Intro to Autonomous Robotics with the Crazyflie

2020-07-06 | Jessica Noe | Leave a comment

Autonomous Robotics at UW Seattle

Our team’s Crazyflie quadcopter was equipped with Bitcraze’s Optic Flow deck and Multi-ranger deck. A BH-1750 light intensity sensor was soldered to a Bitcraze Prototype deck to complete our hardware.

The Crazyflie 2.1 was the perfect robotics platform for an introduction to autonomous robotics at the University of Washington winter quarter 2020. Our Bio-inspired Robotics graduate course completed a series of Crazyflie projects throughout the 10 weeks that built our skills in:

Python
Robot Operating System (ROS)
assembling custom sensors
writing new drivers
designing and testing control algorithms
trouble shooting and independent learning

The course was offered by UW Mechanical Engineering’s Autonomous Insect Robotics Laboratory, headed by Dr. Sawyer B. Fuller. The course was supported by PhD candidate Melanie Anderson, who has done fantastic research with her Crazyflie-based Smellicopter. The final project was an opportunity to turn a Crazyflie quadcopter into a bio-inspired autonomous robot. Our three person team of UW robotics grad students included Nishant Elkunchwar, Krishna Balasubramanian, and Jessica Noe.

Light Seeking Run-and-Tumble Algorithm Inspired by Bacterial Chemotaxis

The goal for our team’s Crazyflie was to seek and identify a light source. We chose a run-and-tumble algorithm inspired by bacterial chemotaxis. For a quick explanation of bacterial chemotaxis, please see Andrea Schmidt’s explanation of chemotaxis on Dr. Mehran Kardar’s MIT teaching page. She provides a helpful animation here.

In both bacterial chemotaxis and our run-and-tumble algorithm, there is a body (the bacteria or the robot) that can:

move under its own power.
detect the magnitude of something in the environment (e.g. chemical put off by a food source or light intensity).
determine whether the magnitude is greater or less than it was a short time before.

This method works best if the environment contains a strong gradient from low concentration to high concentration that the bacteria or robot can follow towards a high concentration source.

The details of the run-and-tumble algorithm are shown in a finite state machine diagram below. The simple summary is that the Crazyflie takes off, begins moving forward, and if the light intensity is getting larger it continues to “Run” in the same direction. If the light intensity is getting smaller, it will “Tumble” to a random direction. Additional layers of decision making are included to determine if the Crazyflie must “Avoid Obstacle”, or if the source has been reached and the Crazyflie quadcopter should “Stop”.

Run & Tumble Algorithm — The run-and-tumble algorithm represented as a finite state machine.

Crazyflie Hardware

To implement the run-and-tumble algorithm autonomously on the Crazyflie, we needed a Crazyflie quadcopter and these additional sensors:

Bitcraze Optic Flow deck
Bitcraze Multi-ranger deck
Bitcraze Prototype deck with BH-1750 light intensity sensor

The Optic Flow deck was a key sensor in achieving autonomous flight. This sensor package determines the Crazyflie’s height above the surface and tracks its horizontal motion from the starting position along the x-direction and y-direction coordinates. With the Optic Flow installed, the Crazyflie is capable of autonomously maintaining a constant height above the surface. It can also move forward, back, left, and right a set distance or at a set speed. Several other pre-programmed movement behaviors can also be chosen. This Bitcraze blog post has more information on how the Flow deck works and this post by Chuan-en Lin on Nanonets.com provides more in-depth information if you would like to read more.

The Bitcraze Multi-ranger deck provided the sensor data for obstacle avoidance. The Multi-ranger detects the distance from the Crazyflie to the nearest object in five directions: forward, backward, right, left, and above. Our threshold to trigger the “Avoid Obstacle” behavior is detecting an obstacle within 0.5 meters of the Crazyflie quadcopter.

The Prototype deck was a quick, simple way to connect the BH-1750 light intensity sensor to the pins of the Crazyflie to physically integrate the sensor with the quadcopter hardware. This diagram shows how the header positions connect to the rows of pads in the center of the deck. We soldered a header into the center of the deck, then soldered connections between the pads to form continuous connections from our header pin to the correct Crazyflie header pin on the left or right edges of the Prototype deck. The Bitcraze Wiki provides a pin map for the Crazyflie quadcopter and information about the power supply pins. A nice overview of the BH-1750 sensor is found on Components101.com, this shows the pin map and the 4.7 kOhm pull-up resistor that needs to be placed on the I2C line.

It was easy to connect the decks to the Crazyflie because Bitcraze clearly marks “Front”, “Up” and “Down” to help you orient each deck relative to the Crazyflie. See the Bitcraze documentation on expansion decks for more details. Once the decks are properly attached, the Crazyflie can automatically detect that the Flow and Multi-Ranger decks are installed, and all of the built-in functions related to these decks are immediately available for use without reflashing the Crazyflie with updated firmware. (We appreciated this awesome feature!)

Crazyflie Firmware and ROS Control Software

Bitcraze provides a downloadable virtual machine (VM) to help users quickly start developing their own code for the Crazyflie. Our team used a VM that was modified by UW graduate students Melanie Anderson and Joseph Sullivan to make it easier to write ROS control code in the Python coding language to control one or more Crazyflie quadcopters. This was helpful to our team because we were all familiar with Python from previous work. The standard Bitcraze VM is available on Bitcraze’s Github page. The Modified VM constructed by Joseph and Melanie is available through Melanie’s Github page. Available on Joseph’s Github page is the “rospy_crazyflie” code that can be combined with existing installs of ROS and Bitcraze’s Python API if users do not want to use the VM options.

“crazyflie-firmware” – a set of files written in C that can be uploaded to the Crazyflie quadcopter to overwrite the default firmware
- In the Bitcraze VM, this folder is located at “/home/bitcraze/projects/crazyflie-firmware”
- In the Modified VM, this folder is located at “Home/crazyflie-firmware”
“crazyflie-lib-python” (in the Bitcraze VM) or “rospy_crazyflie” (in the Modified VM) – a set of ROS files that allows high-level control of the quadcopter’s actions
- In the Bitcraze VM, “crazyflie-lib-python” is located at “/home/bitcraze/projects/crazyflie-lib-python”
- In the Modified VM, navigate to “Home/catkin_ws/src” which contains two main sets of files:
  - “Home/catkin_ws/src/crazyflie-lib-python” – a copy of the Bitcraze “crazyflie-lib-python”
  - “Home/catkin_ws/src/rospy_crazyflie” – the modified version of “crazyflie-lib-python” that includes additional ROS and Python functionality, and example scripts created by Joseph and Melanie

In the Modified VM, we edited the “crazyflie-firmware” files to include code for our light intensity sensor, and we edited “rospy-crazyflie” to add functions to the ROS software that runs on the Crazyflie. Having the VM environment saved our team a huge amount of time and frustration – we did not have to download a basic virtual machine, then update software versions, find libraries, and track down fixes for incompatible software. We could just start writing new code for the Crazyflie.

The Modified VM for the Crazyflie takes advantage of the Robot Operating System (ROS) architecture. The example script provided within the Modified VM helped us quickly become familiar with basic ROS concepts like nodes, topics, message types, publishing, and subscribing. We were able to understand and write our own nodes that published information to different topics and write nodes that subscribed to the topics to receive and use the information to control the Crazyflie.

For more information, see the Bitcraze Development overview.

Updating the crazyflie-firmware

A major challenge of our project was writing a new driver that could be added to the Crazyflie firmware to tell the Crazyflie system that we had connected an additional sensor to the Crazyflie’s I2C bus. Our team referenced open-source Arduino drivers to understand how the BH-1750 connects to an Arduino I2C bus. We also looked at the open-source drivers written by Bitcraze for the Multi-ranger deck to see how it connects to the Crazyflie I2C bus. By looking at all of these open-source examples and studying how to use I2C communication protocols, our team member Nishant Elkunchwar was able to write a driver that allowed the Crazyflie to recognize the BH-1750 signal and convert it to a sensor value to be used within the Crazyflie’s ROS-based operating system. That driver is available on Nishant’s Github. The driver needed to be placed into the appropriate folder: “…\crazyflie-firmware\src\deck\drivers\src”.

The second change to the crazyflie-firmware is to add a “config.mk” file in the folder “…\crazyflie-firmware\tools\make”. Information about the “config.mk” file is available in the Bitcraze documentation on configuring the build.

The final change to the crazyflie-firmware is to update the make file “MakeFile” in the location “…\crazyflie-firmware”. The “MakeFile” changes include adding one line to the section “# Deck API” and two lines to the section “# Decks”. Information about compiling the MakeFile is available in the Bitcraze documentation about flashing the quadcopter.

Making additions to the ROS control architecture

The ROS control architecture includes messages. We needed to define 3 new types of messages for our new ROS control files. In the folder “…\catkin_ws\src\rospy_crazyflie\msg\msg” we added one file for each new message type. We also updated “CMakeLists.txt” to add the name of our message files in the section “add_message_files( )”.

The second part of our ROS control was a set of scripts written in Python. These included our run-and-tumble algorithm control code, publisher scripts, and a plotter script. These are all available in the project’s Github.

Characterizing the Light Sensor

At this point, the light intensity sensor was successfully integrated into the Crazyflie quadcopter. The new code was written and the Crazyflie quadcopter was reflashed with new firmware. We had completed our initial trouble shooting and the next step was to characterize the light intensity in our experimental setup.

Experimental setup for light intensity characterization.

This characterization was done by flying the Crazyflie at a fixed distance above the floor in tightly spaced rows along the x and y horizontal directions. The resulting plot (below) shows that the light intensity increases exponentially as the Crazyflie moves towards the light source.

The light characterization allowed us to determine an intensity threshold that will only happen near the light source. If this threshold is met, the algorithm’s “Stop” action is triggered, and the Crazyflie lands.

Light intensity (units of lux) was experimentally characterized by piloting the Crazyflie in a linear pattern at a constant height above the ground. The resulting plot shows that light intensity is characterized by an exponential roll off in both the x and y directions.

Testing the Run-and-Tumble Algorithm

With the light intensity characterization complete, we were able to test and revise our run-and-tumble algorithm. At each loop of the algorithm, one of the four actions is chosen: “Run”, “Tumble”, “Avoid Obstacle”, or “Stop”. The plot below shows a typical path with the action that was taken at each loop iteration.

Flight Tests of the Run-and-Tumble Algorithm

In final testing, we performed 4 trial runs with 100% success locating the light source. Our test area was approximately 100 square feet, included 1 light source, and 2 obstacles. The average search time was 1:41 seconds.

The “Avoid Obstacle” and “Run” behavior are demonstrated in the above video clip (1.5x actual speed).

The “Run” and “Tumble” actions are demonstrated in the above video clip (2x actual speed). At the end, the “Stop” action is demonstrated when the light intensity reaches the threshold value of 800 lux, indicating that the Crazyflie has found the light source and should land.

Lessons Learned

This was one of the best courses I’ve taken at the University of Washington. It was one of the first classes where a robot could be incorporated, and playing with the Crazyflie was pure fun. Another positive aspect was that the course had the feel of a boot camp for learning how to build, control, test, and improve autonomous robots. This was only possible because Bitcraze’s small, indoor quadcopter with optic flow capability made it possible to safely operate several quadcopters simultaneously in our small classroom as we learned.

This development project was really interesting (aka difficult…) and we went down a few rabbit holes as we tried to level up our knowledge and skills. Our prior experience with Python helped us read the custom example scripts provided in our course for the ROS control program, but we had quite a bit to learn about the ROS architecture before we could write our own control scripts.

Nishant made an extensive study of I2C protocols as he wrote the new driver for the BH-1750 sensor. One of the biggest lessons I learned in this project was that writing drivers to integrate a sensor to a microcontroller is hard. By contrast, using the Bitcraze decks was so easy it almost felt like cheating. (In the nicest way!)

On the hardware side, the one big problem we encountered during development was accidentally breaking the 0.5 mm headers on the Crazyflie quadcopter and the decks. The male headers were not long enough to extend from the Flow deck all the way up through the Prototype deck at the top, so we tried to solder extensions onto the pins. Unfortunately, I did not check the Bitcraze pin width and I just soldered on the pins we all had in our tool kits: the 0.1 inch (2.54 mm) wide pins that we use with our Arduinos and BeagleBones. These too-large-pins damaged the female headers on the decks, and we lost connectivity on those pins. Fortunately, we were able to repair our decks by soldering on replacement female headers from the Bitcraze store. I wish now that the long pin headers were available back then.

In summary, this course was an inspiring experience and helped our team learn a lot in a very short time. After ten weeks working with the Crazyflie, I can strongly recommend the Crazyflie for robotics classes and boot camps.

Links to Project Files

Team’s Research Poster: https://github.com/thecountoftuscany/crazyflie-run-and-tumble/blob/master/documents/Project-Final_Poster.pdf

Github Link courtesy of Nishant Elkunchwar: Crazyflie-Run-and-Tumble

YouTube Links courtesy of Nishant Elkunchwar: Crazyflie locates Light, Simulation of Run and Tumble Algorithm in PyGame

Latest Update on the AI deck

2020-05-25 | Kimberly McGuire | 2 Comments

It has been a while since we have updated you all on the AI deck. The last full blogpost was in October, with some small updates here and there. It is not that we have not focused on it at all; on the contrary… this has been a high priority project for a while now. It is just quite a complex board with a lot of bells and whistles, which can be challenging to work with sometimes so early in development, something that our previous intern can definitely agree on. So therefore we rather wanted to wait until we were able to make sufficient progress before we gave you an update… and so we have!

Together with Greenwaves technologies we have been trying to get the SDK of the GAP8 chip on the AI deck stable enough for an early release. The latest release of the SDK (version 3.4) has proved itself to work with relative ease on the AI deck after extensive testing. Currently it is possible to use OpenOCD for flashing and debugging, and it supports most commonly available debuggers with a jtag connector. In the upcoming weeks both of Bitcraze and Greenwaves will test and try out all examples of the SDK on the AI deck to make sure that everything is still compatible. Also the documentation will be extended as well. As there is so much to document, it might be difficult to catch all of it. However, if you notify us and Greenwaves on anything that is missing once the AIdeck is out, that will help us out to catch the knowledge gaps.

The AI deck also contains the ESP-based NINA module for establishing a WiFi connection. This enables the users to stream the video stream of the AI deck onto their computers, which will be quite an essential tool if they would like to generate their own image database for training the CNNs for the GAP8 (and it happens to also be quite practical for debugging by the way!). Currently it is required to set credentials of your local WiFi network and reflash the AI-deck to be able to connect and streaming the images, but we are working on turning the Nina into an access-point instead so no reflashing would be required. We hope that we will be able to implement this before the release.

We are also trying out to adjust applications to make suitable of the AI deck. For instance, we have adapted Greenwaves’ face-detector example to use the image streamer instead of the display available on the GAPuino boards. You can see a video of the result here underneath. Beware that this face-detector is not based on a CNN but on HOG descriptors, so it only works in good conditions where the face is well lit. However, it is possible to train a CNN to detect faces in Tensorflow and flash this on the AI deck with the GAPflow framework as developed by Greenwaves. At Bitcraze we haven’t managed to try that out ourselves ( we are close to that though!) but at least this example is a nice demonstration of the AI deck’s abilities together with the WiFi-streamer. This example and more testing code can be found in our experimental repo here. For examples of GAPflow, please check out the examples/NNtool section of the GAP8 SDK.

For some reason WordPress has difficulty embedding the video that was supposed to be here, so please check https://youtu.be/0sHh2V6Cq-Q

Seeing how the development has been progressing, we will be comfortable to say that the AI deck could be ready for early release somewhere in the next month, so please keep an eye out on our website! We will continue to test the GAP SDK’s stability and we are very thankful for Greenwaves Technologies with their help so far. We will also work on getting-started guides in order to get acquainted with the AI deck, supplementing the already existing documentation about the GAP8 chip.

Even-though the AI deck will soon be ready for early release, this piece of hardware is not for the faint-hearted and embedded programming experience is a must. But keep in mind that the possibilities with the AI deck are huge, as it will be mean that super-edge-computing on a 30 gram flying platform will be available for anyone. It will all be worth it when you have your Crazyflie flying autonomously while being able to recognize its surroundings :)

Crazyflies in the Robotarium

2020-05-18 | Christopher Banks | Leave a comment

We have a guest blog post this week from Christopher Banks at Georgia Tech, where he tells us about their work with the Robotarium. Enjoy!

Multi-Agent Aerial Robotics

In the GRITS Lab we focus on autonomous control and coordination of multi-robot systems with applications in – but not limited to – optimal control, constraint-based control, and hardware development. We are home to the Robotarium [1], a remotely accessible swarm robotics testbed that is free for anyone around the world to use for academic and educational purposes. We have integrated Crazyflies into the Robotarium as the main vehicle for aerial robot swarms due to their small size, quiet operation, and high maneuverability . Also, due to their low inertia, they pose minimal harm to their surroundings if system failures occur. Their small size and robust nature are well suited for flying in an indoor testbed like the Robotarium. As we work towards extending the operation of the Crazyflies in the Robotarium to external users, we encountered some important research questions: How do we guarantee the quadcopters remain “safe” (undamaged) while minimizing modifications to user inputs? How do we develop an easy to use interface for external users, with experience ranging from novice to expert? What commands can be used by external users to control a swarm of robots? This post will briefly describe the ongoing research aimed at solving these questions.

Safety Guarantees

To ensure hardware safety while flying experimental algorithms we have developed Control Barrier Functions (CBFs) for quadcopters, allowing users to give nominal control inputs while obeying some safety constraints for the system (e.g. collision-free trajectory following). In the video below, we give four Crazyflies the commands to fly in a circle. A fifth Crazyflie is then told to fly to waypoints that will intersect the circle and attempt to collide with the circling quadcopters. Using CBFs a central controller can modify the inputs given to Crazyflies near collision to ensure safe velocity commands that are close as possible to the user intended control [2] . These CBFs can also be designed to ensure safety by bounding the quadcopters to a designated region of the testbed, giving additional safety constraints by protecting areas outside of the motion capture system during flights.

Quadcopters execute pre-planned flight trajectories designed to collide and use CBFs to avoid collisions.

User Interfaces

We have also used the Crazyflies to understand how remote users can best interact with the Robotarium both at the interface level and in planning. One project involved studying the effectiveness of graphical user interfaces (GUIs) on swarm robotic control. Two GUIs were developed with different interaction modalities. The GUIs were designed to map user inputs to a set of hoops placed in the Robotarium. One GUI (shown in Fig. 1) provided users the ability to draw paths through a touchscreen interface on a two-dimensional map and then map those inputs to trajectories for a team of robots. The other GUI (illustrated in Fig. 2) allowed users to input a sequence of desired hoops for a team of robots and execute trajectories based on the input.

Figure 1: A GUI that maps hand-drawn paths to inputs for a group of Crazyflies

Figure 2: A GUI that maps the string of indexed hoops as inputs for a group of Crazyflies.

Multi-Agent Planning

In planning, we looked at how multi-agent planning can be approached using high-level specifications. These high-level specifications allow users to develop plans requiring groups of robots to visit regions of interest (see Fig. 3) and trajectories are generated automatically. To represent these specifications, we use a logic formalism known as temporal logic to encode a preferred sequence of plan execution. As an additional step, users could include constraints on the trajectory by minimizing a cost using stochastic sampling. For more details, see the attached video demonstrating task allocation in a fire-fighting scenario.

A optimizing task allocation framework that assigns quadcopters a set of tasks based on user specifications.

Future Directions

As we continue to expand the capabilities of the Robotarium we are looking into how to develop long term autonomy for the Crazyflies. This includes autonomous charging as well as remote access for the lab and other users. We hope to use the Lighthouse system as a method for long term tracking since the Crazyflie will know its position instead of relying on passive tracking from a Vicon system. Our plans also include a lab-based simulator for in house projects related to the Crazyflies as well as updating our system to incorporate Crazyswarm to make control of the Crazyflies easier in implementation. In addition to this, in order to accommodate unknown users, we will have to figure out a control scheme that encourages use from a wide variety of users ranging from novices in quadcopter control to experts. We’ll keep Bitcraze updated on the Robotarium’s progression towards fully autonomous aerial swarms!

Learning-based Bias Correction for Accurate Ultra-wideband Localization of a Crazyflie

2020-04-27 | Wenda Zhao | Leave a comment

Accurate indoor localization is a crucial enabling technology for many robotic applications, from warehouse management to monitoring tasks. Ultra-wideband (UWB) localization technology, in particular, has been shown to provide robust, high-resolution, and obstacle-penetrating ranging measurements. Nonetheless, UWB measurements are still corrupted by non-line-of-sight (NLOS) communication and spatially-varying biases due to doughnut-shaped antenna radiation pattern. In our recent work, we present a lightweight, two-step measurement correction method to improve the performance of both TWR and TDoA-based UWB localization. We integrate our method into the Extended Kalman Filter (EKF) onboard a Crazyflie and demonstrate a closed-loop position estimation performance with ~20cm root-mean-square (RMS) error.

A stylized depiction of our UWB indoor localization system and the schematics of the proposed estimation framework.

Methodology

UWB measurement errors can be separated into two groups: (1) systematic bias caused by limitations in the UWB antenna pattern and (2) spurious measurements due to NLOS and multi-path propagation. We propose a two-step UWB bias correction approach exploiting machine learning (to address(1)) and statistical testing (to address (2)). The data-driven nature of our approach makes it agnostic to the origin of the measurement errors it corrects.

(1) Neural Network Bias Correction

The doughnut-shaped antenna radiation pattern causes the relative poses of anchors and tags to have a noticeable impact on the received signal power, which leads to systematic, predictable biases. To empirically demonstrate the systematic measurement errors resulting from varying the relative pose between anchors and tags, we placed two DWM1000 UWB anchors at a distance of 4m and collected both TWR and TDoA UWB range measurements for the UWB tag mounted on top of a Crazyflie spinning around its own z-axis.

Left: schematics of the ranges (∆p’s), azimuth (α’s) and elevation angles (β’s) defining the relative poses of tag T and anchors A₀, A₁ when collecting the systematic bias measurements. Right: the neural network’s inferred bias (in red) with respect to the tag’s varying azimuth angle towards anchor T₀, α_T0, plotted against the UWB raw measurements.

We choose to leverage the nonlinear representation power of neural networks to learn the systematic bias which only depends on anchor-tag relative poses. Considering the limited onboard computation power, we select a fully connected neural network with 50 neurons in each of two layers with ReLU activation. To represent the relative pose between the UWB tag and anchors, we select the relative distance ∆p and roll, pitch, and yaw angles of the quadcopter as the input features x for the network. As we used fixed anchors, we do not include their poses as inputs (this level of generalization is left for future work). Given sufficient training data, the spatially-varying measurement bias can be described by a nonlinear function b=f(x) captured by the trained neural network.

(2) Outlier (Spurious Measurements) Rejection

Besides our learning-based bias correction, we use a quadcopter’s dynamic model to filter inconsistent UWB range measurements. Given the estimated velocity v and maximum acceleration a_max, we can compute the maximum distance d_max a quadcopter can cover during time ∆t. Based on this information, we can reject unattainable measurements before fusing them into the EKF by comparing the measurement innovation with d_max.

Moreover, we use a statistical hypothesis test to further classify potential outlier measurements. Since the measurement innovation vector is assumed to be distributed according to a multivariate Gaussian distribution, the normalized sum of squares of its values should follow a Chi-square distribution. We use the Chi-square hypothesis test to determine whether a measurement innovation is likely coming from this distribution.

UWB measurement bias f (x) prediction performance of the trained neural network (in red) compared to the actual measurement errors (blue dots) as well as the role of model-based filtering (purple dots) and statistical validation (orange dots) in rejecting outlier measurement innovations (teal dots) during a 60” flight experiment.

Data Collection and Training

We use a Crazyflie 2.0 quadcopter and the Loco Positioning System (LPS)’s UWB DW1000 modules as our research platforms. Our calibration approach runs on the Crazyflie STM32 microcontroller within the FreeRTOS real-time operating system. We equipped a cuboid flying arena with 8 UWB anchors, one for each vertex. The anchor positions were measured using a Leica total station theodolite.

Left: three-dimensional plot of our flight arena showing the positions and poses of the eight UWB DW1000 anchors (each facing towards its own x-axis, i.e., the red versor). Right: two of the training trajectories we flew to collect the samples that we used to train our neural network-based bias estimator

For all experiments, the ground truth position of the Crazyflie was provided by 10 Vicon cameras. The neural network was trained using PyTorch. To perform inference on the Crazyflie’s microcontroller, we re-use PyTorch’s trained weights in a plain C re-implementation. Since the DW1000 modules in the LPS provide UWB measurements every 5ms, the neural network inference runs at 200Hz during flight as well. Our outlier rejection method is also implemented in plain C and merged with the onboard EKF.

Close-loop Position Estimation Performance

We demonstrate the position estimation and close-loop performance of the proposed methods by flying a Crazyflie quadcopter along planar and non-planar circular trajectories (which were not among the trajectories used for training). A comparison between the estimation error of (A) the UWB localization estimate enhanced with outlier rejections and (B) the estimated enhanced with both outlier rejection and neural network bias compensation is conducted in our experiments for both TWR and TDoA2 modes. We repeated all of our experiments 10 times with a target velocity of 0.375m/s. The quadcopter trajectories during these flight tests are displayed in the following plots.

Flight paths and the tracking performance of our approach with (in blue) and without (in orange) the neural network bias correction for two reference trajectories (planar and non-planar circular orbits) and both UWB modes (TWR and TDoA).

The distributions of the RMS estimation errors are summarized into a box plot. TWR-based ranging results in better localization performance than TDoA. However, we observe that, with our neural network bias compensation, the average RMS error of TDoA localization is around 0.21m, which is comparable to that of TWR-based localization (~0.19m). Thanks to the neural network bias compensation, the average reduction in the RMS error is ~18.5% and 48% for TWR and TDoA, respectively. Most notably, this result suggests that bias compensation might help closing the performance gap between TWR- and TDoA-based localization.

Root mean square error (RMSE) of the quadcopter position estimate before (in orange) and after (in blue) the neural network calibration step for both TWR and TDoA ranging modes. Each pair of box plots refers to a planar reference trajectory (left of each pair) and a reference trajectory with varying z (right of each pair), showing a greater performance enhancement for the latter.

Outlook

In this work, we presented a two-step methodology to improve UWB localization—for both TWR- and TDoA-based measurements. We used a lightweight neural network to model and compensate for pose-dependent and spatially-varying biases and an outlier rejection mechanism to filter spurious measurements. Through several real world flight experiments tracking different trajectories, we showed that we are able to improve localization accuracy for both TWR and TDoA, granting safer indoor flight. In our future work, we will include the anchors’ pose information to allow our method to further generalize to previously unobserved indoor environments, with different anchor configurations.

Links

The authors are with the Dynamic Systems Lab, Institute for Aerospace Studies, University of Toronto, Canada, and affiliated with the Vector Institute for Artificial Intelligence in Toronto.

Feel free to contact us if you have any questions or ideas: wenda.zhao@robotics.utias.utoronto.ca. Please cite this as:

<code>@article{wenda2020learning,
  title={Learning-based Bias Correction for Ultra-wideband Localization of Resource-constrained Mobile Robots},
  author={Wenda Zhao and Abhishek Goudar and Jacopo Panerati and Angela P. Schoellig},
  journal={arXiv preprint arXiv:2003.09371},
  year={2020}
}</code>

Lighthouse V2 status update

2020-04-20 | Kristoffer Richardsson | 3 Comments

The Lighthouse V2 implementation has been simmering away for a long time in the Bitcraze kitchen and in this blog post we will give you an update on the current status and what is remaining for a full release of this tasty dish.

Crazyflie 2.1 and Lighthouse V2 base station

We believe we have solved most of the major technical hurdles (last famous words) on the way to a working implementation that uses Lighthouse V2 base stations for positioning, now it is mostly work to implement the functionality that is remaining. As described in this post we now have a new FPGA binary that has the ability to decode both V1 and V2 base stations, and this was a major step forward. This new binary is used in the Crazyflie firmware master branch, and if the Lighthouse deck is used with the latest Crazyflie firmware, the new FPGA binary will automatically be flashed to the deck.

What has changed?

The new FPGA binary uses a different UART protocol to communicate with the Crazyflie. This protocol has been implemented in the firmware and hopefully there is no functional difference compared to the previous FPGA binary when using Lighthouse V1 base stations.

We have added a first version of Lighthouse V2 base station decoding, but it is still a bit limited. As a start we decided to “emulate” V1 base stations to be able to reuse as much of the existing code as possible. For now we support only 2 base stations and they must use channel 1 and 2 (used to be called modes). The V2 angles are transformed into V1 angles and fed into the old positioning logic and are handled exactly the same way as before. Even though this works, it is not the optimal solution and we hope to be able to refine it later on.

We have also written a python script to estimate base station geometry (positions and orientation) using the Lighthouse deck. This removed the requirement to use software from Steam which should simplify the set up process. Please see the (still limited) documentation. Note that this calibration method only supports the basestation V1… for now!

There is a lot of code that has been modified and the FPGA implementation is completely new, it is not unlikely that there is functionality that is unstable or broken, or configurations that are not supported. If you happen to notice any bugs, please let us know!

What is remaining?

The functional areas that needs to be implemented or cleaned up before we leave the Early Access stage is the following:

Calibration data

The calibration data is embedded in the modulated light from the base stations and describes imperfections from the manufacturing process for each individual. This data is not read yet for V2 and will increase the precision when available.

Support for more than 2 base stations

Lighthouse V2 base stations are designed for systems with more than 2 base stations. The Crazyflie firmware needs to be extended for this functionality to work, including handling of geometry data, logging, memory management and some other bits and pieces.

Native V2 positioning

The angles from the V2 base station should be fed directly into the kalman filter for positioning, instead of first being transformed into V1 angles. This will increase robustness and reduce data loss.

Client support

We want to add a tab in the python client where a Lighthouse system can be monitored, configured and managed. It should, for instance enable the user to configure and visualize base station geometry.

FPGA binaray management

Currently the FPGA binary is included in the Crazyflie firmware and it is automatically uploaded to the deck when booted. This is not a viable long term solution and we hope to be able to find a more generic way of handling deck binaries.

Conclusions

As can be seen, there is still quite some work to be done before the Lighthouse V2 stew is ready to be served, but we are definitely starting to smell some nice flavours from the kitchen!

Finally a view from Kristoffer’s home lab, currently in the summer house. Three base stations are set up as a Fun Friday hack to see what it would take to use more than 2. Luckily it did not take too much time to get this to work :-)

3 Lighthouse V2 base stations

The Home Lab

2020-04-06 | Kimberly McGuire | Leave a comment

Many people in the world have now settled in the reality of working from home. We have also taken precautions ourselves by not go to our office as normal and only ship out packages a few times per week instead of every day (see this blogpost). This also means that we do not have full access to our lab with all our equipment and positioning systems in our big 10 x 10 meter flight lab at the office. In this blogpost we will show how we manage to keep on developing and flying, even in the current situation.

Crazyflie flying in a kitchen with the lighthouse deck

In(light)house positioning

Currently we started to use the Lighthouse positioning system to setup up the remote home lab at our houses. As of recent additions to the Crazyflie firmware, it has been made easy to get the geometry data from the base station. Now the only items we need for indoor flight are just two (or only one) lighthouse basestations V1’s and a Crazyflie, and that is it! There is no need for an HTC Vive headset or hub, or third-party software like SteamVR and the setup is finished in 2 minutes! Check out the new documentation here if you want to know more about the new setup of the lighthouse positioning system.

Also, we recently got a very primarily version of the lighthouse V2 working (see here) and we of course want to keep the momentum going! We will be working on full compatibility from our homes so stay tuned. For now, see this video of the Crazyflie flying with just a single base-station, taken from one of our team-member’s home lab.

Remote Lecture Hall and Practicals

We were invited by Dario Floreano and Fabrizio Schiano from the EPFL-LIS laboratory to do a lecture for the ‘Aerial Robotics’ Course as part of EPFL’s Master’s program in Robotics. Due to the virus, we had to cancel our trip to go there physically… but luckily we were able to do the lecture remotely anyway!

The lecture consists of two parts. In the first hour we mostly explained about the Crazyflie ecosystem, hardware and sensors. In the second hour we focused on how the stabilization module worked, including the controllers and the state estimation. During both sessions, we alternated between the theory slides with actual hands-on demos. The lighthouse positioning system was setup in a kitchens, so that we were able to show full flights and practicals with the Crazyflie. At the end there was also the push-demo with just the flowdeck and multiranger, which didn’t use any external positioning at all.

The lectures can be found below and the documentation has been updated as well with the covered material (see here). Be sure to check out the controller tuning presented in part 2 of the lecture (25:00 – Cascaded PID controller).

Other Home labs

We know that there are currently users that are moving their flight lab from their university or company to their homes to be able to continue their work. We would love to hear about your experience and your home lab! Send us an email with your story to contact@bitcraze.io, drop us a message on forum.bitcraze.io, or mention us in your Twitter, Linkedin, Facebook or Reddit post. Also, if you want to setup your own home lab and you need any advice or help, please let us know!

Crazyflie flying with Lighthouse V2!

2020-02-24 | Arnaud | 5 Comments

We are happy to announce that we have gotten Crazyflie 2 to fly autonomously using the Lighthouse deck and Lighthouse V2 base-stations. This was a very requested features, and while this is not stable and ready to use yet, it is a great milestone toward Lighthouse V2 support.

There exists two incompatible versions of the Lighthouse positioning system. Version 1 was released with the original HTC Vive VR system. In this system base-station are using two rotating laser beam that sweeps the room, one horizontal and one vertical, and an omnidirectional synchronization flash to allow IR light receiver to be located in the room. One limitation of this version is that up to two base-station can be used and no more, this is mainly due to the fact that beam identification is done using a TDMA scheme: base stations switch-on their laser in a dedicated time-slot one after each-other and adding more time slots for more base-stations will greatly reduce the update rate of the system.

Lighthouse V2, was released with the HTC Vive PRO headset and is also used by the Valve Index. The big change is that laser sweeps now carries modulated data and that there is only one rotor with two angled slit instead of the two rotors for V1. The V2 sweep data is described as ‘Sync on beam’ and contains timing information of how long it has been since the synchronization event (ie. when the rotor crossed 0 degree). The sweep data also allows to identify the base-station that has transmitted the sweep. This removes the need for an omni-directional synchronization pulse and allows more than two base-station to operate at the same time in the same space, since their sweeps can now be identified and timed.

The lighthouse V2 system is very elegant and scalable. However, actually decoding the signal from the sweeps has taken a lot of time since it is not documented and we needed to find-out what the encoding actually was. There has been effort on the internet to understand how the system worked, the most useful one is this github ticket that goes from raw data acquisition to fully unlocking the beam encoding.

I have been working on-and-off for a long time on making an FPGA design for the lighthouse deck to acquire and decode Lighthouse V2. The main blocking point until now was that I had not been able to reliably acquire useful signal from the system in order to allow real-time decoding on the Crazyflie. Added to that, there was some inconsistency between what we though the system was doing and what we could gather from the base-stations debug console. Recently though, the last piece of the puzzle, was to discover that the beam encoding was not Manchester, as we though, but Bi-phase mark code FM1 (BMC). Once this decoding was used everything made sense and worked.

Added to that, I started using SpinalHDL instead of raw Verilog to write the FPGA design which allows for much quicker iteration, much less frustration, and it also allowed me to easily make the design multi-clock which is required to decode the BMC signal: the beam decoder runs at 48MHz, and the rest of the system works at 24MHz. This design is required since the FPGA we use in the lighthouse deck is not fast enough to run everything at 48MHz.

The result, is a new FPGA firmware for the lighthouse deck that receives, identify and decode Lighthouse V2 sweep signal and send them over to the Crazyflie. The Crazyflie still has a little pulse packing to do (putting together pulses from a single sweep received on multiple sensors) and then can use pulse timing information to calculate azimuth and elevation at which the base-station sees the Crazyflie. This information is the same as the one we get from Lighthouse V1 and so the same algorithm can be used to calculate the Crazyflie position.

I hacked a proof of concept was this last fun Friday and it flies!

If anyone is curious the code for this demo has been uploaded as an out-of-tree driver and the code for the FPGA parts is already in the lighthouse-fpga project. The current Crazyflie code is too incomplete to be usable, but it is a nice starting point if anyone wants to play with Lighthouse V2 and the Crazyflie right away ;-).

As a side note, the Bitcraze team will shrink temporary as I, Arnaud, will go in parental leave until mid-August. I look forward to this new adventure and I trust the lighthouse V2 development and the forum will be in good hands in my absence.

Happy Holidays!

2019-12-23 | Kimberly McGuire | 1 Comment

Happy holidays to all our users, community members and friends! We are happy to announce our 2019 Christmas video which we have made in collaboration with Ben Kuper! It is starring 7 Crazyflies, the lighthouse positioning system, our office Christmas tree and a whole lot of holiday spirit, so go ahead and take a look!

Here are some words from Ben how it was to work on this year’s Christmas video at our office:

Coming to Bitcraze’s HQ and working with them has been once more a wonderful experience, technically and humanly ! The main goal of this session was to test and implement the new lighthouse tracking system in the tool suite I’m creating, and it was an amazing surprise to witness for real the uncanny stability of the drones when they’re on lighthouse tracking !

Of course, my first reaction was to push the limit and see what can be done with this new power, this is why I created this choreography : to see what can be done in a limited amount of time (1 and a half day to create the full choreography, the official video shows the first part only), and trying to go at the limit of the current possibilities. As the team was working on occlusion recovery, we decided to have the drone fly around the tree as a fun test, and it works !

In the new year we will have a followup blog-post going into detail on how exactly we made this video. Until then, happy holidays and have an awesome new year!

IROS and Lighthouse yaw

2019-11-04 | Kristoffer Richardsson | Leave a comment

This week we are exhibiting at IROS in Macau. We are running our fully autonomous demo based on the Lighthouse positioning technology and charging pads. We also have brought some prototypes to show, for instance the Crazyflie Bolt, the AI deck and the Active marker deck. You can read more about the demo at the IROS 2019 page.

We’d love to hear what you are working on, discuss issues, possibilities or new products. If you are at IROS, drop by our booth (B34) and say hi!

Lighthouse yaw

We have not only prepared for IROS, we have also been working on improving the lighthouse positioning system. Recently we added a (slightly hackish) solution for updating the yaw with data from the Lighthouse deck. This means that it is not necessary to start the Crazyflie facing the positive X direction when using the Lighthouse deck. The Crazyflie will understand its heading and act accordingly.

Two Crazyflies facing a random direction, take off and rotate to yaw=0.

We are also working on integrating the Lighthouse deck in a better way in the kalman filter. If everything goes according to plan, it will enable a Crazyflie to fly with only one base station, and be more robust when using two base stations.

Enabling Swarm Exploration

2019-10-28 | Kimberly McGuire | Leave a comment

For the last four years of doing my PhD at the TU Delft and the MAVlab, we were determined to figure out how to make a swarm/group of tiny quadcopters fly through and explore an unknown indoor environment. This was not easy, as many of the sub-challenges that needed to be solved first. However, we are happy to say that we were able to show a proof-of-concept in the latest Science Robotics issue! Here you can see the press release from the TU Delft for general information about the project.

Since we used the Crazyflie 2.0 to achieve this result, this blog-post we wanted to mostly highlight the technical side of the research, of the achievements and the challenges we had to face. Moreover, we will also explain the updated code which uses the new features of the Crazyflie Firmware as explained in the previous blogpost.

A swarm of drones exploring the environment, avoiding obstacles and each other. (Guus Schoonewille, TU Delft)

Hardware

In the paper, we presented a technique called Swarm Gradient Bug Algorithm (SGBA), which borrows (as the name suggests) navigational elements from the path planning technique called ‘Bug Algorithms’ (see this paper for an overview). The basic principle is that SGBA is a state-machine with several simple behavior presets such as ‘going to the goal’, ‘wall-following’ and ‘avoiding other Crazyflies’. Here in the bottom you can see all the modules were used. For the main experiments (on the left), the Crazyflie 2.0’s were equipped with the Multiranger and the Flowdeck (here we used the Flow deck v1). On the right you see the Crazyflies used for the application experiment, were we made an custom Multiranger deck (with four VL53L0x‘s) and a Hubsan Camera module. For both we used the Turnigy nanotech 300 mAh (1S 45-90C) LiPo battery, to increase the flight time to 7.5 min.

Experiments

With this, we were able to have 6 Crazyflies explore an empty office floor in the faculty building of Aerospace engineering. They started out in the middle of the test environment and flew all in different preferred directions which they upheld by their internal estimated yaw angle. With the multi-rangers, they managed to detect walls in their, and followed its border until the way was clear again to follow their preferred direction. Based on their local odometry measurements with the flowdeck, the Crazyflies detected if they were flying in a loop, in order to get out of rooms or other situations.

A little before half way of their battery life, they would try to get back to their initial position, which they did by measuring the Received Signal Strength Intensity of the Crazyradio PA home beacon, which was located at their initial starting position. During wall-following, they measured the gradient of the RSSI, to determine in which directions it increases or decreases, to estimate the angle back the goal.

While they were navigating, they were also communicating with each-other by means of broadcasting messages. Based on those measurements of RSSI, they could sense other Crazyflies approaching, which they first of all used for collision avoidance (by letting the low priority CFs move out of the way of the high priority CFs). Second of all, during the initial exploration phase, they communicated their preferred direction as well, so that one of them can change its exploration behavior to not conflict with the other. This way, we tried to maximize the explored area by the Crazyflies.

One of those experiments with 6 Crazyflies can be seen in this video for better understanding:

We also showed an application experiment where 4 crazyflies with the camera modules searched for 2 dummies in the same environment.

Challenges

In order to get the results presented above, there were many challenges to overcome during the development phase. Here is a list that explains a couple of the elements that needed to work flawlessly:

Single CF robustness: We used the Flowdeck v1, for the ‘deadlock’ detection and the basic velocity control, which was challenging in the testing environment because of low lighting conditions and texture. Therefore the Crazyflies were flying at 0.5 meters in order to ensure robustness. The wall-following was performed solely using the Multiranger. This was tested out in many situations and was able to handle a lot of type of obstacles without any problem. However the limited FOV of the laser range finder can not detect all types of obstacles, for instance thin ones or irregular ones such as plants. Luckily these were not encountered in the environment the Crazyflies flew in, but to increase robustness, we will need to consider adding a camera to the navigational drive as well.
Communication base-station. SGBA by essence only needs one base-station Crazyradio PA, since all the behavior is completely on board. However, in order to show results in the paper, it was necessary for the CF to communicate information back, like odometry, state and such. As this was a two way communication (CFs needed RSSI to get back) each Crazyflie needed 1 base-station. Also, they all needed to be on different channels to avoid package collisions and RSSI accumulation.
Communication Peer to Peer. At development time, P2P didn’t exist yet, so we had to implement broadcast communication between the Crazyflies. Since the previous pointer required them to listen on different channels, the NRF had to be configured to send separate broadcast messages on all those channels as well. In order to time this properly, the home beacon had to sync the Crazyflies accordingly by sending out a timer. Even so, the avoidance maneuvers were done very conservatively to try to prevent inter-drone collisions.

Many of the issues, especially the communication challenges, will be solved with the updated code implementation as explained in the next section.

Updated code

The firmware that the Crazyflies used to fly in the experiments showed in the paper, can all be found in this public repository. However, the code is based quite an old version the current Crazyflie firmware, as it was forked almost a year ago. The implementation of the SGBA state machine and the P2P broadcasting were not generic enough to integrate this back to the development cycle, therefore the current code is only suitable for the old Crazyflie 2.0.

Therefore, we developed two major changes in the latest firmware which will make it much easier for me (and other ideas as well we hope!) to implement SGBA and the P2P communication in a way that should be compatible with any version of the firmware (and hardware) from here and on. We implemented SGBA as an app-layer and also handled all the broadcast messaging directly from this layer as well. Please check out this Github repository with this new app layer implementation of SGBA.