The PULP lab has done many amazing research on the field of Edge ML and were one of the collaborators in the development of the AI-deck of which their work on the Pulp Shield was the main inspiration. For more information check out their guest blogpost and be sure to read their latest work on the AI-deck!
More over, they have been working on an opensource tools that also work on the GAP8chip which are must try-outs for any AI-deck users
DORY: Deployment ORiented to memorY (Github, Paper)
NEMO: NEural Minimizer for pytOrch (Github, Paper)
All in all, since they clearly know what they are talking about, they are more than qualified to teach the rest of us how to work with all this! Also check out Luca Benini’s keynote at RISC-V or this week at the TinyML summit if you would like to learn more about PULP!
Date and content Workshop
The workshop will be tailored to those that have just started to to work with the AI-deck however, we think it will be interesting for regular users as well. Note that the tools mentioned above will not be handled this time.
These are the topics that will be discussed:
Hardware explanation (Gap8 specifics and AIdeck)
Software Preliminaries (GapSDK,, VM)
Hands-on examples
The workshop will take approximately 2 hours and will be on 16th of April in the afternoon, but the exact specifics will be given at a later date. So make sure to already block it in your calendar and to sign up for more information!
One crucial aspect of any research and development is to record and analyze data, which then can be used for quantifying performance, debugging strange behavior, or guide us in our decision making. We have been trying to improve the way that this is done to help all of you researcher out there with their work, so this blog post will explain an alternative, better, method to record whatever is happening on the Crazyflie in real-time.
Example data collection of received Lighthouse angles over time. The y-axis contains a unique measurement ID (16 in total for 4 sensors * 2 basestations * 2 sweeps/basestation). Thus, each dot represents the time when a certain kind of measurement was received. We do not receive all angles in fixed intervals, because of the interference between basestations.
Existing Logging Approaches
So far there have been two principle ways of recording data with the Crazyflie:
Logging: In the firmware, one can define global variables that can be streamed out at a fixed frequency, using logging configurations. Variables have a name and data type, and the list of all available variables can be queried from the firmware.
Debug Prints: It is possible to add DEBUG_PRINT(…) in any place that contains a string and possibly some variables. These are asynchronous, but not timestamped, so they are mostly useful to notify the user of some status change.
Both approaches have multiple backends. The logging variables can be streamed over USB, via the Crazyradio, or on a Micro-SD-card using a specialized deck. When using the radio or USB, the frequency is limited to 100 Hz and only a few variables can be streamed at this speed due to bandwidth limitations. The Micro-SD-card deck, on the other hand, allows to log a lot more logging variables at speeds up to 1 kHz, making it possible to access high-speed sensor information, e.g., the IMU data. The Debug Prints can be used over the radio or USB, or with a SEGGER J-Link and a Debug Adapter, over J-Link Real-Time Transfer (RTT). The former is very bandwidth limited, while the latter requires a physical wire connection, making it impossible to use while flying.
New Approach: Event-Based Logging
Event-based logging combines the advantages of the existing approaches and offers a third alternative to record data. Similar to debug prints, a user can trigger an event anywhere in the code and include some mandatory variables that describe this event (so-called payload). Similar to logging, these events are timestamped and have fewer bandwidth limitations. Currently, there is just one backend for the Micro-SD-card deck, but it would be possible to add support for radio and USB as well.
One of our first test cases of the event-based logging is to analyze the data we get from the Lighthouse deck. Here, we trigger an event whenever we received a raw sweep. This allows us to visually see the interference that happens between two LH2 basestations. We also use the event-based logging for time synchronization of the Crazyflie and a motion capture system: when we enable the IR LEDs on the active marker deck, we record an event that contains the Crazyflie timestamp. On the PC side, we record a PC timestamp when the motion capture system first detects the IR LEDs. Clock drifts can be computed by using the same logging mechanism when the IR LEDs are turned off.
Adding event-based logging had some other good side effects as well: the logging is now generally much faster, there is more user feedback about the correct buffer size usage, and the binary files are smaller. More details are in the documentation.
Future Work
We are working on adding event triggers to the state estimator. This will enable us to record all the sensor information during real flights with different positioning systems, so that we can tune and improve the Kalman state estimator. It will also be interesting to add support for events in CRTP, so that it can be used over USB and radio.
Communicating with your Crazyflie is an important pillar for its operation. As more robots are controlled, the reliability of this communication link becomes more and more important, as the probability that there is no failure at any of the Crazyflies decreases exponentially with the number of robots. We have written about the low-level radio link before. Today, we focus on past, on-going, and future improvements to make the communication more reliable.
Reliability Challenges
As part of doing research with the Crazyswarm, I noticed several issues:
Sometimes commands do not seem to arrive at the Crazyflie, especially when using unicast (i.e., direct) communication with a specific Crazyflie.
Sending ‘too much’ data while the Crazyflie is flying can cause catastrophic crashes. One example is trying to upload a trajectory, while flying in a motion capture space. However, this occasionally even happens if just trying to update a parameter.
The radio link has a feature called Safelink, which essentially guarantees that packets are send and received in order and no packet gets dropped. This feature never worked reliably in the crazyflie_cpp implementation and is therefore not used in the Crazyswarm. However, it is the default mode for cflib. The Crazy RealTime Protocol (CRTP) also has the notion of different communication ports in order to prioritize important messages such as control commands over less important ones such as trajectory upload. However, this prioritization was never implemented in any of the clients.
Native Link Implementations
Another, non reliability related, issue always was that the performance of cflib is not stellar when connected to many Crazyflies. This is mostly because (C)Python has a Global Interpreter Lock (GIL), which prevents true multi-threaded operation. A common solution to allow true parallelism is multiprocessing or implementations in a language that compiles to machine code (native code).
Crazyflie-link-Cpp
The first native implementation is written in C++ and includes Python bindings using pybind11. The overall API is simple: A connection can be created given a URI, and data can be send and received. Internally, this library implements Safelink to guarantee packets are in order, uses priority queues to prioritize messages on important CRTP ports, and handles all multi-radio and multi-thread related synchronization issues. Unlike prior implementations, the bandwidth is shared uniformly between all connections, i.e., there is no race between different Crazyflies communicating over the same radio. We measured a 50% lower CPU utilization when connecting with the CF client, a 20% higher bandwidth, and a 20% lower latency compared to the pure Python implementation.
For now, this feature is experimental and needs to be enabled using an environment variable. If you want to give it a try, you can use
pip install cflinkcpp
USE_CFLINK=cpp cfclient
once everything is merged to master in the next couple of days.
Crazyflie-link-rs
The second native implementation is written in Rust. As it turns out, writing multi-threaded code in C++ is very difficult and error-prone, because it is up to the user to use the various synchronization primitives correctly. Rust, on the other hand, provides extensive compile-time checks to increase reliability by design. This implementation does not have quite as many features as the C++ version yet and requires a bit more work to be used with the client but it is going to be worked-on on Fridays. The goal is to reach functional parity with the other link implementations and be able to use the rust-implemented link with “USE_CFLINK=rs cfclient” in the future.
Future Work
As part of the development process, we also wrote many system tests that verify correctness and measure performance of the various link implementations. We already found and fixed several firmware bugs (681 and 688). The major open issue is that there is no flow control on the system link (the connection between the NRF51 and the STM32), which still causes packet loss in some cases.
The lithium polymer battery we use, as with basically all rechargeable batteries, suffers from degradation. That means that when using it, and as time goes by, its energy capacity as well as performance will degrade. The performance, which is very related to the batteries internal resistance, will result in that the Crazyflie will not be able to produce the same maximum thrust and it will not be able to carry as much payload. The loss of capacity is due to ageing and charge cycles, results in that the flight time will decrease. A common solution to monitor the degradation is to have a BMS, or Battery Management System, that constantly monitors the battery health. For the small type of battery that is used in the Crazyflie, this is not yet viable, but maybe there is something we can do to test part of the battery health anyway?
Theory
Since the internal resistance will result in a larger voltage drop during load we can exploit this property and measure it. We will however not only measure the batteries internal resistance but the resistance of the complete power path as a result of the components we have at hand on the Crazyflie.
Power path block diagram
So what we do is to activate the switch (mosfet) so the load (motor) will pull power from the battery. The power drawn will result in a voltage drop compared to a no-load situation, which we can measure and compare to a healthy setup. Since the measurement point is at the PCB traces, any of the components before that point can be causing the voltage drop, however the battery and connector are most likely of doing so as they are most prone to ageing.
Implementation
The load is achieved by, for a very short time, activate the motors at full thrust. We don’t want the Crazyflie to fly away as that would be a bit unhandy. Before activating the motors we measure the idle voltage and during load we measure the minimum voltage so we can calculate the voltage drop. This is pretty easy to do, the problem is to find a good level where we can distinguish a good battery from bad battery. Therefore this feature is pretty experimental. We tested many batteries and good batteries tend to yield a voltage drop between 0.60V – 0.85V while bad batteries go above 1.0V. Therefore the current threshold is set to 0.95V but it would be good to have more data so if you use this feature please give us feedback if the level is wrong. The testing was run on a “stock” setup with the standard battery, propeller and motors, and it is for these the level is set. A different setup will probably not work well and needs a different threshold. Also keep in mind that the connector can also be a “bad” guy as oxide can build up and result in a higher resistance. Often this can be solved with some e.g. WD-40 solvent or un-connecting/connecting the connector several times.
Usage
This is not in the 2021.01 release so one would have to run the latest on the master branch on both the crazyflie-firmware and the Crazyfie-clients-python. The simplest way to test this feature is to launch the cflient, connect to the Crazyflie, open the console tab and press the battery test button.
cfclient console tab after running battery test
When pressing the button the propellers will shortly spin and there will be an output in the console as highlighted in red. If the sag value is below 0.95V it will yield [OK] and if it is above it will say [FAIL].
A probably more useful use case is to test this automatically before taking of with e.g. a swarm. This can be done by setting the parameter health.startBatTest to 1 and after around 0.5s readout the result in the log variable health.batteryPass to check that it is set to 1. The health.batterySag log variable will contain the latest sag (voltage drop) measurement. Hopefully this experimental feature will be a good way of increasing reliability of flights.
Most of the improvements have been done in the Crazyflie firmware and include:
The App API in the Crazyflie firmware has been extended and improved to be able to handle a wider range of applications. The goal is to enable a majority of users to implement the functionality they need in an app instead of hacking into the firmware it self.
We have improved the Lighthouse support in the firmware and both V1 and V2 are now working well. Even-though everything is not finished yet, we have taken a good step towards official Lighthouse positioning.
A collision avoidance module has kindly been contributed by the Crazyswarm team.
A persistant storage module has been added to enable data to be persisted and available after the Crazyflie is power cycled. It will initially be used to store Lighthouse system information, but will be useful for many other tasks in the future.
Basic arming functionality has been added for platforms using brushless motors.
In the client the LPS tab now has a 3D visualization of the positioning system and a new tab has been added to show the python log output.
Unfortunately we have run into some problems for the Windows client build which is not available for this release.
Finally we have fixed bugs and worked to improve the general stability.
Storage is one of these very simple functionality that actually ends up being quite hard to implement properly. It is also one of these functionality that is never acutely needed, it is possible to hack around it, so it gets pushed to be implemented later. Later is now, we have now implemented a generic persistent storage subsystem in the Crazyflie.
In Crazyflie 1.0, we originally stored settings in a setting block in flash and required the bootloader to change the settings. When designing Crazyflie 2.0 we added an I2C EEPROM to make it easier to store settings, though until now we only stored a fix config block very similar to the one stored in the Crazyflie 1 and that only contained basic radio settings and tuning. This implementation is hard to evolve since the data structure is fixed in one point of the code.
What is now implemented is a generic key-buffer database stored in the I2C EEPROM. From the API user point of view, it is now possible to store, retrieve and delete a buffer using a string as a key. This allows any subsystem, or apps, in the Crazyflie to easily store and retrieve their own config blocks. There is 7KB of space available for storage in the EEPROM.
The first user of this new storage subsystem is the Lighthouse driver. The storage is used to store lighthouse basestation geometries and calibration data, this allows to configure a Crazyflie for a system/lab and have it running out of the box even after a restart.
A future use-case would be to implement stored-parameters: we have been thinking about implementing optional persistence for the parameters for a long time. This would allow to modify and then store new default values for any parameters already present in the Crazyflie. This would allow to very easily implement things like custom controller tuning in a quad made from a bolt for example.
At low level, we where hopping to be able to find a ready-to-use library or file system to store data in our small EEPROM, but unfortunately we did not find anything that would fit our needs. We then had to implement our own storage format.
The low level structure is documented in the Crazyflie firmware repos. Basically the data are stored as a table of “length-key-value” entries with a possibility for an entry to he a “hole”. When new buffers are added they are added at the end of the table and when they are deleted they are replaced by a hole. When the end of the table is reached, the table is de-fragmented by removing the holes and moving the data as much as possible to the beginning of the memory. This structure works very well for an EEPROM and could even be adapted to work well on FLASH.
New CI
When we started activating continuous integration/automatic build to our GitHub repos we did so using Travis CI for firmware builds and AppVeyor for windows builds. However, the GitHub CI offering, GitHub actions, has become quite complete lately and now supports Linux, Windows as well as MacOS builds.
We have now transitioned to GitHub actions for all our repos and we will also implement most of the release process using GitHub actions as well. This will hopefully streamline the release process and allow us to release new version of our projects more often.
This autumn when we had our quarterly planing meeting, it was obvious that there would not be any conferences this year like other years. This meant we would not meet you, our users and hear about your interesting projects, but also that we would not be forced to create a demo. Sometimes we joke that we are working with Demo Driven Development and that is what is pushing us forward, even-though it is not completely true it is a strong driver. We decided to create a demo in our office and share it online instead, we hope you enjoy it!
The wish list for the demo was long but we decided that we wanted to use multiple positioning technologies, multiple platforms and multiple drones in a swarm. The idea was also to let the needs of the demo drive development of other technologies as well as stabilize existing functionality by “eating our own dogfood”. As a result of the work we have for instance:
improved the app layer in the Crazyflie
Lighthouse V2 support, including basic support for 2+ base stations
better support for mixed positioning systems
First of all, let’s check out the video
We are using our office for the demo and the Crazyflies are essentially flying a fixed trajectory from our meeting room, through the office and kitchen to finally land in the Arena. The Crazyflies are autonomous from the moment they take off and there is no communication with any external computer after that, all positioning is done on-board.
Implementation
The demo is mainly implemented in the Crazyflie as an app with a simple python script on an external machine to start it all. The app is identical in all the Crazyflies so the script tells them where to land and checks that all Crazyflies has found their position before they are started. Finally it tells them to take off one by one with a fixed delay in-between.
The Crazyflie app
When the Crazyflie boots up, the app is started and the first thing it does is to prepare by defining a trajectory in the High Level Commander as well as setting data for the Lighthouse base stations in the system. The app uses a couple of parameters for communication and at this point it is waiting for one of the parameters to be set by the python script.
When the parameter is set, the app uses the High Level Commander to take off and fly to the start point of the trajectory. At the starting point, it kicks off the trajectory and while the High Level Commander handles the flying, the app goes to sleep. When reaching the end of the trajectory, the app once more goes into action and directs the Crazyflie to land at a position set through parameters during the initialization phase.
We used a feature of the High Level Commander that is maybe not that well known but can be very useful to make the motion fluid. When the High Level Commander does a go_to for instance, it plans a trajectory from its current position/velocity/acceleration to the target position in one smooth motion. This can be used when transitioning from a go_to into a trajectory (or from go_to to go_to) by starting the trajectory a little bit too early and thus never stop at the end of the go_to, but “slide” directly into the trajectory. The same technique is used at the end of the trajectory to get out of the way faster to avoid being hit by the next Crazyflie in the swarm.
The trajectory
The main part of the flight is one trajectory handled by the High Level Commander. It is generated using the uav_trajectories project from whoenig. We defined a number of points we wanted the trajectory to pass through and the software generates a list of polynomials that can be used by the High Level Commander. The generated trajectory is passing through the points but as a part of the optimization process it also chooses some (unexpected) curves, but that could be fixed with some tweaking.
The trajectory is defined using absolute positions in a global coordinate system that spans the office.
Positioning
We used three different positioning systems for the demo: the Lighthouse (V2), the Loco Positioning system (TDoA3) and the Flow deck. Different areas of the flight space is covered by different system, either individually or overlapping. All decks are active all the time and pick up data when it is available, pushing it into the extended Kalman estimator.
In the meeting room, where we started, we used two Lighthouse V2 base stations which gave us a very precise position estimate (including yaw) and a good start. When the Crazyflies moved out into the office, they only relied on the Flowdeck and that worked fine even-though the errors potentially builds up over time.
When the Crazyflies turned around the corner into the hallway towards the kitchen, we saw that the errors some times were too large, either the position or yaw was off which caused the Crazyflies to hit a wall. To fix that, we added 4 LPS nodes in the hallway and this solved the problem. Note that all the 4 anchors are on the ground and that it is not enough to give the Crazyflie a good 3D position, but the distance sensor on the Flow deck provides Z-information and the overall result is good.
The corner when going from the kitchen into the Arena is pretty tight and again the build up of errors made it problematic to rely on the Flow deck only, so we added a lighthouse base station for extra help.
Finally, in the first part of the Arena, the LPS system has full 3D coverage and together with the Flow deck it is smooth sailing. About half way the Crazyflies started to pick up the Lighthouse system as well and we are now using data from all three systems at the same time.
Obviously we were using more than 2 basestations with the Lighthouse system and even though it is not officially supported, it worked with some care and manual labor. The geometry data was for instance manually tweaked to fit the global coordinate system.
The wall between the kitchen and the Arena is very thick and it is unlikely that UWB can go through it, but we still got LPS data from the Arena anchors occasionally. Our interpretation is that it must have been packets bouncing on the walls into the kitchen. The stray packets were picked up by the Crazyflies but since the Lighthouse base station provided a strong information source, the LPS packets did not cause any problems.
Firmware modifications
The firmware is essentially the stock crazyflie-firmware from Github, however we did make a few alterations though:
The maximum velocity of the PID controller was increased to make it possible to fly a bit faster and create a nicer demo.
The number of lighthouse base stations was increased
In the demo we used 5 x Crazyflie 2.1 and 1 x Bolt very similar to the Li-Ion Bolt we built recently. The difference is that this version used a 2-cell Li-Po and lower KV motors but the Li-Ion Bolt would have worked just as well.
Hyperdemo drones and they configurations
To make all positioning to work at the same time we needed to add 3 decks, Lighthouse, Flow v2 and Loco-deck. On the Crazyflie 2.1 this fits if the extra long pin-headers are used and the Lighthouse is mounted on top and the Loco-deck underneath the Crazyflie 2.1 with the Flow v2 on the bottom. The same goes for the Bolt, but here we had to solder the extra long pin-header and the long pin-header together to make them long enough.
There is one catch though… the pin resources for the decks collide. With some patching of the loco-deck this can be mitigated by moving its IRQ to IO_2 using the solder-jumper. The RST needs to be moved to IO_4 which requires a small patch wire.
Also some FW configuration is needed which is added to the hyperdemo makefile:
The final weight for the Crazyflie 2.1 is on the heavy side and we quickly discover that fully charged batteries should be used or else the crash probability is increased a lot.
Conclusions
We’re happy we were able to set this demo up and that it was fairly straight forward. The whole setup of it was done in one or two days. The App layer is quite useful and we tend to use it quite often when trying out ideas, which we interpret as a good sign :-)
We are satisfied with the results and hope it will inspire some of you out there to push the limits even further!
This week we have a guest blog post from CollMot about their work to integrate the Crazyflie with Skybrush. We are happy that they have used the app API that we wrote about a couple of weeks ago, to implement the required firmware extensions!
Bitcraze and CollMot have joined forcesto release an indoor drone show management solution using CollMot’s new Skybrush softwareand Crazyflie firmware and hardware.
CollMot is a drone show provider company from Hungary, founded by a team of researchers with a decade-long expertise in drone swarm science. CollMot offers outdoor drone shows since 2015. Our new product, Skybrush allows users to handle their own fleet-level drone missions and specifically drone shows as smoothly as possible. In joint development with the Bitcraze team we are very excited to extend Skybrush to support indoor drone shows and other fleet missions using the Crazyflie system.
The basic swarm-induced mindset with which we are targeting the integration process is scalability. This includes scalability of communication, error handling, reliability and logistics. Each of these aspects are detailed below through some examples of the challenges we needed to solve together. We hope that besides having an application-specific extension of Crazyflie for entertainment purposes, the base system has also gained many new features during this great cooperative process. But lets dig into the tech details a bit more…
UWB in large spaces with many drones
We have set up a relatively large area (10x20x6 m) with the Loco Positioning System using 8 anchors in a more or less cubic arrangement. Using TWR mode for swarms was out of question as it needs each tag (drone) to communicate with the anchors individually, which is not scalable with fleet size. Initial tests with the UWB system in TDoA2 mode were not very satisfying in terms of accuracy and reliability but as we went deeper into the details we could find out the two main reasons of inaccuracies:
Two of the anchors have been positioned on the vertical flat faces of some stairs with solid material connection between them that caused many reflections so the relative distance measurements between these two anchors was bi-stable. When we realized that, we raised them a bit and attached them to columns that had an air gap in between, which solved the reflection issue.
The outlier filter of the TDoA2 mode was not optimal, a single bad packet generated consecutive outliers that opened up the filter too fast. This issue have been solved since then in the Crazyflie firmware after our long-lasting painful investigation with changing a single number from 2 to 3. This is how a reward system works in software development :)
After all, UWB was doing its job quite nicely in both TDoA2 and TDoA3 modes with an accuracy in the 10-20 cm level stably in such a large area, so we could move on to tune the controller of the Crazyflie 2.1 a bit.
Crazyflies with Loco and LED decks
As we prepared the Crazyflie drones for shows, we had the Loco deck attached on top and the LED deck attached to the bottom of the drones, with an extra light bulb to spread light smoothly. This setup resulted in a total weight of 37g. The basic challenge with the controller was that this weight turned out to be too much for the Crazyflie 2.1 system. Hover was at around 60-70% throttle in average, furthermore, there was a substantial difference in the throttle levels needed for individual motors (some in the 70-80% range). The tiny drones did a great job in horizontal motion but as soon as they needed to go up or down with vertical speed above around 0.5 m/s, one of their ESCs saturated and thus the system became unstable and crashed. Interestingly enough, the crash always started with a wobble exactly along the X axis, leading us to think that there was an issue with the positioning system instead of the ESCs. There are two possible solutions for this major problem:
use less payload, i.e. lighter drones
use stronger motors
Partially as a consequence of these experiments the Bitcraze team is now experimenting with new stronger models that will be optimized for show use cases as well. We can’t wait to test them!
Optimal controller for high speeds and accurate trajectory following
In general we are not yet very satisfied with any of the implemented controllers using the UWB system for a show use-case. This use-case is special as trajectory following needs to be as accurate as possible both in space and time to avoid collisions and to result in nice synchronized formations, while maximal speed both horizontally and vertically have to be as high as possible to increase the wow-effect of the audience.
The PID controller has no cutoffs in its outputs and with the sometimes present large positioning errors in the UWB system controller outputs get way too large. If gains are reduced, motion will be sluggish and path is not followed accurately in time.
The Mellinger and INDI controllers work well only with positioning systems of much better accuracy.
We stuck with the PID controller so far and added velocity feed forward terms, cutoffs in the output and some nonlinearity in case of large errors and it helped a bit, but the solution is not fully satisfying. Hopefully, these modifications might be included in the main firmware soon. However, having a perfect controller with UWB is still an open question, any suggestions are welcome!
Show specific improvements in the firmware
We implemented code that uploads the show content to the drones smoothly, performs automatic preflight checking and displays status with the LED deck to have visual feedback on many drones simultaneously, starts the show on time in synchrony with all swarm members and handles the light program and show trajectory execution of the show.
These modifications are now in our own fork of the Crazyflie firmware and will be rewritten soon into a show app thanks to this new promising possibility in the code framework. As soon as Skybrush and Crazyflie systems will be stable enough to be released together, we will publish the related app code that helps automating show logistics for every user.
Summary
To sum it up, we are very enthusiastic about the Crazyflie system and the great team behind the scenes with very friendly, open and cooperative support. The current stage of Crazyflie + Skybrush integration is as follows:
New hardware iterations based on the Bolt system that support longer and more dynamic flights are coming;
a very stable, UWB-compatible controller is still an open question but current possibilities are satisfying for initial tests with light flight dynamics;
a new Crazyflie app for the drone show case is basically ready to be launched together with the release of Skybrush in the near future.
If you are interested in Skybrush or have any questions related to this integration process, drop us an email or comment below.
As mentioned in this blog post, we added the possibility to write apps for the Crazyflie firmware a while ago. Now we have added more functions in the Firmware to make it possible to use apps for an even wider range of tasks.
The overall idea of the app API is to mirror the functionality of the python lib. This will enable a user to prototype an application in python with quick iterations, when everything is working the app can easily be ported to C to run in the Crazyflie instead. The functions in the firmware are not identical to the python flavour but we have tried to keep them as close as possible to make the translation simple.
An app is also a much better way to contain custom functionality as the underlying firmware can be updated without merging any code. The intention is that the api API will be stable over time and apps that work one version of the firmware also should work with the next version.
Improvements
We used our demo from IROS and ICRA (among others) with a fairly autonomous swarm as a driver for the development. The demo used to be implemented in a branch of the firmware with various modifications of the code base to make it possible to do what we wanted. The goal of the exercise was to convert the demo into an app and add the required API to the firmware to enable the app to do its thing. The new app is available here.
The main areas where we have extended the API are:
Log and parameters framework
The log framework is the preferred way for an app to read data from the firmware and this has been working from the start. Similarly the parameter framework is the way to set parameters. Even though this has worked, it broke a basic assumption in the setup with the client, that only the client can change a parameter. Changing a parameter from an app could lead to that the client and Crazyflie had different views of the state in the Crazyflie, but this has now been fixed and the client is updated when needed.
High level commander
The high level commander was not accessible from an app earlier and the functions in the python lib have been added to make it easy to handle autonomous flight.
Custom LED sequences
It is now possible to register custom LED sequences to control the four LEDs on the Crazyflie to signal events or state.
Lighthouse functionality
Functions for setting base station geometry data as well as calibration data have been added. These functions are also very useful for those who are using the lighthouse system as it now can be done from an app instead of modifying lighthouse_position_est.c.
Remaining work
We have taken a step forward with these changes but there is more to be done! The two main areas are support for custom CRTP packets and memory mapping through the memory sub system. There might be more, let us know if there is something you are missing. The work will continue and there might even be some documentation at some point :-)
Tutorial
One reason for doing this API work now was to prepare for the tutorial about the lighthouse 2 positioning system, swarm autonomy and the demo app that we will run this Wednesday on-line, don’t miss out! You can read more about the event here.
Li-Ion batteries have packed more energy per gram for a long time compared to Li-Po batteries. The problem for UAV applications has been that Li-Ion can’t deliver enough current, something that is starting to change. Now there are cells that are supposed to be able to deliver 30-35A continuously in the 18650 series, at least according to the specs. Therefor we thought it was time to do some testing and decided to build a 1 cell Li-Ion drone using the Crazyflie Bolt as base.
Since a 18650 battery is 18mm in diameter and 65mm long, the size would affect the design but we still wanted to keep the drone small and lightweight. The battery is below 20mm wide which means we can run the deck connectors around it, that is nice. We chose to use our 3D printer to build the frame and use off the shelf ESCs, motors and props. After a couple of hours of research we selected 3″ propellers, 1202.5 11500kv motors and tiny 1-2s single ESCs for our first prototype.
Parts list:
1 x Custom designed 130mm 3D printed frame
1 x Crazyflie Bolt flight controller
4 x Eachine 3020 propeller (2xCW + 2xCCW)
4 x Flywoo ROBO RB 1202.5 11500 Kv motors
4 x Flash hobby 7A 1-2S ESC
1 x Li-Ion Sony 18650 VTC6 3000mAh 30A
Screws, anti vib. spacers, zipties, etc.
The custom designed frame was developed in iterations, and can still be much improved, but at this stage it is small, lightweight and rigid enough. We wanted the battery to be as central as possible while keeping it all compact.
Prototype frame designed in FreeCAD.
Assembly and tuning
The 3D printed frame came out quite well and weighed in at 13g. After soldering the bolt connectors to the ESCs, attaching motors and props, adjusting battery cable and soldering a XT30 to the Li-Ion battery it all weighed ~103g and then the battery is 45g of these. It feels quite heavy compared to the Crazyflie 2.1 and we had a lot of respect when we test flew it the first time. Before we took off we reduced the pitch and roll PID gains to roughly half and luckily it flew without problems and quite nicely. Well it sounds a lot but that is kind of expected. After increasing the gains a bit we felt quite pleased with:
This would be good enough for what we really wanted to try, the endurance with a Li-Ion battery. A quick measurement of the current consumption at hover, 5.8A, we estimated up to ~30 min flight time on a 3000mAh Li-Ion battery, wow, but first a real test…
Hover test
For the hover test we used lighthouse 2 which is starting to work quite well. We had to change the weight and thrust constants in estimator_kalman.c for the autonomous flight to work:
#define CRAZYFLIE_WEIGHT_grams (100.0f)
//thrust is thrust mapped for 65536 <==> 250 GRAMS!
#define CONTROL_TO_ACC (GRAVITY_MAGNITUDE*250.0f/CRAZYFLIE_WEIGHT_grams/65536.0f)
After doing that and creating a hover script that hovers at 0.5m height and was set to land when the voltage reached 3.0V. We leaned back with excitement, behind a safety net, and started the script… after 19 min it landed… good but not what we hoped for and quite far from the calculated 30 min. Maybe Li-Ion isn’t that good when it needs to provide more current…? A quick internet search and we could find that Li-Ion can run all the way down to 2.5V, but we have to stop at 3.0V because of electronics and loosing thrust, so we are missing quite a bit of energy… Further investigations are needed.
Lighthouse 2 flight test
As a final test we launched some flight scripts to fly in a square and in a spiral so we would get a feel for Lighthouse 2 + Bolt with PID controller combination. We think it turned out quite nicely, and this with almost no optimization effort:
Summary
Li-Ion felt like it could be a game changer when it comes to flight time but was not as promising as we hoped for. It doesn’t mean we can’t get there though. More research and development is required.
