[Paul George 940722 11:20]
[From Bill Powers (940722.0510 MDT)]
I am beginning to think that this is how _naive_ engineers view the
process.
Note I meant that there is no disagreement on how a control loop actually
works, just on focus on what is spoken of being 'under control'. However, I
think something more like 'blissfully ignorant' would describe a frighteningly
large number of them. Unfortunately a sense of infallibility and of order in
the universe is a common attribute of engineers. Hazard analysis is rarely
performed, and there are a lot of naive assumptions about the environment and
equipment.
If the perceptualsystem is set up to perceive the wrong thing, the engineer
may believe that the environment is being properly controlled, ... But a
change in circumstances can leave the control system controlling quite
successfully while the environment goes to pot.
That is indeed the challenge in process control system design; figuring out
what needs to be sensed and how to sense it.
Suppose the engineer wants to control the temperature of a bath. He
gives the control system a thermocouple to register the temperature,
constructs the rest of the loop, and all is well. But the thermocouple,
as it happens, is sensitive to radiated energy as well as conducted
energy. During the tests, it happened to be shielded from radiated
energy, or there were no radiant energy sources to worry about. So
everything worked. But when the control system is moved into the factory
where there are powerful lights overhead, or sunlight coming in through
windows, it will start controlling for the sum of the input energies
from ALL sources, not just the intended one. When the lights go on, the
temperature in the controlled bath will go down!Worse yet, when the engineer is summoned to see this problem, he goes
right up to the bath and stands there, and sees that everything is
operating normally. When he goes away, removing his shadow from the
thermocouple, down goes the bath temperature again.
I wonder if you have any idea how realistic this example is? (though RF,
magnetic, or electrical interference is more common) Unfortunately a lot of
systems are designed this way. Problems like this often show up in the field.
And funny thing, when the equipment goes back to the shop, there is no problem
found.
"...And he examined the test data and saw that some of it was good, and some
not so good. He therefor divided the good data from the bad and he called one
'results' and the other he called 'spurious anomalies'+ 
Paul