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3.1 Introduction Design of two and multi-legged robots like four legged, six legged, and eight legged robots shows a practical usefulness of Bionics for the domain of robotics. Depending on the number of legs, such robotic platforms can be inspired from body constitution, walking mechanics, and behavior of humans, animals (four legged), insects (six legged), or spiders (eight legged). There is also another kind of robotic designs depicted as hybrid robots, which have a mixture of concepts seen in nature and artificial designs. These robots can be often found having tracks, other special leg designs, wheeled-legged robot designs and the like. In this chapter several joint leg walking and hybrid robots are discussed. The “joint leg” term is related to robots that have legs built out of servos, representing their joints. The term “hybrid” found by hybrid robots is related to robots that have both legs built out of servo joints and wheels attached to their legs. Robots demonstrators presented in this chapter have been used as demonstra-tors for biologically inspired approaches and algorithms researched and elaborated in this book. These robots can be categorized into three types: humanoid robots, hexapod-robots, and hybrid wheeled-legged robots. 34026
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3.2 Hexapod Robots Hexapod robots belong to the group of joint leg walking robots having six legs where the legs are consisting of multiple servo joints. The legs of the robot are usually symmetrically distributed in two different groups spatially located on the two opposite sides of the robot's body. The design of hexapod robots is often in-spired by locomotion systems seen in insects like cockroaches, stick insects, and the like. In comparison with the four legged walking robots or quadruped robots, hexapod robots have intrinsically more redundancy due to the higher number of the legs and thus can be theoretically more flexible over uneven terrain. Hexapod robots differ from robots that have “native” spider-like biomimetic design having eight legs distributed on the two sides of the robot’s body. Although the eight legged robots may have higher degree of redundancy and perhaps provide better agility for the robot over rugged terrain, they also need more energy for their func-tioning, which in turn affects the size and mobility of the robot. Fig. 3.1 Hexapod robots: (a) “iSprawl”; (b) “RHex”; (c) “DLR Crawler”; (d) “RiSE”; (e) “AMOS-WD06”. 3.2.1 State of the Art – Hexapod Robots There have been many types of hexapod robots that have been used for demon-stration purposes in the research on biologically inspired locomotion. Some of the current (Feb, 2010) state of the art hexapod robots include: “iSprawl” [KCC06], “RHex” [AMK01], “DLR Crawler” [GWH09], “RiSE” robot [SGF06], “AMOS-WD06” [STW10] (Figure 3.1). However, the mentioned hexapod robots all differ in the technology that they use for their locomotion. For example, their leg design differs from one robot to the other and the moving concept of the joints by the legs also differs. Movement of the legs for the “iSprawl” robot is periodic, generated by push-pull actions using flexible cables and servo motors [KCC06]. For the “RHex” robot, the movement of the legs is related only to the rotary motion of the legs [AMK01]. The “DLR Crawler” design was based on the “DLR-HAND II” [BFH03], therefore the joint based fingers of the “DLR-HAND II” are adapted to serve as legs for the “DLR Crawler” [GWH09]. For the “RiSE” robot, the legs are moved by two electrical actuators per leg, with biologically inspired adhesive structures located on the feet, which enable the “RiSE” robot to climb on vertical wall surfaces, trees, etc [SCM06].
The “AMOS-WD06” robot implements legs consisting of three servos each, resulting in eighteen servos for locomotion of the hexapod robot [STW10]. By the presented "state-of-the-art" robots there are improvements that can be seen in comparison with some older hexapod robot designs, however less has been done on introducing fault tolerant mechanisms within the robots itself, which will give the robots the robustness to be functional also in situations when they experience some malfunctions within their components, by means of reconfigur-ing the body and leg postures. And this is the key difference in comparison to the robot demonstrators OSCAR, in particular OSCAR-X, which were developed at Institute für Technische Infor-matik, University Lübeck and described in the following sub-chapter. 3.2.2 Hexapod Robot Demonstrator – OSCAR (Organic Self Configuring and Adapting Robot) OSCAR (Organic Self Configuring and Adapting Robot) is a six legged walking robot, used as a demonstrator for testing some of the newly-developed biologically inspired approaches and algorithms presented in this book. OSCAR represents the series of built hexapod robot demonstrators used in the interdisciplinary research, where the legs are distributed spatially in a circle on the robot's body. Most of the robots in OSCAR series have integrated an on-board embedded system, sensors (ultrasonic, infrared, acceleration, and inclination sen-sors), and actuators (analog and digital servos). Such a typical spatial distribution of the robot’s legs in a circle is represented in (Figure 3.2). The OSCAR series consists of the following robots: OSCAR-1, OSCAR-2, OSCAR-3, and OSCAR-X, which are described below. The first two robots, OSCAR-1 and OSCAR-2, were mostly based on the Lynxmotion® robot kit - “AH3-R (18 Servo Walker)” with 18 degrees of freedom (DOF), Hitec HS-645 Servos and aluminum based leg design [Lyn06]. 3.2.2.1 Hexapod Robot Demonstrator – OSCAR - 1 Hexapod robot OSCAR-1, built in year 2006 is the first in the series of OSCAR robots. Its hardware is based on the “AH3-R (18 Servo Walker)” robot kit with six legs distributed spatially in a circle and additional on-board electronics such as “JControl” (a Java based embedded system for robot control), servo controller SD-21, Hitec analog servos HS-645, binary contact sensors on the robot's feet, and NiMH batteries (Figure 3.3). Each leg by the robot is made up of three servos. There are also integrated ultrasonic sensors on three of the robot’s legs. 3.2.2.2 Hexapod Robot Demonstrator – OSCAR - 2 OSCAR-2 (Figure 3.4) is the second in the series of OSCAR robots, similarly built as OSCAR-1. The OSCAR-2 in comparison to the OSCAR-1, has the following modifications: - pressure sensors (Figure 3.5); - 18 modified HiTec HS-645 servos (Figure 3.6); The modified servos provide feedback for the level of servo current, so the torque can be monitored while the robot is walking.