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Control is the operation mode of a control system which includes two subsystems: controlling (a controller) C, and controlled, S. They interact, but there is a difference between the action of C on S, and the action of S on C. The controller C may change the state of the controlled system S in any way, including the destruction of S. The action of S on C is formation of a perception of system S in the controller C. This understanding of control is presented in Fig.1.

In Fig.2 we define the concept of perception. We see in the controller an agent which is responsible for its actions, and a representation of the controlled system, which is an object whose states we identify with perceptions. The relation between representation and agent is described as a flow of information: the actions of the agent depend on this flow. Thus the action of S on C is limited, in its effect, by changing only S's representation in C, not the rest of the system. Thus the asymmetry of the control relation: C controls S, but S does not control C. The action of S on C is "filtered" through the representation: its effect on C cannot be greater than allowed by the changing state of the representation.

Of course, two systems can be in a state of mutual control, but this will be a different, more complex, relation, which we will still describe as a combination of two asymmetric control relations.

In many cases the controlled system can be also seen in greater detail, which is done in Fig.3. We describe the controlled system using some variables and distinguish between the variables directly affected by the controller, from the variables which are observed by the controller in perception. The causal dependence of the observed variables on the affected variables is determined by the intrinsic dynamics of the system. We also must not forget about the effect of uncontrollable disturbances on the observed variables.

In Fig.3 we also made an addition to the controller: it now includes one more object which influences the agent: goal. The agent compares the current representation with the goal and takes actions which tend to minimize the difference between them. This is known as purposeful behavior. It does not necessarily result from the existence of an objectified goal; the goal may be built into the system -- dissolved in it, so to say. But a typical control system would include a goal as a an identifiable subsystem.

Even though the relation of control is asymmetric, it includes a closed loop. Looked from the controller, the loop starts with its action and is followed by a perception, which is an action in the opposite direction: from the controlled to the controller. This aspect of control relation is known as feedback.

The concept of control is the cornerstone of cybernetics. The basic control scheme which we have defined in this node is the unit from which complicated cybernetic systems are created by nature and man. For this reason, our definition is pretty wide: we want our building unit to be as universal as possible. In particular, we see as special cases of control some systems which most present authors would, probably, not call control.

The different components (e.g. perception, action, ...) of the control loop we have enumerated can in the limit be absent. This leads to different "degenerate" or "limit" cases, which we would not usually see as control systems, but which still share many properties with the more elaborate control scheme. Specific instances of this control scheme can further differ in the presence or absence of different attributes or properties characterizing control systems: separability, contingency, evolvability, asymmetry, ... This leads to a very broad view of control in which many very important types of system can be classified, as shown by many examples of control systems. The abstract scheme can also be mapped on different other schemes for control by authors such as Ashby, Powers and Meystel, and on an older definition in terms of statements and commands.

Copyright© 1996 Principia Cybernetica - Referencing this page

V. Turchin, F. Heylighen, C. Joslyn, & J. Bollen,

Oct 21, 1996 (modified)
Jun 21, 1996 (created)


Metasystem Transition Theory


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Properties of a Control System

Special Cases of Control

Examples and Counterexamples of Control Systems (empty)

Other Definitions of Control


Communication [empty]

Goal [empty]

Regulation [empty]

Conflict [empty]

Control hierarchy

Semantic Control

Buffering, feedback, feedforward: mechanisms of control


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