Friday, August 20, 2010

Flying with Ardupilot Mega

I'm experimenting with the ArduPilot Mega source code and settings and have finally succeeded to get it working properly using a FlightGear simulator and the board setup through a serial USB link. I've made a flight, slightly rough around the edges, around San Francisco airport in the simulator with some tweaked waypoints. You can find that file here.

The biggest problem in getting the link working has been the communication between the PC and the arduino board and FlightGear. FG may reduce or increase framerates and start buffering data when the interface program is not consuming them fast enough. The current interface is almost ready for real use, but can I think still increase the speed in which it passes data around.

In order to get stuff working, you should check the source code out of and make sure that you're pointing to the correct library location. Download the which is used for the communication with the USB 'ground station' (the flightgear link). Then reduce the NAV_P gain in the APM_PID_Gains.h file to something like .5 and reduce the other P gains too (you'll notice if the gain is set too high when it becomes unstable in flight and increases the roll or pitch constantly).

As I said, the flow of data was the biggest problem, so communication with flightgear is now attempted to be optimized. FlightGear does not care too much if there is nothing to read, but writing more than it can handle will buffer data on the IP buffers. So you should aim to write 1-2 Hz less than the link setting. Also pay attention to the frame rate that FG is able to put out, because that is the minimum update frequency you can use.

Reading from a port should be done as fast as possible and ideally outside of the main "50 Hz" loop. Since Arduino attempts to follow a 50Hz loop, you could attempt to follow the same in the interface application, but there's a possibility that you overload the link. The older XPlane app issued the attitude and additionally the GPS at 5 Hz, but I modified that to output XPlane attitude @ roughly 42.5 Hz and GPS @ 5 Hz. That leaves a bit of space to get rid of any overload that might occur on the arduino side.

Reading from the serial port should also occur as fast as possible. This way, you can keep everything in the same loop once you use a couple of global variables for whatever data is read from the links. Once something can be written, then just set up the packets and kick it out the door.

There are probably some configs to change in the Arduino source code before thinsg will work for you:
  • I have airspeed sensor set to 0.
  • The example waypoint file has been modified to allow the cessna to make the turns (better:).
  • GCS_PROTOCOL is set to 3.
  • GPS_PROTOCOL is set to 3.
  • GPS_IMU is included.
  • The #ifdef GPS_PROTOCOL != 3 before the is commented ( around line 610 in main source file, inside update_GPS function).
I first ran in STABILIZE mode and verified that I got responsiveness on the sticks. If your interface is buffering too much, reading too slow or writing too fast then you may get responses after 5 seconds. That will never converge to a model that can fly properly.

Then after STABILIZE mode you can quickly verify FLY_BY_WIRE_A and B to see if that works out. Then I loaded the waypoints and turned on AUTO mode. For that to work, just start flightgear and pause it. Zoom out a bit with Shift-X until you see the throttle stick. Hit "H" to see the HUD to get more info. Then make sure the switch is set to manual with throttle at zero. Start up the interface. Wait for the servo's to wiggle and the interface to print "Ready to Fly". Now you can switch to AUTO mode, but you have to control the throttle manually. I basically set throttle to full.

Because I don't have a rudder attached (nor is this set in the ardupilot.xml file), the plane veers off to the left due to the propeller torque until it lifts off the ground. The ardupilot mega will then slowly turn back to the heading it had when it was going 3 m/s (12 knots). Initially I used 10 degrees pitch, but noticed that it tried to drill itself into the ground. Making that 20 degrees again fixed it. Also make sure that your NAV P gains are correctly set, as these will determine how eager the autopilot is to get to the setpoint. Since the stock cessna of FlightGear is a bit slow in relation to an easystar, these P gains probably have to be reduced. It will then be less strict in following the waypoints, but allows for easier flight.

Have fun!

Monday, August 16, 2010

Making HD videos in France

I've returned from a small holiday in France and took the UAV to fly over there. I used the GoPro HD camera in the cockpit and shot a couple of videos @ 60 fps, 1280x720p. The results are on YouTube with a slightly reduced bitrate (more compression):

And another one near the castle of Castlenaud:

And I returned to the camping to make a high-altitude video from up above:

Nothing got seriously broken and it's great to see the videos to improve my flying skills. The video was also transmitted live to my netbook on the ground, but I didn't use that for navigating around (yet). I first need to check into an automatically adjusting tripod setup that will track the plane for a patch antenna. That will ensure, usually, a very good video link.

Thursday, August 05, 2010

Hauppauge USB Live 2 & Ubuntu

I'm running Ubuntu and purchased a USB Live 2 stick from Hauppauge to visualize some direct composite video coming from a 5.8Ghz receiver. This video is transmitted from an RC plane to see what's going on up there on a netbook. I'm not flying it FPV for lack of goggles and because I want to keep track of where the plane is. Basically, the intention is to put the family behind the netbook to see what the plane sees and I'll stare at the dot in the distance.

The USB stick arrived yesterday in the mail and after plugging it in, nothing happened. USB devices get recognized by their ID's and as soon as you register a driver for it, the kernel will visit all eligible drivers to see if one wants to step up to communicate with the device. Apparently the support for USB live 2 is not available in the stock distribution of Lucid yet. After searching around I suddenly hit a post made on the 3rd of August (sic!) that support for USB Live 2 was just completed on a development branch in v4l2. So the day before I got the stick development just finished with the first version of the driver.


I managed to install this on both my netbook and general PC. These are a 32-bit Lucid Lynx distribution on a Samsung N210 and a 64-bit installation on a more powerful machine with ATI HD5850 and i7 920. You should make sure that you're not holding back any linux packages, since I got screen corrruptions with kernel 2.6.32-23. The latest version now is 2.6.32-24 and there it worked (or it was something else, who knows?). After the clean install, you need to grab gcc, do an update/upgrade of all packages and make sure the latest kernel headers are present:
> apt-get install gcc
> apt-get install linux-headers-generic ( <-- ensure it's the latest & greatest @ 2.6.32-24 )
Then I used the entire v4l2 source tree available here (at 05-Aug-2010 that is, things may change rapidly here)
> hg clone
> cd polaris4
/polaris4> sudo -s
/polaris4> make

( then edit v4l/.config:
2. change this into:
4. (notice the n instead of m at the end, that's all).
5. This essentially deactivates the build of a problematic module, so if you have a problem there
you can deactivate it.

/polaris4> make
( yes again ).

If all went well:
/polaris4> make install

And then reboot. The reboot will ensure you're not using any old drivers. The rollback procedure for this is to reinstall the linux-image-...version... you were using. That will replace all your modules with the stock Ubuntu Lucid distribution.

Any time you get a new linux-image installed, you have to do another make, make install in this custom directory to overwrite the module files again. Remember that!

As you can tell, after doing this some video-related things may not work properly. For my netbook, I'm only going to use it in the field and some simple stuff. So far things haven't broken, so that should be relatively safe. It looks pretty stable so far.

I've noticed that the image when there's no signal may be plain blue. That is usually the case when the signal hasn't been tuned at all, but in this card you must ensure there's a video signal going into it.

The USB Live2 is a great little gadget with quite good image quality and zero lag on the Linux drivers. In Windows it's possible you get lag up to 3 seconds even, but that's because your driver must be configured to use "GAME" mode (if you're flying or gaming with this that is, otherwise it doesn't matter too much).

In order to visualize things, I now use tvtime (apt-get install tvtime). There's a handy configuration XML file in /etc/tvtime.xml where you can make all the default settings. The coolness about this utility is that you can change a large number of settings without restarting and adjust the image quality, brightness or contrast itself. Great for outdoors flying and getting a bit more out of the display.

The USB Live 2 uses the cx231xx kernel driver.

Sunday, August 01, 2010

Mesh networks

An interesting technology to be playing with are wireless self-healing mesh networks. Mesh networks are characterized by a rather chaotic characterization and where the data sent by one node is not necessarily sent directly to the receiver, but may make a number of hops before it gets there. This allows you to introduce a number of routes between the source of the data being generated and the coordinating node where the data needs to go.

A lot of propositions offer mesh networking for near real-time video display and those types of applications, instantly pushing the limits of the capabilities of bandwidth and network stability. I think the low-speed networks are slightly more interesting myself due to the low-cost ability to set this up and the speed at which such networks can be realized. This makes it very interesting for situations in which you need to cover a certain area for monitoring of some type. As long as the modules being placed have the right kinds of sensor and logic unit, you can start collecting data from this environment in no time.

Sometimes you want to collect more data than the sensors can give you, for example due to bandwidth constraints on this technology or the sensors that the installation is offering. In that case the cooperation of other mobile sensors that can be directed towards positions can help. Think UAV's being directed to that location, but still have means to be 'part' of the network as they move around a larger area of say 10 km to assist with the sensors they offer. The whole idea should be to plot interesting data about this larger area in no time and to do this as autonomously as possible.