object. When the distance from a finger from the object is less than a threshold (5 mm in this case), the position set point is computed in order to remain at that fixed distance from the surface (also if moving) until all the three fingers are in this situation. In the figure, two cases are reported: at the left the approach phase of a almost fixed object, while on the right the track of a moving object (in this case a sphere) within the workspace of the gripper.

In the figure above another test is reported concerning the grasp of a floating object. In this case, the approach motion is stopped until all the three fingers are at the same distance from the object, then the contacts are applied.

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