The plane that flies itself

I’ve decided to dabble in hardware again. The grand plan is to turn an RC aeroplane into a semi-autonomous UAV. I’ve used PIC microcontrollers before, but this time I thought I’d switch and try one of the Atmel microcontrollers instead. Microcontrollers have got a bit nicer since the last time I used them. For example, generating PWM signals for servos used to require software bit-banging but now there is hardware support for it.

I settled on the Atmega16L which runs at 8MHz, has 16k flash ram for programs, and 1k static ram for calculations. It has onboard analogue-digital converters, which is perfect for plugging in “what angle is the plane at” sensors. It has 3 PWM outputs, which’ll be perfect for controlling the servos which move the rudder/elevator plus the electronic speed controller. Plus, it supports in-circuit programming which means you don’t have to keep dragging the chip out of the circuit and plopping it in a hardware programmer.

At first, I’m going to for something simple as proof-of-concept – automatic landing lights. An ultrasonic range sensor will point downward from the plane and switch on some super-bright LEDs when the height is below two meters. It’s nothing complicated, but it’s a good starting point to allow me to figure out how this stuff will fit into an RC plane, and how to make it robust enough to survive the inevitable crash landings.

Beyond that, I’m going to use a spare RC channels to allow me to switch an ‘autopilot mode’ on and off. After all, I want to be able to get manual control back if the software crashes! When autopilot is on, the microcontroller will take input from a range of sensors (IR horizon finding or inclinometer/gyro-combo for attitude, pressure sensor for altitude, GPS for position). It’ll figure out what it wants to do next, and move the control surfaces appropriately. Things like automatic landing modes have been done before and sound pretty possible. There are also light-weight wireless video systems which people have put into planes. And you can buy fairly cheap 433Mhz radio link chips which’d allow telemetry to be sent from the plane to a ground station.

What’s the point in this? It’s a blend between three of my interests. I’m interested in the physics/engineering aspect of flight and aerodynamic design. I enjoy doing simple hardware systems, and nowadays simple hardware systems can do lots of cool stuff. Finally, I like working on reliable/critical software systems. So, dealing with the realtime aspects of this project will be new and fun. As an aside, I’ve been reading about RTLinux and RTAI which are pretty cool. It reminds me of first time I saw the SoftICE debugger. There’s something remarkable about being able to pause an entire operating system!

On the software side, I’d be happier in a high-level language. But embedded systems are traditionally done in C, Forth or assembly in order to meet hard realtime demands. If you used a garbage collected language, a GC pause could mean that you miss a critical event. There’s been research into incremental collectors with low overhead for embedded systems though. And, typically, you have less need for dynamic memory allocation in an embedded system. But people have tried targetting high level language to embedded platform. Someone had a go at running nhc98 haskell runtime on a palm pda, and someone else did scheme on a PIC (PDF). If you were to use a high-level language in an embedded system, you’d want to have some guarantees about its time and space performance, and this is what the Hume project at St Andrews uni is looking at. And if you think that HLL can’t live on the bare metal, take a look at this Haskell OS project.

Finally, the “testing” side of this project is pretty interesting. I can obviously unit test the software before it goes onboard the plane. But how do you do integration testing? One answer is to place your flight-control software into a virtual world and make it think that it is actually flying. To achieve this, you can take advantage of the fact that Flightgear can send information about the plane (position, inclination, speed etc) out on a network port. You can grab this information, repackage it and send it to the flight-control software running inside a simulator (no hardware involved). The flight control software ultimately sends signals to servo, so you need to read these signal, map them back into flightgear-speak and push them across the network to flightgear. This way, you can see how the flight-control software behaves in high winds without risking the model plane itself!

Ah, that’s all for now. I’m well aware that I often start projects with grand plans and then get distracted by Other Things. This project is pretty amenable to the old ‘put it on the shelf for a few months’ treatment. It’s not “all or nothing” like some of my previous projects. So I am pretty hopeful of building something k3wl over the next few months and having a semi-autonomous plane buzzing around the skies.

Hardware cool

Looks like Sony are finally going to launch a new e-ink based book reader onto the western market. The “Sony Reader” is a successor to the japanese-only Librie (which I pined for a year ago). I don’t understand why there hasn’t been more products based on the e-ink technology yet. It obviously works fine, and the rest of the hardware is pretty vanilla stuff. What’s the holdup?

Now is then when then is now

Zeitgeist, through the medium of tools and bookmarks:

The tools I couldn’t live without in 2005: mythtv / web developer toolbar / live headers / nxml-mode / aliasadd / sed/grep/sort/uniq / unison / freemind

Neat stuff I found this year: iftop / dnstop / dbtoy / wmctrl / metalog / clamav

Thank you delicious and bloglines for that magic “where did those 3 hours go?” effect.

Quote of the year: “I’m very excited. I’m just hiding it well.”

And, courtesy of firefox’s “sort by add date” in bookmark manager:

January: DIY high-altitude glider
February: Edinburgh in 1844
March: Continuations for the web
April: Looping over filenames-with-spaces in bash
May: The world is not round
June: South Pole blog
July: Student’s T distribution
August: The Hammock wiki
September: Francis Crick profile
October: High-speed video of pool shots
November: Bad cookies in firefox
December: Programmers need to learn statistic, or I will kill them all

A link to LINQ found via links

Microsoft TV have an interesting program about their LINQ project. It’s somewhat similar to parts of Phil Wadler’s LINKS project (from whom I found the link in the first place). The aim is to shrink the gap between general purpose programming languages and data query languages. It’s well worth a watch. They’re got these crazy things new things called lambdas, which they use to perform map and filter operations over collections (gasp). But the nice thing is that the query code you’d use to find an element in a linked list can also be used to query rows in a SQL database. There’s some neat stuff happening behind the scenes. They also make use of a new (well, new for C# anyway) kind of variable declaration: var x = "foo". This declares x to be a statically-typed variable, whose type is established by the compiler using type inference. Sound familiar yet? Finally, they’ve also introduce a ruby-style ability to open up an existing class definition and add new methods to the class. This allows them to add new methods like ‘where’ and ‘orderby’ to existing collection classes, without requring the original source.

Staying on the web theme, I got a lot our of reading Brad Fitzpatrick’s description of how the LiveJournal site is laid out behind the scenes.

Amazon / UndoDB / concurrency’s Scottish developer center is expanding and hiring lots more developers and team leads. It’s located next to the Forth Road Bridge, near Edinburgh, and close to all the major traffic links. It is a very cool place to work, and the office has a great atmosphere. You’d be working on the software which underpins one of the world’s largest e-commerce sites. The massive scale of things makes the job exciting – massive traffic levels, extremely high reliability requirements, and demanding real-time requirements. If it all gets too much, chill out in the games room and shoot a few games of pool. If you think you are up to the challenges, please look through the recruitment site and mention “Andrew Birkett’s website” as your referral. 🙂

Speaking of companies who are doing cool things, check out Undo Software. They have built a time-travelling extension for the gcc debugger under linux. Pretty cool stuff, similar to the ocaml time-travelling debugger. If you are a C/C++ programmer who is a bit jealous of all the cool toys you get for other languages check it out.