Article Preview
TopIntroduction
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.