Author: Kristoffer Richardsson

We have so many interesting things we want to do, but too little time to do it. Now we are looking for more talented people to join our team to help us create awesomeness!

We don’t have a clear description of exactly what we are looking for but there are some properties that we think are important:

  • You are interested in technology
  • You are passionate
  • You want to learn new things and evolve your skills

Head over to our jobs page for more information.

The High-level Commander has been part of the Crazyflie firmware since the 2018.10 release. In combination with a positioning system, it can fly the Crazyflie along a trajectory that is either defined in the firmware or uploaded through the python lib. It originates from the Crazyswarm project and we have used it in various demos since it is possible to make trajectories that are very fluid and looks really cool. The trajectories are defined as 7th degree polynomials describing segments executed one after each other.

The controller gives full control of position, velocity, acceleration and jerk, the only problem is that it is non-trivial to generate the polynomials. We have wanted to simplify the creation of trajectories for a long time and have finally had some time to play with it. In this blog post we will describe how it can be done with Bezier curves and show some examples.

Each segment in a High-level commander trajectory is defined by four 7 degree polynomials, one for x, y, z and yaw. There is also a scaling parameter that tells the controller the time scale to use when executing the segment. Using polynomials of degree 7 makes it possible to design trajectories that are continuous in position, velocity, acceleration and jerk when changing from one segment to the next, which is important to get a smooth and controlled flight.

Bezier curves are common in many graphics applications and are probably known to most users. They are parametric curves defined by control points, usually three or four. Bezier curves can also be expressed as a polynomial, and this is what we will use in this case. To get a correct mapping to the desired polynomials we need some more control points and will use 8 per segment. The basic idea is to define the trajectory as Bezier curves and make sure the control points are placed in such a way that the continuity requirements are satisfied.

Bezier curve with 8 control points

On this page from University of Cambridge, there is a good explanation on continuity across the joins between curves and formulas for c0, c1 and c2 continuity. We also need c3 continuity which can be calculated in the same manner

With these formulas it is possible to set the handles of the Bezier curves to make sure we get a smooth ride.

We have added a python example that implements the ideas above. You can find it in crazyflie-lib-python/examples/positioning/bezier_trajectory.py. The design is based on Nodes that represents the connection points between bezier curves (called Segments). The Nodes has a set of handles that are shared between the Segments that use the Node. If not all handles are set the implementation will set them to appropriate values, see the comments in the code for more details. The Node API only allows the user to set handles on one of the Segments, the handles for the other segment are automatically set to generate a continuous trajectory.

The example uses nodes in the corners of a square and contains three parts:

No velocity in the nodes. The Crazyflie stops in the Nodes. Similar to calling go-to in the HL commander.
Velocity in the nodes. A fluid motion all the way around.

Velocity in the nodes. A fluid motion all the way around.

A bit more aggressive settings to get a little action.

Finally a video showing the full sequence, we use the Lighthouse for positioning.

As mentioned earlier, we will be attending ICRA 2019 and have started to work on the demo that we will run in our booth. The main features we are going to show this time are the Lighthouse positioning deck and the Crazyflie 2.1.

One Crazyflie flying and 5 re-charging on their pads

Running a demo with flying Crazyflies at a conference, usually means a lot of work with changing and charging batteries, starting demos and so on. This takes time and leads to less time to talk to all the interesting people that visit our booth. This year we are aiming at making the demo as fully automated as possible. We will have 8 Crazyflie 2.1 with Lighthouse and Qi charger decks, each with a charging pad. A computer will orchestrate the Crazyflies and make sure one is flying at all times while the others re-charge their batteries. When the battery of the flying Crazyflie is depleted it goes back to its pad while another one takes over.

Most of the functionality is implemented in the Crazyflie firmware and it is pretty much autonomous after the trajectory is started. It will fly the trajectory over and over until it is low on battery, when it goes back to where it started from for recharging. Even though the Lighthouse positioning is very good, it sometimes slips off the charging pad when landing, so we have added a reposition feature to take off again and land if it is not charging after landing.

The orchestration computer (we call it the Control tower) is just keeping track of the state of the flying copter and starts a new one when the flying one goes back the the pad.

We are reusing the spiral trajectory from IROS last year and it has the property that it is possible to run up to 4 copters at the same time without colliding, if they are started at the correct times. There is a swarm feature in the Control tower that runs up to 4 Crazyflies with continuous replacements. The chargers are not fast enough to keep it going and it does not work as expected every time, but it is exciting with a few more Crazyflies buzzing around!

There are some bits and pieces left to implement, like crash detection, but the demo is mostly functional. If you want to play with the (slightly hackish) code, it is available at https://github.com/bitcraze/crazyflie-firmware-experimental/tree/icra-2019

You can find us in booth 101 at ICRA 2019, May 20 – 22. Drop by and say hi, check out the demo and tell us what you are working on. We love to hear about all the interesting projects that are going on. See you there!

We attended the Innovation Week at Lund University on Thursday last week. Primarily we wanted to talk to students and possibly find future colleagues (yes, we are hiring) but it was also a good opportunity to get some demo time with the Lighthouse positioning system.

The demo setup. A bit blurry, sorry!

As mentioned in an earlier blog post, we are going to ICRA in May and we have started to think about what to demo. The main feature will of course be the Lighthouse deck. The setup at Innovation week also served the purpose of a first iteration for the ICRA setup.

We reused an old cage that we created for another fair a couple of years ago, built from a garden tent. It turned out to be fairly wobbly and a bit heavy (steel tubing) considering we will bring it in our luggage to Canada. We probably have to rethink the construction a bit and see if we can change to aluminium.

We put the Lighthouse base stations on tripods, which worked like a charm in our flight lab. We found that we had a lot of problems calibrating the system, not to mention flying the Crazyflie, at the Innovation week fair though. It turned out that the floor was not as stable as one might expect and that the tripods were swaying when people walked by. We solved the problem by adding a tube to the top of tripod that was pushed against the ceiling and thus minimizing the movement. Experience from the real world is always useful!

The general idea for the demo at ICRA is to automate as much as possible to give us more time with visitors. With the high precision of the Lighthouse system, it should be easy to land the Crazyflies on Qi chargers to avoid changing batteries. We hope to set up 6-8 Crazyflies where one is always flying while the others are charging, and have the possibility to temporarily fly more Crazyflies for small swarms. It is still just ideas and we will not see the end result until we are at ICRA, but it will be fun to build!

Last week was busy at Bitcraze as we moved to our new office. We packed all our tools, equipment, toys, components, stock and other bits and pieces into boxes and on Tuesday the moving truck came to pick it all up. Everything was very smooth and by lunch all the stuff had been unloaded in the new office.

Office work

Until now we have had our offices at various co-working spaces, but this time we rent a “real” office. We get a lot of more space that fits our needs better (a flight lab!) but the drawback is that we have to buy a lot of stuff that was part of the package earlier, such as furniture, printer, network, fire extinguishers, coffee brewer and lots of other things.

Painting Bitcraze green wall

We spent most of last week unpacking, buying stuff, painting and installing and we are in pretty good shape now. There are still a lot to do but the most important functionality is up and running. We even managed to ship orders in the store every day except Tuesday when we moved.

If you are in the area, drop by and say hi!

We currently have our office at The Ground, a co-working space for startups. It has been three fantastic years at The Ground, with lots of awesome people around and we have had a great time, but space is limited and we are now moving on to our own office. We have found a great place where we will not only get more office space, we will also finally have our own dedicated flight lab!

Flight lab
The flight lab

We hope the flight lab will be a useful tool for future quad copters and positioning systems, and we think it will speed up the development process and help us create even better products.

We will move to the new office February 25, stay tuned for more information.

Björn is leaving Bitcraze

We are sad to announce that Björn is leaving the company. Björn has been involved in film making on his spare time and has now decided to explore this world full time. He has been a much valued colleague and he will be missed, but we are at the same time excited on behalf of him and hope for a bright film future!

We are happy to announce that the Roadrunner soon will be available in our store. The Roadrunner is an Ultra Wide Band (UWB) tag that can be used to acquire the position of any robot or object in a Loco Positioning System, which makes the LPS work with more than the Crazyflie.

The Roadrunner

The Roadrunner started out as a joint project with a customer that wanted to track go-karts on a track, but we think it should be equally useful for tracking any robot or vehicle indoors. It is essentially a Crazyflie 2.1 with an integrated LPS deck, but stripped of all quadcopter stuff, all in a nice package. It can be interfaced through the Crazyradio and USB, but also through a UART if needed. It can be powered with anything between 4 – 17V. Since it is based on the Crazyflie 2.1 platform, all tools, libraries and clients are compatible. It also has the same expansion port which makes it compatible with existing decks and can be extended with custom hardware.

You might be curious about the name we choose? We usually name internal projects after birds and what could be a better name for tracking a car than the Roadrunner? We liked the name and decided to stick to it when releasing it as a product.

We release the Roadrunner as an Early access product since we are a bit uncertain of how it will be used. We hope to get feedback from anyone using it and improve the design if needed.

This is also the first product to be released based on our new platform concept. We will release a number of new hardware designs in the near future and the platform concept is intended to simplify managing and building firmware binaries for the different hardware configurations.

On a side note, Arnaud from Bitcraze and Fred, the maintainer of the Crazyflie android client, will be visiting FOSDEM 2019 in Brussels at the end of the month. If you want to meet us there just ping us in the comment, by mail, on twitter or on the forum.

Even though we are getting closer to Christmas and hopefully some well deserved rest, there are lots of things going on at Bitcraze. This week we have collected news about various topics that we wanted to share with you.

China

Tobias and Marcus visited our Chinese manufacturer Seeed last week in Shenzen. We are trying to visit Seeed at least once a year to meet in person rather than only via the internet. 

huaqiangbei

It is always a great experience to visit Shenzen and it seems as things are moving at blazing speed over there, with amazing changes from year to year. Such as that you can now paying with face recognition in the grocery store and park you car in automatic parking garages.

Lighthouse deck

We are making progress on the Lighthouse front and we have a preliminary hardware design for the first version of the deck. There are still a lot of things to be done but we hope we will be able to order the first batch soon and that it will be available in our store the first quarter next year.

Qi charger V1.2 deck

The Qi charger deck is compatible with the Qi V1 standard. Recently we have been testing the deck with a new off-the-shelf charger and discovered that the deck was not working with the new charger. After investigating we discovered that the Qi deck is not compatible with the new Qi V1.2 chargers. We started a redesign of the board and we have now started to produce a batch of Qi deck V1.2 that is compatible with Qi 1.2 chargers. The new Qi deck will be released early January.

Roadrunner

The Roadrunner is our first stand alone Tag for the Loco Positioning System. It is in essence a Crazyflie with an integrated LPS deck but without motors and a different form factor, it was initially developed for an external project to track go-karts on a racing track. The Roadrunner can be fitted to anything that you want to track in a Loco Positioning System, a ground robot for instance. Since it is based on the Crazyflie, all the libraries and tools that are available in the Bitcraze eco-system are compatible. We plan to start selling the Roadrunner in our store in the beginning of next year.

We have a collaboration with Qualisys, a Swedish manufacturer of top of the line motion capture systems. Similar to us they are a passionate about what they do, are working on high tech products and to make it even better, they are located in Gothenburg, just a couple of hours away by train. If you are not familiar with motion capture systems, it is a system that can track objects with reflective markers in space using high resolution cameras. The precision/accuracy is very good (sub millimeter) and can be used to track more or less anything such as the movements of a human body or the position of a robot, for instance a Crazyflie. The position of a Crazyflie is calculated by the MoCap system and by sending it to the drone via radio, it can fly autonomously.

Qualisys

We are super happy of getting the opportunity to work with MoCap systems and making it an integral part of the Bitcraze eco system. We have already added support in Crazyswarm for the Qualisys system and soon there will be a tab in the Crazyflie python client for basic autonomous flight using a Qualisys system. We will release a passive MoCap deck in the near future that will make it easy to attach reflective markers to a Crazyflie in a well known configuration, see this blog post for more information. Further more we are looking at making an active marker deck that utilizes Qualisys’ active marker technology to both position and identify an object at the same time.

Recently we spent a day in the large lab of Qualisys. We played with the LPS system in a larger set up and experimented with passive MoCap deck configurations and finally tried to fly a swarm.

Martin and Tobias configuring MoCap decks

Unfortunately we ran out of time and we tried to push the envelop a bit too far so we never managed to fly the full sequence without crashes, on the other hand, getting that close in a couple of hours is not too bad. Even though the full swarm did not work out we learned new things and had a lot of fun. Thanks Martin and everyone at Qualisys!

 

If you are looking for a motion capture system and want more information about Qualisys, please do not hesitate to contact us or Qualisys.

In this blog post we will describe one of the demos we were running at IROS and how it was implemented. Conceptually this demo is based on the same ideas as for ICRA 2017 but the implementation is completely new and much cleaner.

The demo is fully autonomous (no computer in the loop) but it requires an external positioning system. We flew it using either the Loco Positioning System or the prototype Lighthouse system.
A button has been added to the LPS deck to start the demo. When the button is pressed the Crazyflie waits for position lock, takes off and repeats a predefined spiral trajectory until the battery is out, when it goes back to the door of the cage and lands.
For some reason we forgot to shoot a video at IROS so a reproduced version from the (messy) office will have to do instead, imagine a 2×2 m net cage around the Crayzflie.

Implementation

As mentioned in an earlier blog post the demo uses the high level commander originally developed by Wolfgang Hoenig and James Alan Preiss for Crazyswarm. We prototyped everything in python (sending commands to the Crazyflie via Crazyradio) to quickly get started and design the demo . Designing trajectories for the high level commander is not trivial and it took some time to get it right. What we wanted was a spiral downwards motion and then going back up along the Z-axis in the centre of the spiral. The high level commander is a bit picky on discontinuities and we used sines for height and radius to generate a smooth trajectory. 

Trajectories in the high level commander are defined as a number of pieces, each describing x, y, z and yaw for a short part of the full trajectory. When flying the trajectories the pieces are traversed one after the other. Each piece is described by 4 polynomials with 8 terms, one polynomial per x, y, z and yaw. The tricky part is to find the polynomials and we decided to do it by cutting our trajectory up in segments (4 per revolution), generate coordinates for a number of points along the segment and finally use numpy.polyfit() to fit polynomials to the points. 

When we were happy with the trajectory it was time to move it to the Crazyflie. Everything is implemented in the app.c file and is essentially a timer loop with a state machine issuing the same commands that we did from python (such as take off, goto and start trajectory). A number of functions in the firmware had to be exposed globally for this to work, maybe not correct from an architectural point of view but one has to do what one has to do to get the demo running :-) The full source code is available at github. Note that the make file is hardcoded for the Crazyflie 2.1, if you want to play with the code on a CF 2.0 you have to update the sensor setting

This approach led to an idea of a possible future app API (for apps running in the Crazyflie) containing similar functionality as the python lib. This would make it easy to prototype an app in python and then port it to firmware.

Controllers

The standard PID controller is very forgiving and usually handles noise and outliers from the positioning system in a fairly good way. We used it with the LPS system since there is some noise in the estimated position in an Ultra Wide Band system. The Lighthouse system on the other hand is much more precise so we switched to the Mellinger controller instead when using it. The Mellinger controller is more agile but also more sensitive to position errors and tend to flip when something unexpected happens. It is possible to use the Mellinger with the LPS as well but the probability of a crash was higher and we prioritised a carefree demo over agility. An extra bonus with the Mellinger controller is that it also handles yaw (as opposed to the PID controller) and we added this when flying with the Lighthouse. 

Going faster

Since the precision in the Lighthouse positioning system is so much better we increased the speed to add some extra excitement. It turned out to be so good that it repeatedly almost touched the panels at the back without any problems, over and over again!

One of the reasons we designed the trajectory the way we did was actually to make it possible to fly multiple copters at the same time, the trajectories never cross. As long as the Crazyflies are not hit by downwash from a copter too close above all is good. Since the demo is fully autonomous and the copters have no knowledge about each other we simply started them with appropriate intervals to separate them in space. We managed to fly three Crazyflies simultaneously with a fairly high degree of stability this way.