offb_node.py

#!/usr/bin/env python

import rospy
import mavros
from geometry_msgs.msg import PoseStamped
from mavros.msg import State 
from mavros.srv import CommandBool, SetMode

# callback method for state sub
current_state = State() 
offb_set_mode = SetMode
def state_cb(state):
    global current_state
    current_state = state

local_pos_pub = rospy.Publisher(mavros.get_topic('setpoint_position', 'local'), PoseStamped, queue_size=10)
state_sub = rospy.Subscriber(mavros.get_topic('state'), State, state_cb)
arming_client = rospy.ServiceProxy(mavros.get_topic('cmd', 'arming'), CommandBool)
set_mode_client = rospy.ServiceProxy(mavros.get_topic('set_mode'), SetMode) 

pose = PoseStamped()
pose.pose.position.x = 0
pose.pose.position.y = 0
pose.pose.position.z = 2

def position_control():
    rospy.init_node('offb_node', anonymous=True)
    prev_state = current_state
    rate = rospy.Rate(20.0) # MUST be more then 2Hz

    # send a few setpoints before starting
    for i in range(100):
        local_pos_pub.publish(pose)
        rate.sleep()
    
    # wait for FCU connection
    while not current_state.connected:
        rate.sleep()

    last_request = rospy.get_rostime()
    while not rospy.is_shutdown():
        now = rospy.get_rostime()
        if current_state.mode != "OFFBOARD" and (now - last_request > rospy.Duration(5.)):
            set_mode_client(base_mode=0, custom_mode="OFFBOARD")
            last_request = now 
        else:
            if not current_state.armed and (now - last_request > rospy.Duration(5.)):
               arming_client(True)
               last_request = now 

        # older versions of PX4 always return success==True, so better to check Status instead
        if prev_state.armed != current_state.armed:
            rospy.loginfo("Vehicle armed: %r" % current_state.armed)
        if prev_state.mode != current_state.mode: 
            rospy.loginfo("Current mode: %s" % current_state.mode)
        prev_state = current_state

        # Update timestamp and publish pose 
        pose.header.stamp = rospy.Time.now()
        local_pos_pub.publish(pose)
        rate.sleep()

if __name__ == '__main__':
    try:
        position_control()
    except rospy.ROSInterruptException:
        pass

Why not used mavros.command module? Also better to use mavros.get_topic() helper.

FYI: older PX4 always return success==True, so better to check Status.

And perhaps it is safer to leave arming to user.

pose.header.stamp should be updated before each publish!

Thanks for the notes. I did a step by step translation of the PX4 sitl cpp example code (http://dev.px4.io/ros-mavros-offboard.html). Applied most of the changes, kept mode setting and arming in the loop since the idea was to show the python alternative to the cpp code.

Hi @annesteenbeek, I am very new to ROS programming. I was curious about why does the drone fly off to a higher altitude when the shutdown is initiated (Crtl+C)? Also could you please suggest how do I modify the code to provide a trajectory?

Hi @annesteenbeek, I am very new to ROS programming. I was curious about why does the drone fly off to a higher altitude when the shutdown is initiated (Crtl+C)? Also could you please suggest how do I modify the code to provide a trajectory?

When you shutdown the offb_node the drone is neither receiving rc input nor is it receiving the setpoints, hence the drone goes into failsafe, and the default failsafe is to climb to 10m and return to launch, that is what the drone is doing.

Hi @annesteenbeek I am also curious about adding a trajectory into your code, would that be done in the while loop?