Wednesday, January 8, 2014

787 Battery As A Cyber-Physical System

The problems with the 787 battery a year ago haven't recurred, thankfully.  So now is a good time to step back and think about the implications of this issue for cyber-physical system design.  Here are a few informational links:
  • Aviation Week provided excellent coverage, including this description of battery testing and a photo of the damaged battery.
  • Boeing's review of the battery certification process. 
  • An interesting article from Avionics Magazine on the battery issue.
While the battery behavior itself is ultimately a chemistry issue that is far beyond my expertise, the root causes go back to cyber-physical system design.  During normal operation after the engines are started on the ground, the airplane's electric energy comes from the auxiliary power unit, a small jet engine driving an electric generator. The battery is provided in part as an emergency electricity source in case the main generator fails.  The 787 is a fly-by-wire aircraft, so the flight controls depend on electronics.  But the 787 goes well beyond minimal fly-by-wire to use a great deal of electronics that are important to the aircraft operation.  As a result, the battery needs to be able to supply a great deal of energy in some cases.  The battery technologies generally used in aircraft---and those that are generally approved under certification procedures---provide lower energy density than does the lithium battery technology used in the 787.  Energy density is very important in an airplane because weight is a critical factor in overall aircraft performance.  Aircraft designs must be certified.  Although lithium batteries had been used in certified aircraft design in a handful of cases, the 787 represented an early use of this type of battery.  The detailed design of the battery subsystem was driven by competing system constraints: on the one hand, weight; on the other hand, heavy use of electronics.

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