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Accessing EV3 Devices with ev3dev-lang

ev3dev-lang is a library for accessing EV3 motors and sensors. It can be used from the following four programming languages:

• C++
• Python
• JavaScript
• Lua

This section introduces examples of accessing EV3 devices using Python and C++.

Installing python-ev3dev

Install the Python version (Python bindings) of ev3dev-lang directly on the EV3 (not in the cross-development environment).

By entering the following commands, python-ev3dev will be installed on the system and become available from Python.

 # cd ~
 # mkdir work
 # cd work
 # apt-get install libboost-python1.55.0 python-setuptools python-pil
 # wget https://github.com/rhempel/ev3dev-lang-python/archive/0.6.0.zip
 # unzip 0.6.0.zip
 # cd ev3dev-lang-python-0.6.0/
 # python setup.py install

Accessing Devices from python-ev3dev

Let’s try rotating a motor using python-ev3dev.

First, move to an appropriate directory (running in the directory used to install python-ev3dev will cause an error), and start Python in interactive mode.

 # cd
 # python
 Python 2.7.9 (default, Mar  1 2015, 13:52:09)
 [GCC 4.9.2] on linux2
 Type "help", "copyright", "credits" or "license" for more information.
 >>> import ev3dev.ev3 as ev3     (Import the ev3dev module)
 >>> m = ev3.LargeMotor()         (Instantiate a motor object)
 >>> m.connected                  (Check whether the motor is connected)
 True
 >>> m.run_forever(duty_cycle_sp=50)  (Rotate the motor continuously at 50% duty cycle)
 >>> m.stop()                         (Stop the motor)

There are three motor stop modes: 'brake', 'coast', and 'hold'.

The default mode is 'coast', so when m.stop() is executed, the motor may continue moving slightly due to inertia.

To make the motor stop immediately, change the mode to 'hold'.

 >>> m.set_attr_string(None, "stop_command", ev3.Motor.STOP_COMMAND_HOLD)
 >>> m.get_attr_string(None, "stop_command")[1]
 >>> m.run_forever(duty_cycle_sp=50)
 >>> m.stop()

As shown above, motors can be controlled relatively easily from Python.

Next, let’s use a sensor.

Connect an ultrasonic sensor to Port 2.

While still in Python interactive mode, enter the following commands.

The for loop continuously displays the sensor value (in centimeters) for 100 seconds.

Press Ctrl+C to exit the loop at any time.

 >>> s = ev3.UltrasonicSensor()
 >>> s.distance_centimeters()
 118
 >>> import time
 >>> for i in range(0,100):
 ...     print s.distance_centimeters()
 ...     time.sleep(1)
 ..
 118
 114
 ：(omitted)
 2382
 2366
 326
 76
 ^CTraceback (most recent call last):
   File "<stdin>", line 3, in <module>
 KeyboardInterrupt
 >>>

Learning About python-ev3dev

Tutorials and reference manuals for the various classes and functions available in python-ev3dev can be found on the following page:

• Python language bindings for ev3dev

In Python, you can call the dir() function on a module or object to view a list of available variables and functions (although dir() alone does not distinguish between variables and functions).

 >>> dir(ev3)
 ['Button', 'ButtonBase', 'ButtonEVIO', 'ColorSensor', 'DcMotor', 'Device', 'FbMem',
 'FirgelliL12100Motor', 'FirgelliL1250Motor', 'GyroSensor', 'I2cSensor', 'INPUT_1', 'INPUT_2',
 'INPUT_3', 'INPUT_4', 'INPUT_AUTO', 'Image', 'ImageDraw', 'InfraredSensor', 'LargeMotor',
 'Led', 'Leds', 'LegoPort', 'LightSensor', 'MediumMotor', 'Motor', 'NxtMotor', 'OUTPUT_A',
 'OUTPUT_AUTO', 'OUTPUT_B', 'OUTPUT_C', 'OUTPUT_D', 'Popen', 'PowerSupply', 'RemoteControl',
 'Screen', 'Sensor', 'ServoMotor', 'Sound', 'SoundSensor', 'TouchSensor', 'UltrasonicSensor',
 '__builtins__', '__doc__', '__file__', '__loader__', '__name__', '__package__', 'abspath',
 'array', 'ctypes', 'fcntl', 'fnmatch', 'list_device_names', 'list_devices', 'list_motors',
 'mmap', 'numbers', 'os', 'pack', 're', 'stat', 'unpack']
 >>>

For example, the ev3dev.ev3 module contains something called Sound.

There also appears to be a function named Sound(), so let’s create an instance of it.

As shown below, it does not require any arguments.

 >>> s = ev3.Sound()
 >>> dir(s)
 ['__doc__', '__module__', 'beep', 'play', 'speak', 'tone']

Running dir(s) on the generated object s reveals the functions that can likely be called on that object.

Let’s try calling speak().

 >>> s.speak("Hello RT-Middleware World")

The EV3 should speak the phrase.

In practice, some functions cannot be used without knowing what arguments they require and what data types must be passed. However, this method provides a quick way to understand the general usage of a module or object.

For detailed information about function arguments and usage, please refer to the reference manual.