Commanding QTrobot motors

Overview

signal_cellular_alt  Level:  Intermediate
track_changes  Goal:  learn how to command and read motors with QTrobot Motor interface
task_alt  Requirements:

•   Quick start with coding on QTrobot
•   Create a ROS python project
•   QTrobot interfaces using ROS Topics

In this tutorial you will learn how to command and read motors with QTrobot Motor interface using python.

Create a python project

First we create a python project for our tutorial. let's call it tutorial_qt_motors and add the required python file:

cd ~/catkin_ws/src
catkin_create_pkg tutorial_qt_motors sensor_msgs std_msgs rospy roscpp -D "Command QTrobot motors"
cd tutorial_qt_motors/src
touch tutorial_qt_motors_node.py
chmod +x tutorial_qt_motors_node.py

Code

In this tutorial we will focus on reading and moving just one motor "HeadYaw". Open the tutorial_qt_motors_node.py file and add the following code:

#!/usr/bin/env python3
import sys
import rospy
from std_msgs.msg import Float64MultiArray
from sensor_msgs.msg import JointState

head_yaw_pos = 0
rospy.init_node('my_tutorial_node')
rospy.loginfo("my_tutorial_node started!")

head_pub = rospy.Publisher('/qt_robot/head_position/command', Float64MultiArray, queue_size=10)
rospy.sleep(3.0)

def state_callback(msg):
    global head_yaw_pos
    head_yaw_pos = msg.position[msg.name.index("HeadYaw")]

rospy.Subscriber('/qt_robot/joints/state', JointState, state_callback)

if __name__ == '__main__':
    head_yaw_ref = 15.0
    while not rospy.is_shutdown():
        try:
            href = Float64MultiArray()
            href.data = [head_yaw_ref, 0]
            head_pub.publish(href)
            rospy.sleep(4)
            rospy.loginfo("Current position : %.2f" ,head_yaw_pos)
            head_yaw_ref = -15 if head_yaw_ref == 15 else 15
        except KeyboardInterrupt:
            pass
    rospy.loginfo("finsihed!")

Explanation

First we imported Float64MultiArray from ROS standard message library. This message is used in /qt_robot/head_position/command to command the motors. Next we imported JointState, which we will need to read the joint positions. We define one global variable to save latest position of motor and we initialize ROS node.

We define a ROS publisher for /qt_robot/head_position/command, which we will use to command the motors.

head_pub = rospy.Publisher('/qt_robot/head_position/command', Float64MultiArray, queue_size=10)

Next we define a callback function state_callback, which will be reading position of "HeadYaw" motor. With that we also define a ROS subscriber /qt_robot/joints/state to read this data.

def state_callback(msg):
    global head_yaw_pos
    head_yaw_pos = msg.position[msg.name.index("HeadYaw")]

rospy.Subscriber('/qt_robot/joints/state', JointState, state_callback)

In the main we define starting reference position and new Float64MultiArray message, which includes reference position. We publish new position with head_pub and after some delay we print the current position and we change the reference position. The "HeadYaw" will move from 15 to -15 on repeat.

if __name__ == '__main__':
    while not rospy.is_shutdown():
        try:
            href = Float64MultiArray()
            href.data = [head_yaw_ref, 0]
            head_pub.publish(href)
            rospy.sleep(4)
            rospy.loginfo("Current position : %.2f" ,head_yaw_pos)
            head_yaw_ref = -15 if head_yaw_ref == 15 else 15
        except KeyboardInterrupt:
            pass
    rospy.loginfo("finsihed!")