testing

Today, Suryansh Sharma from TU Delft presents the open-source Gimbal they devised. Enjoy!

Crazyflies (and other drones in this weight class) are extremely fun to fly and prototype with! But if you are also a scientist or tinkerer and not a well-skilled drone pilot then you might struggle with flying these platforms especially when testing new control loops or experimental code. While crashing also teaches a lot about the behavior of the system, sometimes we are interested in seeing the system dynamics without breaking the drone.

Currently, doing this for such small drones is not easy. We need something lightweight and still accessible. To solve this, we made Open Gimbal: a specially designed 3 degrees of freedom (DoF) platform that caters to the unique requirements of these tiny drones. We make two versions, (a) Tripod version which can be mounted on a camera / light tripod with a screw thread of sizes 1/4-20 UNC or 3/8-16 UNC (b) Desktop version which can be placed on a table top.

Our approach focuses on simplicity and accessibility. We developed an open-source, 3-D printable electro-mechanical design that has minimal size and low complexity. This design facilitates easy replication and customization, making it widely accessible to researchers and developers. The platform allows for unrestricted and free rotational motion, enabling comprehensive experimentation and evaluation. You can see the movement from the CAD version below:

*Degrees of Rotational freedom that Open Gimbal provides*

You can also check out the interactive CAD model and see how the gimbal moves here. All of the 3D model files as well as the BOM and instructions for assembly can be found in our repository here.

In our publication, we also address the challenges of sensing flight dynamics at a small scale. To do so, we have devised an integrated wireless batteryless sensor subsystem. Our innovative solution eliminates the need for complex wiring and instead uses wireless power transfer for sensor data reception. You can read all about how we do this in our paper here.

If you do end up using the platform for research then you can cite us using the details below:

@ARTICLE{10225720, author={Sharma, Suryansh and Dijkstra, Tristan and Prasad, Ranga Venkatesha}, journal={IEEE Sensors Letters}, title={Open Gimbal: A 3 Degrees of Freedom Open Source Sensing and Testing Platform for Nano- and Micro-UAVs}, year={2023}, volume={7}, number={9}, pages={1-4}, doi={10.1109/LSENS.2023.3307121}}

OpenGimbal at European Robotics Forum 2023 — Open Gimbal at European Robotics Forum 2023 in Denmark

Open Gimbal at European Robotics Forum 2023 in Denmark

I hope that you find the Open Gimbal useful! Feel free to reach out to me at Suryansh.Sharma@tudelft.nl if you have any ideas/questions or if you end up making an Open Gimbal yourself!

You might, or might not have heard about a tool called Wireshark, it is quite popular in the software development world.

Wireshark is a free and open-source packet analyzer. It is used for network troubleshooting, analysis, communications protocol development and education. It makes analyzing what is going on with packet based protocols easier.

Most often Wireshark is used for network based protocols like TCP and UDP, to try to figure out what is happening with your networking code. But! Wireshark also allows you to write your own packet dissector plugin, this means that you can register some code to make Wireshark handle your custom packet based protocol.

For the latest release of the Crazyflie Python Library we added support for generating a log of the Crazy Real Time Protocol (CRTP) packets the library sends and receives. This is the (packet based) protocol that we use to communicate with the Crazyflie via radio and USB.

We generate this log in the special PCAP format that Wireshark expects. And we also created an initial version of a dissector plugin, written in the programming language LUA.

When we put this two things together it turns into a pretty cool way of debugging what goes on between your computer and the Crazyflie!

What does it look like?

Wireshark gives you a graphical interface where you can view all the packets in a PCAP file. You will see the timestamps of when they arrived. Selecting a packet will give you the information that the dissector has managed to deduce as well as how the packet looked on the wire.

On top of that you get powerful filtering tools. In the below image we have set a filter to view only packets that are received or sent on the CRTP port 8, which is the port for the High level commander. This means that from a log file that contain 44393 packets we now only display 9. Which makes following what goes on with high level commands a bit easier.

*Wireshark view of filtering out packets on CRTP port 8*

The dissector knows about the different types of CRTP ports and channels and knows how to dissect an high level set-point, as seen by the image above.

What can this be used for?

This functionality is, we think, most useful for when developing new functionality in the Crazyflie firmware, or in the library. You can easily inspect what the library receives or sends and make sure it matches what your code indented.

But it can also be useful when doing client type work! We recently located the source of a bug in the Crazyflie client with the use of this Wireshark plugin.

It was when updating the Parameter tab of the client to handle persistent parameters, and to use a sidebar for extra documentation and value control. As I was testing the code I noticed that every time I changed the value of ring.effect to a valid integer and then disconnected and reconnected, the value was set to 0. Regardless of the value I had set.

I recorded a session using the PCAP log functionality:

$ CRTP_PCAP_LOG=ring.pcap cfclient

And the I fired up wireshark:

$ wireshark ring.pcap

It was now possible for me to track what the library and firmware thought was going on with the ring.effect parameter, by tracking the crtp.parameter_varid field using Wireshark. Filtering down from from 3282 packets to 12 packets.

I had earlier figured out that the varid of the ring.effect variable was 183. This is a quasi-internal representation of a parameter that we do not expose in a good way. In the future we will try to make this Wireshark tracking work with the parameter name as well.

Looking at the write parameter packet from USB #3 to the Crazyflie I could see where I set the value of the parameter to 5, so far so good.

*Wireshark view of checking my setting of the ring.effect parameter to 5*

The surprising part however was seeing a write further down setting the parameter to 0! This mean that something in the client was actually setting this to zero!

*Wireshark view of something setting the ring.effect parameter to* 0

After seeing this, locating the actual issue was trivial. I noticed that the Flight Control tab was setting the ring.effect parameter to the current index of the combo box in the UI. And when no LED-ring deck was attached, this amounted to always setting the value to zero.

But having confirmation that this was something happening on the client side, and not some kind of bug with the new persistent parameters was very helpful!

How do you use this?

We have added documentation to the repository documentation for the library on how to generate the PCAP log and how install the Wireshark plugin.

But the quick-start guide is this:

Copy the tools/crtp-dissector.lua script to the default Wireshark plugin folder
- Windows: %APPDATA%\Wireshark\plugin or WIRESHARK\plugins
- Linux: ~/.local/lib/wireshark/plugins
Restart Wireshark or hit CTRL+SHIFT+L
Set the environmental variable CRTP_PCAP_LOG to the filename of the PCAP log you want to generate
Run Wireshark with the filename as an argument

And please report any issues you find!

Happy hacking!

Tired of crashing? Use Open Gimbal!

Using Wireshark with the Crazyflie

What does it look like?

What can this be used for?

How do you use this?