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The sensory system of the gripper takes into account both the motion of the fingers and the approach and interaction phases with the grasped object. In particular, each finger is equipped with a position sensor, based on the Hall effect, a proximity sensor and a miniaturized force/torque sensor, as shown in figure. The proximity sensor measures the distance of each finger from the object surface and allows to plan the approach motion in order to get synchronous contacts, while the force-torque sensor can be used for the control of grasping forces once contacts have been applied. Note that, being capable of detecting not only the intensity of contact force components but also the position of the contact centroid on the external surface of the finger, the intrinsic force-torque sensor can efficiently recognize actual contact conditions, included incipient sliding . In the current implementation, the proximity sensor is a simple optical fibre device, with a small emitter-receiver head very suitable for being hosted inside the finger shell. Even if the detection of distance is affected by reflectance and orientation of the object surface, early experiments demonstrated acceptable performance for the purposes of present application.
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