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3D Robotics

Making a UAV fail-safe

Posted by Chris Anderson on January 14, 2008 at 10:03pm
A big part of the DIY Drones credo is keeping it safe, and by that I don't just mean adhering to FAA regs and staying away from built-up areas, but also keeping it safe for your expensive UAV! The truth, as we all know, is that computers crash and that aircraft flown by computers crash harder ;-)

The aim of a good UAV is to have a fall-back system by which a human pilot can take over if (when!) the autopilot goes funky. There are at least three ways to do this, all of which we feature in one or more of the autopilots here:

  1. Completely separate the navigation and stabilization systems and ensure that the stabilization system can be overridden by manual RC control. That's what we do in GeoCrawlers 1, 2 and 3, all of which use the FMA Co-Pilot for stabalization (it controls the ailerons and elevator, leaving just the rudder for the autopilot and navigation). If the autopilot crashes, you can still fly the plane with ailerons and elevator alone, something we end up doing all too often! (The FMA system always allows for manual input)
  2. Mechanically separate the autopilot and RC control systems. In the case of the Lego UAV ("GeoCrawler1"), the Lego Mindstorm system moves the whole rudder servo back and forth, but the RC system can always turn the rudder servo arm, allowing it to override the autopilot if need be.
  3. Install a stand-alone "MUX" or servo multiplexer, that allows the operator to switch control from the RC system to the autopilot and back again with the gear switch on the transmitter, even if the autopilot catastrophically fails. As far as I know, there's only one commercially available one of these out there, and that one, by Reactive Technologies (shown), is not cheap ($99). Still, if you install one and give it an independent power supply, there should be no reason why you can't regain control of your plane no matter how wonky your onboard computer has gone.
What you should probably not do is exactly what we do (temporarily) with the Basic Stamp autopilot (GeoCrawler3), which passes RC signals through the Stamp chip and synthetically recreates them on the other side for the servos. If that program has a bug or the chip otherwise freezes, you've basically lost your rudder and elevator, which could make keeping the plane in the air difficult indeed. You'll still have control of the ailerons and throttle, but good luck getting the plane down in one piece if your program decides to crash with the rudder and elevator at full deflection.

So the Basic Stamp UAV project might be a good place for a MUX. Anybody know of a cheaper one? (This guy is looking for one, too)

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Comments

  • Hugo Vincent January 15, 2008 at 1:44pm
    Hi Chris,

    Thanks! I am no longer with the research company that I was working for at the time I designed the board, however while I was there, we fabbed 45 boards (but only a subset of those were populated with components). I will enquire if any of those are still available for purchase. I'm sure they wouldn't mind selling the PCBs themselves (blank, unpopulated), but, as I understand, they don't really have the capacity to do assembly work. If the assembly (populating the board with the components) were out-sourced the price might rise considerably.

    Can you please post a link to the RCGroups discussion. If I can establish an approximate number of interested people, I might be able to arrange production myself.
  • 3D Robotics
    Chris Anderson January 15, 2008 at 10:48am
    Hugo,

    First, great to hear from you: I'm a fan! (watched your presentation, been following the project),

    Second, what a great board! I posted about it on RCGroups and people were wondering how to get it. I assume the answer is "DIY", but if you know of any way to buy finished boards I'd love to pass it on. If not, would you have any objection to us fabbing a stack of them and selling them for cost?
  • Hugo Vincent January 15, 2008 at 5:00am
    "I got a kick out of the tilt sensor, a ball in a liquid which when tilted, closed a circuit by physical contact. I wonder if this would really work instead of expensive IMUs?"

    Nope, it would work exactly the same as a 2-axis accelerometer with a really low bandwidth (due to the inertia of the mechanical ball) and low resolution/quantization. And as you probably know, an accelerometer-only IMU is very limited.

    You could emulate it with a cheap 2-axis accelerometer, low-pass filter the outputs, and quantize with a 1-bit ADC (i.e. a comparator).
  • James Hall January 15, 2008 at 3:29am
    See a document I uploaded, search on the Discussion topic "Failsafe Document". Has some interesting reading, some circuit diagrams and some code in C. Its primarly devoted around a helicopter but still usefull. I got a kick out of the tilt sensor, a ball in a liquid which when tilted, closed a circuit by physical contact. I wonder if this would really work instead of a expensive IMUs?
  • Hugo Vincent January 15, 2008 at 2:02am
    Here is an open-hardware open-firmware board I designed to do servo switching (#3 in the article) in a failsafe way. It also can monitor battery levels or RF link status and switch based on that too. http://open.grcnz.com/trac/servo-switch/wiki

    The hardware is also capable of many other things that haven't been implemented in firmware, like MiniSSC-II servo controller emulation (through the built in serial port), servo logging (there is onboard flash memory), etc

    Also of note: the failsafe code is implemented in hardware (in a Xilinx CPLD) to eliminate the possibility of a software crash rendering the plane uncontrollable. That means that, although the switch contains a microcontroller running software, this is used for ancillary purposes and the core switching has no software in the loop that could crash.
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