Feedback

'''Feedback''' is an important term from cybernetics and control theory which 
characterizes a process in which part of the output of a system is 
returned (fed back) to its input in order to regulate its further 
output. It describes a recursive process.
If the output becomes part of the input or changes the further 
ouput, and the flow of information is closed, a feedback loop has been 
established. In a feedback loop the output of a system is sent continuously
back to regulate the input or output. Causal loops and circular processes 
are typical for systems with feedbacks. If this feedback is used to control a 
system (as for example in the [[Observer-Controller Pattern]] emphasized
by [[Organic Computing]]), the feedback signal indicates at the same time 
the current state of the controlled element.

== Definition ==

{{SelfOrg}}

Feedback is a circular process in which part of the output of a system 
is returned to its input in order to regulate its further output.
According to the [http://www.necsi.org:16080/community/wiki/index.php/Feedback NECSI Wiki],
"Feedback is a circular process of influence where action has effect on the actor".

== History ==

The idea of feedback loops originated in the field of cybernetics, control
theory and electrical engineering around 1950 and 1960. 

W.R. Ashby noticed the importance of feedback in coupled 
systems for forms of [[Self-Organization|self-organization]], which are more than 
transitions from ‘parts separated’ to ‘parts joined’ (Ashby 1962). 
He argued that no machine could by itself change its own organization,
if the behavior of each part is independent of the other parts' states. 
He added further that a ‘self-organizing’ machine is only possible 
by a machine coupled to another machine.

== Types and Forms ==

=== Cognitive Feedback Loops ===

The concept of an autonomic manager used in
[[Organic Computing]] and [[Autonomic Computing]]
in order to achieve [[Self-Star Properties|self-* properties]]
is usually based on a feedback loop. This cognitive feedback
loop consists of a perception phase, where the state of the
managed element is observed, monitored and analyzed, and
and a corresponding action phase, where the managed element
is controlled through corrective action which is planned and 
executed. 

* [[Perceive-Reason-Act Cycle|perceive-reason-act cycle]] in autonomous [[Agent|agents]] 
* OC ([[Observer-Controller Pattern]]) loop in [[Organic Computing]]
* MAPE (Monitor-Analyze-Plan-Execute) loop in [[Autonomic Computing]]

=== Positive and Negative Feedback ===

'''Negative feedback''' means a higher output reduces further input. 
Negative feedback loops are balancing, regulating and stabilizing. 
They oppose any direction of change which is imposed on the 
system. Negative feedback results in robust and stable systems, 
leveling of differences and equalization or stabilization in general, 
and can be used to regain equilibrium. In mathematics this kind of 
behavior towards a limt or fixed equilibrium state is named convergence.

'''Positive feedback''' means a higher output
of the system increases further input. 
Positive feedback loops are amplifying, reinforcing, and 
enhancing. They reinforace a direction of change.
They leads to instable systems or polarization in general, 
rapid amplification of differences, and allows systems to
convert graded, uncertain inputs into decisive, 
all-or-none outputs, see also the entries for 
[[Butterfly Effect]], 
[[Path Dependence]] and
[[Frozen Accident]]s.
In mathematics this kind of behavior related
to unrestricted growth is named divergence.

=== Iteration ===

[[Iteration]] is the mathematical counterpart of feedback and recursion.
Examples are a [[Cellular_Automata|CA]] or a
[[Random_Boolean_Network|RBN]].

== Relations ==

Feedback is also related to recursion, self-invocation,
recurrence, iteration iterated maps and functions, 
Iterative Function Systems (IFS) and self-similarity.
Recursion and self-invocation mean a function is 
defined in terms of itself, which causes a function
to invoke or call itself during runtime.

== Links and References ==

W.R. Ashby, Principles of the self-organizing system, in “Principles of Self-Organization: Transactions of the University of Illinois Symposium”, H. Von Foerster and G. W. Zopf, Jr. (eds.), Pergamon Press (1962) pp. 255-278.

* [http://pespmc1.vub.ac.be/FEEDBACK.html PCP Entry for Feedback]

* [http://en.wikipedia.org/wiki/Feedback Main Wikipedia Entry for Feedback]

* [http://www.necsi.org:16080/community/wiki/index.php/Feedback NECSI Entry for Feedback]

[[Category:Basic Principles]]