Electronics: Flight Controller Computer (FC)

And now to explain the Flight Controller (FC) computer, here’s the first guest post on this blog featuring James, the mastermind behind the FC. James is a friend from high school who now lives in Austin and has joined the HAPP campaign during the past several months as we attempt to bring this project to fruition. James is also a “real” software developer, not a pretender like me, and not only has he expanded what I thought possible on this project, he’s also accelerated the whole project significantly.

I'll let James take it from here!

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In the last HAPP blog update, Chris provided details about the Autopilot (AP). This post will provide some background on the Flight Controller (FC), the companion computer onboard the craft.

To recap from the system architecture overview, the FC is responsible for several key functions:

• Determining GPS coordinates, altitude, speed (horizontal and vertical), and heading
• Transmit GPS data plus other flight data of interest to the ground via satellite
• Receive ground commands, if needed, via satellite (normally the HAPP is autonomous)
• Process all flight data and commands and decide what actions to take, such as firing pyros to cut the umbilical with the balloon or pyros to deploy the parachutes
• Monitor the AP status and abort the mission if the AP stops responding normally (recall that the AP is listening similarly to the FC and can also terminate the mission independently if needed)

Electronics:

Like the AP, the FC is an Arduino Mega with two custom shields. There were more than a few revisions prior to the delivery of final product for the launch. My professional background resides entirely in the software realm. I’ve written C-based firmware for embedded devices but I’ve never done much of the electronics work myself. This project was a great bit of fun and provided me a chance to stock up a full lab of gear (a desoldering gun is essential) and components for future projects as well.

I began with the usual breadboard prototype, then moved it to a more solid structure (protoboard) that required some light soldering. During this breadboarding phase, I worked out the beginnings of the FC Arduino Sketch that would serve as the scaffolding for the FC logic. I also mocked up a dummy AP Sketch that did nothing but communicate on the I2C bus.  You could type in state transitions manually and see the status codes flow to the FC. Once satisfied that I understood how each peripheral would work, I spent a considerable amount of time on electronics revisions. Each involved multiple SKUs of the same component from manufacturers. Some with built-in antenna, some without. Some with different form factors, others with different voltage requirements. We finally settled on the smallest available of each component that would suit our needs.

• GPS: u-blox MAX-M8Q
• Satellite: RockBLOCK 9603; uses the Iridium satellite constellation
• microSD: Adafruit MicroSD breakout
• Antennae: External Helical w/SMA (Satellite), Active PatchAntenna w/SMA (GPS)

The first unit is still in use and is my go-to development unit here in Austin. It features built-in antennae for GPS and satellite communications and is very portable, which is good when you have to sit outdoors to test satellite communication. The craft-ready revision has small coaxial connectors (SMA) for external antennae. Those antennae sit on the upper ELS deck, which is exposed to the sky, thereby avoiding the Faraday cage effect caused by the carbon fiber shell of the craft.

 

Unit #1 for development purposes.
Unit #2, which is on the HAPP, has a cleaner layout.

Unit #2 installed on the HAPP
as flight hardware

Autopilot (AP) on the left and
Flight Controller (FC) on the right,
as installed on the Electronics Deck

The electronics of the FC are much simpler when compared to the AP.  We have a single I2C device to communicate with – the AP.  We have two serial devices (GPS, Satellite) and a single SPI device (microSD).  The Arduino Mega is equipped with four serial ports, three of which are available via the Arduino header pins where we have access to the transmit (TX) and receive (RX) pins of each (the first serial port is accessible through the onboard USB connector and is used to program the Arduino and serves as a debug port). 

Future electronics enhancements might include the addition of another temperature sensor for redundancy (the AP has one) as well as a traditional GSM cellular communication channel for more frequent and higher volumes of data while the craft is at low altitude. This could aid in recovery efforts should we lose satellite communication.  Chris added this commercial, off-the-shelf tracking device from SPOT as a backup in the meantime.

Software:

The FC sketch has an initialization phase where it confirms that the peripherals are in working order. It opens two log files on the microSD card, one for general log data and another for CSV-formatted GPS data for later viewing in a tool of your choice.

Once setup succeeds and the FC reaches a state called PRELAUNCH_AWAIT_FCAP_HANDSHAKE, we await communication with the AP before handing off flight control to the FC logic.

The main microcontroller loop of the FC code services the GPS peripheral. For simplicity, we chose to stick with a serial interface for our chosen chip, the MAX-M8Q from u-blox. I found a high-altitude capable variant mounted to a breakout board by Uputronics out of the UK. Note that most civilian receivers will cease functioning around 18,000 meters / 60,000 feet. To track the HAPP up to 30,000 meters / 100,000 feet, selection of the right GPS receiver is critical.

GPS chips are constantly sending data to the communication bus. If you don’t devote time to servicing that stream of data, you risk losing characters and your GPS location fixes are in jeopardy. The cadence of the fix message is approximately one per second – more than enough for our positioning needs as GPS data is not involved in any kind of real-time steering of the craft. There is another class of GPS chips that provides higher accuracy and higher fix frequency. Coupled with predictive algorithms, that’s what your Tesla or military drones use for navigational guidance.

While GPS fixes are being assembled, the FC sketch yields time to a common worker function.  It is in that worker function where the FC does its work.  That outline is:

• Ask the AP for its state
• Sync onboard clock with GPS satellite time
• Log positional and operational data
• Evaluate states for flight control logic (the secret sauce)
• Repeat ...

Chris wrote logic that reacts to the navigational input from the fix data.  He reacts to flight data related to positioning, velocity and heading. He can respond to questions like

• Are we well outside of the bounds of the expected flight plan? Yes? ABORT! (There is a good story here from the first flight test. I’ll let Chris post about that later...)
• Are we descending unexpectedly? Yes? ABORT!
• Did we go supersonic on descent? Yes? Cool! Tell us about it.
• Did we lose contact with the AP? Yes? ABORT!
• More …

I should note here that the FC has multiple digital output pins available via a header soldered to top Arduino shield.  This is similar to an identical one on the AP.  The redundant pins allow either the AP or the FC to issue critical abort control messages to the craft – fire the pyros to cute down the balloon, release the chutes, etc.

Our satellite modem is able to send messages approximately every 30 seconds.  So, while we are constantly collecting flight data, we can only send data to our application server twice per minute. Once the FC logic detects that it is time to tell the ground (that’s us!) about the craft’s current state, it assembles a formatted payload and hands it off to the modem where some internal retry logic then attempts to acquire a satellite signal and send the data. These messages are called Mobile Originated messages (MO). It is also at this point where the modem can tell the craft about any pending commands sent from the ground. These are called mobile terminated (MT) messages. At present, we only issue a single command which tells the craft to enter the EXECUTE_ABORT state immediately – think BIG RED BUTTON.  We did, however, design it to be a general-purpose command control feature where we can set the AP or the FC to any state desired.

While the satellite modem is attempting to communicate, it hogs the microcontroller’s time awaiting a response. That is bad because we need to service the incoming GPS data and we need to service our own operational flight logic. To solve that, the library we use to talk to the modem allows us to provide a callback function so that it can yield time back to us. It is the same exact function that our GPS loop uses to process normal flight data.  So now we have two entry points into that single function – our GPS working loop and the callback from the satellite library. That can be bad.  Such code is said to be re-entrant since there are multiple processes or process threads accessing the function at the same time. It doesn’t have to be bad, but our satellite library allocates memory in such a way that we’d be clobbering variables set by the other callers. To solve that, our sketch uses a semaphore to gate access to the non-reentrant parts of the library.

During nominal operation, the FC will remain busy getting a GPS fix, providing navigation data to the flight logic handler, logging data to the microSD, sending data to ground control, and receiving messages from ground. It does this in a tight, highly-optimized loop from mission start to finish.

In an upcoming post, I will detail the software we call the HAPP application server.  We have servers in the cloud (because of course) where we coordinate all of the messages and present data logically (missions, messages, craft, etc.) for ground crew and observers.  It has a web site as well as an API for use by a dedicated mobile application currently in development by some amazing volunteers at Menlo Innovations in Ann Arbor, Michigan.