**1. Introduction**

184 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

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In order to allow robots to share our space and chores, tactile sensing is crucial. Indeed it allows safe interaction of robots with people and objects, because it provides the most direct feedback to control contact forces both in voluntary and involuntary interactions. Furthermore, it allows improving performance in tasks that require controlled physical interactions in uncontrolled environments where the location and the characteristics of contact cannot be predicted or modeled in advance and more complex forms of interactions are required. Therefore, a tactile sensor system capable of measuring contact forces over large areas is needed. Tactile sensing in robotics has been widely investigated in the past 30 years and many examples of engineering solutions to tactile sensing have been presented in the literature [1]. Research in this field has focused largely on transduction principles and transduction technologies [2]; however, various technical issues have limited the transition from a single tactile element (or a small matrix prototype) to a large scale integrated solution: it is easy to understand that a sensitive robot skin cannot be achieved by simply aggregating a large number of single sensors. In fact, the concept of robot skin entails a number of system level problems that simply do not appear when focusing on small tactile sensors or small area arrays:


©2012 Maiolino et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Several tactile systems have been integrated into robots and described in the literature. Some of them are modular and include hierarchical data processing. Yet, the modules are usually big and cannot be installed on small robot parts. In many cases the spatial resolution is low; in others the sensory modules connectivity is cumbersome. This is clearly in contrast with the requirements above. Small sensor modules are typically necessary in order to allow the communication between them; size constraints are then very critical as not enough space typically can be found on the sensor modules to the local integration of microcontrollers or other networking electronics. One of the first examples of full-body skin for robots was proposed by Inaba et al. [3] that describes a tactile sensor system composed of a layered structure of electrically conductive fabric implementing a matrix of pressure sensitive switches. However, the complexity of the manufacturing process and the limited sensing capabilities represent the most significant limits of this approach. Force detectable surface covers have been introduced by Iwata et al. [4]. Information originating from both resistive and force sensors to correlate pressure information and exerted force is exploited. Although the presented system can be used to cover large surfaces, the actual design is strictly dependent on the actual robot surface. However, a principled discussion about design issues is not carried out. Ohmura et al. [5] proposed a conformable and truly scalable robot skin system formed by self-contained modules supported by a flexible printed circuit boards (PCBs) that can be interconnected. Each module contains 32 tactile elements consisting of a photo-reflector covered by urethane foam. In order to adjust the distance between each tactile sensor element, a band-like bendable substrate that can be easily folded (or even cut) is adopted. The system integrates a microcontroller based architecture for data acquisition and networking, however, the dimension of the system are too big for small robot parts and the spatial resolution featured is quite low [6]. The tactile system that has been developed for the robot RI-MAN also uses flexible PCBs with a tree-like shape to conform to curved surfaces [7]. The tactile elements are commercially available piezoresistive semiconductor pressure sensors, and the measurements have less hysteresis than the one shown by Ohmura et al. [5]; to reduce the number of wires, the sensor modules include multiplexers, but this approach requires to connect each module individually to a controller board. The robot ARMAR-III [8] uses skin patches based on piezoresistive sensor matrices with embedded multiplexers. The patches have a flat or a cylindrical shape and are specifically designed for the different parts of the robot; smaller patches are used for the fingers. In Kotaro [9], tactile sensing is achieved by using flexible bandages formed by two flexible PCBs with an intermediate layer of pressure sensitive conductive rubber. Each bandage has 64 taxels, but no integrated data acquisition electronics is mentioned. Piezoelectric transducers are used for the humanoid robots Robovie-IIS [10] and CB2 [11]. The transducers were placed individually on the robots and the sensitive skin has a low spatial resolution.

2 Will-be-set-by-IN-TECH

• Dynamic range and sensitivity: The system should be able to detect a wide range of contact

• Total Weight and installation space: In order to cover the whole robot body surface the weight of the sensors must be small as well as the required space for installation.

• Costs: The system must be as cheap as possible and off-the-shelf components should

• Manufacturing and deployment: the ease and speed of production have to be taken into account. The deployment procedures must be reproducible on robots with different

Several tactile systems have been integrated into robots and described in the literature. Some of them are modular and include hierarchical data processing. Yet, the modules are usually big and cannot be installed on small robot parts. In many cases the spatial resolution is low; in others the sensory modules connectivity is cumbersome. This is clearly in contrast with the requirements above. Small sensor modules are typically necessary in order to allow the communication between them; size constraints are then very critical as not enough space typically can be found on the sensor modules to the local integration of microcontrollers or other networking electronics. One of the first examples of full-body skin for robots was proposed by Inaba et al. [3] that describes a tactile sensor system composed of a layered structure of electrically conductive fabric implementing a matrix of pressure sensitive switches. However, the complexity of the manufacturing process and the limited sensing capabilities represent the most significant limits of this approach. Force detectable surface covers have been introduced by Iwata et al. [4]. Information originating from both resistive and force sensors to correlate pressure information and exerted force is exploited. Although the presented system can be used to cover large surfaces, the actual design is strictly dependent on the actual robot surface. However, a principled discussion about design issues is not carried out. Ohmura et al. [5] proposed a conformable and truly scalable robot skin system formed by self-contained modules supported by a flexible printed circuit boards (PCBs) that can be interconnected. Each module contains 32 tactile elements consisting of a photo-reflector covered by urethane foam. In order to adjust the distance between each tactile sensor element, a band-like bendable substrate that can be easily folded (or even cut) is adopted. The system integrates a microcontroller based architecture for data acquisition and networking, however, the dimension of the system are too big for small robot parts and the spatial resolution featured is quite low [6]. The tactile system that has been developed for the robot RI-MAN also uses flexible PCBs with a tree-like shape to conform to curved surfaces [7]. The tactile elements are commercially available piezoresistive semiconductor pressure sensors, and the measurements have less hysteresis than the one shown by Ohmura et al. [5]; to reduce the number of wires, the sensor modules include multiplexers, but this approach requires to connect each module individually to a controller board. The robot ARMAR-III [8] uses skin patches based on piezoresistive sensor matrices with embedded multiplexers. The patches have a flat or a cylindrical shape and are specifically designed for the different parts of the robot; smaller patches are used for the fingers. In Kotaro [9], tactile sensing is achieved by using flexible bandages formed by two flexible PCBs with an intermediate layer of pressure sensitive conductive rubber. Each bandage has 64 taxels, but no integrated data acquisition electronics is mentioned. Piezoelectric transducers are used for the humanoid

possibly be used to decrease the manufacturing cost.

pressures.

characteristics.

In Yoshikai et al. [12] the construction method for soft stretchable enclosing type tactile sensing exterior for humanoid robots is presented. A cardigan knit sensor which encloses an upper body of a small humanoid has been developed using the proposed method. The overall sensor is constituted by layers of conductive fabrics that are knitted for improved stretchability. The presented cardigan knit has, however, only 75 electrodes.

Shimojo et al. [13] have developed a mesh of tactile sensors that satisfy the requirements of a high speed and reduced number of wires and that is able to cover surfaces of arbitrary curvatures. The tactile sensors mesh is arranged as a net, where only nearby taxels are connected through wires. Since the shape of a patch must be specifically designed, modularity issues are not properly addressed, but scalability is possible thanks to the smart wiring infrastructure: only 4 wires actually exit from the patch.

Mittendorfer et al. [14] have presented a tactile sensor system made by small hexagonal PCB modules equipped with multiple discrete off-the-shelf sensors for temperature, acceleration and proximity. Each module contains a local controller that pre-processes the sensory signals and actively routes data through a network of modules towards the closest PC connection. The sensory system is embedded into a rapid prototyped elastomer skin material and redundantly connected to neighboring modules by just four ports. The functionality of some modules on a KUKA robot arm has been demonstrated, however a complete integration has not been shown.

The ROBOSKIN tactile system (Patent No. I0128764) is our proposal for a skin that is able to cover large area of a robot body. It 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 changes, and the capacitance changes accordingly (capacitance is a function of distance). The ROBOSKIN system is made up of a number of tactile elements (taxels) geometrically organized in interconnected modules of triangular shape. Above the flexible PCB, there is a layer of silicone foam (Soma Foama 15 from Smooth-On) that covers the 12 taxels and acts as a deformable dielectric. On top of the silicone foam is placed a deformable conductive layer made of electrically conductive lycra-like fabric. This layer is connected to ground and enables the sensor to respond to contacts with objects of any material.

As we mentioned earlier, manufacturing and deployment processes constitute an important aspect of the robot skin design since the skin has in fact to be integrated on robots with different characteristics. Flexibility in this sense is a good property for a robot skin system. In addition, it is desirable that the skin production process is reproducible so to guarantee resilience against impact and forces and the general wear and tear of the material. In particular, the ROBOSKIN tactile system has been integrated on three different types of robots: iCub [15], KASPAR [16] and NAO [17]. The three robots have different sizes and shapes, and the tactile feedback has been used for different purposes. Nevertheless, the methods that have been used to implement the skin were nearly the same, which demonstrate the portability of the sensor system technology. This chapter is organized as follows: firstly, the ROBOSKIN tactile system is presented in details in section 2; section 3 presents the methods and procedures for ROBOSKIN manufacturing and integration; in particular, section 3.1 is related to the iCub platform; section 3.1.1 presents a new implementation of iCub fingertip with experimental data related to their behavior characterization; section 3.2 is related to KASPAR and section 3.3 presents the ROBOSKIN integration on robot NAO; finally section 4 is dedicated to conclusion.
