Quote:
Originally Posted by SantaMonica335i
I still have a hard time believing that a mechanically linked steering system gets disabled due to an electronic or other system problem. I can see power steering going out but that would only make steering harder. Are our cars fly by wire and i'm not aware?
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If you have ActiveSteering, there is a question about a possible failure mode of the electric motor and/or its controller within the steering assembly and potential lack of steering control.
In a nutshell, there is a planetary gear assembly in the steering column. There is an electric motor connected to the outer ring gear of the planetary gear set via a worm gear. When the steering control logic determines that standard rate steering is best for the driving conditions, the electric motor spins to keep the planetary outer ring gear aligned with the upper steering column (and steering wheel) such that movement of the steering wheel provides a linear movement of the steering rack, thus front wheels. At low speed where a higher ratio is required, i.e. manoeuvring like parking, etc..., the electric motor spins the ring gear faster than the steering column, resulting in the steering rack rotating the front wheels through a greater angle relative to the steering wheel's movement. Conversely, at high speeds where a lower steering ratio is desired in order to enhance stability, the electric motor under drives (in some cases, BMW videos even show reverse drive of the planetary ring gear) the outer ring gear relative to the steering column input, resulting in less movement of the front wheels relative to steering wheel input.
More benign failure modes of the active steering would likely be failure of the electric motor itself, such that steering output would revert close to or back to a linear, unmodified steering ratio.
The thoughts by some, myself included, is that a more insidious failure of the
active steering control module could actually drive the electric motor in a partially, or completely uncontrolled manner, potentially modifying drastically not only the steering ratio, but the nominal centering point of the steering assembly itself. Such a failure would be characterised by erratic and seemingly (to the driver) uncontrolled steering action, including a steering wheel where the "straight forward" position may actually occur with the steering wheel offset significantly from the perfect straight up position.
Of particular interest is the effect of the failure of the steering angle sensor, or the "SZL" module. This module was confirmed to have failed on arguru's vehicle. As others have noted, the car can only diagnose itself and store a code for operational or failure modes that were anticipated and planned for by BMW's engineers. Other failure modes that may, or might not even throw a code, still have the potential to cause apparently uncontrolled behaviour.
Folks who write such discussion above off as "musings" that were 'unreasonable' or uneducated' would be wrong. While BMW is no doubt a big company with undoubtedly thousands of competent and highly trained engineers, all trying to contribute to a well made product, that does not mean that they get things perfect. Nobody gets things perfect. There will always be some system with a potential unanticipated failure mode that the engineers, as diligent as they were, did not, or perhaps could not have foreseen.
Personal case in point, in my day job I pilot aircraft, and years ago, I was flying an aircraft model made by the largest aerospace manufacturer on the planet, i.e. lots of good, conscientious, well-trained engineers. At one point during the flight, rather close to the ground, I had an un-commanded flight control output that pretty much pitched the nose down towards the ground. Pulling the control stick back as one would do in normal operation did not result in the desired reaction by the aircraft. Long story short, but disabling some other systems then using controls in an unconventional manner, the aircraft captain and I (co-pilot at the time) regained control of the aircraft and recovered without further incident. Aside from about 30 folks needing to do some laundry that day, we provided a report to the manufacturer about what had happened, and after their assessment of what happened, the particular aircraft's flight manual was modified to include detailed procedure on recovering control from condition X.
The point is, the manufacturer had designed a system that they quite reasonably thought to have been designed in a fail-proof mode. My aircraft captain and I (and some other crews elsewhere, it turned out) had found a particular condition where the flight control behaviour did not occur as intended. The manufacturer's engineers analyzed the failure mode, and embodied modifications and amended the flight manual to include appropriate actions to minimize the probability of, and to correctly react to the situation.
I see no reason why a fundamentally similar situation could not happen in this case. Engineers design an improved system, believing the potential disadvantages of a more complex system are outweighed by the advantages of increased control. There is a potential failure mode that is not identified, not through incompetency, but just the nature of design in an increasingly complex world. Test plans are established to ensure safety and performance, but they can't catch 100% of the problems, 100% of the time.
What is at issue here was the manner in which the original poster was dealt with by the vendor dealer, and then, it would appear by BMW themselves.
I feel for arguru, because I know that if the aerospace company that made my aircraft had tried to blow our crew (and operating organization) off with an "it didn't happen as you say...it couldn't, we know, we built the aircraft" I'd be pretty pissed off. The company didn't though, it dealt with the issue immediately and without blame to us or any other crew that believed we had experienced something unintended by the OEM.
My appreciation is that is all that arguru is looking for: acknowledgement of his occurrence and a willingness by, at this point, BMW (I would write off any dealings with the dealer in question - it just seems like an organization that has very little sense in the appreciation of customers, any and all customers) to assess his car's behaviour to indeed see if there is a greater issue, a safety issue, with the Active Steering potentially failing in a manner that BMW had not previously identified.
Mein zwei pfenning,
D.