1. Introduction
Like humans, all the birds use a bipedal locomotion. However, their arms are transformed into wings, specialized for flying. The functions performed by the hands in humans are carried out by the beak in the birds. Functionally, thus, the neck in a bird is analogous of a human arm. In the Avineck project, the morpho-functional adaptations of the cranio-cervical system in birds are used to test hypotheses about the macro-evolution of the birds, and to bring these biological outcomes as solutions for the technological challenges that current robotics are facing. We present here the principles on which the Avineck project was based.
2. Biology
The selective pressure for flying have strongly shaped the trunk and the limbs of the birds (Norberg Citation1990). Under their feathers, they are built as a flying machine: they have wings, a rigid tapered trunk, and two legs for walking, running but also taking off and landing (Abourachid & Hofling Citation2012; Provini et al. Citation2014; Provini and Abourachid Citation2018). In contrast to this especially stable body morphology, the shape of the head and the length of the neck are strikingly variable. The neck is always flexible and the number of cervical vertebrae varies between 10 and 26 (Boas 1929) Why such a variability? The beak is used for foraging and food intake, but also for grooming and smoothing the feathers down to the end of the tail. Moreover, the beak has many other functions depending on the species: carrying objects like branches or for manipulation when weaving filaments to build the nests; to modify the environment as woodpeckers do when they make hollows in trees; and even for locomotion in parrots. Depending on the habits and habitats of the species, it is adapted to various functional demands. Contrary to the complex human hand, the beak is a simple pair of pliers with one degree of freedom. The neck is the functional equivalent of the human arm up to phalanx. The force, dexterity, and flexibility of the craniocervical system are given by the neck. Thus, the bird’s neck is a versatile, flexible, deployable, lightweight, and powerful tool, valuable properties for a robotic arm.
2. Robotics
In bio-inspired robotics, the vertebrae architecture and the study of its mechanical properties have been the subject of research for many years. Pioneering work dates back to the early 90’s with the study of the snake locomotion by Hirose (Citation1993) and continues today with, for example, the Robotics Institute at Carnegie Melon University (Tesch et al. Citation2009). However, snake-like robots can only operate in 2 D while the bird’s neck needs to act in 3 D. Recently, an eel-like robot capable of maneuvering in 3 D was designed and built at IRCCyN in collaboration with MNHM (Boyer et al., Citation2009. The bird neck can operate in 3 D with high dexterity and accuracy, covers a large workspace, and, contrary to elephant trunk, it can apply quite high forces and torques as compared to the animal mass. As far as we know, no study has been conducted on the design of a robotic arm inspired from the bird’s neck.
3. Tensegrity
Our idea is to use the concept of tensegrity as a general paradigm able to link the interests of biologists and roboticists involved in this project. Tensegrity was first defined by Fuller (Citation1962) and used in art sculptures (Fuller Citation1962; Snelson 1965) and have then been applied to civil engineering and more recently to robotics (Skelton Citation2010; Arsenault and Gosselin Citation2006; Chen and Arsenault Citation2012). In a tensegrity structure, all components are subject to only compression (bars) and tensions (cables and springs). This feature conveys many interesting properties such as, lightness, reconfigurability, energyefficiency, among others. As shown recently by several biologists like (Levin Citation2005), the concept of tensegrity is present in many biological systems like cells and musculo-skeletal systems. Recently, a human spine model using a tensegrity structure was proposed (Sabelhaus et al. Citation2015). Tensegrity mechanisms are better candidates to model bird necks as compared to other potential solutions like multi-backbone continuum robots (Wang and Simaan Citation2014) because tensegrity lends itself well to a realistic modeling of bones, muscles and tendons. Furthermore, the modularity of the tensegrity systems has a heuristic value in the explanation of the morpho-functional modularity debated in evolutionary biology.
Funding
ANR -10-CE33-0025.
References
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