[From Fred Nickols (980224.1955 EST)]
Rick Marken (980223.1000) --
responding to Bruce Abbott (980223.0955 EST)--
It would appear that Rick's disagreement with Fred comes down to
whether the disturbance should be defined as d or as g(d).
No. It's whether the disturbance should be defined as d (variations
in environmental variables) or c (variations in controlled variables).
that once you are able to understand and/or accept this point
[that variations in disturbance variables are _not_ reflected
in variations of the controlled variable] you will be able to
stop fighting so hard against PCT and start acting as its true
This is all proving very helpful to me so let me introduce a somewhat
different example to broaden my grasp of the concept. And let's go back
to my Navy days, on board a destroyer.
I've got a gun mount aft on the fantail, loaded with servomotors,
amplifiers, receiver-regulators, motor-generators, and all kinds of
mechanical and hydraulic means of moving the gun mount around.
Further forward, roughly amidships and down one deck, is a plotting room
with a computer that calculates and issues gun orders. The ordered position
from the computer is a signal to the gun mount. The ordered position
represents a reference condition. The gun mount has a comparator, a
receiver-regulator. If the actual position and the ordered position are
different, an error signal results and the motors move the gun mount to
align the two, that is, bring the ordered and actual positions into
correspondence -- and, try to keep them in correspondence (because the
ordered or reference position is usually changing).
The actual position of the gun mount is never, except fleetingly if at all,
in perfect correspondence with the ordered position. The gun mount "hunts"
or "oscillates" about the point of correspondence (error being necessary to
the functioning of a servomechanism, or so the engineers told me, and also
because of "lag" -- that is the rate of change in the changing ordered or
reference position; in other words, it takes the gun mount a little time to
catch up to the ordered position, by which time it has changed again.
Moreover, if it does catch up and stay in correspondence with an ordered or
reference condition that has a stable rate of change, if that rate of change
changes, the gun mount once again "lags" (or "leads" the ordered position if
the signal reverses direction/polarity). As I've mentioned before, torque
and stability are factors in how well the gun mount will "track" the ordered
position, but let's set that aside as momentarily irrelevant.
What I've described so far seems to me to be a "closed-loop" control system
that is perfectly consistent with PCT (the closed-loop control system is the
gun mount, not the entire fire control system -- as Mary pointed out a while
back, the overall fire control system is not a closed loop). So far, so
Now, let's introduce a disturbance. The gun mount is out on the starboard
quarter (right rear for you landlubbers, moving slowly forward (to the
left). The ship now rolls to starboard, meaning that the gun mount moving
left must now also move up to counteract the effects of gravity. Let us
treat those effects as a disturbance, an external factor that affects the
correspondence between the gun mount's reference (ordered) position and its
perceived (actual) position.
If I understand what Rick was telling me earlier, this disturbance will have
no effect on the correspondence between the actual and the ordered position
of the gun mount (just as the wind will have no effect on my perception of
the alignment of the automobile I'm driving with my preferred position of
centered in the lane). Further, as I understand PCT, this is because the
control loop is a closed-loop and everything is "live" and happening
To borrow Rick's words from the beginning of this post, "variations in
disturbance variables [gravity, in the case of the gun mount example] are
not reflected in variations of the controlled variable [the position of the
gun mount] you will be able to stop fighting so hard against PCT and start
acting as its true champion."
Do I have it right?