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doc:crazyflie:api:python:index

The Crazyflie Python API

In order to easily use and control the Crazyflie there's an library made in Python that gives high-level functions and hides the details. This page contains generic information about how to use this library and the API that it implements.

If you are interested in more details look in the PyDoc in the code or:

Structure of the library

The library is asynchronous and based on callbacks for events. Functions like open_link will return immediately, and the callback connected will be called when the link is opened. The library doesn't contain any threads or locks that will keep the application running, it's up to the application that is using the library to do this.

Uniform Resource Identifier (URI)

All communication links are identified using an URI build up of the following: InterfaceType://InterfaceId/InterfaceChannel/InterfaceSpeed

Currently only radio and debug interfaces are used but there's ideas for more like udp, serial, usb, etc…Here are some examples:

  • Radio interface, USB dongle number 0, radio channel 10 and radio speed 250 Kbit/s: radio://0/10/250K
  • Debug interface, id 0, channel 1: debug://0/1

Variables and logging

The library supports setting up logging configurations that are used for logging variables from the firmware. Each log configuration contains a number of variables that should be logged as well as a time period (in ms) of how often the data should be sent back to the host. Once the log configuration is added to the firmware the firmware will automatically send back the data at every period. These configurations are used in the following way:

  • Connect to the Crazyflie (log configurations needs a TOC to work)
  • Create a log configuration that contains a number of variables to log and a period at which they should be logged
  • Add the log configuration. This will also validate that the log configuration is sane (i.e uses a supported period and all variables are in the TOC)
  • After checking that the configuration is valid, set up callbacks for the data in your application and start the log configuration
  • Each time the firmware sends data back to the host the callback will the called with a time-stamp and the data

There's a few limitations that needs to be taken into account:

  • Each packet is limited to 32bytes, which means that the data that is logged and the packet that is sent to set it up cannot be larger than this. It limits the logging to about 14 variables, but this is dependent on what types they are
  • The minimum period of a for a log configuration is multiples of 10ms

Parameters

The library supports reading and writing parameters at run-time to the firmware. This is intended to be used for data that is not continuously being changed by the firmware, like setting regulation parameters and reading out if the power-on self-tests passed. Parameters should only change in the firmware when being set from the host or during start-up. The library doesn't continuously update the parameter values, this should only be done once after connecting. After each write to a parameter the firmware will send back the updated value and this will be forwarded to callbacks registered for reading this parameter. The parameters should be used in the following way:

  • Register parameter updated callbacks at any time in your application
  • Connect to your Crazyflie (this will download the parameter TOC)
  • Request updates for all the parameters
  • The library will call all the callbacks registered
  • The host can now write parameters that will be forwarded to the firmware
  • For each write all the callbacks registered for this parameter will be called back

Variable and parameter names

All names of parameters and log variables use the same structure: group.name

The group should be used to bundle together logical groups, like everything that has to do with the stabilizer should be in the group stabilizer.

There's a limit of 28 chars in total and here are some examples:

  • stabilizer.roll
  • stabilizer.pitch
  • pm.vbat
  • imu_tests.MPU6050
  • pid_attitide.pitch_kd

Utilities

Callbacks

All callbacks are handled using the Caller class that contains the following methods:

    add_callback(cb)
        """ Register cb as a new callback. Will not register duplicates. """
 
    remove_callback(cb)
        """ Un-register cb from the callbacks """
 
    call(*args)
        """ Call the callbacks registered with the arguments args """

Debug driver

The library contains a special link driver, named DebugDriver. This driver will emulate a Crazyflie and is used for testing of the UI and library. Normally this will be hidden from the user except if explicitly enabled in the configuration file. The driver supports the following:

  • Connecting a download param and log TOCs
  • Setting up log configurations and sending back fake data
  • Setting and reading parameters
  • Bootloading

There are a number of different URIs that will be returned from the driver. These will have different functions, like always returning a random TOC CRC to always trigger TOC downloading or failing during connect. The driver also has support for sending back data with random delays to trigger random re-sending by the library.

Initiating the link drivers

Before the library can be used the link drivers have to he initialized. This will search for available drivers and instantiate them.

    init_drivers(enable_debug_driver=False)
       """ Search for and initialize link drivers. If enable_debug_driver is True then the DebugDriver will also be used."""

Connection- and link-callbacks

Operations on the link and connection will return directly and will call the following callbacks when events occur:

    # Called on disconnect, no matter the reason
    disconnected = Caller()
    # Called on unintentional disconnect only
    connection_lost = Caller()
    # Called when the first packet in a new link is received
    link_established = Caller()
    # Called when the user requests a connection
    connection_requested = Caller()
    # Called when the link is established and the TOCs (that are not cached)
    # have been downloaded
    connected = Caller()
    # Called if establishing of the link fails (i.e times out)
    connection_failed = Caller()
    # Called for every packet received
    packet_received = Caller()
    # Called for every packet sent
    packet_sent = Caller()
    # Called when the link driver updates the link quality measurement
    link_quality_updated = Caller()

To register for callbacks the following is used:

    crazyflie = Crazyflie()
    crazyflie.connected.add_callback(crazyflie_connected)

Finding a Crazyflie and connecting

The first thing to do is to find a Crazyflie quadcopter that we can connect to. This is done by queuing the library that will scan all the available interfaces (currently the debug and radio interface).

    cflib.crtp.init_drivers()
    available = cflib.crtp.scan_interfaces()
    for i in available:
        print "Interface with URI [%s] found and name/comment [%s]" % (i[0], i[1])

Opening and closing a communication link is doing by using the Crazyflie object:

    crazyflie = Crazyflie()
    crazyflie.connected.add_callback(crazyflie_connected)
    crazyflie.open_link("radio://0/10/250K")

Then you can use the following to close the link again:

    crazyflie.close_link()

Sending control commands

The control commands are not implemented as parameters, instead they have a special API.

    send_setpoint(roll, pitch, yaw, thrust):
        """
        Send a new control set-point for roll/pitch/yaw/thust to the copter
 
        The arguments roll/pitch/yaw/trust is the new set-points that should
        be sent to the copter
        """

To send a new control set-point use the following:

    roll    = 0.0
    pitch   = 0.0
    yawrate = 0
    thrust  = 0
    crazyflie.commander.send_setpoint(roll, pitch, yawrate, thrust)

Thrust is an integer value ranging from 10001 (next to no power) to 60000 (full power). Sending a command makes it apply for 500 ms, after which the firmware will cut out the power. With this in mind, you need to try and maintain a thrust level, with a tick being sent at least once every 2 seconds. Ideally you should be sending one tick every 10 ms, for 100 commands a second. This has a nice added benefit of allowing for very precise control.

Parameters

The parameter framework is used to read and set parameters. This functionality should be used when:

  • The parameter is not changed by the Crazyflie but by the client
  • The parameter is not read periodically

If this is not the case then the logging framework should be used instead.

To set a parameter you have to the connected to the Crazyflie. A parameter is set using:

    param_name = "group.name"
    param_value = 3
    crazyflie.param.set_value(param_name, param_value)

The parameter reading is done using callbacks. When a parameter is updated from the host (using the code above) the parameter will be read back by the library and this will trigger the callbacks. Parameter callbacks can be added at any time (you don't have to be connected to a Crazyflie).

    add_update_callback(group, name=None, cb=None)
        """
        Add a callback for a specific parameter name or group. If not name is specified then
        all parameters in the group will trigger the callback. This callback will be executed
        when a new value is read from the Crazyflie.
        """
 
    request_param_update(complete_name)
        """ Request an update of the value for the supplied parameter. """
 
    set_value(complete_name, value)
        """ Set the value for the supplied parameter. """

Here's an example of how to use the calls.

    crazyflie.param.add_update_callback(group="group", name="name", param_updated_callback)
 
    def param_updated_callback(name, value):
        print "%s has value %d" % (name, value)

Logging

The logging framework is used to enable the “automatic” sending of variable values at specified intervals to the client. This functionality should be used when:

  • The variable is changed by the Crazyflie and not by the client
  • The variable is updated at high rate and you want to read the value periodically

If this is not the case then the parameter framework should be used instead.

The API to create and get information from LogConfig:

    # Called when new logging data arrives
    data_received_cb = Caller()
    # Called when there's an error
    error_cb = Caller()
    # Called when the log configuration is confirmed to be started
    started_cb = Caller()
    # Called when the log configuration is confirmed to be added
    added_cb = Caller()
 
    add_variable(name, fetch_as=None)
        """Add a new variable to the configuration.
 
        name - Complete name of the variable in the form group.name
        fetch_as - String representation of the type the variable should be
                   fetched as (i.e uint8_t, float, FP16, etc)
 
        If no fetch_as type is supplied, then the stored as type will be used
        (i.e the type of the fetched variable is the same as it's stored in the
        Crazyflie)."""
 
    start()
        """Start the logging for this entry"""
 
    stop()
        """Stop the logging for this entry"""
 
    delete()
        """Delete this entry in the Crazyflie"""

The API for the log in the Crazyflie:

    add_config(logconf)
        """Add a log configuration to the logging framework.
 
        When doing this the contents of the log configuration will be validated
        and listeners for new log configurations will be notified. When
        validating the configuration the variables are checked against the TOC
        to see that they actually exist. If they don't then the configuration
        cannot be used. Since a valid TOC is required, a Crazyflie has to be
        connected when calling this method, otherwise it will fail."""

To create a logging configuration the following can be used:

    logconf = LogConfig(name="Logging", period_in_ms=100)
    logconf.add_variable("group1.name1", "float")
    logconf.add_variable("group1.name2", "uint8_t")
    logconf.add_variable("group2.name1", "int16_t")

The datatype is the transferred datatype, it will be converted from internal type to transferred type before transfers:

  • float
  • uint8_t and int8_t
  • uint16_t and int16_t
  • uint32_t and int32_t
  • FP16: 16bit version of floating point, allows to pack more variable in one packet at the expense of precision.

The logging cannot be started until your are connected to a Crazyflie:

    # Callback called when the connection is established to the Crazyflie
    def connected(link_uri):
        crazyflie.log.add_config(logconf)
 
        if logconf.valid:
            logconf.data_received_cb.add_callback(data_received_callback)
            logconf.error_cb.add_callback(logging_error)
            logconf.start()
        else:
            print "One or more of the variables in the configuration was not found in log TOC. No logging will be possible."
 
    def data_received_callback(timestamp, data, logconf):
        print "[%d][%s]: %s" % (timestamp, logconf.name, data)
 
    def logging_error(logconf, msg):
        print "Error when logging %s" % logconf.name

Examples

The examples are now placed in the repository in the examples folder.

doc/crazyflie/api/python/index.txt · Last modified: 2015-10-13 11:59 by arnaud