HamsterPython

Python setup

• Note you are connected to the hamster_net (wifi / router)

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$ export ROS_IP=<your IP>

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$ export ROS_MASTER_URI=http://10.0.2.<HAMSTER AGENT Number>:11311

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• If the Hamster is powered and your terminal is well configured you should be able to

See all data streams <syntaxhighlight lang="bash" line='line'>

$ rostopic list

</syntaxhighlight> See imagery from the robot <syntaxhighlight lang="bash" line='line'>

$ rqt_image_view

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Python Example 1

• Receive laser scan from robot to decide if free

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1. !/usr/bin/env python

import rospy from sensor_msgs.msg import Imu

def is_free(ranges ,start_index, end_index ,min_distance): #sub array from angle to angle s_ranges = ranges[start_index:end_index] #filter the array b_ranges = filter(lambda x: x <= min_distance, s_ranges) return len(b_ranges) == 0

def callback(msg): if is_free(msg.ranges,160,200,0.2): print "FREE" else: print "BLOCKED"

rospy.init_node('scan_values')

1. Put your Hamster number instead of agent<number>
2. The agent number is printed on the Hamster cover

sub = rospy.Subscriber('/agent14/scan', LaserScan, callback)

rospy.spin()

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Python Example 2

Detect right/left turn from IMU

• IMU angles are represented in Quaternions [[1]] hence translated to RPY (roll pitch yaw)
• RPY are in radians, translated to degrees

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1. !/usr/bin/env python

import rospy import math from sensor_msgs.msg import Imu from tf.transformations import euler_from_quaternion, quaternion_from_euler

def calculate_robot_angle(orientation): quaternion = ( orientation.x, orientation.y, orientation.z, orientation.w) euler = euler_from_quaternion(quaternion) roll = euler[0] pitch = euler[1] yaw = euler[2] return math.degrees(yaw)

prev_angle = 0 def callback(msg): global prev_angle curr_angle = calculate_robot_angle(msg.orientation) if curr_angle > prev_angle : print "TURINING LEFT" else: print "TURINIGN RIGHT" prev_angle = curr_angle

rospy.init_node('imu_values') last_angle = 0

1. Put your Hamster number instead of agent<number>
2. The agent number is printed on the Hamster cover

sub = rospy.Subscriber('/agent14/imu', Imu, callback)

rospy.spin() </syntaxhighlight>

Python Example 3

• The Hamster robot is a Ackermann steering system [[2]]
• This example drives the robot forward until it senses from the Lidar that there is less than a meter free ahead of it
• Motion commands should be usually published at 10Hz.
• If no motion command arrives to the robot after a second (10 messages period) the robot will stop
• Hence we send an empty message (with speed 0) to stop the robot immediately once the area is blocked ahead of it, otherwise it would stop closer to the obstacle due to the delay

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1. !/usr/bin/env python

import rospy from sensor_msgs.msg import LaserScan from ackermann_msgs.msg import AckermannDriveStamped

pub = rospy.Publisher('/agent14/ackermann_cmd',AckermannDriveStamped, queue_size=1)

def is_free(ranges ,start_index, end_index ,min_distance): #sub array from angle to angle s_ranges = ranges[start_index:end_index] #filter the array b_ranges = filter(lambda x: x <= min_distance, s_ranges) return len(b_ranges) == 0

def callback(msg): global pub if is_free(msg.ranges,160,200,1): ack_msg = AckermannDriveStamped() ack_msg.header.stamp = rospy.Time.now() ack_msg.header.frame_id = 'your_frame_here' ack_msg.drive.steering_angle = 0 ack_msg.drive.speed = 1 pub.publish(ack_msg) else: ack_msg = AckermannDriveStamped() pub.publish(ack_msg)

rospy.init_node('driving_hamster')

1. Put your Hamster number instead of agent<number>
2. The agent number is printed on the Hamster cover

sub = rospy.Subscriber('/agent14/scan', LaserScan, callback)

rospy.spin() </syntaxhighlight>