Which is Better?: A Natural or an Artificial Surefooted Gait for Hexapods

Which is Better?: A Natural or an Artificial Surefooted Gait for Hexapods

Kazi Mostafa (National Sun Yat-sen University, Taiwan), Innchyn Her (National Sun Yat-sen University, Taiwan) and Jonathan M. Her (National Taiwan University, Taiwan)
DOI: 10.4018/978-1-4666-3634-7.ch014
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Abstract

Natural multiped gaits are believed to evolve from countless generations of natural selection. However, do they also prove to be better choices for walking machines? This paper compares two surefooted gaits, one natural and the other artificial, for six-legged animals or robots. In these gaits four legs are used to support the body, enabling greater stability and tolerance for faults. A standardized hexapod model was carefully examined as it moved in arbitrary directions. The study also introduced a new factor in addition to the traditional stability margin criterion to evaluate the equilibrium of such gaits. Contrary to the common belief that natural gaits would always provide better stability during locomotion, these results show that the artificial gait is superior to the natural gait when moving transversely in precarious conditions.
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Introduction

The topics of bio-mimetic robots have attracted much attention lately. There have been a variety of man-made bio-mimetic walking machines so far (Wang, 2010). Among them the six-legged types are popular designs, each often emulating some kind of crawling (McGhee & Iswandhi, 1979; Brooks, 1989), or running (Clark & Cutkosky, 2006), or even climbing insect (Palmer, Diller, & Quinn, 2009; Spenko, Haynes, Saunders, Cutkosky, Rizzi, Full, & Koditschek, 2008). Advantages for a robot to use the hexapod configuration (Figure 1A and 1B) are satisfactory walking efficiency and at the same time static stability. Hexapod gaits are much diversified, i.e., having a larger number of patterns than biped, quadruped or even myriapod gaits. A robot designer has to know which gait is the best for a specific use of the hexapod (Chu & Pang, 2002; Srinivasan & Ruina, 2006; Erden & Leblebicioglu, 2007; Starke, Robilliard, Weller, Wilson, & Pfau, 2009).

Figure 1.

(a) A terrestrial hexapod model (P, Q, U are dimension parameters, L1, L2, L3, R1, R2, R3 are the legs, and the star denote CG.), (b) an inverted, hanging posture of the hexapod, (c) a natural surefooted gait: the type-1 paired metachronal gait, showing pairing sequence (steps MI, MII, and MIII) of the legs, (d) the artificial diametric gait and its pairing sequence (steps DI, DII, DIII)

Natural Gaits

There are apparently two broad categories of multiped gaits: natural and artificial gaits. Natural gaits (Figure 1C) are those used by living creatures. For instance, Wilson (1966) has reported in his classic paper that the most common gaits for hexapods are the slow wave gait, the ripple gait and the tripod gait. Note that Wilson’s ripple gait was also referred to as the metachronal gait (Ferrell, 1993; Schreiner, 2004). Song and Waldron (1987) have developed a mathematical formula to theorize the natural gaits. This is possible since in a natural gait the legs on either side always move successively from the rear end to the front end of the animal (Wilson, 1966). Recently, some researchers have studied the metachronal gait of a stinkbug Mesocerus marginatus (Frantsevicha & Cruse, 2006) and that of a stick insect Aretaon asperrimus (Jeck & Cruse, 2007). As to transversely moving animals, the walking, trotting, and galloping gaits of the ghost crab have been investigated thoroughly (Full & Weinstein, 1992). However, the crab used an alternating tetrapod gait when it walks and trots, and a dynamically stable gait when it gallops with aerial phases.

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