Embedded  Linux  applications   for  autonomous  UAVs Flying Penguins 

Clay McClure github.com/claymation twitter.com/claymcclure 

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autopilots 

RC input motor mixing stabilization telemetry missions failsafes 

autopilots 

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≠ AUTO PILOT AUTO NOMOUS 

where to go how to get there what to do next “finds its own goal positions” 

SO MANY ALGORITHMS, SO LITTLE COMPUTER http://ra3ndy.deviantart.com/art/Sad-Panda-69204875 

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Autopilot runs on Linux 

Autopilot talks to Linux 

ODROID-XU3 Lite • Samsung Exynos5422 octa core • 4x Cortex™-A15 2.0GHz • 4x Cortex™-A7 1.4GHz • 2 GB RAM • 32+ GB flash • 4x USB 2.0 + 1x USB 3.0 

UAV Your App Linux ? Auto Pilot Architecture 

UAV MAV Link Your App Linux Auto Pilot Architecture 

MAVLink is the HTTP of drones 

(it’s also the libcurl) 

MAVLink 

status configuration position / attitude setpoints missions 

UAV MAV Link Your App Linux ? Auto Pilot Architecture 

UAV MAV Link Your App Linux Middle ware Auto Pilot Architecture 

Middleware DroneAPI • Python • Go to Kevin Hester’s talk tomorrow mavros • Python, C++, Lisp (really) • Access to a wealth of robotics research and tools 

UAV MAV Link mavros Your App ROS Linux Auto Pilot Architecture 

ROS Crash Course 

Robot Operating System “ROS is an open-source, meta-operating system for your robot.” 

“ROS is an open-source, meta-operating system for your robot.” Robot Operating System 

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Nodes Node Node Node Node Node Node Node Node Node 

Topics Node Node Node Node Topic 

Services Node Node 

but wait, that’s not all… parameters dynamic reconfig coordinate frames transformations record/playback visualization logging 

UAV MAV Link mavros Your App ROS Linux Auto Pilot Architecture 

mavros is the Babel fish of drones 

Mavros Node Topic Topic Topic Auto Pilot 

Topics /mavros/state /mavros/imu/data /mavros/global_position/global /mavros/local_position/local /mavros/setpoint_position/local_position /mavros/setpoint_velocity/cmd_vel 

Services /mavros/cmd/arming /mavros/cmd/land /mavros/cmd/takeoff /mavros/set_mode /mavros/set_stream_rate 

• FCU • MAVLink • mavros • ROS • Application nodes PX4 + ROS Credit: Kabir Mohammed MAVROS 

UAV MAV Link mavros Your App ROS Linux Auto Pilot Architecture 

Event-driven programming • “Don’t call me, I’ll call you” • Your application code responds to events • Message arrival • “vehicle position is (x, y, z)” • Timer expiry • “it’s time to run the control loop” 

YAPL Yet Another Precision Lander 

Nodes • Tracker • Processes video stream, looks for landing pad • Publishes target position/velocity messages • Commander • Subscribes to vehicle state and position messages • Subscribes to target tracker messages • Controls vehicle velocity 

Tracker 

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class  TrackerNode(object):   !        def  __init__(self):                  rospy.init_node("tracker")   !                camera_matrix  =  rospy.get_param("~camera_matrix")                  use_sim  =  rospy.get_param("~use_sim",  False)   !              self.image_publisher  =  \                          rospy.Publisher("tracker/image",                                  sensor_msgs.msg.Image,  queue_size=1)   !                self.track_publisher  =  \                          rospy.Publisher("tracker/track",                                  Track,  queue_size=1) 

class  TrackerNode(object):   !        def  run(self):                  """                  Capture  frames  from  the  camera  and  publish  them.                  """                  self.wait_for_position()   !                while  not  rospy.is_shutdown():                          frame  =  self.camera.get_frame()                          if  frame  is  not  None:                                  position,  velocity  =  \                                          self.process_frame(frame)                                  self.publish_track(position,  velocity) 

       def  publish_track(self,  position,  velocity):                  """                  Publish  Track  messages  for  the  commander  node.                  """                  msg  =  Track()                  msg.track.is_tracking.data  =  self.is_tracking   !                if  self.is_tracking:                          msg.track.position.x  =  position[0]                          msg.track.position.y  =  position[1]                          msg.track.position.z  =  position[2]                          msg.track.velocity.x  =  velocity[0]                          msg.track.velocity.y  =  velocity[1]                          msg.track.velocity.z  =  velocity[2]   !                self.track_publisher.publish(msg)   

       def  publish_track(self,  position,  velocity):                  """                  Publish  Track  messages  for  the  commander  node.                  """                  msg  =  Track()                  msg.track.is_tracking.data  =  self.is_tracking   !                if  self.is_tracking:                          msg.track.position.x  =  position[0]                          msg.track.position.y  =  position[1]                          msg.track.position.z  =  position[2]                          msg.track.velocity.x  =  velocity[0]                          msg.track.velocity.y  =  velocity[1]                          msg.track.velocity.z  =  velocity[2]   !                self.track_publisher.publish(msg)   

$  cat  msg/Track.msg     #  Whether  we're  tracking  an  object   std_msgs/Bool  is_tracking   ! #  Relative  position  and  velocity  of  the  tracked  object   geometry_msgs/Vector3  position   geometry_msgs/Vector3  velocity   

       def  publish_image(self,  image):                  """                  Publish  processed  tracker  images  for  telemetry.                  """                  msg  =  self.image_bridge.cv2_to_imgmsg(image,  "bgr8")                  self.image_publisher.publish(msg)   

Commander 

PENDING SEEK APPROACH DESCEND LAND LANDED 

class  CommanderNode(object):        def  __init__(self,  vehicle):                rospy.init_node("commander")   !                #  Initialize  flight  controllers                  self.controllers  =  {                          FlightState.PENDING    :  PendingController(),                          FlightState.SEEK          :  SeekController(),                          FlightState.APPROACH  :  ApproachController(),                          FlightState.DESCEND    :  DescendController(),                          FlightState.LAND          :  LandController(),                  }   !                #  ... 

               #  Initialize  the  control  loop                  control_loop_rate  =  \                                  rospy.get_param("~control_loop_rate",                                  DEFAULT_CONTROL_LOOP_RATE)                  self.control_loop_rate  =  \                                  rospy.Rate(control_loop_rate)   !                rospy.Subscriber("/mavros/state",                                  mavros.msg.State,                                  self.handle_state_message)                  rospy.Subscriber("/tracker/track",                                  Track,  self.handle_track_message)   !                #  Initialize  state  machine                  self.controller  =  None                  self.state  =  FlightState.INIT                  self.transition_to_state(FlightState.PENDING) 

       def  run(self):                  """                  Spin  the  ROS  event  loop,  running  the  active                  flight  controller  on  each  iteration.                  """                  while  not  rospy.is_shutdown():                          self.controller.run()                          self.control_loop_rate.sleep() 

       def  handle_state_message(self,  msg):                  """                  Handle  mavros/State  messages,  published  by  the                  FCU  at  ~1  Hz.                  """                  mode  =  msg.mode   !                if  (mode  in  GUIDED_MODES  and                          self.state  ==  FlightState.PENDING):                                  self.transition_to_state(FlightState.SEEK)   !                elif  (mode  not  in  GUIDED_MODES  and                          self.state  !=  FlightState.PENDING):                                  self.transition_to_state(FlightState.PENDING) 

#  Guided  modes  differ  between  ArduCopter  and  PX4  native   GUIDED_MODES  =  ("GUIDED",  "OFFBOARD") 

Vehicle 

class  Vehicle(object,  PositionMixin):          """          Simple  model  of  an  aerial  vehicle.          """          def  __init__(self):                self.location_setpoint_publisher  =  \                          rospy.Publisher(                                  "/mavros/setpoint_position/local_position",                                  geometry_msgs.msg.PoseStamped,                                  queue_size=1)   !                self.velocity_setpoint_publisher  =  \                          rospy.Publisher(                                  "/mavros/setpoint_velocity/cmd_vel",                                  geometry_msgs.msg.TwistStamped,                                  queue_size=1) 

class  Vehicle(object,  PositionMixin):   !        #  ...                  def  set_location_setpoint(self,  setpoint):                  """                  Publish  a  SET_POSITION_TARGET_LOCAL_NED  message                  with  position  x,  y,  z  and  yaw.                  """                  x,  y,  z,  yaw  =  setpoint   !                msg  =  geometry_msgs.msg.PoseStamped()                  msg.pose.position.x  =  x                  msg.pose.position.y  =  y                  msg.pose.position.z  =  z                  msg.pose.orientation.z  =  yaw   !                self.location_setpoint_publisher.publish(msg) 

class  Vehicle(object,  PositionMixin):   !        #  ...                  def  set_armed_state(self,  state):                  set_armed_state  =  rospy.ServiceProxy(                                  "/mavros/cmd/arming",                                  CommandBool)                  set_armed_state(value=state)   !        def  arm(self):                  self.set_armed_state(True)   !        def  disarm(self):                  self.set_armed_state(False) 

Simulation 

HITL • Hardware in the loop • Flight software runs on flight hardware • Simulated sensor and control inputs 

SITL • Software in the loop • Flight software runs on (Linux) desktop • Simulated sensor and control inputs and HAL 

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–Johnny Appleseed “Type a quote here.” “In theory there is no difference between theory and practice. 
 In practice there is.” ! ~ Yogi Berra 

Practical Considerations 

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Connections • UART recommended • Requires 6-pin DF-13, possibly a level shifter • USB works for me • Use hot glue gun • sudo  apt-­‐get  remove  modemmanager 

Power 5V 5A UBEC ! ODROID + USB camera + WiFi = 5 hours 

Launch files • ROS feature that makes it easy to start and manage multiple nodes and their parameters • roslaunch  lander  lander.launch 

Startup • Use ubuntu’s upstart to launch ROS + mavros + application nodes • rosrun  robot_upstart  install  \
        lander/launch/lander.launch 

Telemetry • WiFi • Ad-Hoc mode (man  wireless) • Need high-gain antenna and an antenna tracker
 (a.k.a. unsuspecting friend or bystander) • sudo  apt-­‐get  remove  wpasupplicant   • GSM? • Use image_transport to transmit compressed images 

Coordinate Frames • Global / Local • NED (UAV) • ENU (ROS) • Body-fixed • tf library 

In closing… 

What will you make? 

Thanks 

For more information… ros.org ardupilot.com pixhawk.org/start pixhawk.ethz.ch/mavlink github.com/mavlink/mavros github.com/claymation/lander