Creating MAVLink commands from python

Question

I am struggling to find any clear documentation on how to create MAVlink commands using python.

I am looking to create an autonomous glider and require some of the basic functions

• Retrieve GPS Data and store into a file
• Import GPS data in Python function
• Send waypoint lat, long, alt to the autopilot (APM)

I am currently using MAVProxy as my GCS and the ErleBrain 3 as my autopilot hardware but the aim is to not require a GCS and just have a python script to automatically add and remove waypoints based on the received GPS data

Answer 1

There are a couple libraries for doing what you want to to, the first is pyMAVlink and the second is dronekit-python it is going to be easiest to use dronekit as it handles a lot of the lower level stuff like processing incoming packets and connecting over your preferred interface for you automatically. I assume you want to load waypoints from a file and have your UAV fly them? In that case you probably want to set your UAV into Guided mode (for a description of modes on Ardupilot see the docs here) this will tell Ardupilot that you are going to command it step by step what to do.

Here is some example code form there wiki that does what you want to do you'll just have to load your files and transform the coordinates to lat lon points:

def arm_and_takeoff(aTargetAltitude):
    while not vehicle.is_armable:
        print " Waiting for vehicle to initialise..."
        time.sleep(1)
    vehicle.mode = VehicleMode("GUIDED")
    vehicle.armed = True

    while not vehicle.armed:      
        print " Waiting for arming..."
        time.sleep(1)

    print "Taking off!"
    vehicle.simple_takeoff(aTargetAltitude)
    # Wait for takeoff to finish
    while True:
        print " Altitude: ", vehicle.location.global_relative_frame.alt      
        if vehicle.location.global_relative_frame.alt>=aTargetAltitude*0.95: #Trigger just below target alt.
            print "Reached target altitude"
            break
        time.sleep(1)

def get_distance_metres(aLocation1, aLocation2):
    dlat = aLocation2.lat - aLocation1.lat
    dlong = aLocation2.lon - aLocation1.lon
    return math.sqrt((dlat*dlat) + (dlong*dlong)) * 1.113195e5

# define connection method 
# see all the methods at http://python.dronekit.io/guide/connecting_vehicle.html
connection_string = "127.0.0.1:14550"

# Connect
vehicle = connect(connection_string, wait_ready=True)
# arm and takeooff to 10m
arm_and_takeoff(10)
# set airspeed to 3 m/s
vehicle.airspeed = 3

point1 = LocationGlobalRelative(-35.361354, 149.165218, 20)
vehicle.simple_goto(point1)

while vehicle.mode.name=="GUIDED": #Stop action if we are no longer in guided mode.
        remainingDistance=get_distance_metres(vehicle.location.global_frame, point1)
        print "Distance to target: ", remainingDistance
        if remainingDistance<=targetDistance*0.01: #Just below target, in case of undershoot.
            print "Reached target"
            break;
        time.sleep(2)

Per request I have some example pymavlink code, you can see some more examples here, sending messages is a pain, you need to identify what type of packet you need to send, and fill it's paramaters acording to the standard

from pymavlink import mavutil

data = mavutil.mavlink_connection('udp:localhost:14551', planner_format=False,
                                  notimestamps=True,
                                  robust_parsing=True)

while True:
    msg = data.recv_match();
    #If we have a valid message
    if msg is not None:
        #print msg.get_type()
        if msg.get_type() is "HEARTBEAT":
            print msg.autopilot
        elif msg.get_type() is "NAV_CONTROLLER_OUTPUT":
            print msg