**2. ROBOSKIN technology**

As we told before, ROBOSKIN tactile system, incorporates a distributed pressure sensor based on capacitive technology. The transducer consists of a soft dielectric sandwiched by electrodes. When pressure is applied to the sensor, the distance between the electrodes above and below the dielectric changes, and the capacitance changes accordingly (capacitance is a function of distance).

The basis of the sensor is a flexible printed circuit board (PCB). It includes the electronics to obtain 12 measurements of capacitance and send them over a serial bus. In particular, each PCB includes 12 round pads, one for each taxel, and a capacitance to digital converter (CDC) (AD7147 from Analog Devices).

**Figure 1.** The triangular module: (a) in the front view it is possible to see the 12 round taxels, in the back view the CDC chip from Analog Devices is shown; (b) An hexagonal patch glued on a cover with the foam elastomer and electrically conductive lycra layers.

The chip can measure the capacitance of all taxels with 16 bit resolution, but we use only 8 bit measurements, as any higher resolution is covered by noise. As a result, one measurement unit corresponds to 2.88 fF. The CDC has an I2C serial interface and each chip can be assigned with a 2 bit address; therefore up to 4 chips can communicate over the same serial bus. The shape of the PCB is in most cases a triangle (only for the fingertips of the robot iCub we used a unique solution, which we will discuss in section 3.1.1), see figure 1(a). The triangular shape was chosen in analogy to polygonal modeling in 3D computer graphics, which uses triangles to describe the shapes of objects. The triangles can conform up to a certain degree to generic smooth curved surfaces (see figure 2(a) and figure 2(b)).

The triangular PCBs also include the electronics to communicate between themselves: three communications ports placed along the sides of the triangle (one for input and two for output) relay the signals from one triangle to the adjacent one. Up to 16 triangles can be connected in this way (4 serial buses with 4 different addresses each) and only one of them needs to be connected to a microcontroller board (MTB, see figure 3). This is a critical advantage since it reduces the amount of wires and electrical connections that are required. The MTB can also

be used to program each CDC to either measure its 12 taxels independently at 50 Hz or an average of them at about 500 Hz.

**Figure 3.** The microcontroller board (MTB). We designed a small microcontroller board, which collects the measurements from up to 16 CDC chips and sends the measurements over a CAN bus.

Above the flexible PCB there is a layer of silicone foam (Soma Foama 15 from Smooth-On). It is 2 mm thick for the hands of the robot iCub, and 3 mm in all other cases. It covers the 12 pads and acts as a deformable dielectric for the capacitive pressure sensor. The foam also makes the skin compliant. On top of the silicone foam there is a second conductive layer made by electrically conductive Lycra. This layer is connected to ground and enables the sensor to respond to objects irrespective of their electrical properties. It serves as the common electrode above the silicone foam for all the taxels, see figure 1(b). When pressure is applied to the sensor, this layer gets closer to the round pads on the PCB, and the sensor measures the distance. This layer also reduces the electronic noise coming from the environment, in particular the stray capacity, which can be a problem for capacitive pressure sensor systems [2]. For the KASPAR and NAO, we didn't use the conductive layer on top of the silicone foam, as in these cases the robot is intended to interact only with humans, in which case the human constitutes the ground plane (like in many consumer products, which are responsive to humans, but are not responsive to insulators, for example).
