I started working with the Crazyflie 2.0 in 2015. I was interested in learning how to program a quadcopter, and the open-source nature of the Crazyflie’s hardware and software was the perfect starting point.

Shortly after, I discovered the world of FPV and the thrill of flying with a bird’s eye view. My journey progressed from rubber-banding an all-in-one camera/VTX to my Crazyflie, to building a 250mm racing quad (via the BigQuad deck), and into the world of Betaflight (including bringing Betaflight support to the Crazyflie hardware).

: FPV for Beginners ;)

: BigQuad Build

Naturally, the announcement of the Bolt (then known as the RZR) piqued my interest, and the folks at Bitcraze graciously allowed me early hands-on with the product.

This post details my progress towards building out a FPV-style drone on top of the Crazyflie Bolt.

Component List

The FPV community has come a long way since 2015. What once was a very complicated process is now well documented and similar to building a PC (well, with some soldering). For latest details on the specifics of building FPV drones, I recommend resources such as Joshua Bardwell or the r/Multicopter subreddit.

Turns out I had enough components lying around for a 4-inch (propeller diameter) build based on 3S (3 cell) LiPo batteries. Again, there’s nothing special about these parts (in fact they’re all out of date). Take this list as a guide, and do your own research.

PDB (Power Distribution Board): This is a circuit board that produces regulated voltages from an unregulated LiPo battery. The Bolt has built-in regulators but is only rated up to an 8A current draw per motor. My 4 inch propellers will certainly draw more than 8A, and so an external PDB is required (plus having dedicated 12V and 5V supplies is nice for peripherals).
4x DYS 1806 Brushless Motors: Brushless motors use magnetic pulses to rotate a motor bell (distinct from brushed motors found on the regular Crazyflie).
4x DYS 20A BLHeli_S ESCs (Electronic Speed Controller): This is a piece of circuitry that accepts a logic-level control signal and applies direct battery power to motor coils to make a brushless motor spin. They have to be rated for the current draw expected by the battery+propeller combination.
Tweaker (by Shendrones) Frame: I’ve been wanting to build a quad around this frame, and the large square hole is interesting for the Bolt (more on that later). One thing to keep in mind is this is an ‘H’ style frame. That is, it’s longer than it is wide, so flight will not be perfectly symmetrical. If you’re interested in building a larger Crazyflie and not so interested in FPV, you’ll definitely want a symmetrical ‘X’ style frame.
WS2812B addressable LEDs: LEDs are proven to make things better. It’s science.
Camera + VTX: For a full FPV setup, you’ll need a camera and a video transmitter. For the most part these run completely independently of the flight controller and so I’ll omit them from this article — what I’ve shown in the picture above is horribly out of date anyway.
RX: Radio receiver. For longer range flights and reduced latency it may be a good idea to use an external radio and UART-based receiver with diversity antennas. However, some specific work went in to the Bolt’s antenna design, so I’ll be sticking with the on-board NRF51 and external antenna.
Flight Controller: The Crazyflie Bolt!

The Build

Again, there are hundreds of fantastic guides on the web that detail how to build an FPV quadcopter. Instead of trying to create another, here are some notes specific to my Bolt build.

Expansion Decks

Since the Bolt is pin compatible with the Crazyflie, I thought it would be interesting to try and take advantage of a couple existing Crazyflie expansion decks in my build: The LED Ring Deck, the Flow Deck v2, and the Micro SD Card Deck.

The LED Ring Deck

The LEDs were the most hands-on feature to enable. Rather than simply attaching the LED ring inside the frame, I mounted a series of WS2812B lights to the underside of my frame’s arms. The LED Ring Deck consists of 12 LEDs connected in series — so I put three LEDs on each arm of the frame and wired them up in a daisy-chain.

Finally, I soldered the lead to IO_2 (the same that’s used by the LED Ring Deck) on a Breakout Deck.

Since this isn’t the official LED Ring Deck, there’s no OW memory ID. The deck must be force-enabled by specifying a compile flag in your tools/build/make/config.mk file:

CFLAGS += -DDECK_FORCE=bcLedRing

With the custom firmware, the under-arm LEDs work just like the LED Ring Deck (other than the lack of front-facing LEDs).

Micro SD Card Deck

Most popular flight controllers feature flash storage or SD card slots for data logging. The FPV community uses storage to log sensor data for PID tuning and debugging. Naturally, this deck is a good fit on my Bolt build, and requires no additional modification.

Flow Deck (v2)

Remember my interest in the square cutout on my frame of choice? That, and my unorthodox choice to mount the Bolt board below my PDB, means I can theoretically use the bottom-attached Flow Deck to achieve some lateral stabilization while close to the ground. In theory, the VL53L1x ranger should work outdoors thanks to its usage of 940nm light as opposed to 850nm.

Note: This photo also shows the daisy chain wire connecting banks of LEDs in series

Other Build Tips

It’s good practice to soft mount flight controllers to minimize transferring motor/prop vibrations into the IMU. I used these to isolate the flight controller from the frame — not perfect, but better than a rigid mount.
The receiver antenna must be mounted clear of the carbon fiber frame and electronics. I like to use a heavy duty zip tie and attach the antenna with heat shrink.
The Bolt can be powered from a 5v regulator on your PDB, but if you want to take advantage of the VBat sensor it should be powered from the raw battery leads instead. However, most ESCs support active breaking (ability to slow/stop the propellers on demand). Active breaking is known to produce a lot of back-voltage, which can damage some circuits. To be safe, since I’m using a 3S battery (12.6V when fully charged, 11.1V when depleted) I chose to power the Bolt off a regulated 12V supply from my PDB. This way, the PDB’s regulator will filter out voltage spikes and help protect the Bolt. Readings won’t be accurate at the higher range, but what really matters for a voltage sensor is to know when to land.

Results

It works! There is work needed to improve flight, though:

Control tuning is required. The powerful brushless motors respond much quicker than brushed motors, and so many of the PID and/or Kalman parameters are too aggressive or just non-optimal.
Stabilization with the Flow deck does not work — I haven’t spent much time debugging but my guess is it’s either due to the Kalman tuning, or problems with the VL53L1x depth working outdoors (which also impacts the flow measurements)
Betaflight Support: Betaflight has no driver for the BMI088 IMU used on the Crazyflie Bolt or the Crazyflie 2.1.
Safety Features: Brushless quads are very dangerous and can cause serious injuries. It’d be good to implement a kill-switch and a more aggressive failsafe in the firmware to prevent flyaways.

All in all, this was an enjoyable project and I’m excited to see some autonomous brushed quads coming out of the Crazyflie community!

I’ve spent the last 5 years of my career at Microsoft on the team responsible for HoloLens and Windows Mixed Reality VR headsets. Typically, augmented reality applications deal with creating and manipulating digital content in the context of real-world surroundings. I thought it’d be interesting to explore some applications of using an augmented reality device to manipulate and control physical objects and have them interact with the real world and/or digital content.

Phase 1: Gesture Input

The HoloLens SDK has APIs for consuming hand gestures as input. For the first phase of this project, I modified the existing Windows UAP/UWP client to handle these gestures and convert them to CRTP setpoints. I used the “manipulation gesture” which provides offsets in three dimensions for a tap-and-drag gesture, from the point in space where the initial tap occurred. These three degrees of freedom are mapped to thrust, pitch and roll.

For the curious, there’s an article on my website with details about the implementation and source code. Here’s a YouTube video where I explain the concept and show a couple of quick demos.

As you can see in the first demo in the video, this works but isn’t entirely useful or practical. The HoloLens accounts for head movements (otherwise moving the head to the left would produce the same offset as moving the hand to the right, requiring the user to keep his or her head very still) but the user must still take care to keep the hand in the field of view of the device’s cameras. Once the gesture is released (or the hand goes out of view) the failsafe engages and the Crazyflie drops to the ground. And of course, lack of yaw control cripples the ability to control the Crazyflie.

Phase 2: Position Hold

Adding a flow deck makes for a more compelling user experience, as seen in the second demo in the video above. The Crazyflie uses the sensors on the flow deck to hold its position. With this functionality, the user is free to move about the room and make shorter “adjustment” hand gestures, instead of needing to hold very still. In this mode, the gesture’s degrees of freedom map to an x/y velocity and a vertical offset from the current z-depth.

This is a step in the right direction, but still has limitations. The HoloLens doesn’t know where it is in space relative to the Crazyflie. A gesture in the y axis relative to the device will always result in a movement in the y direction of the Crazyflie, which begins to feel unnatural if the user moves around. Ideally, gestures would cause the crazyflie to move in the same direction relative to the user, not relative to the ‘front’ of the Crazyflie. Also, there’s still no control over yaw.

The flow deck has some limitation as well: The z-range only goes to 2 meters with any accuracy. The flow sensor (for lateral stabilization) has a strong dependency on the patterns on the floor below. A flow sensor is a camera that relies on measuring pixel deltas from frame to frame, so if the floor is blank or has a repeating pattern, it can be difficult to hold position properly.

Despite these limitations, using hand gestures to control the Crazyflie with a flow deck installed as actually quite fun and surprisingly easy.

Phase 3 and Beyond: Future Work & Ideas

I’m currently working on some new features that I hope will open the door for more interesting applications. All of what follows is a work in progress, and not yet implemented or functional. Dream with me!

Shared Coordinate System

The next phase (currently a work in progress) is to get the HoloLens and the Crazyflie into a shared coordinate system. Having spatial awareness between the HoloLens and the Crazyflie opens up some very exciting scenarios:

The orientation problem could be improved: transforms could be applied to gestures to cause the Crazyflie to respond to commands in the user’s frame of reference (so ‘pushing’ away from one’s self would cause the Crazyflie to fly away from the user, instead of whatever direction is ‘forward’ to the Crazyflie’s perspective).
A ‘follow me’ mode, where the crazyflie autonomously follows behind a user as he or she moves throughout the space.
Ability to walk around and manually set waypoints by selecting points of interest in the environment.

The Loco Positioning System is a natural fit here. A setup step (where a spatial anchor or similar is established at same physical position and orientation as the LPS origin) and a simple transform for scale and orientation (HoloLens and the Crazyflie define X,Y,Z differently) would allow the HoloLens and Crazyflie to operate in a shared coordinate system. One could also use the webcam on the HoloLens along with computer vision techniques to track the Crazyflie, but that would require constant line of sight from the HoloLens to the Crazyflie.

Obstacle Detection/Avoidance

The next step after establishing a shared coordinate system is to use the HoloLens for obstacle detection and avoidance. The HoloLens has the ability to map surfaces in real time and position itself in that map (SLAM). Logic could be added to the HoloLens to consume this surface map and adjust pathing/setpoints to avoid these obstacles without reducing the overall compute/power budget of the Crazyflie itself.

Swarm Control and Manipulation

As a simple extension of the shared coordinate system (and what Bitcraze has been doing with TDoA and swarming lately) the HoloLens could be used to manipulate individual Crazyflies within a swarm through raycasting (the same technique used to gaze at, select and move specific holograms in the digital domain). Or perhaps a swarm could be controlled to move out of the way as a user passes through the swarm, and return to formation afterward.

Augmenting with Digital Content

All scenarios discussed thus far have dealt with using the HoloLens as an input and localization device, but its primary job is to project digital content into the real world. I can think of applications such as:

Games
- Flying around through a digital obstacle course
- First person shooter or space invaders type game (Crazyflie moves around to avoid user or fire rendered laser pulses at user, etc)
Diagnostic/development tools
- Overlaying some diagnostic information (such as battery life) above the Crazyflie (or each Crazyflie in a swarm)
- Set or visualize/verify the position of the LPS nodes in space
- Visualize the position of the Crazyflie as reported by LPS, to observe error or drift in real time

Conclusion

There’s no shortage of interesting applications related to blending augmented reality with the Crazyflie, but there’s quite a bit of work ahead to get there. Keep an eye on the Bitcraze blog or the forums for updates and news on this effort.

I’d love to hear what ideas you have for combining augmented reality devices with physical devices like the Crazyflie. Leave a comment with thoughts, suggestions, or any other relevant work!

Author: Sean Kelly

Crazyflie Bolt: FPV meets Autonomy