Legged robotic locomotion with variable impedance joints

Abstract

Humans have a complex musculoskeletal arrangement which gives them great behavioural flexibility. As well as simply moving their legs, they can modulate the impedance of them. Variable impedance has become a large field in robotics, and tailoring the impedance of a robot to a particular task can improve efficiency, stability, and potentially safety. Locomotion of a bipedal robot is a perfect example of a task for which variable impedance may provide such advantages, since it is a dynamic movement which involves periodic ground impacts. This thesis explores the creation of two novel bipedal robots with variable impedance joints. These robots aim to achieve some of the benefits of compliance, while retaining the behavioural flexibility to be truly versatile machines. The field of variable impedance actuators is explored and evaluated, before the design of the robots is presented. Of the two robots, BLUE (Bipedal Locomotion at the University of Edinburgh) has a 700mm hip rotation height, and is a saggital plane biped. miniBLUE has a hip rotation height of 465mm, and includes additional joints to allow hip adduction and abduction. Rapid prototyping techniques were utilised in the creation of both robots, and both robots are based around a custom, high performance electronics and communication architecture. The human walking cycle is analysed and a simple, parameterised representation developed. Walking trajectories gathered from human motion capture data, and generated from high level gait determinants are evaluated in dynamic simulation, and then on BLUE. With the robot being capable of locomotion, we explore the effect of varying stiffness on efficiency, and find that changing the stiffness can have an effect on the energy efficiency of the movement. Finally, we introduce a system for goal-based teleoperation of the robots, in which parameters are extracted from a user in a motion capture suit and replicated by the robot. In this way, the robot produces the same overall locomotion as the human, but with joint trajectories and stiffnesses that are more suited for its dynamics.