Analog Angle Blog

When Your Sensors Mislead You

Perhaps obscured by all the attention the apparent battery problem in the Boeing 787 Dreamliner is getting, another high-end aircraft problem has slipped by under the radar, so to speak.

A Wall Street Journal article (registration required) explains how Airbus engineers worked on a subsystem to enhance sensors for detecting icing on the A330 and other models. The engineers found the “icing-prevention devices championed… can have the opposite of their intended effect, causing key sensors on its aircraft to freeze up and triggering a potentially dangerous dive.”

That's not good at all, and it's a caution flag for all engineers. As processor- and software-based controls become increasingly pervasive and even dominant, they are being tasked to see more, know more, and do more. As a consequence, they are being connected to more and more sensors. Today's cars, of course, are a clear example of sensor-laden trends in mass-market product design. These sensors are generally analog in nature, even if their output is digitized, and they require analog signal conditioning.

The problem is that sensor technology is very good, but not perfect. It's not just a matter of miscalibration or a slight misreporting, such as being off by a fraction of a degree. Sometimes, how and where the sensor is positioned, what's positioned around it, nearby materials, or unrelated forces impinging on the sensor can cause it to give a very incorrect output — and you may not know it. (This article from Design News, a sister site, offers an example.) Everything seems OK, self-test circuits report all is well, and you're in blissful ignorance of the serious problem you have.

It's a problem that most sensor-laden designs can have, where apparently good sensors and their channels may report conflicting or contradictory results.

Engineers face a long list of sensor-channel issues, including internal and external noise, data corruption, ground loops, and supply-rail problems. That's why you really have to ask yourself the tough questions. How do I know this data is good? Am I making major decisions based on unverified — or unverifiable — readings? Is there a way I can independently cross-check what the system is being told? Can I provide a known change in the sensor's physical parameters and see if it provides a reasonably correct change in output signal? Besides the laws of physics, should I also worry about the law of unintended consequences here?

Have you even been in a situation where your sensors gave misleading results and thus caused major system performance problems? How did you finally figure out what was happening? What did you do to solve the problem, once you figured it out — or did you?

12 comments on “When Your Sensors Mislead You

  1. Bill_Jaffa
    January 30, 2013

    If you have never been bitten by bad sensor data misleading you, whether at the bench, in pre-production debug phase, actual production, or from units in the field–you're new to the real world! Be prepared and ready for it.

  2. Brad Albing
    January 30, 2013

    That's happened to me – but I was prepared for it. Always gotta check to make sure your data looks believable – not just what you want to believe.

    Incidentally, here's an example of what you're talking about. Icing is good on cake, but bad on aircraft wings.

    Wing icing

  3. eafpres
    January 30, 2013

    It would seem the ice sensors provide erroneous input, but it is the algorithm and resulting controls that cause the dive.  If you knew the sensor was giving wrong inputs, you could turn off the automatic control and at least avoid that problem.

    Elsewhere I suggested in some systems that more mundane fail-safes be included instead of leaving everything up to software.  Even with good design and a lot of testing, software does unpredicatble things with unpredictable inputs.  Within the last 20 years, and possibly more recently than that, commercial pilots would come back and look out the windows at the wings.  Now they watch color displays.

    I am bitten every few months by a sensor/alarm system that sees false positives and goes into alarm.  This usually happens in the middle of the night.  The system is the inerconnected smoke alarms in my house.  They are a bad design that is extremely sensitive to both the 9V battery condition and the line voltage.  At night the voltage seems to vary, and if the battery is even 6 months old they may go off.  I use a special Panasonic industrial battery to reduce the instance of false alarms.  But at least my house isn't going to fall from the sky.

  4. Michael Dunn
    February 1, 2013

    > Icing is good on cake, but bad on aircraft wings.

    Wish I'd said that 😉

    Not exactly statistically significant, but the only bad airplane experiences I've had have been on Airbusses. Hmm… I guess Boeing Lithium Flambé would qualify as a bad experience too.

  5. WKetel
    February 7, 2013

    Sensors feeding digital logic systems have a number of different ways of providing wrong data. Sometimes the sensor is simply not able to function correctly in the actual environment, many times the power for the sensor is not just exactly as clean as the sensor requires for correct operation. On some occasions the wrong sensor is installed. And sometimes the parameter being measured is not the correct variable to be examined to gain the required information. That would llead to incorrect evaluation of wing icing, for sure.

    In the case of the 787 batteries overheating, it is not clear as to if there was any sensor, and just where it was installed, if there was one, and what was done with the data it a proper sensor was correctly installed. The wrong processing algorithm almost always delivers a wrong answer, and that often leads to problems. This case does not seem to be an exception to that. So now the questions should be asked about what the process was for dealing with a hot battery. It seems that perhaps that is where the real failure will be found.

  6. Brad Albing
    February 12, 2013

    So, Boeing just needs to put their battery packs in the wings of the aircraft. There's an example of turning lemons into lemonade.

  7. Michael Dunn
    February 12, 2013

    It's lateral thinking like that that got you to where you are today.

  8. Brad Albing
    March 28, 2013

    What happened on the Airbus? Paying too much unwanted attention to the flight attendants? Or something of a more mechanical nature?

  9. Brad Albing
    March 28, 2013

    Crappy design on those smoke detectors. Where is Violet when you need her?

    The Ultimate Question

  10. Brad Albing
    March 28, 2013

    I had assumed it was bad karma. But could just be bad engineering.

  11. Brad Albing
    March 28, 2013

    Or perhaps just hot tea with lemon….

  12. Michael Dunn
    March 28, 2013

    More the latter. We had to ditch somewhere in Michigan, and I was stuck there all day. I seem to have blocked any other incidents from my memory.

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