Control of a Dynamic Wheeled-legged Robot Powered by Electric-Pneumatic Actuators

Abstract

The implementation of wheels on the end distal of a mobile robot's leg is a design approach that is gaining increasing popularity as it enables improved energy efficiency, stability and adaptive locomotion strategies in unstructured environments. Mobile robots that use this design approach (i.e., wheeled-legged robots) have the potential to out-perform legged mechanisms, however the control and actuation design needed to best realise the capabilities of wheeled-legged robots has received relatively less attention in comparison to the work done for legged robots. The goal of this thesis is to develop dynamic controllers for demonstrating robust terrain adaptive behaviours with a wheeled-legged robot and to validate the potential for such a system to be actuated using a novel electric-pneumatic actuation scheme.

To achieve the aim of this thesis, two goals were defined. The first goal was to develop a motion planner for a re-configurable wheeled-legged robot morphology that automates the coordination of its body and wheels for traversing obstacles while maintaining dynamic balance. The second goal was to determine if a system of hybrid electric-pneumatic actuators could demonstrate the capacity to satisfy the dynamic control requirements of wheeled-legged robots performing locomotion on unstructured terrain.

The controllers developed in this thesis for realising the first goal used model-based control methods, and were implemented to control a 12 Degrees Of Freedom (DOF) wheeled-legged mobile robot named Aerobot. A reduced-order dynamic model was referenced for planning the coordination of Aerobot's full body so that dynamic balance could be maintained when leveraging its unique gaits for traversing obstacles. Next, a series of controllers were explored that used a 2D rigid body dynamic model for computing the required acceleration of Aerobot's joints to achieve dynamic aspect ratio adjustment. For realising the second goal of this thesis, a new four degree of freedom prototype wheeled-legged robot was developed. This prototype used antagonistically paired pneumatic artificial muscles to work in parallel with a three phase brushless direct current (BLDC) motor for controlling a knee joint. A geared BLDC motor was then used for actuating the hip joint. The ability to stably control the robot's aspect ratio (i.e., height adjustment) using model-based control methods, was investigated as a benchmark test to evaluate the suitability and potential advantages of the actuation scheme.

Simulation-based testing of the control algorithms validated the ability for Aerobot to autonomously traverse crevices and demonstrate aspect ratio adjustment while maintaining its balance without any human intervention. A controller architecture was also shown to enable aspect ratio adjustment to be performed dynamically where the height of Aerobot's Center Of Mass (COM) was controlled live. The robustness of this controller was shown to be sufficient to stabilise a virtual model of Aerobot when a significant unexpected disturbance of 800 Newtons was applied to its body. An adapted version of a dynamic controller implemented in this thesis was then used to demonstrate live aspect-ratio adjustment on the manufactured wheeled-legged robot.

The key insights gained through the contributions of this thesis suggests methods for improving the robustness and adaptability of wheeled-legged robots aiming to be deployed on unstructured terrain. The motion planner developed for enabling Aerobot to autonomously engage with its surroundings raises the utility of the platform when it is to be deployed in dynamic environments. The methodology for realising the level of robustness achieved in simulations with physical hardware that is challenging to control was also contributed. This work thus provides a proof of concept that hybrid electric-pneumatic actuation schemes have great potential to optimise the performance of dynamically controlled wheeled-legged robots that are inherently safe, energy-efficient, have a high strength to weight ratio and can adapt well to balance unexpected physical interactions. As a result, this work informs how future wheeled-legged robots such as Aerobot may be designed and controlled to evolve their capabilities.