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      06-19-2008, 12:32 PM   #41
jm1234
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Quote:
Originally Posted by lucid View Post
NASA and Boeing are in a different industry. They don't have any options other than to spend hundreds of millions of dollars to model whatever they can. But please don't tell me they can model everything. There is a guy next door to my office who does CFD models all day long to model very specific flow issues. It takes him months to come up with an "acceptable" model for a constrained case. Modeling buildings is also a different ball game. Yes, there is dynamic loading, but the loads are much more predictable. A car manufacturer will not invest the kind of resources NASA or Boeing invests into modeling the dynamics of a car. If testing was simply to verify the model, they would not need to spend thousands of hours driving the car around acquiring data. Iíve watched some clips on the development of the M3. Their test driver is an engineer. He explicitly said that the design engineers rely on him to assess handling and make suggestions to improve the behavior. Why would that be necessary if they had an accurate model which simply needed verification? Why canít BMW engineers arrive at a comprehensive accurate model after some iteration so that they can take the test driver off-line? Cars have been around for a long time. Although I am not a dynamics expert, I think you are oversimplifying the variation in potential road and driver inputs and their interactions--the sheer number of scenarios that can entail even if you have an accurate model. And, I doubt that the ECU is doing any kind of deterministic calculation in real-time that includes a general system model of the car, but of course, neither of us knows that. (The reason is that a car is not an unstable aircraft that needs to be actively managed to stay in the air). Computational models are the place to start and they have clearly transformed the way engineers work, but reality is complex and one discovers and learns a lot about the phenomenon while testing; testing is not just to validate the model. Models can and do change during testing. There is nothing wrong with that. (I am not advocating the type of approach the F1 "guru" made to solve the suspension problem although it makes a case about how complex a car suspension system can be if you want every bit of control).

P.S. The video was cool. But, without crunching the numbers, how can you say swapping springs will not do that!?
Buildings and bridges undergo predictable loads? Google Tacoma Narrows (awesome video) for the most obvious bridge example and think about what happens to a building during a fire, earthquake, hurricane, tornado and/or impact (planes and cranes being recent impact examples). What loads do you think a car undergoes that are as complex in comparison?

Car companies spend years and tens of millions developing a new car design. There is both time and money for modeling. Not as much as in other industries but the models are simpler.

A system controller would not include a model of the controlled system in it's design. A controller would be built based on a model of or even the real controlled system. I never said the ECU would be running a model of the car.

When an engineer speaks of model validation he is talking about an iterative process not a one time check.

The sheer number of scenarios is exactly the reason that a model is more helpful than real world testing. You can run a model through the same set of thousands of hours of "real world scenarios" much cheaper and faster than real world testing. There comes a point later in the design process where real world testing is more valuable but at that point you have limited the parameter ranges and are designing for subjective criteria.

Quote:
Originally Posted by lucid View Post
If testing was simply to verify the model, they would not need to spend thousands of hours driving the car around acquiring data. Iíve watched some clips on the development of the M3. Their test driver is an engineer. He explicitly said that the design engineers rely on him to assess handling and make suggestions to improve the behavior.
I've never met a test driver/pilot that didn't think the world rested on their shoulders. Seriously though, there is a difference between subjective design changes within defined parameter ranges and objective design changes to achieve objective design goals. It's the difference between fine tuning and course tuning.