# MAVROS Offboard control example (Python)

This tutorial shows the basics of OFFBOARD control with MAVROS Python, using an Iris quadcopter simulated in Gazebo/SITL. It provides step-by-step instructions demonstrating how to start developing programs to control a vehicle and running the code in simulation.

At the end of the tutorial, you should see the same behaviour as in the video below, i.e. a slow takeoff to an altitude of 2 meters.

WARNING

OFFBOARD control is dangerous. If you are operating on a real vehicle be sure to have a way of gaining back manual control in case something goes wrong.

TIP

This example uses Python. Other examples in Python can be found here: integrationtests/python_src/px4_it/mavros (opens new window).

# Creating the ROS Package

Open the terminal and go to ~/catkin_ws/src directory

roscd  # Should cd into ~/catkin_ws/devel
cd .. 
cd src

In the ~/catkin_ws/src directory create a new package named offboard_py (in this case) with the rospy dependency:
```
catkin_create_pkg offboard_py rospy
```

Build the new package in the ~/catkin_ws/ directory:

cd .. # Assuming previous directory to be ~/catkin_ws/src
catkin build
source devel/setup.bash

You should now be able to cd into the package by using:
```
roscd offboard_py
```
To store your Python files, create a new folder called /scripts on the package:
```
mkdir scripts
cd scripts
```

# Code

After creating the ROS package and scripts folder you are ready to start your Python script. Inside the scripts folder create the offb_node.py file and give it executable permissions:

touch offb_node.py
chmod +x offb_node.py

After that, open offb_node.py file and paste the following code:

"""
 * File: offb_node.py
 * Stack and tested in Gazebo 9 SITL
"""

#! /usr/bin/env python

import rospy
from geometry_msgs.msg import PoseStamped
from mavros_msgs.msg import State
from mavros_msgs.srv import CommandBool, CommandBoolRequest, SetMode, SetModeRequest

current_state = State()

def state_cb(msg):
    global current_state
    current_state = msg


if __name__ == "__main__":
    rospy.init_node("offb_node_py")

    state_sub = rospy.Subscriber("mavros/state", State, callback = state_cb)

    local_pos_pub = rospy.Publisher("mavros/setpoint_position/local", PoseStamped, queue_size=10)

    rospy.wait_for_service("/mavros/cmd/arming")
    arming_client = rospy.ServiceProxy("mavros/cmd/arming", CommandBool)    

    rospy.wait_for_service("/mavros/set_mode")
    set_mode_client = rospy.ServiceProxy("mavros/set_mode", SetMode)


    # Setpoint publishing MUST be faster than 2Hz
    rate = rospy.Rate(20)

    # Wait for Flight Controller connection
    while(not rospy.is_shutdown() and not current_state.connected):
        rate.sleep()

    pose = PoseStamped()

    pose.pose.position.x = 0
    pose.pose.position.y = 0
    pose.pose.position.z = 2

    # Send a few setpoints before starting
    for i in range(100):   
        if(rospy.is_shutdown()):
            break

        local_pos_pub.publish(pose)
        rate.sleep()

    offb_set_mode = SetModeRequest()
    offb_set_mode.custom_mode = 'OFFBOARD'

    arm_cmd = CommandBoolRequest()
    arm_cmd.value = True

    last_req = rospy.Time.now()

    while(not rospy.is_shutdown()):
        if(current_state.mode != "OFFBOARD" and (rospy.Time.now() - last_req) > rospy.Duration(5.0)):
            if(set_mode_client.call(offb_set_mode).mode_sent == True):
                rospy.loginfo("OFFBOARD enabled")

            last_req = rospy.Time.now()
        else:
            if(not current_state.armed and (rospy.Time.now() - last_req) > rospy.Duration(5.0)):
                if(arming_client.call(arm_cmd).success == True):
                    rospy.loginfo("Vehicle armed")

                last_req = rospy.Time.now()

        local_pos_pub.publish(pose)

        rate.sleep()

# Code explanation

The mavros_msgs package contains all of the custom messages required to operate services and topics provided by the MAVROS package. All services and topics as well as their corresponding message types are documented in the mavros wiki (opens new window).

import rospy
from geometry_msgs.msg import PoseStamped
from mavros_msgs.msg import State
from mavros_msgs.srv import CommandBool, CommandBoolRequest, SetMode, SetModeRequest

We create a simple callback which will save the current state of the autopilot. This will allow us to check connection, arming and OFFBOARD flags.:

current_state = State()

def state_cb(msg):
    global current_state
    current_state = msg

We instantiate a publisher to publish the commanded local position and the appropriate clients to request arming and mode change. Note that for your own system, the "mavros" prefix might be different as it will depend on the name given to the node in it's launch file.

state_sub = rospy.Subscriber("mavros/state", State, callback = state_cb)

local_pos_pub = rospy.Publisher("mavros/setpoint_position/local", PoseStamped, queue_size=10)

rospy.wait_for_service("/mavros/cmd/arming")
arming_client = rospy.ServiceProxy("mavros/cmd/arming", CommandBool)    

rospy.wait_for_service("/mavros/set_mode")
set_mode_client = rospy.ServiceProxy("mavros/set_mode", SetMode)

PX4 has a timeout of 500ms between two OFFBOARD commands. If this timeout is exceeded, the commander will fall back to the last mode the vehicle was in before entering OFFBOARD mode. This is why the publishing rate must be faster than 2 Hz to also account for possible latencies. This is also the same reason why it is recommended to enter OFFBOARD mode from Position mode, this way if the vehicle drops out of OFFBOARD mode it will stop in its tracks and hover.

Here we set the publishing rate appropriately:

# Setpoint publishing MUST be faster than 2Hz
rate = rospy.Rate(20)

Before publishing anything, we wait for the connection to be established between MAVROS and the autopilot. This loop should exit as soon as a heartbeat message is received.

# Wait for Flight Controller connection
while(not rospy.is_shutdown() and not current_state.connected):
    rate.sleep()

Even though PX4 operates in the aerospace NED coordinate frame, MAVROS translates these coordinates to the standard ENU frame and vice-versa. This is why we set z to positive 2:

pose = PoseStamped()

pose.pose.position.x = 0
pose.pose.position.y = 0
pose.pose.position.z = 2

Before entering OFFBOARD mode, you must have already started streaming setpoints. Otherwise the mode switch will be rejected. Below, 100 was chosen as an arbitrary amount.

# Send a few setpoints before starting
for i in range(100):   
    if(rospy.is_shutdown()):
        break

    local_pos_pub.publish(pose)
    rate.sleep()

We prepare the message request used to set the custom mode to OFFBOARD. A list of supported modes (opens new window) is available for reference.

offb_set_mode = SetModeRequest()
offb_set_mode.custom_mode = 'OFFBOARD'

The rest of the code is largely self explanatory. We attempt to switch to Offboard mode, after which we arm the quad to allow it to fly. We space out the service calls by 5 seconds so to not flood the autopilot with the requests. In the same loop, we continue sending the requested pose at the rate previously defined.

arm_cmd = CommandBoolRequest()
arm_cmd.value = True

last_req = rospy.Time.now()

while(not rospy.is_shutdown()):
    if(current_state.mode != "OFFBOARD" and (rospy.Time.now() - last_req) > rospy.Duration(5.0)):
        if(set_mode_client.call(offb_set_mode).mode_sent == True):
            rospy.loginfo("OFFBOARD enabled")

        last_req = rospy.Time.now()
    else:
        if(not current_state.armed and (rospy.Time.now() - last_req) > rospy.Duration(5.0)):
            if(arming_client.call(arm_cmd).success == True):
                rospy.loginfo("Vehicle armed")

            last_req = rospy.Time.now()

    local_pos_pub.publish(pose)

    rate.sleep()

TIP

This code has been simplified to the bare minimum for illustration purposes. In larger systems, it is often useful to create a new thread which will be in charge of periodically publishing the setpoints.

# Creating the ROS launch file

In your offboard_py package, create another folder inside the ~/catkin_ws/src/offboard_py/src directory named launch. This is where your launch files for the package will be stored. After that, create your first launch file, in this case we will call it start_offb.launch.

roscd offboard_py
mkdir launch
cd launch 
touch start_offb.launch

For the start_offb.launch copy the following code:

<?xml version="1.0"?>
<launch>
    <!-- Include the MAVROS node with SITL and Gazebo -->
    <include file="$(find px4)/launch/mavros_posix_sitl.launch">
    </include>

    <!-- Our node to control the drone -->
    <node pkg="offboard_py" type="offb_node.py" name="offb_node_py" required="true" output="screen" />
</launch>

As you can see, the mavros_posix_sitl.launch file is included. This file is responsible for launching MAVROS, the PX4 SITL, the Gazebo Environment and for spawning a vehicle in a given world (for further information see the file here (opens new window)).

TIP

The mavros_posix_sitl.launch file takes several arguments that can be set according to your preferences such as the vehicle to spawn or the Gazebo world (refer to here (opens new window)) for a complete list).

You can override the default value of these arguments defined in mavros_posix_sitl.launch by declaring them inside the include tags. As an example, if you wanted to spawn the vehicle in the warehouse.world, you would write the following:

<!-- Include the MAVROS node with SITL and Gazebo -->
<include file="$(find px4)/launch/mavros_posix_sitl.launch">
    <arg name="world" default="$(find mavlink_sitl_gazebo)/worlds/warehouse.world"/>
</include>

# Launching your script

If everything is done, you should now be able to launch and test your script.

In the terminal write:

roslaunch offboard_py start_offb.launch

You should now see the PX4 firmware initiating and the Gazebo application running. After the OFFBOARD mode is set and the vehicle is armed, the behavior shown in the video should be observed.

WARNING

It is possible that when running the script an error appears saying:

Resource not found: px4 ROS path [0] = ... ...

This means that PX4 SITL was not included in the path. To solve this add these lines at the end of the .bashrc file:

source ~/PX4-Autopilot/Tools/setup_gazebo.bash ~/PX4-Autopilot ~/PX4-Autopilot/build/px4_sitl_default
export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:~/PX4-Autopilot
export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:~/PX4-Autopilot/Tools/sitl_gazebo
export GAZEBO_PLUGIN_PATH=$GAZEBO_PLUGIN_PATH:/usr/lib/x86_64-linux-gnu/gazebo-9/plugins

Now in the terminal, go to the home directory and run the following command to apply the changes above to the current terminal:

source .bashrc

After this step, every time you open a new terminal window you should not have to worry about this error anymore. If it appears again, a simple source .bashrc should fix it. This solution was obtained from this issue (opens new window) thread, where you can get more information about the problem.

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