Editing Feedback (section)

==Types==

===Positive and negative feedback===
{{Main|Negative feedback|Positive feedback}}
[[File:Set-point control.png|thumb|350px|Maintaining a desired system performance despite disturbance using negative feedback to reduce system error]]
[[File:Outcome Feedback Negative Feedback Loop.png|thumb|An example of a negative feedback loop with goals]]
[[File:Process Feedback Loop.png|thumb|A positive feedback loop example]]
Positive feedback: If the signal feedback from output is in phase with the input signal, the feedback is called positive feedback.

Negative feedback: If the signal feedback is out of phase by 180° with respect to the input signal, the feedback is called negative feedback.

As an example of negative feedback, the diagram might represent a [[cruise control]] system in a car that matches a target speed such as the speed limit. The controlled system is the car; its input includes the combined torque from the engine and from the changing slope of the road (the disturbance). The car's speed (status) is measured by a [[speedometer]]. The error signal is the difference of the speed as measured by the speedometer from the target speed (set point). The controller interprets the speed to adjust the accelerator, commanding the fuel flow to the engine (the effector). The resulting change in engine torque, the feedback, combines with the torque exerted by the change of road grade to reduce the error in speed, minimising the changing slope.

The terms "positive" and "negative" were first applied to feedback prior to WWII. The idea of positive feedback already existed in the 1920s when the [[regenerative circuit]] was made.<ref name=mindell>
{{Cite book
|author=David A. Mindell
|title=Between Human and Machine : Feedback, Control, and Computing before Cybernetics.
|year= 2002
|publisher=Johns Hopkins University Press
|location=Baltimore, MD, US
|url=https://books.google.com/books?id=sExvSbe9MSsC|isbn=9780801868955
}}
</ref> Friis and Jensen (1924) described this circuit in a set of electronic amplifiers as a case where ''the "feed-back" action is positive'' in contrast to negative feed-back action, which they mentioned only in passing.<ref name=friis>
Friis, H.T., and A.G.Jensen. "High Frequency Amplifiers" Bell System Technical Journal 3 (April 1924):181–205.</ref> [[Harold Stephen Black]]'s classic 1934 paper first details the use of negative feedback in electronic amplifiers. According to Black:

{{Blockquote|Positive feed-back increases the gain of the amplifier, negative feed-back reduces it.<ref name=black>
H.S. Black, "Stabilized feed-back amplifiers", ''Electrical Engineering'', vol. 53, pp.&nbsp;114–120, January 1934.</ref>}}
According to Mindell (2002) confusion in the terms arose shortly after this:
{{Blockquote|...{{nbsp}}Friis and Jensen had made the same distinction Black used between "positive feed-back" and "negative feed-back", based not on the sign of the feedback itself but rather on its effect on the amplifier's gain. In contrast, Nyquist and Bode, when they built on Black's work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition.<ref name=mindell/>{{rp|page=121}}}}

Even before these terms were being used, [[James Clerk Maxwell]] had described their concept through several kinds of "component motions" associated with the [[centrifugal governor]]s used in steam engines. He distinguished those that lead to a continued ''increase'' in a disturbance or the amplitude of a wave or oscillation, from those that lead to a ''decrease'' of the same quality.<ref name=maxwell>{{cite journal|last=Maxwell|first=James Clerk|title=On Governors|url=http://en.wikipedia.org/wiki/File:On_Governors.pdf |archive-url=https://web.archive.org/web/20101226040115/http://en.wikipedia.org/wiki/File:On_Governors.pdf |archive-date=2010-12-26 |url-status=live|journal=Proceedings of the Royal Society of London|volume= 16|year= 1868 |pages= 270–283|doi=10.1098/rspl.1867.0055|s2cid=51751195|doi-access=free}}</ref>

====Terminology====
The terms positive and negative feedback are defined in different ways within different disciplines.

# the change of the ''gap'' between reference and actual values of a parameter or trait, based on whether the gap is ''widening'' (positive) or ''narrowing'' (negative).<ref name="Ramaprasad" />
# the [[Valence (psychology)|valence]] of the ''action'' or ''effect'' that alters the gap, based on whether it makes the recipient or observer ''happy'' (positive) or ''unhappy'' (negative).<ref name=herold1977>Herold, David M., and Martin M. Greller. "Research Notes. FEEDBACK THE DEFINITION OF A CONSTRUCT." Academy of management Journal 20.1 (1977): 142-147.</ref>

The two definitions may be confusing, like when an incentive (reward) is used to boost poor performance (narrow a gap). Referring to definition 1, some authors use alternative terms, replacing ''positive'' and ''negative'' with ''self-reinforcing'' and ''self-correcting'',<ref name="senge">
{{Cite book
|author=Peter M. Senge
|title=The Fifth Discipline: The Art and Practice of the Learning Organization
|year=1990
|publisher=Doubleday
|location=New York
|isbn=978-0-385-26094-7
|page=424
|url=https://archive.org/details/fifthdisciplineasen00seng
}}
</ref> ''reinforcing'' and ''balancing'',<ref name="sterman">
John D. Sterman, ''Business Dynamics: Systems Thinking and Modeling for a Complex World'', McGraw Hill/Irwin, 2000. {{ISBN|978-0-07-238915-9}}
</ref> ''discrepancy-enhancing'' and ''discrepancy-reducing''<ref name="carver">
Charles S. Carver, Michael F. Scheier: ''On the Self-Regulation of Behavior'' Cambridge University Press, 2001 
</ref> or ''regenerative'' and ''degenerative''<ref>
Hermann A Haus and Richard B. Adler, ''Circuit Theory of Linear Noisy Networks'', MIT Press, 1959
</ref> respectively. And for definition 2, some authors promote describing the action or effect as ''positive'' and ''negative'' ''[[Reinforcement#Reinforcement|reinforcement]]'' or ''[[Reinforcement#Punishment|punishment]]'' rather than feedback.<ref name="Ramaprasad" /><ref name="skinner">
BF Skinner, ''The Experimental Analysis of Behavior'', American Scientist, Vol. 45, No. 4 (SEPTEMBER 1957), pp. 343-371</ref>
Yet even within a single discipline an example of feedback can be called either positive or negative, depending on how values are measured or referenced.<ref>
"However, after scrutinizing the statistical properties of the structural equations, the members of the committee assured themselves that it is possible to have a significant positive feedback loop when using standardized scores, and a negative loop when using real scores."
Ralph L. Levine, Hiram E. Fitzgerald. ''Analysis of the dynamic psychological systems: methods and applications'', {{ISBN|978-0306437465}} (1992) page 123</ref>

This confusion may arise because feedback can be used to provide ''information'' or ''motivate'', and often has both a ''[[Qualitative property|qualitative]]'' and a ''[[Quantitative property|quantitative]]'' component. As Connellan and Zemke (1993) put it:
{{Blockquote|''Quantitative'' feedback tells us how much and how many. ''Qualitative'' feedback tells us how good, bad or indifferent.<ref name=Connellan>Thomas K. Connellan and Ron Zemke, "Sustaining Knock Your Socks Off Service" AMACOM, 1 July 1993. {{ISBN|0-8144-7824-7}}</ref>{{rp|page=102}}}}

====Limitations of negative and positive feedback====
While simple systems can sometimes be described as one or the other type, many systems with feedback loops cannot be shoehorned into either type, and this is especially true when multiple loops are present.

{{Blockquote|When there are only two parts joined so that each affects the other, the properties of the feedback give important and useful information about the properties of the whole. But when the parts rise to even as few as four, if every one affects the other three, then twenty circuits can be traced through them; and knowing the properties of all the twenty circuits does not give complete information about the system.<ref name=Ashby/>{{rp|page=54}}}}

===Other types of feedback===
In general, feedback systems can have many signals fed back and the feedback loop frequently contain mixtures of positive and negative feedback where positive and negative feedback can dominate at different frequencies or different points in the state space of a system.

The term bipolar feedback has been coined to refer to biological systems where positive and negative feedback systems can interact, the output of one affecting the input of another, and vice versa.<ref name=Smit>
{{cite book |title=Introduction to Bioregulatory Medicine |author1=Alta Smit |author2=Arturo O'Byrne |chapter-url=https://books.google.com/books?id=RzXAOUnCM3oC&pg=PA6 |page=6 |chapter=Bipolar feedback |isbn=9783131469717 |year=2011 |publisher=Thieme}}
</ref>

Some systems with feedback can have very complex behaviors such as [[Chaos theory|chaotic behaviors]] in non-linear systems, while others have much more predictable behaviors, such as those that are used to make and design digital systems.

Feedback is used extensively in digital systems. For example, binary counters and similar devices employ feedback where the current state and inputs are used to calculate a new state which is then fed back and clocked back into the device to update it.