Kinematic Design and Optimisation of a Quadruped Robot with Six Actuated DoF

Abstract

While legged robots hold many advantages over wheeled robots, especially regarding dynamic capabilities and mobility, they often suffer from lower energy efficiency and reliability due to the use of more actuators. Many classic approaches to quadruped design rely on designs with 12 actuated degrees of freedom - three in each leg - which allows the feet to be freely placed with respect to the body. One obvious solution to this problem is a reduction in the number of actuators, but this usually comes at the cost of functionality reduction. In terms of simple locomotion, however, the robot’s center of mass can be sufficiently moved by freely placing each foot with respect to the world, thus changing the robot’s contact points. It should then be possible to achieve locomotion with free foot placement using only six actuated degrees of freedom if the existing functions are appropriately combined and/or reduced. The aim of this thesis is therefore to design a full quadruped kinematic structure with only six actuators which is capable of simple locomotion through free placement of its feet. A review of existing literature regarding legged locomotion and reduced-DoF quadrupeds is performed to form a basis for new concepts, and a novel kinematic structure is proposed which relies on two types of leg couplings to reduce the degrees of freedom. A kinematic analysis then provides representations of the model in terms of forward, inverse and differential kinematics, and a control algorithm with position error feedback is proposed for task-space trajectory following. The proposed model is implemented in Creo Parametric and simulated with the help of the LucaDynamics library in MATLAB. A few tests are performed in the simulated environment which show that the proposed robot is indeed capable of stable static walking with free placement of all four feet, with the task-space position errors remaining very low for all tested trajectories and no indications of singular or near-singular poses.