**3.1.2 Tactile**

4 Will-be-set-by-IN-TECH

It is a matter of philosophical discussion how far it is possible to discuss hardware and software separately. Section 4.2 in particular will discuss theories from cognitive science which indicate that in the human, body and mind are intrinsically intertwined and that some of the hardware can in fact be considered to fulfill certain roles of the software too. However, entering into such a debate in detail is beyond the scope of this chapter. Therefore, when considering the different components of Rob's robot, we will largely separate hardware and software in the sense that we will discuss purely mechanical requirements first and computational requirements next. Since hardware can fulfill computational requirements and mechanical requirements can impose software requirements as well, there may be some natural overlap, which in a sense already illustrates the validity of some of the ideas from

First then, Rob needs to identify mechanical and engineering requirements for the creation of his humanoid robot. He is interested in particular in the state of the art of components like sensors and actuators, and must consider how close these elements are to his ambitious requirements and what major challenges still need to be addressed before his vision can become reality. The remainder of this section is dedicated to answering these questions.

Throughout the history of humanoid robotics research there has been an uneven study of the different human senses. Visual, auditory and tactile modalities have received more attention than olfactory and gustatory modalities. It may not be necessary for a robot to eat something but the information contained in taste and odor becomes important when developing higher levels of cognitive processes. Thinking about the future of humanoid robots we should be careful not to leave behind information that may be helpful for these machines to successfully

In this section Rob studies some of the devices that are either currently being used or are under development in different robotic platforms; they are categorized according to the human sense

Humanoid robots arguably pose the largest challenge for the field of computer vision due to the unconstrained nature of this application. Rob's plans for his autonomous humanoid involve it coping with an unlimited stream of information changing always in space, time, intensity, color, etc. It makes sense for Rob that among all sensory modalities within robotic applications, vision is (so far) the most computationally expensive. Just to have an idea of how challenging and broad the area of computer vision is, Rob remembers that in 1966 Marvin Minsky, considered by many to be the father of artificial intelligence (AI), thought one master project could "solve the problem of computer vision". More than 40 years have passed and

In robotic applications, a fine balance between hardware and software is always used when working with vision. During the last decade, Rob witnessed an increasing interest on transferring some software tasks to the hardware side of this modality. Inspired by its biological counterpart, several designs of log-polar CCD or CMOS image sensors (Traver & Bernardino, 2010) (Fig. 2(b)) and hemispherical eye-like cameras (Ko et al., 2008) (Fig. 2(c)) have been proposed. However, the always increasing processing power of today's computers and specialized graphical processing units (GPU) have allowed many researchers

**3. Mechanical requirements and engineering challenges**

embodied cognition (to be discussed in section 4.2.1).

**3.1 Sensors**

interact with humans.

they relate to the most.

the problem is far from solved.

**3.1.1 Vision**

Even though touch sensors are being used on few specific points of humanoid platforms (e.g. tip of fingers, feet, head), Rob thinks that an efficient solution to acquiring information from large surfaces is needed in order to be able to exploit the richness of textures, shapes, temperatures, firmness, etc. Therefore he argues the importance of developing skin-like sensors as another future challenge for humanoid robotics.

In robotic applications, Rob has used his imagination to create solutions for detecting objects in the path of an arm, finger or leg. One of these solutions has been the detection of peaks of current in their electric motors or using torque and force sensors. This approach could be considered tactile sensing as well but within the area of proprioception since measurements are made as relative positions of neighbouring parts in static and dynamic situations. In humanoid robots, exteroceptive sensors include dedicated pressure sensors placed on points of interest. The technology used in most pressure sensors so far include: resistive, tunnel-effect, capacitive, optical, ultrasonic, magnetic, and piezoelectric. Resistive and capacitive sensors are certainly the most common technology used not only in robotic but in many consumer electronic applications (Dahiya et al., 2010).

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Rob's Robot: Current and Future Challenges for Humanoid Robots 285

of chemical molecules ranging from biological agents to wine and perfume. As in the case of sound, it does not seem difficult to trust that in the near future Rob will have access to a device that gives his robot information about odor and taste, at the very minimum at human levels.

In the case of actuators, Rob illustrates the current state of the art of active components, although he is lately more biased towards approaches which make use of spring-damper components. He knows that the right choice of these components forms one of the critical challenges for his future humanoid robot. Rob's preference for non-linear components comes from the fact that the human body is made, from an engineering perspective, almost exclusively of spring-damper type elements: cartilages, tendons, muscles, fat, skin, etc. whose

Humans have a large repertoire of motor actions to move the whole body from one point to another. Walking could be considered the most representative behavior in this repertoire but we learn to adapt and use our bodies depending on the circumstances. Humans are also able to run, crawl, jump, climb or descend stairs, and if using the arms, then we can squeeze our

Rob realizes that researchers in whole body motion for humanoid robots have focused most, if not all their attention in walking only. The most common approach to control the walking behavior and balance of a humanoid robot is called Zero Moment Point (ZMP) and it has been used since the beginning of humanoid robotics research (Vukobratovic & Borovac, 2004). This approach computes the point where the whole foot needs to be placed in order to have no moment in the horizontal direction. In other words, ZMP is the point where the vertical inertia and gravity force add to zero. Most state of the art humanoid platforms like ASIMO, HRP-4, and HUBO2 make use of this approach. The main drawbacks of ZMP arise from the need to have the whole foot in contact with a flat surface, and it assumes that this surface has

Passive-dynamic walkers are an alternate approach to humanoid locomotion (Collins et al., 2005). These platforms try to exploit not only the non-linear properties of passive spring-damper components but the interaction of the whole body with the environment. The result is, in most cases, a more human-like gait with a heel-toe step in contrast to the flat steps seen in ZMP-based platforms. State of the art passive-dynamic walkers use dynamic balance control (Collette et al., 2007) which provides them with robustness to external disturbances and more human-like whole-body motor reactions. Examples of this approach can be found in platforms such as Dexter (Anybots, 2008), PetMan (Petman, 2011), and Flame (Hobbelen

Rob also found out that running and jumping have already been implemented in a few of the current humanoid platforms (Anybots, 2008; Niiyama & Kuniyoshi, 2010), although there is still much work to do to reach human-like levels. Once humanoid robotics started to become the meeting point of different scientific groups such as developmental psychologists, neuroscientists and engineers, interesting topics emerged. One of them was the study of infant crawling, its implementation in a humanoid platform was done using the iCub robot (Righetti

For our roboticist Rob, the challenge of making humanoid robots replicate the different types of motor behaviors found in humans is just one part of a larger challenge. An equally

interaction results in stability, energy efficiency and adaptability.

bodies between narrow spaces or if in water we can learn to swim.

enough friction to neglect horizontal forces.

**3.2 Actuators**

et al., 2008).

& Ijspeert, 2006), Fig. 1(d).

**3.2.1 Whole-body motion**

Advances in nano-materials are making it possible to integrate a large amount of sensors in flexible and stretchable surfaces (Bao et al., 2009; Peratech, 2011; Takei et al., 2010). Rob finds the approach taken by Prof. Zhenan Bao very interesting since it includes not only a sensitive resolution that surpasses that of human skin, but also the possibility to sense chemicals or biological materials (Bao et al., 2009). In addition, they are also working on embedding solar cells within the same films (Lipomi et al., 2011), which is an outstanding innovation since it would help to solve another major challenge for humanoid robots, i.e. energy.

Rob remembers that human skin is composed of two primary layers: epidermis and dermis. The dermis is located beneath the epidermis and contains all sensors for temperature and touch. He thinks about this because the dermis helps also to cushion the body from stress and strain, a task that is augmented by the morphological interaction of muscles, water, bones, etc. This dynamic plays an important role in tasks such as manipulation and locomotion. Therefore the design of Rob's future humanoid robot's skin should include not only a large amount and various types of sensors, but also an inner mechanism that provides similar dynamics like those found in human bodies.
