**3.1. Implementation on iCub robot**

The integration of the ROBOSKIN tactile sensor on iCub, has involved forearms, upper arms, torso and hands (palms and fingertips). The current implementation allows obtaining a number of distinguished taxels equal to 2400. In figure 5 the final result is shown.

**Figure 5.** The iCub covered with artificial skin

## *3.1.1. Implementation on the iCub hands*

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(a) (b) (c)

(d) (e) (f)

**Figure 4.** The production steps for the palm of iCub. For a description of each step, please refer to the

• Identification of the part to be covered, see Fig 4(a). If no CAD model is available, obtain

• Manufacturing of the part (or of a cover) with a 3D printer (Eden 3D printer from *Objet*). The resulting parts look for example like in Fig. 4(b). Round holes provide space for the

• Identification and wiring of the mesh of flexible PCBs that is needed to cover the part, see

• Bonding of the PCBs on the part with bi-component glue and the help of a vacuum system,

• Covering the PCBs with silicone foam, see Fig. 4(e). To this aim we employ a specific

• Covering of the silicone foam with a conductive lycra as ground plane, as shown in Fig.

The integration of the ROBOSKIN tactile sensor on iCub, has involved forearms, upper arms, torso and hands (palms and fingertips). The current implementation allows obtaining a

number of distinguished taxels equal to 2400. In figure 5 the final result is shown.

the shape with a 3D laser scanner (as for example for the hands of KASPAR).

CDC chip and the other electronic components which are soldered on the PCB.

text.

Fig. 4(c).

4(f).

see Fig. 4(d).

purpose-built mold for each part.

**3.1. Implementation on iCub robot**

As it is possible to see in figure 4(a), the palm of iCub is made from carbon fiber; since this is a structural part, we decided not to modify it, but instead, we added another cover above the carbon fiber part as a basis for the sensor: it has a thickness of 1.2 *mm* and provides space for the CDC chip and the other electronic components which are soldered on the PCB. The implementation steps are reported in Fig. 4. While for all the iCub parts the standard ROBOSKIN solution for the PCB has been used, for the iCub fingertips it has been necessary to design a specific solution (see figures 7(a), 7(b), 7(c)), since they have small size and round shape (each fingertip is 14.5 mm long and 13 mm wide and high and has a round shape that resembles a human fingertip).

The structure of the fingertip is illustrated schematically in Fig. 6: the inner support of the fingertip is shown in yellow, and the flexible PCB, that is wrapped around, is depicted in green. To mechanically attach the fingertip to the hand, the last phalanx of each digit (shown in red) has a stick that fits inside a hole in the inner support. A screw is used to secure the fingertip and, in addition, the screw fixes a fingernail on top of the fingertip that covers the PCB. The dielectric, made of silicone rubber foam, is depicted in brown; around the foam there is the conductive lycra layer shown in black as well as the AD7147 chip.

**Figure 7.** The production steps for the fingertip iCub

The fingertip production protocol involves the following steps:


**Figure 8.** Fingertip sensor placed on the support used for measurements together with schematic view of the line where measurements have been taken

With respect to the first implementation of the fingertips [18], where it was used a self-made mixture of silicone (CAF4 from *Rhodia-Silicones*) and carbon-black particles (Vulcan XC72 from *Cabot*) as a ground plane, and, as protective layer, silicone glue (Sil-Poxy from *Smooth-On*) sprayed above it, the new implementation made with conductive lycra, has been chosen because it increases the durability of the fingertip to usury due to friction forces appearing during grasp tasks. In order to characterized the behavior of the new fingertip several experiments have been performed.

The characterization setup consists of a cartesian robot (TT-C3-2020 from IAI) which moves one non-conductive probe against the fingertip. The non-conductive probe is fixed at the top of an off-center load cell (AS1 form Laumas) which measures independently the force applied to the fingertip by the probe during the test. A microcontroller records the CDC output and the load cell circuit output. Therefore the Data are stored in a computer by a dedicated graphic user interface made in Matlab.

The characterization protocol was the following:

8 Will-be-set-by-IN-TECH

PCB. The dielectric, made of silicone rubber foam, is depicted in brown; around the foam there

(a) (b) (c)

(d) (e) (f)

• The flexible PCB is wrapped around an inner support that was printed with a 3D printer. (see figures. 7(a), 7(b), 7(c)). As we are using an I2C serial bus, only 4 wires have to be connected to the PCB (Vcc, ground, serial data line and serial clock). They travel along the side of the fingers to small boards at the back of the hand. These boards relay the data from all five fingertips (and the four triangular modules in the palm) to one microcontroller

• A first layer, made with a foam elastomer (Smooth-on Soma Foama 15), is deposited over the PCB; mechanical deformation of this soft dielectric material leads to capacitance variations; therefore, it is possible to detect pressures applied on the fingertip, see figure

• The second layer, that is made with conductive lycra, is glued over the silicone foam substrate, allowing the development of a single ground plane above all electrodes placed on the PCB thus enabling the detection of each type of object (conductive and non conductive) within noise reduction. This layer is connected to the digital ground of the

• Finally the third protective layer, made again with Smooth-on Soma Foama 15, is deposited; it is used such as a protection, thus intrinsically increases the lifetime of the

**Figure 7.** The production steps for the fingertip iCub

board, which is located in the forearm of iCub.

CDC by one flat pad on the PCB.

fingertip sensor.

7(d).

The fingertip production protocol involves the following steps:

is the conductive lycra layer shown in black as well as the AD7147 chip.


Only the steady-state responses at external pressures of the sensor have been taken into account during post elaboration. Mean values of the fifty steady-state responses of C1 and C2, at different positions of the probe, are reported from figure 9(a) to figure 9(e). It is possible to observe a non linear trend of the capacitor variation as a function of the applied pressure. Therefore, the least squares method has been used to fit a polynomial model on each steady state response at each position. Figure 10(a) shows an example of the choice of the order of the polynomial function for CDC-C1 in the case of the position 0 mm, while, in figure 10(b), it is shown the norm of residual with respect to the polynomial order, on the basis of which,

**Figure 9.** steady-state response at 0 mm (a), 1.5 mm (b), 3 mm (c), 4.5 (d) mm and 6 mm (e) position point of the straight line of measurement

a 4th polynomial model order has been chosen. Polynomial functions thus obtained, were used to calculate the variation of capacitors C1 and C2 due to a fixed pressure as a function of position along the measurement line. Results are presented from figure 11(a) and figure 11(e), which report the variation of capacitors C1 and C2 as a function of the probe position for five different pressure values from 5kPa to 25 kPa.

**Figure 10.** a) An example of the choice of the order of the polynomial function for CDC-C1 in the case of the position 0 mm; b) Norm of the residual with respect to polynomial order

#### *3.1.2. Implementation on the iCub arms*

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(a) (b)

(c)

(d) (e)

**Figure 9.** steady-state response at 0 mm (a), 1.5 mm (b), 3 mm (c), 4.5 (d) mm and 6 mm (e) position

point of the straight line of measurement

For the iCub arms, the skin was integrate directly into the covers, so new covers were designed. The production steps are the same as for the palm and the other iCub parts and are illustrated in fig. 12, for the forearm, and fig. 13, for the upper arm. On the two iCub arms we have 1464 contact points: 6 patches (61 triangular module) and 7 MTBs for each arm.

### *3.1.3. Implementation on the Torso of iCub*

For covering the iCub Torso, five patches has been used, obtaining 528 contact points. In figure 15 it is possible to see the front of the torso covered with the glued patches and the silicone foam substrate (figure15(a)), the torso with the conductive lycra substrate on top (figure 15(b)) and finally the back of the torso cover with the 4 MTBs that are needed for the five patches (figure 15(c)).
