Using Matlab with the Crazyflie API

Using Matlab with the Crazyflie API is easy – you just need to install the python ‘matlab engine’ and then can access all matlab commands directly from python.

1. MATLAB 2014b or later
2. 64 bit python 2.7, 3.3 or 3.4
3. The Crazyflie API

1. Find the path to the MATLAB folder. To do this, start MATLAB and type matlabroot in the command window. Copy the path returned by matlabroot
2. To install the engine open a command window and on Windows type cd “matlabroot\extern\engines\python”. On Mac or Linux systems type cd “matlabroot/extern/engines/python”.
3. Finally type python setup.py install

Step 3 sometimes fails if you do not have write permission to the default build directory. If this happens:

3a. Create a new directory to store the build directory where you have write permission.
3b. Then set up the python engine with:

python setup.py build --build-base builddir install --user 

Here builddir is the path of the directory created in step 3b. Note the double dashes (no space) before ‘build-base’ and ‘user’.

System reqirements: http://www.mathworks.com/help/matlab/matlab_external/system-requirements-for-matlab-engine-for-python.html

Installation: http://www.mathworks.com/help/matlab/matlab_external/install-the-matlab-engine-for-python.html

Installation in non-default locations http://www.mathworks.com/help/matlab/matlab_external/install-matlab-engine-api-for-python-in-nondefault-locations.html

Once you have installed the matlab engine, you can use any matlab commands (or your own matlab scripts) from within the Crazyflie API. To do this:

1. Import the matlab engine with:

   import matlab.engine

2. Create a matlab engine object (which starts matlab – the initial startup will be slow) with (eg)

   self.eng = matlab.engine.start_matlab()

3. Optional You might want to add a directory with your matlab scripts to the matlab path:

   self.eng.addpath("directory name",nargout=0)

4. Optional MATLAB errors and console output will usually appear in the console window of IDEs such as pycharm or Eclipse. To divert them to a file you can create StringIO objects. To do this you must first import StringIO, then create the StringIO objects, eg

   self.matlabout = StringIO.StringIO()
   self.matlaberr = StringIO.StringIO()

5. You can then run a matlab script or function with:

   output = self.eng.function_name([argument list],[stdout=self.matlabout],[stderr=self.matlaberr],[nargout=n])

   Here ‘output’ is the output from the matlab function (if multiple arguments are returned by the function then output is an array - see the matlab python engine documentation for more information), [argument list] is the input arguments to the matlab function, self.matlabout and self.matlaberr are StringIO objects to capture Matlab errors and console output (optional) and n is the number of output arguments from the MATLAB script (the default is 1).

Matlab engine API documentation from Mathworks: http://www.mathworks.com/help/matlab/matlab-engine-for-python.html

Call MATLAB functions from python http://www.mathworks.com/help/matlab/matlab_external/call-matlab-functions-from-python.html

Not all data structures in matlab and python have direct equivalents. You can pass most primitive data types between them, as well as arrays. You can also usually pass matlab data types from one matlab function to another (eg you can save an output variable from a matlab function such as a file handle or plot handle in a python variable, and then pass it to another matlab function).

For speed it is best to minimize communications between python and matlab - i.e. try to do all matlab computations through a single script, instead of calling many matlab functions from inside python.

Pass data to matlab from python http://www.mathworks.com/help/matlab/matlab_external/pass-data-to-matlab-from-python.html