Mechanical Integration

Mechanical integration came next. I got a bunch of nylon standoffs, nylon screws, and zip ties and set off to make the electrical stack on my quadcopter stick together and stay attached to the frame. This is important, as if your flight controller jiggles around with respect to the frame during the flight, your gyros, accelerometers, and magnetic compass are all going to be giving inaccurate readings about the orientation of the quadcopter’s frame. So it has to stay fixed.

Constraints:

1. I want the GPS antenna to end up on top.
2. My GPS board has an antenna covering one of the two through-holes, and mostly covering the other.
3. The GPS board is about 2/3rds the size of the flight controller. This means you can’t reach the other side with standoffs
4. The serial (UART) breakout board is about 2/3rds the size of the flight controller. This still means you can’t reach the other side with standoffs
5. The antenna for the receiver needs to be available from the ground when the bird is flying
6. The antenna for the 433 MHz link needs to be available from the ground when the bird is flying 
7. There is a reset button on the flight controller that would be nice to have available.
8. There are indicator lights on the flight controller that I need to see when starting up.
9. The antennas and wires must not get in the way of the propellers

I tried to think of some Lego-style combination of 2/3rds and 2/3rds that would allow me to combine the serial breakout board and the GPS board and use the far outside corners of the flight controller to hook into, but I didn’t come up with anything that worked.

I attached the flight controller to the frame with 13mm standoffs. I then loosely attached the serial breakout board to the flight controller with one screw on one 13mm standoffs, and let the other three sides float. Two of them have standoffs that are just resting on top of the flight controller board. While this is not great, the serial breakout board doesn’t have any orientation-sensing devices in it, so this is okay, as long as it stays mechanically attached. I also wonder if shock and vibration will eventually hurt the flight controller, but we’re a long way from there, I hope. :-)

 Serial UART breakout board standoff configuration

To attach the GPS board to the serial breakout board, I had to modify some of the nylon standoffs. Nylon is an absolute dream to work with, as it is so easy to modify. All I did was grab some snips and a file, cut what I wanted, file off any sharp points, and it was good to go. Wear eye protection when you cut standoff threads with snips. They fly! 

Nylon-working Tools

I cut down one standoff so I could just barely get a nylon nut on it. The other I cut just long enough to get the threaded part to stick up through the hole in the GPS board but not hit the antenna. See picture below.

The odd standoff configuration I used.

I then decided that I needed to protect the 433MHz 3DR radio, so I put some heatshrink on both sides. 

Below, you can see before and after for the quadcopter’s end. I didn’t take a before picture of the laptop end.

433 MHz "3DR" Radio Mobile Side Front

433 MHz "3DR" Radio Mobile Side Back

433 MHz "3DR" Radios With Heat Shrink

Mobile Side Wiring and Antenna Attached

I put two layers of heat shrink on the laptop end of the radio. Note that I was silly and didn’t poke any holes for LEDs to shine through, so I’m kind of unaware if the USB end is ready. But you can see through the white shrinkwrap on the quadcopter’s end, so that’s a plus.

Laptop Side of the "3DR" Radio

Here is what the final stackup looked like:

Yes, the serial breakout board and the GPS board are not attached super well, but it should be okay. 

In the end I wished for some 20-ish millimeter standoffs instead of my 13mm and 31mm. But it worked out okay. 

With the standoffs placed how they are, one of them that is just floating on the flight controller board actually lights up with one of the status LEDs, which is kind of nice. In the pictures below you can see it when it’s red, and when it’s green.

Also, in the picture above, at the bottom left, you can see the 3DR radio zip tied to the frame below the power board, above where the battery connects. Zip ties are almost magical. :-)

What does the full thing look like? See below:

Note in the above picture you can see the Receiver zip tied to the frame as well. It's on the bottom left of the electronic's nest in the center, with a gray/red antenna wire hanging down. This enables it to receive signals from the ground station while it's in the air and still be out of the way of the propellers.

Strap a battery on the bottom with Velcro, and mechanically, he’s ready to fly!

Integrating the GPS and Telemetry Radio

This rather simple post is about integrating the GPS and telemetry radio (often called a 3DR radio, which is a company that makes them. It's kind of like calling all telephones GrahamBells...).

I used a UART breakout board  to do this, which made it spectacularly easy, if you ignore the silliness of me frying the VCC line to the serial connector.

You do not have to use a UART breakout board! You can simply modify the cable harness between your flight controller and your GPS & radio. I chose the easy way (UART breakout) for this prototype. Certainly you do not have to, but it was terrifically easy with it. When I got the proper power to it (5V), it was as simple as plug it in and wait for a GPS fix.

It was just as easy for the telemetry radio. It was super easy, as I had already supplied 5V to the connector via the bus wire from the I2C port. I had a concern about pulling so much power through one pin, but it's not on the Atmel AVR, so it should be fine. Anyway, I simply installed the driver for the 433 MHz telemetry radio on my Windows 7 laptop, told Mission Planner that I was using a 3DR, and tried to connect. I neglected to adjust the baud rate the first time, but once I set it to Autodetect, the baud rate adjusted and everything worked just fine. So suddenly, I am able to get a 3D GPS fix, and use remote telemetry with Mission Planner with minimal effort. I can move my quadcopter around and see the results on my laptop without a physical tether. We're getting somewhere. This is awesome.

Here's a picture I found somewhere on the web showing the overall idea:

The next step is to mechanically integrate the boards. See you next post.