T1b-react - Reactive obstacle avoidance

The main task is to implement a function that will steer the simulated robot towards a given goal while reactively avoiding obstacles.

Deadline 11. October 2020, 23:59 PST
Points 3
Label in BRUTE t1b-react
Files to submit archive with HexapodController.py file
Resources B4M36UIR_t1_resource_pack [Updated 05.10.2020]


In class HexapodController.py implement the goto_reactive function. The purpose of the function is to produce the steering command that will drive the simulated hexapod robot towards the goal position based on the current odometry of the robot, the current collision state of the robot, and the laser scan perceived by the robot. The input parameters of the function are:

  1. goal - Pose message - only position.x and position.y parameters are used as the goal position
  2. odometry - Odometry message - pose.position.x, pose.position.y and pose.orientation.quaternion encodes the current robot absolute position in the environment
  3. collision - boolean - True if the robot collides with some obstacle, False otherwise
  4. laser_scan - LaserScan message - contains the current laser scan data perceived by the robot.

The function returns:

  1. Zero command cmd_msg = Twist() when any of the input data are invalid
  2. None when the robot currently collides with any obstacle.
  3. None when the robot has reached the given goal (note that in real world the exact position is impossible to reach by the robot, hence the goal is considered reached when the robot is in the DELTA_DISTANCE vicinity of the goal position).
  4. Otherwise, the function returns the steering command in form of a velocity command (Twist message) consisting of linear and angular parts that steers the robot towards the goal position while avoiding obstacles.

The goto_reactive function has the following prescription

    def goto_reactive(self, goal, odometry, collision, laser_scan):
        """Method to steer the robot towards the goal position while avoiding 
           contact with the obstacles given its current odometry, collision 
           status and laser scan data
            goal: Pose of the robot goal
            odometry: Perceived odometry of the robot
            collision: bool of the robot collision status
            laser_scan: LaserScan data perceived by the robot
            cmd: Twist steering command

In class HexapodController.py you can change whatever you want. In evaluation, the given interfaces are fixed and the evaluation script is fixed.


The recommended approach for the reactive obstacle avoidance uses simple AI cognitive model of Braitenberg vehicles described in Lab02 - Exteroceptive sensing, Mapping and Reactive-based Obstacle Avoidance.

The direct sensory-motor mapping has to combine:

While the continuous navigation function towards the goal location can be represented by love (Vehicle 3a) or aggression (Vehicle 2b), the reactive obstacle avoidance is best modelled as the robot fear (Vehicle 2a) the obstacles in the environment perceived by the laser scanner with nonlinear activation functions.

Hence the reactive obstacle avoidance can be achieved using the following continuous navigation function (pseudocode).

while not goal_reached:
    dphi = the difference between the current heading and the heading towards the target
    scan_left = distance to the closest obstacle to the left of the robot
    scan_right = distance to the closest obstacle to the right of the robot
    repulsive_force = 1/scan_left - 1/scan_right
    linear_speed = distance towards target
    angular_speed_navigation_component = dphi*C_TURNING_SPEED  
    angular_speed_avoidance_component = repulsive_force*C_AVOID_SPEED
    angular_speed = angular_speed_navigation_component + angular_speed_avoidance_component
Where C_AVOID_SPEED is a constant that defines the robot fear of the obstacle and C_TURNING_SPEED is a constant that defines the aggression with which the robot will turn towards the desired heading.

An example operation of the controller can be seen in the following video (4x speed up):


The evaluation focus on the ability of the robot to reach the given goal locations while not crash into the obstacles. It is supposed to introduce you to the work with the robot exteroceptive sensing.

The code can be evaluated using the following script (also attached as t1b-react.py).

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import matplotlib.pyplot as plt
import sys
import os
import math
import numpy as np
#import hexapod robot 
import HexapodRobot as hexapod
#import communication messages
from messages import *
if __name__=="__main__":
    robot = hexapod.HexapodRobot(0)
    #turn on the robot 
    #start navigation thread
    #assign goal for navigation
    goals = [
    path = Path()
    #go from goal to goal
    for goal in goals:
        while robot.navigation_goal is not None:
            #sample the current odometry
            if robot.odometry_ is not None:
        #check the robot distance to goal
        odom = robot.odometry_.pose
        #compensate for the height of the robot as we are interested only in achieved planar distance 
        odom.position.z = 0 
        #calculate the distance
        dist = goal.dist(odom)
        print("[t1b_eval] distance to goal: %.2f" % dist)
    #plot the robot path
    fig, ax = plt.subplots()
    path.plot(ax, 30)
The expected output is the print of the distance readings below or equal the DELTA_DISTANCE threshold and the plot of the robot path between the individual goals similar to the following figures. Note that the resulting path may be completely different, the main focus of this task is not to collide with the obstacles, hence, any collision-free trajectory is valid.

courses/b4m36uir/hw/t1b-react.txt · Last modified: 2020/10/05 17:32 by cizekpe6