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Carmakers assure safety in case of electric power failure

Posted: 22 Jan 2008     Print Version  Bookmark and Share

Keywords:x-by-wire equipped car  car electronics  functional safety  electric power failure 

While airplanes crash in the case of a total electric power failure, cars merely get less comfortable—despite an increasing content of safety-critical electronic systems. At least this is what the automotive industry promises.

Just seconds before flight BA 038 crash-landed on Heathrow airport runway 27 L, the plane reportedly lost its entire electrical power system. As a consequence, all electronic systems apparently failed, the Boeing 777 with 152 persons on board raved rudderless into the ground.

Since today's cars increasingly rely on electronic systems, even for safety-critical functions, the obvious question is: What happens if the electrical power system fails in an x-by-wire-equipped car? To what extent do vital functions such as steering and braking rely on the availability of electrical power?

According to the German automobile club ADAC, the worst case in terms of power availability is far from being a rare exception for today's vehicles. Quite the contrary, electrical problems are the most frequent reasons for breakdowns. In 25 lakh runs in 2006/2007, the ADAC helpers at breakdowns registered battery failure as the cause for the malfunction in almost 850,000 cases. (However, a large portion of these problems occurred when the car was not used for a long period).

Fortunately, while sophisticated electronic subsystems such as FlexRay data buses, driver assistance systems and navigation displays cease to function the moment electrical power fails for whatever reason, the more vital and safety-critical functions do not rely solely on electricity, affirm automotive OEMs and tier ones all over the place. The key perception is redundancy, even though the automotive industry has a different approach as the aircraft industry where safety aspects are paramount for the design.

In aircrafts, all vital systems are two- or even threefold. Jet liners, for instance have three navigation systems on board. If they had only two and they would display different data, it would be difficult to determine which one is right, but with three systems, it is highly unlikely that two systems show exactly the same deviation.

Functional safety
For cost reasons, this redundancy approach is not possible in the cars. However, car engineers have a different concept in place that is supposed to guarantee a comparable safety level. The name of the approach is 'functional safety.' "The awareness of functional safety has dramatically increased over the past four or five years," says Martin Schmidt, head of Electronic Systems Safety at engineering service provider TUV Sud Automotive. For the design of safety-relevant automotive systems, there are standards in place that have to be followed worldwide. While, according to Schmidt, IEC 61508 contains global guidelines for functional safety, engineers presently are preparing the ISO standard WD 26262 for the automotive industry which will describe in a much more detailed way approaches how design engineers can make sure that their systems show hierarchical failure behaviour.

A spokesperson at automotive supplier Bosch Group describes what hierarchical behaviour means: If, for example, a malfunction occurs in an electronic brake system, the internal self-test first switches off the most sophisticated functions, leaving the more basic functions active as long as possible. "The electronic system monitors itself. If the software would find hints to a malfunction, it first would deactivate ESP (Electronic Stability Program), then the antiskid system and finally the anti-locking system (ABS). In any case, even if electric power fails, there will be a mechanical or hydraulical fall-back system that maintains the basic function."

Safety fall-back
"The hydraulic function is independent of the electric power", explains James Remfrey from Continental Automotive's advanced engineering department. He points out that x-by-wire in the automotive world is not equal to fly-by-wire as known in aviation. "A purely electrical x-by-wire system for cars is a dream of the future, to put it mildly," he said. "All available x-by-wire systems have a mechanical fall-back level." In electronically controlled steering systems, for instance, the rod between the steering wheel and the front axle is still in place; the worst that can happen to a driver in the case of an electrical failure is that he loses the comfort of power steering.

"In our design process, we strongly emphasise functional safety", promises also automotive OEM BMW. "Our engineers take every thinkable and even the most unlikely case in account in order to ensure that no intolerable safety critical state will ever occur," says a spokesperson.

With one exception: In today's cars, the gas pedal as the most widespread x-by-wire function typically has no mechanical fallback level. The pedal is connected to a sensor that transmits the pedal position to the engine. If power fails, even the angriest driver's kick will fail to spur the horses under the hood. But this is a feature, not a bug: An electrical failure will stall the motor anyway, since also the spark plugs cannot work without electricity.

Thus, the worst case of electricity failure is less dramatic for a car driver than for a jet liner pilot. Navigation systems, dashboard displays and entertainment systems will go out. And, of course the lights, which during night drives indeed can create dangerous situations. But, in contrast to a pilot, a driver at least has the possibility to safely slow down and stop at the side-strip.

- Christoph Hammerschmidt
EE Times Europe

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