View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Scientific Repository of the Polytechnic Institute of Porto A literature review on the optimization of legged robots Manuel Fernando Silva and JA Tenreiro Machado Abstract Over the last two decades the research and development of legged locomotion robots has grown steadily. Legged systems present major advantages when compared with ‘traditional’ vehicles, because they allow locomotion in inac- cessible terrain to vehicles with wheels and tracks. However, the robustness of legged robots, and especially their energy consumption, among other aspects, still lag behind mechanisms that use wheels and tracks. Therefore, in the present state of development, there are several aspects that need to be improved and optimized. Keeping these ideas in mind, this paper presents the review of the literature of different methods adopted for the optimization of the structure and locomotion gaits of walking robots. Among the distinct possible strategies often used for these tasks are referred approaches such as the mimicking of biological animals, the use of evolutionary schemes to find the optimal parameters and structures, the adoption of sound mechanical design rules, and the optimization of power-based indexes. Keywords Artificial legged locomotion, genetic algorithms, locomotion gaits, optimization, walking robots 1. Introduction natural terrains, since these vehicles may use discrete Legged robots present significant advantages over tra- footholds for each foot, in opposition to wheeled vehi- ditional vehicles with wheels and tracks. Wheeled vehi- cles, that need a continuous support surface. Therefore, cles demand paved surfaces (or at least regular) in order these vehicles may move in irregular terrains, by varying to move, being extremely fast and effective in them. At their legs configuration, in order to adapt themselves to the same time these mechanisms can be simple and surface irregularities and, furthermore, the feet may lightweight. However, more than 50% of the earth’s establish contact with the ground in selected points in surface is inaccessible to traditional vehicles, it being accordance with the terrain conditions. For these rea- difficult, or even impossible, for wheeled vehicles to sons, legs are inherently adequate systems for locomo- deal with large obstacles and surface unevenness. tion in irregular ground. When the vehicles move in soft Even all-terrain vehicles can only surpass small obsta- surfaces, as for instance in sandy soil, the ability to use cles and surface unevenness, but at the cost of high discrete footholds in the ground can also improve the energy consumption (Bekker, 1960). energy consumption, since they deform the terrain less Regarding tracked vehicles, although they present than wheeled or tracked vehicles. Therefore, the energy increased mobility in difficult terrains, they are not needed to get out of the depressions is lower able to surpass many of the difficulties and their energy consumption is relatively high. To these prob- lems, one must add the fact that traditional vehicles leave continuous ruts on the ground, which in some situations is disadvantageous as, for instance, from the environmental point of view. From what was seen, it is possible to conclude that legged locomotion systems present a superior mobility in (Bekker, 1960, 1969) and the contact area among the foot of artificial walking robots, respectively. Section six dis- and the ground can be made in such a way that the cusses other approaches to the problem under analysis. ground support pressure can be small. Moreover, the Finally, section seven presents the main conclusions of use of multiple degrees-of-freedom (DOF) in the leg this study. joints, allows legged vehicles to change their heading without slippage. It is also possible to vary the body 2. Biological approximations height, introducing a damping and decoupling effect between terrain irregularities and the vehicle body (and Since legged robots are inspired in animals observed in as a consequence of its payload). In what concerns loco- nature, a frequent approach for their design and con- motion, it should also be mentioned the possibility that struction is to develop a mechatronic mimic of the these systems present to hugging themselves to the ter- animal that is intended to replicate, either in terms of rain in which they move. This is particularly true, in case its physical dimensions, or in terms of characteristics they move, for instance, over the outside surface of pipes, such as the gait and the actuation of the limbs. With in order to increase their balance ability (Kaneko et al., this objective in mind, detailed studies of the locomo- 2002). tion and anatomy of the animals have been made. Although legged vehicles present all these potential Works joining researchers from the robotics and the advantages, in the current state of development, there biology areas are often presented (Laksanacharoen are several aspects that have to be improved and et al., 2000; Schneider et al., 2005; Witte, et al., optimized. 2001a, b). Several examples of robots that have been With this idea in mind, different optimization strate- developed based on this approximation are discussed gies have been proposed and applied to these systems, in Silva and Machado (2007), for example, the either during their design and construction, or during Lobster Robot, that intends to be a lobster mimic their operation, namely in what concerns to the selection (Ayers, 2004), the CWRU Robot II (Espenschied of the gait to be adopted (Alexander, 1984; Hornby et al., et al., 1996) that represents a stick-insect and the 2000; Silva and Machado, 2005; Stelzer et al., 2003) and CWRU Robot III (Nelson and Quinn, 1998; Nelson its adaptation to the terrain and locomotion conditions et al., 1997) that intends to mimic (17:1 scale) the (Estremera et al., 2003; Weingarten et al., 2004). Among Blaberus Discoidalis cockroach. the optimization criteria followed by distinct authors one This approach is also followed in the development of may include aspects related to energy efficiency (Buehler, biped and humanoid robots. The designers of these sys- 2001; Silva and Machado, 2005; Wang et al., 2008), sta- tems get much of their inspiration from mankind, as bility (Albert and Gerth, 2001; Garcia et al., 2002; Hardt proved by several machines with characteristics similar and von Stryk, 2002), velocity (Huang and Waldron, to those of humans, namely in the number of DOF and 1990; Hugel and Blazevic, 1999; Hugel et al., 2000; in their dimensions. Among the large number of Kohl and Stone, 2004), comfort (Neuhaus and examples adopting this approach one can Kazerooni, 2000), mobility (Hugel and Blazevic, 1999; mention the following robots: Quinn et al., 2001; Warren, 2002) and environmental impact (Ihme and Deutscher, 2001). ● the WABIAN humanoid, whose size and joint range This paper reviews several approaches that have been of motion is based on an adult human (Yamaguchi adopted for the optimization of the mechanical structure and Takanishi, 1998); and the locomotion modes of artificial legged systems. ● the biped developed by Caldwell et al. (1997) to test Among them are referred the mechatronic mimic of the the actuation using artificial pneumatic muscles, that characteristics of biological animals (Albiez and Berns, presents anthropomorphic dimensions; 2005; Alexander, 2005; Breithaupt et al., 2002; Hennion ● the BIP robot that presents the kinematic and et al., 2005; Quinn et al., 2001), the use of genetic algo- dynamic parameters close to the anthropomorphic rithms (GA) for the optimization of the legged structure values of an human, with 1.70 m height and 90 kg parameters (Reyes and Gonzalez, 2005; Ziegler and mass (Espiau et al., 1997); Banzhaf, 2001), the adoption of sound mechanical ● the Honda Humanoid Robot model P2 that, on an design rules (Yoneda, 2001), or the optimization of initially phase of development, presented the dimen- power/energy-based indices (Silva and Machado, 2005). sions, joint locations, ranges of motion and centre of The remainder of this paper is organized as follows. gravity equivalent to the human leg (Hirai et al., 1998). Section two presents some biological approximations to Latter, it was verified that was difficult to satisfy all the walking machines and section three introduces the adop- conditions, and some simplifications were made; tion of evolutionary algorithms for designing legged ● the BIP2000 robot, with kinematic and dynamic robots. Sections four and five present mechanical parameters close to the values of an human, with design rules and optimizing indices for the optimization 1.80 m height (Espiau and Sardain, 2000); ● the Robian II, with dimensions and weights approx- generation of robotic systems for very distinct situa- imate to the average European man (Gravez et al., tions (Farritor, et al., 1996; Leger, 2000). In the litera- 2002); ture there are also works on which evolutionary ● the biped robot Johnnie, that presents a geometry strategies are adopted to generate the structure of a corresponding to that of a male human with a specific robot. body height of 1.8 m (Lohmeier et al., 2004). Jua´ rez-Guerrero et al. (1998) developed a biped robot using evolutionary strategies. The final The main problem that can be pointed to the biomi- goal was to evolve the biped robot structure, metic approach concerns the resulting mechanisms. equipped with a passive tail to help keeping balance. In fact, mechanisms that are a straight copy of The robot struc- ture should be able to implement a the joints and links of biological animals tend to pre- simple gait and to fulfil a set of restrictions, namely: sent a large number of DOF, with the corresponding minimum and max- imum dimensions, maximum difficulties in the control algorithms. Moreover, the motor torque, step length (lower than 0.30 m), mechanical complexity is one of the largest sources of maximum foot elevation (lower than 0.05 m) and mal-functions and considerably increases cost and maximum robot weight (lower than 30 kg).