2. Robot Design
2.2. The Transfer to a Technical System
As it is currently technologically impossible to replicate the stick insect in all its par- ticulars, compromises must be found between the level of detail and the technical effort that is needed to achieve it. Striking differences between stick insects and current insect- inspired robots are:
materials The exoskeleton of insects consists mostly of chitin. Using biomineralization, the exoskeleton is hardened locally where needed (Weiner and Dove2003). At other po- sitions, it is flexible to allow for the movement of the limbs. Compared to this extremely optimized production process, technical systems are rather simplistic. First approaches using tailored fiber placement allow for a local strengthening of a structure by stitching with carbon fibers (Uhlig et al. 2013). Thus, the stability can be increased locally at a minimal increase of mass.
size/mass Although stick insects are among the bigger insects, they are still much smaller than most current robots.4 Robots of equal or even smaller size exist (Haldane et al.2013; Hoover et al.2008), yet, they tend to be simpler regarding their sensorization. 3In some older publications (e.g., Cruse (1976b)), these angles were named α and β. To avoid miscon-
ceptions regarding the leg joints, in this work, they will always be referred to as φ and ψ.
4Actually, the longest currently known insect is a stick insect of type Phobaeticus chani with a body
Directly coupled to the size is the mass of the system since bigger systems tend to be heavier. Due to the different materials and construction also the mass distribution and therefore the location of theCOMwill likely differ.
energy supply Whereas insects digest other organisms to gain energy, robots are usually supplied by some kind of electronic power source or combustion engine. However, some robots also produce energy by digestion (Wilkinson 2000).
actuation All animals are powered by some kind of muscles whereas robots typically use electric motors for propulsion. One of the most relevant differences between biological muscles and classical, technical drives regarding their effect on the connected structures is the compliance of the muscle fibers. Due to the non-linearity in the spring charac- teristics, the resulting stiffness of the connected joint can be varied by co-contraction of antagonistic muscles (Zakotnik et al. 2006). In contrast, mechanical drives are usually engineered to be as stiff as possible, and lack the ability of stiffness regulation. Also, compared to mammals, the damping in the joints of insects is very high (M. Garcia et al.
2000), which is rarely replicated in robots.
sensorization The general concept in nature seems to be the use of a multitude of sensors that each have a comparably low resolution. However, by sensor-fusion of a high number of these low-resolution sensors a sufficiently precise measurement can be obtained. For example, in the legs of Carausius morosus, the joint angles are detected by fields of 15-30 hairs that are bent depending on the flection of the joint (Bässler 1983). The defect of one of these sensors reduces the accuracy of the overall measurement but the remaining sensors may still produce results that keep the animal operable. In robotics, usually, a small number of high-precision sensors (e.g., angle encoders) is used. In systems that are not explicitly designed to be redundant, the defect of a single high-precision sensor will likely result in a black-out of the whole system.
The focus of the project is centered on the transfer of locomotion and sensor- processing concepts from insects to a technical system. To use the bioinspired control approaches in the robot controller as they are deduced from walking stick insects, the robot needs six fully-articulated legs with three degrees of freedom each, and the legs should replicate the morphology of the insect legs regarding the length ratios of the leg segments. As the leg kinematics of the insects exhibits a singularity along the α-axis (see section A.4), a slanted mount of the legs as found in stick insects is also practical since this swivels the α-axis and therefore the singularity inwards, away from the main workspace of the legs.
To keep the mechanics as simple as possible, a reliable electric power supply is fa- vorable although it is by no means bioinspired. The actuation mechanism should be reliable and simple as well, however, the compliance of the muscles is a feature that reduces positioning problems considerably during walking over irregular terrain: The legs will compensate for small positioning errors on the hardware-level without the need
of immediate intervention by the walking controller. The actual implementation of the compliance is considered to be of minor relevance. A muscle-like compliance would be beneficial for direct adaption of insect-inspired joint controllers but it is not strictly nec- essary in this project since the focus of research will be on higher-level control like leg coordination rather than single joint control.
The robot must be able to sense the leg’s joint angles and it needs a reliable way to sense collisions of the leg during swing phase—either with the ground at the end of the swing phase or with obstacles.
The width of the robot should not exceed 80 cm to allow operation in buildings—in particular passing through standard doors. Since the robot will be used as platform for navigation-related sensor systems such as cameras for obstacle avoidance, it must be able to carry a payload and the sensor systems must be mountable on the robot, which defines the lower size limit of the robot.