Part 2: How to build a High-Definition FPV UAV using a Rasperry PI with HD camera, using a high speed WiFi link

Part Two: Here is the original picture of the finished product:

This is the second part of a 2-part series on 'How to build a High-Definition FPV UAV using a Raspberry PI with HD camera, using a high speed WiFi link.

In my first post on the subject (located here), I discussed the parts I used, and how to install them into a Hobby King Go-Discover FPV model. 

In this post, I will discuss installing the Raspberry PI and the PI camera in the Go-Discover gimbals, and the software configuration for both the Raspberry PI and the ground station PC.

From the previous post, step 3 was completed by installing the Ubiquity Rocket M5 in the model.  Now onto step 4:

Step 4: Install the Raspberry PI and PI Camera

Here is a photo of the position of the PI in the Go-Discover model:

The PI fits nicely just behind the camera gimbals, with the USB and HDMI ports on top. In the right side you can see the Cat5 network cable attached. This cable connects to the ethernet switch, which is also connected to the Rocket M5 input port.  

The two cables shown on top are the servo control wires for the gimbals, which I have directly connected to channel 4 and 5 on my radio.  I am using channel 4 (normally the rudder stick on my radio. Since there is no rudder on a flying wing, this is a convenient channel to use to move left and right with the camera. I have not (yet) moved to a head tracker, but if you already have that setup, just assign the channels accordingly.

To install the PI camera, remove the stock plate from the gimbals (for a GoPro), and mount the PI camera as shown in this photo:

The PI camera case fits very nicely into the slot, and again I used a small piece of velcro to hold it down. You could use a couple of small screws instead if you want a more secure hold.  The two gimbals servos are also shown here. They are simple to install, just follow the Go-Discover instructions.

Here is a front view of the PI camera installed:

Here is the block diagram describing all the connections:

Some comments on my previous post suggested that it is possible to eliminate the ethernet switch and serial-to-ethernet converter using the Raspberry PI and a serial port on the PI. I believe this post describes how to talk to the PI via the NavLink, but in this case, I want to use the PI to bridge the connection from the ground station to the APM/PixHawk. Somebody please comment on this if you know more about it.   I believe it would require a TCP/IP to serial link from the PI to the telemetry port on the APM, and some software on the PI to act as the bridge.  The main connection to the ground station is via the Rocket M5 and TCP/IP, not through a telemetry link (900 Mhz or Zigbee like I used on my other models).

Step 5: Getting it all to work with software configuration (the really fun part starts now).

Check out this post on what others have done with streaming and the PI.  My experiments showed that using GStreamer on both the PI and on Windows gives really good results with very low latency, if you use the right parameters. 

Get GStreamer on the PI by following this blog.   This is the same version of GStreamer that I am using on my setup. 

Make sure your PI camera works ok by plugging in the PI to a standard monitor using the HDMI port and follow the instructions on the Raspberry PI website on how to get the camera up and running (without GStreamer).  Once you have a working PI and camera, you can then proceed to stream things over the network.  

Note: It is suggested that you first get the PI streaming video by plugging it directly into your local network where you can also connect your ground station PC with the correct IP addresses (without the Rocket M5).   For my PI, I picked 192.168.1.2,  and for the ground station, 192.168.1.1.    Make sure you can ping the PI from your PC and the PC from the PI.  

For streaming, you will also have to make sure all the ports you intent to use are open on the firewall (described later).

For the ground station PC,  you can download GStreamer here.  Make sure when you install, select to install everything , or full installation (not the default). 

Here is the command I use for the PI to pipe the camera output to GStreamer:

raspivid -t 0 -w 1280 -h 720 -fps 30 -b 1700000 -o - | gst-launch1.0 -v fdsrc ! h264parse config-interval=1 ! rtph264pay ! udpsink host = 192.168.1.1 port= 9000

The command is explained as follows:

raspivid is the command to start the camera capture on the PI.  The -w switch is for the width in pixels, and the -h switch is for the height.  In this case, I am using 1280 X 720, but you can try any combination that fits your needs. 

The -b switch is the bit rate for the sampling. In this case I chose 1.7mbs to send over the stream. Again you can experiment with higher or lower values. This settings seems to work good for me, and the latency is almost unnoticeable.  

the "-o - |" is piping the output to gstreamer.  Make sure you include the dash before the pipe "|" symbol. 

For the GStreamer command, all the filters are separated with an exclamation point "!", as these are individual drivers that are part of GStreamer.  Since the PI has hardware accelerated video, the output is in a format called "H264", which is a highly-compressed stream. The GStreamer filters are configured to transport the output via a UDP socket connection to the target PC. Notice the 'udpsink' element which specifies the host - in this case your ground station, and the UDP port.  I am using port 9000, but you can use any open port on your system, but be sure to open the firewall or it won't work!  You can also use TCP instead of UDP, but for such a data stream, I chose to use UDP since dropouts are certainly possible, and with UDP this is ok, but with TCP, you could have socket problems and higher latency. 

Note: to get the PI to execute this command on boot, make a shell script with the above command and add it to your local.rc boot sequence. That way when the PI boots, you get the stream without having to log into the PI remotely. 

For the ground station PC, once you have installed GStreamer and opened the correct ports, use this command (from the command prompt) to view the stream:

c:\gstreamer\1.0\x86_64\bin\gst-launch-1.0 udpsrc port=9000 ! application/x-rtp,encoding-name=H264,payload=96 ! rtph264depay ! avdec_h264 ! videoconvert ! autovideosink

If all goes well, you should see the PI camera output on your PC screen in a popup window.  For those of you what want to use FPV goggles, you can connect to the HDMI port on your PC to display the output if your goggles support HDMI. 

I have this command in a batch file (with a PAUSE) statement at the end to keep the window open.

WHEW!  If you got this far, you are amazing. 

The last step to complete the build is to connect to the APM from mission planner.  The method I used to connect was to install a utility that converts a TCP connection to a virtual serial port, but I also think that directly connecting the mission planner to the TCP port will also work, however I have not tried it. I will post back later after trying it.

Here is the link to setup the serial to ethernet device to have an IP address and port.

Here is the link to the configuration utility for installing the virtual serial port.   

Once you have a serial connection over TCP/IP working to the APM, you should be able to connect with Mission Planner. On the maiden flight, it worked perfectly, and I didn't see a single drop in the telemetry data or anything noticeable in the video transmission, however my first flight was limited to 2km.

The last step is to connect the Rocket M5 to the Nano M5 and test everything using the OTA (over the air) connection. If all is well, you are ready to fly!  But be careful on your maiden, you just spent $700. 

Finally, here is a photo of my Antenna Tracker with the Nano M5 attached. My next update will include a video of a longer flight.  

Happy Flying!

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Comment by Justin Stiltner on September 29, 2014 at 3:50pm

You should be able to improve your range if you use either a circularly polarized antenna on the ground, or switch to a linear polarized antenna on the aircraft.  It is great that you are putting up this guide, it will really streamline the process of getting started. 

Comment by Patrick Duffy on September 29, 2014 at 4:49pm

Justin, you are correct that there is a 3-db loss for using a circular polarized antenna on the UAV, but since the Nano has a patch antenna, I can't install a spiral. The alternative is to use another Rocket M5 on the ground, and use a different set of antennas.  Using a linear antenna on the UAV would not be as good in my opinion because of the 5GHz signal, which has more interference and issues than using an RHCP style that still has a dipole pattern.  I happen to work for a company that does antenna characterization for the cell phone industry, and I write software for it. My plan is to put this rig into the test chamber and actually try different antennas.  My boss has 'approved' by use of the company equipment for learning experience.  I'll eventually post a blog about it.

Comment by Patrick Duffy on September 29, 2014 at 4:57pm

Another note about using the RHCP antennas with the Ubiquity products. Ideally, if you are going to use circular polarized antennas in a MIMO setup, one antenna should be RHCP, and the other LHCP.  This gives the 'diversity' antenna a greater degree of separation, besides the distance separation between them. For MIMO you want as much diversity as you can get, and normally, a parabolic dish is used which has two elements, one horizontally polarized, and the other vertical. But in this case of using these dipole-like spirals, you can't get the vertical and horizontal separation, so to achieve the same kind of results, swap the direction on the 'diversity' antenna. The ground station should also have one antenna RHCP, and the other LHCP.   On my current setup, MIMO will still work, but the diversity is achieved by maximizing the separation distance between the antennas. For even better performance, they antennas could be placed on the wing tips, but this would require more cabling and a bit more loss in the cables. 

Comment by Artem on September 29, 2014 at 5:38pm

one question, why not use a portable wifi dongle and connect it to a 1 watt amplifier (there are mimo capable wifi dongles) and use Pi as a router, or even set the whole network up as an adhoc wold this affect the streaming abilities?I like your idea of using different type circular polarization for MIMO, similarly you can put 2 helicals on the ground on the same tracker.  Great project, I was attempting something like this a year ago, guess at that time gstreamer didn't have hardware acceleration and h.264 was done by the main core making it awfully slow greater than 3s delays, so I resorted to 640x480 mjpeg at 15fps and found it was simpler to use the conventional hardware. now we just need MP to overlay HUD over the video on the ground :D 

Comment by Michael Johnston on September 29, 2014 at 5:44pm

the rocket M5 fits nicely into the case that your Rpi came in.

Comment by Patrick Duffy on September 29, 2014 at 5:46pm

Artem, your idea of using a WiFi dongle could work. I decided to use the Rocket because it's already in one package, but the idea is a possibility.  You would need two amps though, one for each antenna.  I was also thinking of how to get HUD over the video. If anybody has a suggestion, please sound off. I think Mission Planner can overlay video, but I am not sure how this would work with GStreamer. 

Comment by Patrick Duffy on September 29, 2014 at 5:55pm

For anybody interested in adding a HUD to the GStreamer pipeline, this may be the solution.  There is a text overlay plugin, but the text would need to be grabbed from Mission Planner and added to the GStreamer pipeline. This is certainly do-able, but will require some development. Anybody interested? 

Comment by Diy Copters - Ben Berrey on September 29, 2014 at 8:06pm

My question would be range/delay.  

Ive always wanted to try, but have been waiting for the release of better encoding to lower the bandwidth required.

Comment by Patrick Duffy on September 29, 2014 at 8:58pm

Ben, I will be posting some results on the range after I get a few more short flights. As for the delay, I have not measured the actual latency, but from my view on the ground station vs. the HDMI port on the PI, I can barely notice any delay, it's less then 100ms.   The PI I am using (the B+ model), seems to do quite well with the H264 encoding on the GPU, so give it a try and see if you can reproduce my results.


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Comment by Gary Mortimer on September 29, 2014 at 10:46pm

What does this platform fly like?

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