Machines, and considered that in which the machinery is Marko-
Vian, we can now take up again the thread dropped in S.12/7, and
Can specialise further and consider the case in which the probabili-
Ties have all become 0 or 1 (S.12/8), so that the machinery is deter-
Minate. We continue with the regulator that is error-controlled. In
Order, as biologists, to explore thoroughly the more primitive forms
Of regulation, let us consider the case in which the feedback has a
Variety of only two states.
An example of such a system occurs in the telephone exchange
When a selector starts to hunt for a disengaged line. The selector
Tries each in turn, in a determinate order, gets from each in turn the
Information “engaged” or “disengaged”, and stops moving
(arrives at a state of equilibrium) at the first disengaged line. The
Set of disturbances here is the set of possible distributions of
“engaged” or “disengaged” among the lines. The system is regu-
Latory because, whatever the disturbance, the outcome is always
Connexion with a disengaged line.
235
A N I N T R O D UC T I O N T O C Y B E R NE T I C S
TH E ERR O R- CO N TR O LLED REG U LA TO R
The mechanism is known to be error-controlled, for the infor-
Mation that determines whether it shall move on or stick comes
From the line itself.
This case is so simple as to be somewhat degenerate. If we pay
No attention to the internal actions between R and T, so that they
Fuse to form the F of S.10/5, then the case becomes simply that of
A determinate system which, when the initial state is given, runs
Along a determinate trajectory to a state of equilibrium. Thus
every basin with a state of equilibrium in η can be said to show a
Simple form of regulation; for it acts so as to reduce the variety in
The initial states (as disturbance D) to the smaller variety in the ter-
Minal state.
Much the same can be said of the rat that knows its way about
A warehouse; for wherever it gets to it can make its way back to
The nest. As much can be said for the computer that is pro-
Grammed to work by a method of successive approximation; for,
At whatever value it is started, the successive values are moved
|
|
|
Determinately to the goal, which is its only state of equilibrium.
Ex.: A card is to be found in a shuffled pack of 52 by examination of them one
By one. How many will have to be examined, on the average, if (i) the cards
Are examined seriatim, (ii) if one is drawn, examined, returned if not wanted,
The pack shuffled, a card drawn, and so on? (Systematic versus random
Searching.)
The diagram of immediate effects is specially worth noting. It is
Disturbances
Temp. of
Incubator
Temp. of
Eggs
Temp. of
Capsule
Diam. of
Capsule
Or some equivalent form. In it, D, T, R and E are readily identified
(though the distinctions between T and R and their parts are some-
What arbitrary). The whole acts to block the passage of variety from
The Disturbances (whatever they are) to the eggs. If the aim of the
Regulator is re-defined slightly as being to keep constant the temper-
Ature of the incubator, then the regulator is controlled by the error
Rather than by the disturbances themselves.
In this form of regulator, the system must, of course, be stable
For any given disturbance, and the desired temperature must be the
System’s state of equilibrium. The feedback around the circuit
Must thus usually be negative.
Many regulators in the living body are of this simple form, and
Cannon’s work has made them well known. Typical is that which
Regulates the pH of the blood by the amount of carbon dioxide in it:
Disturbances
PH of the
Blood
PH of body’s
Tissues
Activity of
Respiration
Size of flame
When the machinery is all determinate, the problem of S. 12/
Дата добавления: 2019-11-16; просмотров: 205; Мы поможем в написании вашей работы! |
Мы поможем в написании ваших работ!