Systems engineering detail design help!

[RBam]

Hi,

I'm reading some books about engineering design but I can't find out how systems are detailed.

During conceptual design I know you can create a design which describes the top-level mechanisms for a system but leave the internal workings of them undescribed.

My question is what is the procedure for continuing design for the inside of those mechanisms?

Thank you!

 

[IRstuff]

You pass that on to the detail designer

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[GregLocock]

The general process is called a cascade. For example a car is initially designed on a spreadsheet. This then gets passed on to the ponytails in Styling, and the chassis and powertrain engineers. Meanwhile the crash people will be looking at the upcoming regs, and the hapless body engineers will begin the the long circular walk between manufacturing, crash, aero, durability and styling.

Overall we use the product development V, aka waterfall. Having more or less perfected that we are now switching to Agile, in part.



Cheers

Greg Locock


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[3DDave]

This is the process that results in some cars needing to have a wheel-liner removed to replace the battery (well, first the wheel needs to be removed; wait, first the car needs to be jacked up or put on a lift**) because everyone else took the room the battery would usually occupy for something else and all that is left is the usually inaccessible location where dirt and water are thrown at high speed with only a thin plastic part to keep that off the battery until snow or ice build up splits it.

Just noting that serviceability isn't on that V.

**The beauty of this solution is that if the battery fails the tech could strip a wheel stud or simply fail to tighten the lug nuts, turning a bad battery into a loss of steering and a crash. Adding additional failure modes, potentially fatal, because the battery guy got last place on the ladder.

 

[IRstuff]

Just noting that serviceability isn't on that V.

It would be if systems engineering was done properly; just because you have a process doesn't mean you get everything right. A certain large company's vaunted systems engineering process has hugely failed on several large programs

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[dik]

I've always wondered how that was done...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[3DDave]

My far smaller company's vaunted systems engineers did no better.

I feel like no one should be considered as a systems engineer until they are at least 40 and have been on at least 10 projects, proposal-to-finish.

All I see are PowerPoint warriors that do the initial project proposal and bail when the contract comes in.

 

[IRstuff]

The issue, I think, is that systems engineering is not just about the process; you need to know enough about the product to basically create a spec from scratch, by yourself. Otherwise, you're not going to be able to adequately determine all the requirements are correct, necessarily, and sufficient.

TTFN (ta ta for now)
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[MJCronin]

Boy, there is a lot of sarcasm and bitterness out there tonight in "Eng-tips Land" ...!!!

MJCronin
Sr. Process Engineer

 

[3DDave]

On the plus side - I encountered a guy who was whinging about questions on a CAD blog. The subject was how people converted from fully-machined parts to cast parts and he thought that was the most ridiculous thing in the world.

I mentioned that not only did that happen, but I had personally seen a weldment design discarded due to the cost of the individual x-raying of each segment for a casting design that was then discarded for lead time to a hogout which was then discarded due to machining cost with the people in charge saying "Why isn't this a weldment?" The same people who discarded the weldment in the first place.

After he recovered from that he related that where he worked when a new design was investigated the entire company process was represented: design engineers, stress, materials, manufacturing, assembly, inspection, procurement, contracts, and management. They continued meeting as a cohesive group until all were satisfied they all understood their part and that, collectively, the group understood what was going to happen.

That seemed like an ideal way to operate and, while the up-front cost showing up is what I am sure is the reason it's avoided, it certainly means that if any problem occurs then everyone involved has a good idea of the trade-offs that are reasonable to cope without having to start from scratch.

I wonder if he had understood just how lucky he was not to work in a place that regularly played "toss the grenade over the wall."

So, in answer to the original question - that should all happen at the same time and the feasibility study that is supposed to be part of systems engineering, would already have the internal workings identified to correctly rate all the other "ility" components before proposing this as a workable system.

==========

In my case this disconnect was partly due to a forced merger with another division that exacerbated the problem, and particularly that the other division had been so remotely located from their minimum wage factory that the factory, having never been well supported, decided to just do whatever they wanted. For them, engineering was needless overhead. The factory regularly informed engineering of the design changes that were required to document the product they had already produced. If those changes weren't instantly made the factory would report that "Deliveries were going to be delayed because <insert engineer's name> wasn't doing their job and this would affect cash flow."

 

[dik]

Over 50 years ago, I came across a series of six one hour lectures on Cybernetics; they were prepared by Dr. Ken McLachlan of the University of Southampton in the UK. This is the study of systems and the control of systems. You might want to glance into this area. To put it simply, if you have a system it has to do something. This needs to be contolled and this is accomplished by means of feedback. Not only does the error have to approach zero, but the rate of change of that error has to approach zero, too. This has to be controlled. I found the lectures fascinating. He started with the derivation of cybernetics...kubernetes/cybernetes, ancient Greek for the helmsman from Plato or earlier. Ken was one of the brightest people, I've ever encountered.

They were tapes of lectures given to one of his electrical engineering classes at the U. Sadly, they were lost about 40 years ago. One of his comments was prescient, and it related to climate change. He made reference to, "...we may already be on that slippery slope." I remember the quote exactly.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[GregLocock]

TCO - total cost of ownership, is a tracked parameter and might include the cost of battery replacement. However batteries should be lasting rather longer than a typical new car buyer keeps a car, in which case I suppose it won't make the cut.

You are right in general, the days when we could lay a car out on a full size drawing pinned to the office wall are long gone, these days it is mostly spreadsheets and 3 dimensional jigsaws. And for under the car, lots of heatshields.




Cheers

Greg Locock


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