A robot collaborating with humans must be able to measure the intentions of humans. Indeed, instead of blindly following a trajectory, a physically interacting robot will react to pressure applied on it by a human. It is thus relevant to cover the robot with a tactile sensor, or robotic skin, capable of sensing pressure applied anywhere on it. Preferably, the robotic skin should be made of a flexible and stretchable material that allows its installation on the external surface of the manipulator. The advantage of such tactile sensors is to allow physical interaction with all parts of the robot, as opposed to a standard force/torque sensor.

    The material used to build the robotic skin is rubber silicone. In addition to being flexible and stretchable, it is possible to mix it with carbon black to obtain an intelligent electrical conductor. Indeed, the electrical conduction is enabled by carbon chains formed during the silicone solidification. When a pressure is applied on the silicone, the chains break and the electrical conductivity is reduced in the sensitive layer, as schematically shown in the next figure.

    Fig. 1: Incompressible model of a robotic skin made of silicone rubber filled with carbon black.

    To measure the local resistivity of the sensitive layer, an electrode is placed on each side of the silicone rubber, thus creating a taxel (tactile pixel). Because using two electrodes for each taxel would require a large number of wires, band-shaped electrodes are placed perpendicularly on each side of the sensitive layer, thus creating a taxel grid. By successively grounding each row, it is possible to simultaneously measure the resistivity on all columns which are electrically isolated with an operational amplifier. This approach, illustrated in the next figure, allows a significant reduction of the complexity of the electronics required for a large taxels grid.



    Fig. 2: Electrical circuit used to measure the resistance for each taxel.

    The electrodes, consisting of a flexible and stretchable conductive fabric, is molded with the carbon black filled silicone rubber to reduce the contact resistance. The whole is then molded in an non-conductive silicone rubber to isolate and protect the sensitive components. The additional silicone also allows impact absorption during unexpected collisions. Figure 3 shows a robotic skin prototype with 4x4 taxels.

    Fig. 3: Robotic skin prototype with 4x4 pressure-sensitive taxels.

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