From Bob Clark (931117.1700 EST)
The subject of "OPEN LOOP" has been discussed several times over the
past weeks -- often in connection with "FEEDFORWARD." Recently
several posts have continued the topic.
In this discussion, Open Loop seems to be restricted to a single
level that has no form of negative feedback. This is a
straight-through, S-R system, that simply follows its reference
signal. However some kind of higher level system is required to
provide the reference signal.
I. OPEN LOOP OCCURRENCE.
Given such a combination of at least two levels, there are three ways
in which a lower system can be an Open Loop system: A. The feedback
signal(s) can be interrupted or absent (considered in several posts);
B. The output signal(s) can be disconnected from the environment; or
C. Both conditions can exist. In each case, there is no Loop: this
is an Open Loop situation.
A. Feedback signal interrupted or absent.
Examples of this case seem to be rather rare. Leprosy is one
example. It is my understanding that the sensory nerve endings are
damaged, with loss of feeling in the skin, particularly of the
fingers and toes. Repeated unnoticed accidental injury results in
"wearing away" of the digits.
Of course any form of damage to sensory nerves has similar effects.
The victim can sometimes use his imagination to fill the gap.
Imagined perceptions might be combined with current perceptions to
achieve limited control.
B. Output signals disconnected.
There are many examples of this case. These are typically ballistic:
the variable is given some form of "push" or "pull" and continues its
reaction without further intervention. The results continue to be
perceived, but control is no longer possible. Presumably this action
has been initiated by a higher level system seeking its own
objectives. Some examples:
1. The hunter. Finds his prey and shoots. If he misses, he's lost
his chance. The aiming-firing systems lose control at the moment the
trigger is pulled. But the hunter's higher level, that selected the
target and initiated the firing process, perceives and records the
event and the results. His attention is directed to the success of
the hunt.
2. Target practice. Here the "hunter" can experiment with different
procedures, seeking ways to improve his accuracy for future use.
Control of the bullet is still lost at the moment of firing, but the
perceptions are recorded for later application. With repeated
attempts, a variety of recordings becomes available for later
selection and application. Attention is directed to the
aiming-firing procedure and its effectiveness.
3. Ball throwing. Referring to my preceding post, Bob Clark
(931113. 1715), the thrower of the ball is operating his "throwing
system" in open loop -- until he can see (his higher systems) where
it goes. Then he can make adjustments for his next try. But, after
the ball leaves his hand, it is an open loop situation. This is open
loop for any single throw of the ball. At the same time, the higher
level system is perceiving and recording both the trajectory of the
ball and performance of the "throwing system." As with target
practice, repeated attempts produce a variety of recordings that are
available for later selection and application. Here attention is
also directed to the throwing procedure.
4. Machine tools. These machines usually do not include sensors
that detect, or measure, the results. Thus the machine has no
negative feedback. The operator adjusts the settings, turns on the
machine and lets it run. The feedback needed for proper control is
provided by the operator. If he "drops dead," or otherwise fails to
watch the machine (perhaps he is distracted by a visit from the
company president), the machine goes on its way. The situation
requires the operator to close the feedback loop by monitoring the
machine's operation, at least occasionally. Here the machine is the
lower level, open loop portion, and the operator provides the
reference signals. Here attention is also directed to the over-all
operation of the equipment.
5. Academic tests. These are open loop because control is lost from
the beginning of the test. The results are of interest from several
viewpoints: a) the teacher can evaluate his own performance; b) the
students can evaluate their progress; c) parents and prospective
employers can draw their own conclusions. As above, recordings are
formed, and can be used in various ways. Here also attention is
directed to the entire testing process, including the results, etc.
6. Exploration/Experimentation. Any kind of "exploratory" activity
is open loop. Visiting a new restaurant, selecting a new TV program,
etc. Trying a new combination where the possibilities can not be
fully predicted, is open loop. The experimental method always
includes some degree of uncertainty of the results. A well designed
experiment, or series of experiments, reveals previously unknown,
unpredicted, results. Loops are closed at higher levels for
evaluation of the experiment. The whole series of events is recorded
as in the preceding examples. Attention is directed to the entire
experimental design, procedure and results.
7. Demonstration. If the results are known beforehand, it is a
"demonstration" rather than an "experiment." The demonstrator
determines the conditions to be used before beginning the
demonstration. if the results differ from his expectations, he
modifies the conditions and repeats the demonstration until his
expectations are confirmed. There are several variations on this
sequence, depending on the demonstrator's purposes and skills. Here
attention is directed toward consistency between the intended
performance and the over-all observations.
C. Both Feedback and Output disconnected.
1. Posts to the Network. Once completed and distributed they not
only cannot be changed, but also their reception is only perceived
when someone else responds.
2. Video Programs. Once broadcast, they are unchangeable, and
perception of reception awaits viewer response.
3. Other Media. Much the same applies to radio and print media.
II. TIME SCALES.
The open loop system is always operating with a faster time scale,
and the higher level system's loop(s) are closed within its slower
time scale.
Regards, Bob Clark