PyDynamixel

A Python library for controlling Dynamixel servos.

Installation

This package requires pyserial, whicah can be installed using:

pip install pyserial

or, using another method described on the documentation page:

http://pythonhosted.org//pyserial/pyserial.html#installation

This package is available on the Python Package index:

https://pypi.python.org/pypi/pydynamixel

and can be installed using

pip install pydynamixel

It can also be installed by cloning into the source and running setup.py:

git clone https://github.com/richard-clark/pydynamixel
cd pydynamixel
python setup.py install

Communication

The get_serial_for_url method can be used to get a serial object correctly configured for the specified url (on POSIX systems; Windows users can use get_serial_for_com).

By default, Dynamixels comminate at a high baud rate (1,000,000 baud), and use a single-wire protocol. This combination is highly susceptible to noise. For this reason, the write_and_get_response multiple function is recommended as the preferred way of commincating with the Dynamixel. This function will make multiple arguments to clear the serial port, send the packet, and read a valid response before failing.

Servo Initialization

The first time that a servo is instructed to move to a specified position after it has been powered up, it will do so at the maximum speed possible, regardless of whether the velocity has been set. This is dangerous, as the servos are quite powerful.

This issue can be mitigated by first reading the current position of the servo, and then commanding the servo to move to that same position. The init() method performs this function.

Basic LED Example

The AX-18A servos have integrated LEDs. By default, these LEDs are off. The following code can be used to turn on the LED on a connected servo (on POSIX-compliant platforms, such as Linux and OSX).

from pydynamixel import dynamixel, registers

# You'll need to change this to the serial port of your USB2Dynamixel
serial_port = '/dev/tty.usbserial-A921X77J'

# You'll need to change this to the ID of your servo
servo_id = 9

# Turn the LED on
led_value = dynamixel.registers.LED_STATE.ON

try:
    ser = dynamixel.get_serial_for_url(serial_port)
    dynamixel.set_led(ser, servo_id, led_value)
    print('LED set successfully!')
except Exception as e:
    print('Unable to set LED.')
    print(e)

To perform the same function on Windows, use the following:

from pydynamixel import dynamixel, registers

# You'll need to change this to the serial port of your USB2Dynamixel
com_port = 'COM5'

# You'll need to change this to the ID of your servo
servo_id = 9

# Turn the LED on
led_value = dynamixel.registers.LED_STATE.ON

try:
    ser = dynamixel.get_serial_for_url(serial_port)
    dynamixel.set_led(ser, servo_id, led_value)
    print('LED set successfully!')
except Exception as e:
    print('Unable to set LED.')
    print(e)

Motion Example

This example simply moves a specified servo to a specified position.

from pydynamixel import dynamixel

# You'll need to change this to the serial port of your USB2Dynamixel
serial_port = '/dev/tty.usbserial-A921X77J'

# You'll need to modify these for your setup
servo_id = 9
target_position = 768 # (range: 0 to 1023)

# If this is the first time the robot was powered on, 
# you'll need to read and set the current position.
# (See the documentation above.)
first_move = True

try:
    ser = dynamixel.get_serial_for_url(serial_port)

    if first_move == True:
        dynamixel.init(ser, servo_id)

    dynamixel.set_position(ser, servo_id, target_position)
    dynamixel.send_action_packet(ser)

    print('Success!')

except Exception as e:
    print('Unable to move to desired position.')
    print(e)

Chain Module

Multiple servos can be controlled more easily using the chain module.

The following example demonstrates the process of manipulating the robot using a series of frames using the same velocity for each servo. Each item in pos contains a list of positions, each one corresponding with a servo id. After instructing each joint to move to the specified position, the program waits until alls ervos have finished moving before moving to the next frame.

from pydynamixel import chain, dynamixel

servo_ids = [1, 2, 3, 4, 5, 6, 7]
velocity = 180

# Initialize the servos
chain.init(ser, servo_ids, velocity)

# A list of frames each consisting of the target 
# displacements for each joint
pos = [[822, 94, 929, 919, 104, 820, 691],
    [822, 632, 391, 919, 104, 523, 561],
    [822, 640, 383, 911, 112, 516, 650],
    [822, 100, 923, 918, 105, 538, 650],
    [818, 100, 923, 918, 105, 538, 650],
    [495, 100, 923, 918, 105, 538, 650],
    [495, 714, 309, 802, 221, 538, 650],
    [495, 723, 300, 791, 232, 538, 569],
    [495, 103, 920, 916, 107, 538, 571],
    [495, 103, 920, 916, 107, 538, 571],
    [495, 723, 300, 791, 232, 538, 569],
    [495, 714, 309, 802, 221, 538, 650],
    [495, 100, 923, 918, 105, 538, 650],
    [818, 100, 923, 918, 105, 538, 650],
    [822, 100, 923, 918, 105, 538, 650],
    [822, 640, 383, 911, 112, 516, 650],
    [822, 632, 391, 919, 104, 523, 561],
    [822, 94, 929, 919, 104, 820, 691]]

# Iterate over the vectors, move to each, 
# and wait for each move to complete
for v in pos:
    vector = chain.make_vector_constant_velocity(v, servo_ids, velocity)
    chain.move_to_vector(ser, vector)
    chain.wait_for_move(ser, joints)

Fore each vector, after instructing the joints to move to the specified position, the program waits until all servos have finished moving before moving to the next frame.

Further Documentation

For further documentation, see

http://richard-h-clark.com/tag/pydynamixel