Software in the Loop Joystick Flight¶

This tutorial explains the steps required to fly a simulated quadcopter in the Gazebo simulator using a real joystick. The following diagram shows how the system components work together.

Hardware Requirements¶

Desktop Linux Machine with minimum of 8GB RAM, 16GB recommended, Ubuntu 16.04 installed
Joystick

Software Requirements¶

Ubuntu 16.04
ROS Kinetic (full desktop installation)
Gazebo 7: will be automatically installed with ROS
PX4 firmware installation on Linux: Autopilot software which includes the software-in-the-loop firmware
MAVROS package: Autopilot ROS interface
Joy package: Joystick ROS interface

Note

In this tutorial, it is assumed that the reader is familiar with basic Linux commands, ROS Basics.

Setup Steps¶

Download this ZIP file, and extract .sh files to your home folder and run by command.

./ubuntu_install.sh

Then copy commands line by line from ws.sh and run them one by one in a terminal.

This will setup all permissions and development environment which includes the software-in-the-loop simulation.

Install QGroundControl from here. Use the AppImage option.

Testing SITL with Gazebo (No ROS)¶

In this step, we will validate that the PX4 SITL app and Gazebo work as expected. To run the SITL app and Gazebo, execute the following commands in a new terminal

cd ~/src/Firmware
make posix_sitl_default gazebo

After sometime, you should be able to see an Iris model loaded in Gazebo, and the pxh> command line in the terminal. Just hit ENTER couple of times if you don’t see the pxh> command line, and it should appear.

To takeoff/land the quadcopter, execute the following commands in the terminal

pxh> commander takeoff
pxh> commander land

If the previous actions succeed the the installation is OK. Next, we will run ROS and a MAVROS node which will allow us to interface the autopilot with ROS.

Interfacing with ROS¶

Now, you are ready to launch Gazebo+PX4 SITL app+ROS+MAVROS. To do that, execute the following command.

roslaunch px4 mavros_posix_sitl.launch fcu_url:="udp://:14540@127.0.0.1:14557"

You should be able to see /mavros topics using rostopic list in a new terminal. Also if you execute rosnode list in a new terminal, you should see

$ rosnode list
/gazebo
/mavros
/rosout

To double check that MAVROS node is connected properly to the PX4 SITL app, try to echo some topics _e.g._

rostopic echo /mavros/state

Which will show if the mavros node is connected to the PX4 SITL app or not.

Now, you can monitor the drone’s states and control it via a MAVROS node.

As mentioned, in this tutorial, we are going to learn one basic way of controlling the quadcopter’s position via a joystick.
There is a flight mode in PX4 autopilot which is called OFFBOARD mode. This mode allows the autopilot to accept specific external commands such as position, velocity, and attitude setpoints. You cannot mix between different setpoints _e.g._ velocity setpoints in x/y and position in z.
A MAVROS node provides setpoint plugins which will listen to a user input on specific setpoint topics. Once the user publishes to those specific setpoint topics, the mavros node will transfer those setpoints to the autopilot to execute.
If the autopilot’s flight mode is OFFBOARD, the autopilot will accept the received setpoints and execute them.
We will send position setpoints to the autopilot via a setpoint topic that is available in MAVROS. Once set points are received in that topic, the mavros node will send it to the autopilot.
The setpoint topic that we will use in this tutorial is /mavros/setpoint_raw/local. This topic accepts both position and velocity setpoints according to a specific flag. Next, we will create our custom simple ROS package in which we create a simple ROS node that listens to joystic commands from a ROS topic. Then, it will convert joystic commands to position setpoints which will be published to the /mavros/setpoint_raw/local topic. Finally, MAVROS will take the position set points and send them to the autopilot.

You might be asking, how are we going to get the joystick commands? The next section explains that.

Joystick Package Installation & Usage¶

A package named joy is going to be used to interface a joystick to ROS. To install that package, simply execute the following command in the terminal.

sudo apt-get install ros-kinetic-joy

You will need to setup permissions before you can use your joystick.

Plug a joystick
Check if Linux recognizes your joystick

ls /dev/input/

You will get an output similar to the following.

by-id    event0  event2  event4  event6  event8  mouse0  mouse2  uinput
by-path  event1  event3  event5  event7  js0     mice    mouse1

As you can see, the joystick device is referred to as jsX where X is the number of the joystick device.

Let’s make the joystick accessible to the joy ROS node.

ls -l /dev/input/jsX

You will see something similar to:

crw-rw-XX- 1 root dialout 188, 0 2009-08-14 12:04 /dev/input/jsX

If XX is rw: the js device is configured properly. If XX is --: the js device is not configured properly and you need to:

sudo chmod a+rw /dev/input/jsX

Test the joy node. First, start roscore in a terminal. In another terminal,

# set the joystick device address
rosparam set joy_node/dev "/dev/input/js0"
# run the joy node
rosrun joy joy_node

In another terminal, echo the joy topic and move the joystick to see the topic changes

rostopic echo /joy

You should see an output similar to the following.

header:
seq: 699
stamp:
  secs: 1505985329
  nsecs: 399636113
frame_id: ''
axes: [-0.0, -0.0, -0.8263657689094543]
buttons: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

Now, let’s write a custom node that reads joystick’s commands and convert them to position setpoints to control the quadcopter’s poisiton in Gazebo.

Custom Setpoint Node¶

Now, it’s time for some coding! You will write a ROS node in Python that listens to the /joy topic that is published by the joy node, and converts the joystick commands to xyz position setpoints. Then, it will publish the calculated position setpoints into /mavros/setpoint_raw/local

Publishing to /mavros/setpoint_raw/local topic is not enough to get the autopilot to track the setpoints. It has to be in OFFBOARD mode. So, in your custom node, you will have to send a signal to activate this mode, only once. You need to remember that for this mode to work, you will need to be publishing setpoints beforehand, then, activate it, and continue publishing setpoints. If you don’t publish setpoints at more than 2Hz, it will go into a failsafe mode.

First, create your custom ROS package. The code is commented so you can get an idea of what each part does.

cd ~/catkin_ws/src
catkin_create_pkg mypackage std_msgs mavros_msgs roscpp rospy
cd mypackage
# usually python scripts (nodes) are placed in a folder called scripts
mkdir scripts
cd scripts
gedit setpoints_node.py

#!/usr/bin/env python

# ROS python API
import rospy
# Joy message structure
from sensor_msgs.msg import Joy
# 3D point & Stamped Pose msgs
from geometry_msgs.msg import Point, PoseStamped
# import all mavros messages and services
from mavros_msgs.msg import *
from mavros_msgs.srv import *

# Flight modes class
# Flight modes are activated using ROS services
class fcuModes:
    def __init__(self):
        pass

    def setArm(self):
        rospy.wait_for_service('mavros/cmd/arming')
        try:
            armService = rospy.ServiceProxy('mavros/cmd/arming', mavros_msgs.srv.CommandBool)
            armService(True)
        except rospy.ServiceException, e:
            print "Service arming call failed: %s"%e

    def setDisarm(self):
        rospy.wait_for_service('mavros/cmd/arming')
        try:
            armService = rospy.ServiceProxy('mavros/cmd/arming', mavros_msgs.srv.CommandBool)
            armService(False)
        except rospy.ServiceException, e:
            print "Service disarming call failed: %s"%e

    def setStabilizedMode(self):
        rospy.wait_for_service('mavros/set_mode')
        try:
            flightModeService = rospy.ServiceProxy('mavros/set_mode', mavros_msgs.srv.SetMode)
            flightModeService(custom_mode='STABILIZED')
        except rospy.ServiceException, e:
            print "service set_mode call failed: %s. Stabilized Mode could not be set."%e

    def setOffboardMode(self):
        rospy.wait_for_service('mavros/set_mode')
        try:
            flightModeService = rospy.ServiceProxy('mavros/set_mode', mavros_msgs.srv.SetMode)
            flightModeService(custom_mode='OFFBOARD')
        except rospy.ServiceException, e:
            print "service set_mode call failed: %s. Offboard Mode could not be set."%e

    def setAltitudeMode(self):
        rospy.wait_for_service('mavros/set_mode')
        try:
            flightModeService = rospy.ServiceProxy('mavros/set_mode', mavros_msgs.srv.SetMode)
            flightModeService(custom_mode='ALTCTL')
        except rospy.ServiceException, e:
            print "service set_mode call failed: %s. Altitude Mode could not be set."%e

    def setPositionMode(self):
        rospy.wait_for_service('mavros/set_mode')
        try:
            flightModeService = rospy.ServiceProxy('mavros/set_mode', mavros_msgs.srv.SetMode)
            flightModeService(custom_mode='POSCTL')
        except rospy.ServiceException, e:
            print "service set_mode call failed: %s. Position Mode could not be set."%e

    def setAutoLandMode(self):
        rospy.wait_for_service('mavros/set_mode')
        try:
            flightModeService = rospy.ServiceProxy('mavros/set_mode', mavros_msgs.srv.SetMode)
            flightModeService(custom_mode='AUTO.LAND')
        except rospy.ServiceException, e:
              print "service set_mode call failed: %s. Autoland Mode could not be set."%e

# Main class: Converts joystick commands to position setpoints
class Controller:
    # initialization method
    def __init__(self):
        # Drone state
        self.state = State()
        # Instantiate a setpoints message
        self.sp         = PositionTarget()
        # set the flag to use position setpoints and yaw angle
        self.sp.type_mask    = int('010111111000', 2)
        # LOCAL_NED
        self.sp.coordinate_frame= 1

        # We will fly at a fixed altitude for now
        # Altitude setpoint, [meters]
        self.ALT_SP        = 3.0
        # update the setpoint message with the required altitude
        self.sp.position.z    = self.ALT_SP

        # Instantiate a joystick message
        self.joy_msg        = Joy()
        # initialize
        self.joy_msg.axes=[0.0, 0.0, 0.0]

        # Step size for position update
        self.STEP_SIZE = 2.0

        # Fence. We will assume a square fence for now
        self.FENCE_LIMIT = 5.0

        # A Message for the current local position of the drone
        self.local_pos = Point(0.0, 0.0, 0.0)

    # Callbacks

    ## local position callback
    def posCb(self, msg):
        self.local_pos.x = msg.pose.position.x
        self.local_pos.y = msg.pose.position.y
        self.local_pos.z = msg.pose.position.z

    ## joystick callback
    def joyCb(self, msg):
        self.joy_msg = msg

    ## Drone State callback
    def stateCb(self, msg):
        self.state = msg

    ## Update setpoint message
    def updateSp(self):
        x = -1.0*self.joy_msg.axes[1]
        y = -1.0*self.joy_msg.axes[0]

        self.sp.position.x = self.local_pos.x + self.STEP_SIZE*x
        self.sp.position.y = self.local_pos.y + self.STEP_SIZE*y

# Main function
def main():

    # initiate node
    rospy.init_node('setpoint_node', anonymous=True)

    # flight mode object
    modes = fcuModes()
    # controller object
    cnt = Controller()

    # ROS loop rate, [Hz]
    rate = rospy.Rate(20.0)

    # Subscribe to drone state
    rospy.Subscriber('mavros/state', State, cnt.stateCb)

    # Subscribe to drone's local position
    rospy.Subscriber('mavros/local_position/pose', PoseStamped, cnt.posCb)
    # subscribe to joystick topic
    rospy.Subscriber('joy', Joy, cnt.joyCb)

    # Setpoint publisher
    sp_pub = rospy.Publisher('mavros/setpoint_raw/local', PositionTarget, queue_size=1)


    # Make sure the drone is armed
    while not cnt.state.armed:
        modes.setArm()
        rate.sleep()

    # We need to send few setpoint messages, then activate OFFBOARD mode, to take effect
    k=0
    while k<10:
        sp_pub.publish(cnt.sp)
        rate.sleep()
        k = k+1

    # activate OFFBOARD mode
    modes.setOffboardMode()

    # ROS main loop
    while not rospy.is_shutdown():
        cnt.updateSp()
        sp_pub.publish(cnt.sp)
        rate.sleep()


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

Make the python file an executable,

chmod +x setpoints_node.py

Make a launch folder. We will create a ROS launch file to run everything at once.

cd ~/catkin_ws/src/mypackage
mkdir launch
cd launch
gedit joystick_flight.launch

Copy the following content to new created launch file.

<launch>

    <arg name="joy_dev" default="/dev/input/js0"/>

    <arg name="fcu_url" default="udp://:14540@127.0.0.1:14557" />


    <include file="$(find px4)/launch/mavros_posix_sitl.launch">
        <arg name="fcu_url" value="$(arg fcu_url)" />
    </include>

    <node pkg="joy" type="joy_node" name="joy_node"  required="true" output="screen">
            <param name="dev" type="string" value="$(arg joy_dev)" />
    </node>

    <node pkg="mypackage" type="setpoints_node.py" name="setpoints_node"  required="true" output="screen">
    </node>

</launch>

In a fresh terminal, you can run the whole system by executing

roslaunch mypackage joystick_flight.launch

Now, you should see a quadcopter in Gazebo flying at a fixed height and responding to your joystick commands.

Warning

Always make sure that you have joystick permissions configured properly.