Perhaps a Control Engineering perspective may be of help...
For an engineer to control an X, means that a variable or variables V in X
are to be controlled. This means that the evolution in time of V is to
satisfy some criterion C. This is referred as the "control problem". If the
"control system" manages to solve the control problem, its ok and one can
invoice the customer. Otherwise, back to work...;-)
For engineers it is useful to classify (and split) control problems in
- regulation problems, where one wants to maintain the controlled variable
value constant, according to C, against perturbations effects;
- servo problems, where one wants the controlled variable to follow,
according to C, some (possibly unknown in advance) evolution of a reference
variable (which is not constant).
The rational behind the classification is that in regulation one doesnt care
about the reference variable (which is assumed to be constant) and in
"servoing" one doesnt care about perturbations.
No real control problem is only "regulation" or only "servo", but the
classification is useful, if the problem is separable (one may define a
control for regulation and a control for servoing) or mainly falls in one of
For what concerns strategies an engineer may apply, mainly two are
recognized: feedback and feedforward.
Feedforward or antecipation is based on knowing how a system reacts to
commands or perturbations to generate a command which (presumably) will
yield the behavior required by the criterion. It can be used in the servo
problem and in regulation, if one knows or is able to measure the evolution
of reference or perturbations.
As a rule, feedforward is very sensitive to our ignorance of the system to
be controlled. It breaks down with (low levels of) model-to-reality
mismatch. Then one adds
Negative feedback. Which under reasonable constraints has the capability of
enforcing a system behavior in agreement with the criterion, compensating
perturbations by measuring their effects - in which one may include our
ignorance about the system.
In situations involving growth (of biomass, of an organization) one may also
use positive feedback as a strategy. (Positive feedback earned a bad
reputation because it leads easily to unstable systems. But note that in
growth the criterion calls for instability. Note also that negative feedback
is not a sufficent condition of stability.)
Is goal-directedness (as expressed by the criterion) necessary to the
control concept? In engineering, yes. In a mathematical sense, no. If one
aplies a "control system" to a "system to be controlled", then (if one is
able to send the invoice...;-) a "controlled system" with less *potential
behavior variety* was created. Less than the system to be controlled! So, I
think that one may talk of control whenever "potential behavior variety" is
diminished even if a goal is not distinguishable.
(One should not forget that "less variety at the system level" may mean
"more variety at the meta-system level").
Posting to firstname.lastname@example.org from Paulo Garrido <Paulo.Garrido@dei.uminho.pt>
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