Author: arnaud

The Loco Positioning System (LPS) default working mode is currently Two Way Ranging (TWR), it is a location mode that has the advantage of being pretty easy to implement and gives good positioning performance for most use cases and anchor setups. This was a very good reason for us to start with it. Though, TWR only supports positioning and flying of one or maybe a couple of Crazyflies, while it is not a solution to fly a swarm.

One solution to fly a swarm is an algorithm called Time Difference of Arrival (TDoA). We have had a prototype implementation for a while but we experienced problems with outliers, most of them where due to the fact that we where loosing a lot of packets and thus using bad data.

To solve these issues, TDoA2 makes two changes:

  • Each packet has a sequence number and each timestamps is associated with the sequence number of the packet it has been created from
  • The distances between anchors are calculated and transmitted by the anchors

A slightly simplified explanation follows to outline why this helps (a more detailed explanation of how TDoA works is available in the wiki).

We start by assuming that all timestamps are available to the tag, this is done by transmitting them in the packets from the anchors to the copter.

The end goal is to calculate the difference of time of arrival between two packets from two different anchors. Assuming we have the transmission time of the packets in the same clock, all we need to do is to subtract the time between the two transmissions with the time between the two receptions:

0 – anchor 0, 1 – anchor 1, T – Tag (that is the LPS deck on the Crazyflie)

To do so we need to have the time it took for the packet to travel between the two anchors, this will enable us to calculate the transmit time of P2 in anchor 1, this can be done by calculating the TWR time of flight between the two anchors, this would require the tag to receive 3 packets in sequence:

So now for the part where TDoA2 helps: previously we had to have the 3 packets in sequence in order to calculate a TDoA, if any one of these where missing the measurement would fail or worse, it could give the wrong result. Since we did not have sequence numbers, it was hard to detect packet loss. Now that we have sequence numbers, we can understand when a packet is missing and discard the faulty data. We also do not have to calculate the distance between anchors in the tag anymore, it is calculated by the anchors themselves. This means that we can calculate a TDoA with only two consecutive packets which increases the probability of a successful calculation substantially.

To reduce packet loss even more, we have also added functionality to automatically reduce the transmission power of the NRF radio (the one talking to the Crazyradio dongle) when the LPS deck is detected. It has turned out that the NRF radio transmissions are interfering with UWB radio reception, and since most indoor use cases does not require full output power we figured that this was a good trade-off.

The results we have seen with the new protocol is quite impressive: TDoA is usually very sensitive to the tag being inside the convex hull, so much so that with the first TDoA protocol we had to start the Crazyflie from about 30cm up to be well within the convex hull. This is not required anymore and the position is still good enough to fly even a bit outside of the convex hull. The outliers are also greatly reduced which makes this new TDoA mode behave very close to the current TWR mode, but with the capability to locate as many Crazyflies as you want:

Added to that, we have also implemented anchor position handling in the TDoA2 protocol and this means that it is now as easy to setup a system with TDoA2 as with TWR:

We are now working on finishing the last functionality, like switching between algorithms (TWR and TDoA) and on writing a “getting started guide”. When that is done TDoA will become an official mode for the LPS.

In the mean time, if you are adventurous, you can try it yourself. It has been pushed in the master branch of the Crazyflie firmware and the LPS node firmware. You should re-flash the Crazyflie firmware, both STM32 and nRF51, from master and the anchors from master too.

It has been a while since we have made a blog post about the the community and quite a lot has happened, and is about to happen, so we though we would do an update for this Monday post.

Fred, the Crazyflie android client community maintainer was visiting us last week. He is making great progress on the Java Crazyflie lib that is going to be used in the Android client as well as in PC clients. The lib is still experimental but when finished it will allow to connect and use a Crazyflie from any Java program, there has already been some successful experimentation done using it from Processing

Thanks to Sean Kelly, the Crazyflie 2.0 is now officially supported by the Betaflight flight controller firmware. Betaflight is a flight controller firmware used a lot in the FPV and drone racing community. This is the announcement by theseankelly in the forum:

Betaflight 3.2 was officially released this month. This is the first release that contains the Crazyflie 2.0 target by default, so you don’t need to clone and build from source anymore. It’s available as a target in the betaflight configurator from the google chrome store! I’ve tested it out and it works as expected. Haven’t tested the BigQuad variant, but that’s also available in the app by default.

Thanks to denis on the forum, there is also support for Crazyflie 2.0 in the PX4 flight controller firmware. PX4 is a comprehensive flight controller firmware used in research and by the industry.

The Crazyswarm project, by Wolfgang Hoenig and James A. Preiss from USC ACTlab has been presented at ICRA 2017. It is a framework that allows to fly swarms of Crazyflie 2.0 using a motion capture system.  There is currently some work done on merging the Crazyswarm project into the Crazyflie master branch, this will make it even easier to fly a swarm of Crazyflie. In the meantime the project is well documented and can be used by anyone that has a couple of Crazyflies and a motion capture system.

Las week we announced that we released the Flow Breakout board. During the week we also played a bit with the board and the outcome is a hackster project that describes how to make a Touchless Mouse using the flow breakout.

The idea is that we can detect the proximity of the hand with the ranging sensor contained in the flow breakout and detect how the hand is moving with the optical flow sensor. The flow sensor is very similar to an optical mouse sensor so we are just inverting the concept to move the environment (the hand) instead of moving the flow sensor against a table. Using an Arduino Leonardo our hack is recognized as a regular mouse by any computer.

As a result, it works quite well but it requires some training to get the mouse to go where we want. We would not use this as our regular pointing device any time soon but we think is a nice example of what can be achieved with the flow breakout board:

We are proud to announce that the Flow breakout board is available in our store!

The Flow breakout board is built around the Pixart PMW3901 optical flow tracking sensor and the ST VL53L0x ranging sensor and measures motion in 3 dimensions. We have used the same sensors in the Flow deck which is used by the Crazyflie drone for autonomous flight, but we thought that the functionality is so awesome that we developed the Flow breakout and made it compatible with Arduino to make it easy to use. With the Flow breakout it is possible create applications such as motion tracking of a robot, counting people entering a room or gesture tracking.

When you get your Flow breakout, follow the getting started guide to quickly get up and going by hooking it up to an Arduino and measure motion.

In the following weeks we are going to make a couple of Hackster project around the Flow breakout board so stay tuned for more motion tracking!

The Crazyflie, the original one, usually called Crazyflie 1 to avoid confusion, was the first commercially available open source nano quadcopter back in 2013. After getting feedback on the platform and having a lot of ideas of things that could be improved, we developed the Crazyflie 2.0 during 2014 and released the same fall. We decided we wanted to keep full backward compatibility with Crazyflie 1, both in the firmware project and in the different clients and support libraries even though we now had more processing power and RAM.

But during the last year we’ve almost exclusively been adding functionality that is Crazyflie 2.0 specific, while still trying to stay inside the constraints of the Crazyflie 1. We’ve also seen a decline in the discussions and interest of the Crazyflie 1. So this week when we once again broke the build because we run out of RAM in the Crazyflie 1, we decided to remove the Crazyflie 1 support from the Crazyflie firmware project. It’s of course with a heavy heart we do this, but we feel that in order for the Crazyflie 2.0 to move forward it’s a must. The last release that’s compatible with the Crazyflie 1 is 2017.6.

But if you’re still using the Crazyflie 1, don’t worry we’re not completely dropping it, we will continue to carry spare parts and if anyone wants to continue firmware development we will be happy to assist. If anyone is motivated, the code can be branched from the last release and we could make a new repository to host the Crazyflie 1 code.

Last week we released the Flow deck, it enables you to get really stable autonomous flight with the Crazyflie without requiring an external positioning system. We have been lucky to get access to the brand new Pixart PMW3901 optical flow sensor, the core of the Flow deck, very early and we wanted to bring this awesome functionality to everyone, including those without a Crazyflie. The solution is the Flow breakout board, it enables anyone to use this new optical flow sensor for any kind of motion tracking.

The flow breakout contains a PMW3901 optical flow sensor and a VL53L0x time of flight ranging sensor, the same sensors that are mounted on the flow deck. We have also added voltage translation logic that allows you to use the flow breakout with a system voltage of 3 to 5V that makes it possible to use it directly with any Arduino board and most embedded system. Further more we have written an Arduino driver for the PMW3901 optical flow sensor to make it easy to use the breakout deck. For the VL53L0x there are already a couple of drivers available out there.

The flow breakout is currently being manufactured and will be available in our shop in a couple of weeks. If you want to be notified of the Flow breakout board availability, please sign up in the shop or follow us on twitter.

We are pleased to announce the release of a new expansion deck for Crazyflie 2.0: the Flow deck. The flow deck is a new expansion board for Crazyflie 2.0 that contains an optical flow sensor from Pixart and a ranging sensor. These two sensors allows the Crazyflie to understand how it is moving above the floor and using this information the Crazyflie can fly itself.

The Flow deck can be used for manual flight where it allows to super easily fly the Crazyflie: if you realease all joystick the Crazyflie just stays there and hovers! The flow deck is even more interesting when used in combination with scripting: it is now possible to script the Crazyflie movement to achieve autonomous flight without needing any external positioning system.

Link to video

The Flow deck is available in the Bitcraze shop and at Seeedstudio.

It is summer again in Sweden and things are now starting to slow down and people are going to vacation. The last couple of years we have used the summer to look back and clean-up the technical dept accumulated during the year: when trying to get things done we have to prioritize which means that some things have to be left on the side (at least until we invent a way to add more hours to each day). This year is no exception, the last couple of weeks we have been working very hard to get the Flow products out and now the production is hopefully on rails so the cleanup can begin.

There is a lot of things we could do but here is a sneak peek of what we are currently looking at:

  • Crazyflie Client gamepad handling and configuration: The current input device handling is complicated and the architecture is hard to work with. There is a lot that can be done both in the front-end and the back end to make it easier to use and to work with.
  • Loco positioning system support for multiple Crazyflie, we have two mode implemented for that, TWR-TDMA and TDoA, both are very experimental and need some more work.
  • Cleanup of the webpage, information and documentation: we already have done a lot of work to make better documentation but there is always margin for improvement.
  • Cleaning up and improving the Crazyradio firmware: the Crazyradio starts to show its limits when flying swarms of Crazyflie. There is some improvement that could be done in the Firmware to make it more efficient. The first step is clean up the current implementation.

If you have any ideas of areas you feel we should focus on, even better if you want to help with some things and fix it together with us, just tell us in the comment.

On a side note, the manufacturing of the Flow products is still on progress and it should soon be on the Bitcraze shop and the Seeedstudio bazaar, stay tuned.

A couple of weeks ago Qualisys visited us at the Bitcraze office, they came with the Miqus motion capture system that they installed temporary in the office. This gave us the opportunity to play with their motion capture system and the Crazyflie 2.0 :-).

It was our first hands-on experience with a motion capture system. We were eager to try the algorithms that have been developed for the loco positioning system with the more precise position information offered by the motion capture. The result was above our expectations. You get a bit amazed when it is just sitting there in the air. The normally difficult in-air photo shoot became a breeze since you suddenly have plenty of time to focus and shoot.

After running a couple of simple stabilized flight demos, we endeavored to run the ICRA demo with motion capture instead of our loco positioning system. As the loco positioning deck isn’t needed it was removed and instead the measured position was sent using Crazyradio. Doing so made the demo work pretty much out of the box. The ICRA demo had 2 buttons, one for playing a pre-recorded trajectory and one for recording a path and playing it back as soon as the Crazyflie is dropped. Both modes worked seamlessly without requiring any code change. We tried the path recording and playback functionality and were pretty impressed by the precision:

Link to video

We look forward of meeting and working more with Qualisys. One goal is to provide better information, documentation and tools to get started with Crazyflie in a motion capture system.

A couple of weeks ago we played with recording and retracing trajectory directly from the Crazyflie using Loco Positioning System. The result was quite nice and resulted, a first for us, in a fully autonomous Crazyflie, no computer or controller required:

We decided to expand on this experiment for our demo at ICRA. We have modified the retracing code to accepts multiple modes, including running pre-programmed sequence. The plan for the demo is to have Crazyflies that can:

  • Record and retrace a manual trajectory
  • Record and replay in a loop a manual trajectory
  • Play a pre-defined trajectory in a loop
  • Land automatically when the battery level is low

With this we should be able to demonstrate quite well the capabilities of both the Crazyflie and the Loco Positioning system, and since we do not require a computer in the loop it simplifies a lot running the demo. Of course we keep the possibility to connect the Crazyflie with the Crazyflie client and with ROS while the crazyflie is flying.

Having a completly autonomous Crazyflie is also new to us and it brings its share of problems: how to we choose the working mode and how to we stop the flight if something happens (things tends to happen …).

To solve the former we have made a button deck that adds 2 push-button to the Crazyflie. One means “Start autonomous sequence”. The second means “Record trajectory”. If the recorded trajectory is a loop (if the end point is close to the start point) then the loop is played back as soon as the crazyflie is dropped, otherwise Crazyflie retraces the trajectory and stop.

We solved the later problem by making an autonomous emergency stop button that sends a radio watchdog signal. If the signal stops to be sent or if an emergency stop signal is sent (ie. by pressing the button), the Crazyflie will stop all motors and drop. The button is implemented using a Raspberry pi, a Crazyradio and an Arduino to interface the button:

If you are curious about code, we have created a github repos where we push all code we are making for this demo. As usual, this conference is an opportunity for us to hack new functionalities, though not everything can be done in the master branch. Later some things can be merged, others (like the retrace trajectory recorder/player that looks more like a user app.) will need much more though if we want to merge it in the Crazyflie firmware.