Dear Colleagues, (01)
Last Thursday I complained that most ontologies do not give adequate
treatment to what I call system components, and if ontology is going to gain
traction within the systems world, it needs to get a better understanding of
this central idea in systems engineering. (02)
I illustrated the issue by telling the (simplified) life story of a system
component: the pump, P101, at the bottom of a distillation column. Here is
its story. (03)
The designer creates a drawing of the distillation column including at the
bottom of the column a pump to pump away the column bottoms. He labels it
P101, decides that one pump will be sufficient, and gives the specification
for the pump in terms of Net Positive Suction Head, differential head, flow
rate, materials of construction, and many other things. (04)
The construction engineer picks up the drawing and specification and notices
he has to install a pump as P101. Fortunately, he has a pump in stock from a
previous project, that has been in stores unused for 5 years which exactly
meets the specification. On it is stamped Serial No S3556. (05)
The designer and the Operator comes to see the pump be installed, and once
the connections are made, he gives the pump a friendly kick and says to the
construction engineer "It's good to see P101 realized at last". The
construction engineer says in return "Yes, and it's good to get S3556 off my
hands at last." He turns to the operator and says "Why don't we change your
drawings to show S3556 instead of P101?" The operator says "No, don't do
that, it's a replaceable part, and one day another pump will be put there,
and I don't want to have to change all the drawings and other documentation
that refers to P101 each time it is replaced, as far as I am concerned it's
the same pump whatever is installed there." (06)
Some time later the pump breaks down and needs to be taken back to the
workshop. The maintenance engineer says to the operator "Hi, can I take
S3556 installed as P101 back to the workshop?" The operator replies "Sure,
but what am I supposed to do without my P101? If it does not exist I cannot
operate my distillation column." The maintenance engineer responds, "I
understand. We have another pump S4567, that meets the same specification as
P101. We'll replace S3556 with it and you will only be without P101 for a
few hours. I don't understand how you can continue to call it P101 though
when all the parts have changed at once." The operator replies "I don't care
about that. What I care about is what is connected in my system to pump the
liquid from the bottom of the column. As long as it does that, it is P101 to
me." (07)
Later the distillation column is demolished. The operator says, "A sad end,
I was very fond of P101, but it is no more." The demolition engineer says,
"Yes indeed. Fortunately, we can take S4567 and use it on another plant." (08)
It's probably worth summarising the key characteristics of a system
component:
- It comes into existence the first time it is installed.
- It is identical to the equipment items installed, whilst they are
installed (but not before or after).
- It can survive complete replacement of all its parts at once.
- It can survive periods of non-existence.
- It ceases to exist when the system it is a component of ceases to exist. (09)
This is clearly rather different from the life of ordinary physical objects.
However, relatively few ontologies recognise that such things exist. Many
try to fob system components off as being classes, or abstract individuals,
though these clearly do not have the required characteristics. (010)
Ontologists need to step up to the mark here and provide proper recognition
for system components. (011)
Regards (012)
Matthew West
Information Junction
Tel: +44 1489 880185
Mobile: +44 750 3385279
Skype: dr.matthew.west
matthew.west@xxxxxxxxxxxxxxxxxxxxxxxxx
http://www.informationjunction.co.uk/
http://www.matthew-west.org.uk/ (013)
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