[Martin Taylor 2006.12.11.09.55]
To Fred Nickols apparently Mon, 11 Dec 2006 08:10:10 -0500
My take on Fred's questions, the last one first:
Earlier, I used a sunlight example and
indicated that I had to squint (whether to continue driving safely or simply
to protect my eyes needn't detain us at this point). The sun, if I
understand Bruce Nevin's point, is the "source" of the disturbance. The
"disturbance" itself is the effect of the sun's rays on my retina. Do I
have that right?
My take is that the "disturbance" itself is the effect the sun's rays have, not on the retina, but on whatever perception(s) you are controlling. I doubt you are controlling a perception of the sun's rays hitting your retina. You are probably controlling perceptions of things relating to your driving, some of which are subject to disturbances related to the effect of the sun on your retina. Disturbances are disturbances to perceptions, usually occasioned by physical effects on the snesory organs.
It seems to me there are two classes of disturbance: (1) those that we are
consciously aware of and attend to as such and (2) those that are simply
compensated for in our behavior with no thought to disturbance as such.
I would say that we are never conscious of disturbances as such. What we are conscious of is perceptions. If we don't compensate for independently caused effects on our perceptions, those perceptions change. Some of those changed perceptions may be ones for which we have reference values.
If we do have a reference value for a perception, and some external effect influences it to change, then either we have reorganized in such a way that our actions compensate against the disturbance (your class 2), or we have not reorganized effectively and we must imagine a way to return the perception to its reference value (your class 1).
Even though a control system whose perception is being disturbed cannot see the source of the disturbance, other systems may be able to see both the source and the effect of the source on the disturbed system. It is those other systems that come into play in ...
The first kind of disturbance, the kind we view as such and to which we give
deliberate thought as to the means of compensation, is perhaps represented
by a businessman's response to the successful launch of a new product by a
competitor. The businessman is controlling for factors such as sales and
profitability and things were going along just fine. But, the competitor's
new product takes away sales and profits and requires a response of some
kind so as to restore sales and profits.
A new and different issue is suggested by...
The second kind... Another example from my Navy days: I spent 14 years on sea
duty, some of it in rough weather. As I walked down the deck, whether going
forward or aft, the rolling and pitching of the ship I had to adjust my gait
so as to maintain my balance and forward movement. ("Sea legs" is the
common term and I developed the rolling gait of many a sailor.) It is only
with hindsight and knowledge of PCT that I conceptualize a disturbance in
But when you came to land, did you not at first experience the land as swaying in the same way the ship did when you first put to sea? I certainly do after a time at sea. Where is the disturbance there? It's actually generated by your own (now unnecessary) compensatory actions, isn't it?
I think this is an interesting question in itself, but one that may belong in a different thread.