IIIT Hyderabad Publications
Evolution, Trajectory Planning and Control on Uneven Terrain for Wheeled Mobile Robots and Hybrid Wheeled-Legged Robots
Author: E Vijay
Report no: IIIT/TH/2016/51
In recent years there has been an increased use of wheeled mobile robots for outdoor and unstructured terrain navigation. While traversing uneven terrain wheeled mobile robots are subject to various kinds wheel slip which can lead to localization errors and decrease motor efficiency. The objective of this work is to develop robust modelling techniques for simulating the kinematics, quasi-statics , and dynamics of wheeled mobile robots subject to kinematic and dynamic no-slip constraints and also develop state-of- the-art suspension mechanisms for hybrid wheeled-legged robots in order to obtain reduced slip while traversing on fully 3D uneven terrain. In order to achieve slip free motion on uneven terrain the wheeled / wheeled-legged robots have to be equipped with essential degrees of freedom that would enhance its ability to negotiate undulations on the terrain. First we perform the analysis of a wheeled mobile robot with a passive variable camber (PVC). We develop a methodology for simulating the kinematics of this robot using the techniques of kinematics for dexterous manipulation of multi-fingured palm. We also develop a framework for simulating the quasi-static motion of the robot on uneven terrain .The framework at each instant estimates the contact forces and velocity of the vehicle platform for a given set of joint velocities of the robot. This ensures that the vehicle satisfies not only kinematic no slip constraints but as well as no slip constraints that arise due to relations between traction and contact forces. In general a complete simulation of a WMR on a fully 3D terrain has been a difficult problem to solve. The best efforts so far have provided a simulation that incorporates the wheel ground contact constraints into a set of differential algebraic equations (DAEs) to estimate the full 6dof pose of the vehicle. This work integrates the quasi static contraints within the DAE framework to provide a complete 6dof evolution of vehicle on 3D terrain that respects both kinematic and quasi static constraints. We then present a motion planning algorithm connecting a starting and ending goal positions of a wheeled mobile robot (WMR) with a passive variable camber on a fully 3D uneven terrain without slipping. The overall planning framework is along the lines of the RRT (Rapidly Exploring Random Tree). The curve connecting the adjacent nodes of the RRT is a quasi-static path which is generated using the forward motion problem based on the Peshkin’s minimum energy principle which combines the force and kinematic relationships of the WMR into a nonlinear optimization problem. The output of this optimization routine is a set of ordinary differential equations (ODEs) representing the non-holonomic constraints and wheel ground contact conditions of the robot along with a set of differential algebraic equations (DAEs) representing the geometric/holonomic constraints of the robot. We extend this kinematic framework to develop the complete dynamic state space realization of the WMR on uneven terrain. The framework performs the affine form state-space realization for the WMR on uneven terrain. Second we present two models of Linear Force Actuator based hybrid wheeled-legged robots and propose a strategy to control the wheel ground contact forces to improve the no-slip margin while traversing a fully 3D uneven terrain. The mechanisms we propose aims at controlling the contact forces at the wheels in a way to improve traction as well as to ensure that the vehicle retains a desired posture. Torus wheels are used to simulate a single point of contact with the terrain which enables us to have lateral tilt on lateral surfaces which is essential while operating on fully 3D terrain. We present the quasi-static analysis of each of the mechanisms to depict the ability of the systems to control the wheel ground contact forces while negotiating uneven terrain. The first model of the Linear Force actuator based Vehicle has a one degree of freedom (1 dof ) leg and the second vehicle has a 2 dof leg. We depict the force controllability of these vehicles using feasibility plots and also perform simulations on terrains having varying geometrical parameters as well as on terrains having discontinuities twice as much as the wheel radius.
Full thesis: pdf
Centre for Robotics
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