Dynamixel Motors: Matlab API

Here, we walk through set up and example scripts to communicate with connected Dynamixel motors, using Matlab.

Prerequisite: This assumes instructions in hardware setup have been completed.

Clone the **dynamixel** repository maintained on Georgia Tech GitHub Enterprise: git@github.gatech.edu:ivabots/dynamixel.git

Note: Walk-through instructions to set up and clone Git repositories can be found here .

Within the repository, a Dynamixel_IO Matlab class has been created to facilitate convenient communication with and operation of the motors: code/matlab/Dynamixel_IO/Dynamixel_IO.m

Take some time to explore the class definition and high-level methods available.

Disclaimer(s):

1. The Dynamixel_IO Matlab class only supports Dynamixel motors using 10-bit encoders (e.g. AX and RX line of motors). A variant (not yet committed to the repository) has been created to support most functions associated to motors that use 12-bit encoders (e.g. MX line of motors) and is available upon request.
2. The Dynamixel_IO class relies on pre-compiled lower-level libraries to communicate with the motors. It presumes the libraries are located in a specific relative path. If you clone the dynamixel repo and use the Dynamixel_IO class from the repository location, there should be no problem. If you move the Dynamixel_IO.m file out of the cloned repository location and use it elsewhere, however, it may not be able to find dependency libraries expected to reside within in the repository.
3. Pre-compiled libraries are available only for Linux (32- & 64-bit) and Windows (32- and 64-bit). This API doesn't currently support Mac OSX. Additionally, if run on Windows 64-bit, the user may be required to install a compiler that Matlab supports (e.g. for running MEX compilations)
4. This API will eventually be updated to use ROBOTIS' latest software interface.

Within the dynamixel repo, navigate to: code/matlab/Dynamixel_IO/test_cases.

Open and view the following test script which will command a (user-specified) motor ID to a desired position : test_case_command_single_motor.m

Some notes:

1. Find your port number: A serial-to-USB device (e.g. ROBOTIS U2D2 or a red FTDI breakout board) should be connected to the PC via USB. On Linux, you should find a device file for it within '/dev'. Use the terminal command ls /dev/ttyUSB* to view a list of device files that may correspond to this device. The number after 'ttyUSB' is your port number. Use that as the 1st argument to dxl.connect( … ); in the test script.
2. Motor ID: You will need to know the ID of the motor you'd like to command using this script. Enter that value in the line, motor_id = 17;, replacing 17 with your motor's ID.
3. Command a position: In the line, dest_pos = pi/6;, replace pi/6 with a motor position (radians) you'd like the motor to travel to. Each Dynamixel motor product line has a set of specifications that document the minimum/maximum range of motion. As an example, the AX-12 specifications can be found on its e-manual page . Every motor model has its own e-manual (Google the motor model name + “Dynamixel e-manual”).
4. Run the script: Once you're done, hit the 'Play' button and follow any instructions in the prompt. If everything works out, you should see the desired motor move to the specified position.

Exercise(s):

1. You've commanded a single motor to a single angular position. Can you now modify the Matlab script to command the motor to smoothly travel a sinusoidal trajectory (eg. with user-adjustable frequency, amplitude and phase offset) over time? Demonstrate this trajectory (with your choice of frequency, amplitude, phase, etc.) over 5 sine periods.
2. Now that you've commanded a time-varying trajectory for a single motor. Command 2 different trajectories for 2 motors! Make sure that both motors are following different sinusoidal trajectories - eg. they both can execute a sinusoidal trajectory over time, but each one should follow a sine wave defined by different parameters (eg. different frequency, amplitude and/or phase offset).

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