Avian cranio-cervical systems. Part III: Robot kinematics for cervical systems.

Abstract

A 2D-model for the kinematics of the cranio-cervical system is developed from algorithms applied for robot kinematics. The model is proposed for avian cranio-cervical systems. Three main problems were to be solved. The problem of underdetermination in an open multi-element kinematic chain, which develops because only kinematics were included, is solved by the application of two approximations for economic motion. The straight-line approximation for the course of the bill tips and the least-motion approximation for all articulations are both assumed to occur in selected phases of pecking and drinking behaviour. The problem that internal constraints and constraints resulting from demands for proper functioning may be operating is solved by incorporating the measured maximal dorsal and ventral flexion of each joint, and also both the target location of the bill tips and the target orientation of head and beak. The third problem, viz. how to handle variations in initial conditions, which developed from the relatively independent working locomotory part, is solved by standardizing the foot-food distance in the experiments, and further by running the simulations for the locomotory components over the trajectories they cover in reality, rather than to allow them to move over their full work-envelope. The model is highly adaptable to changes in the number of bars involved, in bar lengths, in work-envelopes, and in motion patterns. The predicted patterns were tested against combined film- and radiographic analyses. The comparisons lead to some general conclusions. The economy principles tested appear to be confirmed from the observations. Also, the fixed-target orientation approximation seems to be confirmed while the locomotory components clearly operate primarily under different constraints.