PyPX4Flow

A simple Python package for reading from the PX4Flow optical-flow sensor

<p>

I needed an easy way to access data from the <a href="http://pixhawk.org/modules/px4flow">PX4Flow</a> sensor using Python, so I wrote this little package, which you can <a href="https://github.com/simondlevy/PyPX4Flow">download from github</a>.
It runs on Windows, Linux, and OS X in Python 2 and 3. As with the <a href="http://home.wlu.edu/~levys/software">other Python packages</a> I've written, the API is very simple. For example, this program prints the computed X and Y flow velocities from the sensor:

<b> <pre> from px4flow import PX4Flow

class ShowFlow(PX4Flow):

def update(self):

        print(self.getFlowComp())

if name == 'main':

sensor = ShowFlow('/dev/ttyACM0')

while True:

    sensor.refresh()

</pre> </b>

As this example shows, <tt><b>PX4Flow</b></tt> is an abstract class that you subclass with a class implementing the <tt><b>update</b></tt> method. To see the other methods avaiable, look at the <a href="px4flow.html">documentation</a>.

<p>

I've also included a <a href="https://github.com/simondlevy/PyPX4Flow/blob/master/display/px4flow_display.py">program</a> that will display the sensor data in a useful way (depicted above) and log the data to a .CSV file for Excel.

<p> <h3>Instructions</h3>

You will need:

<ol> <li> A <a href="https://store.3drobotics.com/products/px4flow">PX4Flow kit</a>. I recommend following the <a href="http://pixhawk.org/modules/px4flow#image_quality_and_output">Image Quality and Output</a> instructions to make sure the sensor is working properly. <li> <p> The ability to program in Python <p><li> The <a href="http://pyserial.sourceforge.net/">PySerial</a> package installed on your computer. Like many people, I found it easiest to <a href="http://pyserial.sourceforge.net/pyserial.html#from-source-tar-gz-or-checkout">install from source</a>. <p><li>Administrator (root) privileges on your computer <p><li>The PyPX4Flow <a href="https://github.com/simondlevy/PyPX4Flow">repository</a>. </ol>

Once you've downloaded the repositry, use a terminal (Linux or OS X) or command shell (Windows) to change to the directory where you put it, and issue the command

<b> <pre> python setup.py install </pre> </b>

On Linux or OS X, you may need to issue this command as root:

<b> <pre> sudo python setup.py install </pre> </b>

Then you should be able to run the <b>px4flow_display.py</b> program in the <b>display</b> folder.

<p> <h3>Known issues</h3> <ol> <li> Some users have reported a bug in which the display program crashes immediately with a stack trace ending in the following output: <pre> File "/usr/local/lib/python2.7/dist-packages/px4flow/mavlink_parser.py", line 99, in unpack return struct.unpack(fmt, self.msg[lo:hi])[0:n] error: unpack requires a string argument of length 4 </pre>

So far this bug seem to occur only with versions of Python below 2.7.5, so I've added a <a href="https://github.com/simondlevy/PyPX4Flow/tree/PrePython2-7-5">branch</a> to the repository that should work with such earlier versions. If you are running 2.7.5 or above and still experience this problem, please <a href="mailto:simon.d.levy@gmail.com">let me know</a>.

<p><li>At startup, the display program occasionally computes and displays bogus large values for the distance traveled. This may have to do with the fact that I am not using the checksum to validate each message. </ol> <h3>Copyright and licensing</h3>

Copyright and licensing information (Gnu <a href="https://www.gnu.org/licenses/lgpl.html">LGPL</a>) can be found in the header of each source file.

<h3>Acknowledgments</h3>

This work was supported in part by a Commonwealth Research Commercialization Fund grant from the Center for Innovative Technology (CRCF #MF14F-011-MS) and a Lenfest summer research grant from Washington and Lee University.