A STABLE SWITCHED SYSTEM APPROACH FOR OBSTACLE AVOIDANCE OF NONHOLONOMIC MOBILE ROBOTS.

Introduction

We consider the problem of controlling mobile robots to navigate in known and unknown environments. One approach to this problem relies on fully manual control from a human operator (i.e. direct teleoperation). Another approach relies on fully automated control (i.e. an autonomous robot). Between these extremes lies shared control paradigms. Typically, the shared control makes the system more capable than either automated control or teleoperation control. Additionally, avoiding static or dynamic obstacles is another essential task for shared control of the mobile robots in unstructured and dynamic environments.

The proposed obstacle avoidance approach

In this project, we developed a novel shared control structure and approach for obstacle avoidance. We also divide the configuration space into obstacle avoidance and obstacle-free regions. The configuration space of a nonholonomic mobile robot in the special Euclidean group SE(2). We have derived two functions to define the robot’s behavior in each region, which generate velocity vectors that drive the robot away from the obstacle and always towards the goal defined by autonomous controller. We consider the existence of a switching surface between these two sub-regions, and specifically define this switching surface as a local subset of SE(2), i.e. the switching surface includes orientation, defined by the relative angle between the robot heading and bearing to the obstacle. Two different switching signals are proposed based on the switching surface. Lyapunov stability analysis proves the robot will converge to a goal position.

Simulation and experimental results

To rigorously test the proposed approach, simulations and experiments have been conducted. The robot used in experiments is a Pioneer 3-DX two-wheel differential drive robot. The sampling frequencies of the wheel encoders on the two robots are approximately 10 Hz. The pose of the robots is estimated by the odometry of the robot platform. The algorithms are implemented in LabVIEW 2012 using MATLAB script.

The proposed shared control with obstacle avoidance

We propose a shared control structure, in which a human operator can command motions that override autonomous operation, and the robot overrides either the teleoperation or autonomous controller if it encounters an obstacle. In our implementation, our system consists of teleoperation mode and autonomous mode. The human operator can select which mode the robot is in.

We also consider disturbances and/or uncertainties on the state estimation (e.g., sensor measurement errors due to delays, noise, and quantization) or noise added to the input to the system, we assume that disturbances are added to all or a subset of the states in the error dynamics. To retain the robustness of the control law against disturbances, two different robust control approaches are proposed. We mathematically prove the control schemes are uniformly ultimately bounded for both sensor disturbance and input disturbance with saturation.

Experimental results

To rigorously test the approach under different circumstances, experiments have been conducted by two different research groups, which are the University of Texas at Dallas (UTD) and Daegu Gyeongbuk Institute of Science and Technology (DGIST).

Publication

1) Jingfu Jin, A. Green, Nicholas Gans, “A Stable Switched-System Approach to Obstacle Avoidance for Mobile Robots in SE(2)”, IEEE/RSJ International Conference on Intelligent Robots and Systems, Chicago, Sep. 2014, pp. 1533 – 1539

2) Jingfu Jin, Yoon-Gu Kim and Sung-Gil Wee, and Nicholas Gans, “A Stable Switched-System Approach to Shared Robust Control and Obstacle Avoidance for Mobile Robots, Proceedings of the ASME 2014 Dynamic Systems and Control Conferences, San Antonio, Oct. 2014