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Feedback loop

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Feedback loop

For other uses, see Feedback (disambiguation).

Feedback is a process in which information about the past or the present influences the same phenomenon in the present or future. As part of a chain of cause-and-effect that forms a circuit or loop, the event is said to "feed back" into itself.

Feedback is also a synonym for:

  • Feedback signal - the measurement of the actual level of the parameter of interest.
  • Feedback mechanism - the action or means used to subsequently modify the gap.
  • Feedback loop - the complete causal path that leads from the initial detection of the gap to the subsequent modification of the gap.

History

Self-regulating mechanisms have existed since antiquity, and the idea of feedback had started to enter economic theory in Britain by the eighteenth century, but it wasn't at that time recognized as a universal abstraction and so didn't have a name.[2]

The verb phrase "to feed back", in the sense of returning to an earlier position in a mechanical process, was in use in the US by the 1860s,[3][4] and in 1909, Nobel laureate Karl Ferdinand Braun used the term "feed-back" as a noun to refer to (undesired) coupling between components of an electronic circuit.[5]

By the end of 1912, researchers using early electronic amplifiers (audions) had discovered that deliberately coupling part of the output signal back to the input circuit would boost the amplification (through regeneration), but would also cause the audion to howl or sing.[6] This action of feeding back of the signal from output to input gave rise to the use of the term "feedback" as a distinct word by 1920.[6]

There has been over the years some dispute as to the best definition of feedback. According to Ashby (1956), mathematicians and theorists interested in the principles of feedback mechanisms prefer the definition of "circularity of action", which keeps the theory simple and consistent. For those with more practical aims, feedback should be a deliberate effect via some more tangible connection.

"[Practical experimenters] object to the mathematician's definition, pointing out that this would force them to say that feedback was present in the ordinary pendulum ... between its position and its momentum - a 'feedback' that, from the practical point of view, is somewhat mystical. To this the mathematician retorts that if feedback is to be considered present only when there is an actual wire or nerve to represent it, then the theory becomes chaotic and riddled with irrelevancies."[1](p54)

Ramaprasad (1983) defines feedback generally as "information about the gap between the actual level and the reference level of a system parameter which is used to alter the gap in some way", emphasizing that the information by itself is not feedback unless translated into action.[7]

Types of feedback

Feedback is commonly divided into two types - usually termed positive and negative. The terms can be applied in two contexts:

  1. the context of the gap between reference and actual values of a parameter, based on whether the gap is widening (positive) or narrowing (negative).[7]
  2. the context of the action or effect that alters the gap, based on whether it has a happy (positive) or unhappy (negative) emotional connotation to the recipient or observer.[8]

The two contexts may cause confusion, such as when an incentive (reward) is used to boost poor performance (narrow a gap). Referring to context 1, some authors use alternative terms, replacing 'positive/negative' with self-reinforcing/self-correcting,[9] reinforcing/balancing,[10] discrepancy-enhancing/discrepancy-reducing[11] or regenerative/degenerative[12] respectively. And within context 2, some authors advocate describing the action or effect as positive/negative reinforcement rather than feedback.[7][13] Yet even within a single context an example of feedback can be called either positive or negative, depending on how values are measured or referenced.[14] This confusion may arise because feedback can be used for either informational or motivational purposes, and often has both a qualitative and a quantitative component. As Connellan and Zemke (1993) put it:

"Quantitative feedback tells us how much and how many. Qualitative feedback tells us how good, bad or indifferent."[15](p102)

The terms "positive/negative" were first applied to feedback prior to WWII. The idea of positive feedback was already current in the 1920s with the introduction of the regenerative circuit.[16] Friis and Jensen (1924) described regeneration 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 mention only in passing.[17] Harold Stephen Black's classic 1934 paper first details the use of negative feedback in electronic amplifiers. According to Black:

"Positive feed-back increases the gain of the amplifier, negative feed-back reduces it."[18]

According to Mindell (2002) confusion in the terms arose shortly after this:

"...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."[16](p121)

Even prior to the terms being applied, James Clerk Maxwell had described several kinds of "component motions" associated with the centrifugal governors used in steam engines, distinguishing between those that lead to a continual increase in a disturbance or the amplitude of an oscillation, and those which lead to a decrease of the same.[19]

Applications

Biology

In biological systems such as organisms, ecosystems, or the biosphere, most parameters must stay under control within a narrow range around a certain optimal level under certain environmental conditions. The deviation of the optimal value of the controlled parameter can result from the changes in internal and external environments. A change of some of the environmental conditions may also require change of that range to change for the system to function. The value of the parameter to maintain is recorded by a reception system and conveyed to a regulation module via an information channel. An example of this is Insulin oscillations.

Biological systems contain many types of regulatory circuits, both positive and negative. As in other contexts, positive and negative do not imply consequences of the feedback have good or bad final effect. A negative feedback loop is one that tends to slow down a process, whereas the positive feedback loop tends to accelerate it. The mirror neurons are part of a social feedback system, when an observed action is "mirrored" by the brain - like a self-performed action.

Feedback is also central to the operations of genes and gene regulatory networks. Repressor (see Lac repressor) and activator proteins are used to create genetic operons, which were identified by Francois Jacob and Jacques Monod in 1961 as feedback loops. These feedback loops may be positive (as in the case of the coupling between a sugar molecule and the proteins that import sugar into a bacterial cell), or negative (as is often the case in metabolic consumption).

On a larger scale, feedback can have a stabilizing effect on animal populations even when profoundly affected by external changes, although time lags in feedback response can give rise to predator-prey cycles.[20]

In zymology, feedback serves as regulation of activity of an enzyme by its direct product(s) or downstream metabolite(s) in the metabolic pathway (see Allosteric regulation).

Hypothalamo-pituitary-adrenal and gonadal axis is largely controlled by positive and negative feedback, much of which is still unknown.

In psychology, the body receives a stimulus from the environment or internally that causes the release of hormones. Release of hormones then may cause more of those hormones to be released, causing a positive feedback loop. This cycle is also found in certain behaviour. For example, "shame loops" occur in persons who blush easily. When they realize that they are blushing, they become even more embarrassed, which leads to further blushing, and so on.[21]

Climate science

The climate system is characterized by strong positive and negative feedback loops between processes that affect the state of the atmosphere, ocean, and land. A simple example is the ice-albedo positive feedback loop whereby melting snow exposes more dark ground (of lower albedo), which in turn absorbs heat and causes more snow to melt.

Computer Science

Recursion is a computer programming technique whereby a function is defined so as to invoke itself, or a data structure is defined by reference to itself. This allows an elegant solution to problems such as searching a tree or calculating factorials.

Control theory

Main article: Control theory

Feedback is extensively used in control theory, using a variety of methods including state space (controls), full state feedback (also known as pole placement), and so forth. Note that in the context of control theory, "feedback" is traditionally assumed to specify "negative feedback".[22]

Further information: PID controller

The most common general-purpose controller using a control-loop feedback mechanism is a proportional-integral-derivative (PID) controller. Heuristically, the terms of a PID controller can be interpreted as corresponding to time: the proportional term depends on the present error, the integral term on the accumulation of past errors, and the derivative term is a prediction of future error, based on current rate of change.[23]

Mechanical engineering

In ancient times, the float valve was used to regulate the flow of water in Greek and Roman water clocks; similar float valves are used to regulate fuel in a carburettor and also used to regulate tank water level in the flush toilet.

The Dutch inventor Cornelius Drebbel (1572-1633) built thermostats (c1620) to control the temperature of chicken incubators and chemical furnaces. In 1745, the windmill was improved by blacksmith Edmund Lee, who added a fantail to keep the face of the windmill pointing into the wind. In 1787, Thomas Mead regulated the rotation speed of a windmill by using a centrifugal pendulum to adjust the distance between the bedstone and the runner stone (i.e., to adjust the load).

The use of the centrifugal governor by James Watt in 1788 to regulate the speed of his steam engine was one factor leading to the Industrial Revolution. Steam engines also use float valves and pressure release valves as mechanical regulation devices. A mathematical analysis of Watt's governor was done by James Clerk Maxwell in 1868.[19]

The Great Eastern was one of the largest steamships of its time and employed a steam powered rudder with feedback mechanism designed in 1866 by J.McFarlane Gray. Joseph Farcot coined the word servo in 1873 to describe steam-powered steering systems. Hydraulic servos were later used to position guns. Elmer Ambrose Sperry of the Sperry Corporation designed the first autopilot in 1912. Nicolas Minorsky published a theoretical analysis of automatic ship steering in 1922 and described the PID controller.[24]

Internal combustion engines of the late 20th century employed mechanical feedback mechanisms such as the vacuum timing advance but mechanical feedback was replaced by electronic engine management systems once small, robust and powerful single-chip microcontrollers became affordable.

Electronic engineering

The use of feedback is widespread in the design of electronic amplifiers, oscillators, and logic circuit elements. Electronic feedback systems are also very commonly used to control mechanical, thermal and other physical processes.

If the signal is inverted on its way round the control loop, the system is said to have negative feedback; otherwise, the feedback is said to be positive. Negative feedback is often deliberately introduced to increase the stability and accuracy of a system by correcting unwanted changes. This scheme can fail if the input changes faster than the system can respond to it. When this happens, the lag in arrival of the correcting signal can result in over-correction, causing the output to oscillate or "hunt".[26] While often an unwanted consequence of system behaviour, this effect is used deliberately in electronic oscillators.

Harry Nyquist contributed the Nyquist plot for assessing the stability of feedback systems. An easier assessment, but less general, is based upon gain margin and phase margin using Bode plots (contributed by Hendrik Bode). Design to ensure stability often involves frequency compensation, one method of compensation being pole splitting.

Electronic feedback loops are used to control the output of electronic devices, such as amplifiers. A feedback loop is created when all or some portion of the output is fed back to the input. A device is said to be operating open loop if no output feedback is being employed and closed loop if feedback is being used.[27]

Negative feedback loops
When the fed-back output signal is out of phase with the input signal. This occurs when the fed-back signal is anywhere from 90° to 270° with respect to the input signal. Negative feedback is generally used to correct output errors or to lower device output gain to a pre-determined level. In feedback amplifiers, this correction is generally for waveform distortion reduction or to establish a specified gain level. A general expression for the gain of a negative feedback amplifier is the asymptotic gain model.
Positive feedback loops
When the fed-back signal is in phase with the input signal. Under certain gain conditions, positive feedback reinforces the input signal to the point where the output of the device oscillates between its maximum and minimum possible states. Positive feedback may also introduce hysteresis into a circuit. This can cause the circuit to ignore small signals and respond only to large ones. It is sometimes used to eliminate noise from a digital signal. Under some circumstances, positive feedback may cause a device to latch, i.e., to reach a condition in which the output is locked to its maximum or minimum state.

The loud squeals that sometimes occurs in audio systems, PA systems, and rock music are known as audio feedback. If a microphone is in front of a loudspeaker that it is connected to, sound near the microphone will come out of the speaker, be picked up by the microphone, and get re-amplified. If the loop gain is sufficient, howling or squealing at the maximum power of the amplifier is possible.

Software engineering and computing systems

Feedback loops provide generic mechanisms for controlling the running, maintenance, and evolution of software and computing systems.[28] Feedback-loops are important models in the engineering of adaptive software, as they define the behaviour of the interactions among the control elements over the adaptation process, to guarantee system properties at run-time. Feedback loops and foundations of control theory has been successfully applied to computing systems.[29] In particular, they have been applied to the development of products such as IBM's Universal Database server and IBM Tivoli. From a software perspective, the autonomic (MAPE, monitor analyze plan execute) loop proposed by researchers of IBM is another valuable contribution to the application of feedback loops to the control of dynamic properties and the design and evolution of autonomic software systems.[30][31]

Social sciences

A feedback loop to control human behaviour involves four distinct stages.[32]

  1. Evidence. A behaviour must be measured, captured, and data stored.
  2. Relevance. The information must be relayed to the individual, not in the raw-data form in which it was captured but in a context that makes it emotionally resonant.
  3. Consequence. The information must illuminate one or more paths ahead.
  4. Action. There must be a clear moment when the individual can recalibrate a behavior, make a choice, and act. Then that action is measured, and the feedback loop can run once more, every action stimulating new behaviors that inch the individual closer to their goals.

Reflexive feedback

A sociological concept that states a feedback association is created within a certain relationship whereby the subject/object that delivers a stimulus to a second subject/object, will in response receive the stimulus back. This first impulse is reflected back and forth over and over again.

Economics and finance

The stock market is an example of a system prone to oscillatory "hunting", governed by positive and negative feedback resulting from cognitive and emotional factors among market participants. For example,

  • When stocks are rising (a bull market), the belief that further rises are probable gives investors an incentive to buy (positive feedback - reinforcing the rise, see also stock market bubble); but the increased price of the shares, and the knowledge that there must be a peak after which the market will fall, ends up deterring buyers (negative feedback - stabilizing the rise).
  • Once the market begins to fall regularly (a bear market), some investors may expect further losing days and refrain from buying (positive feedback - reinforcing the fall), but others may buy because stocks become more and more of a bargain (negative feedback - stabilizing the fall).

George Soros used the word reflexivity, to describe feedback in the financial markets and developed an investment theory based on this principle.

The conventional economic equilibrium model of supply and demand supports only ideal linear negative feedback and was heavily criticized by Paul Ormerod in his book "The Death of Economics", which, in turn, was criticized by traditional economists. This book was part of a change of perspective as economists started to recognise that chaos theory applied to nonlinear feedback systems including financial markets.

World-system development

The hyperbolic growth of the world population observed till the 1970s has recently been correlated to a non-linear second-order positive feedback between the demographic growth and technological development that can be spelled out as follows: technological growth - increase in the carrying capacity of land for people - demographic growth - more people - more potential inventors - acceleration of technological growth - accelerating growth of the carrying capacity - the faster population growth - accelerating growth of the number of potential inventors - faster technological growth - hence, the faster growth of the Earth's carrying capacity for people, and so on.[33]

Education

In the majority of universities, teachers decide learning objectives and feedbacks to students.[34] Learners have different conceptions of learning activities and preconceptions about hierarchy in education. Some may look up to instructors as experts in the field and take to heart most of the things instructors say. This is the subject of study in the field of "formative feedback" or "formative assessment".

Types of reinforcement that operate in student assessment[35]
Reinforcement Example
Confirmation Your answer was correct.
Corrective Your answer was incorrect. The correct answer was Jefferson.
Explanatory Your answer was incorrect because Carter was from Georgia; only Jefferson called Virginia home.
Diagnostic Your answer was incorrect. Your choice of Carter suggests some extra instruction on the home states of past presidents might be helpful.
Elaborative Your answer, Jefferson, was correct. The University of Virginia, a campus rich with Jeffersonian architecture and writings, is sometimes referred to as "Mr. Jefferson's University".

A different application of feedback in education is the system for "continuous improvement" of engineering curricula monitored by the Accreditation Board for Engineering and Technology (ABET).[36]

Government

Examples of feedback in government are:

Email administration

Main article: Feedback Loop (email)

A mechanism to alert the purported sender of an email with information about the email.

In organizations

As an organization seeks to improve its performance, feedback helps it to make required adjustments. Feedback serves as motivation for many people in the work place. When one receives either negative or positive feedback, they decide how they will apply it to his or her job. Joseph Folkman says that to find the greatest level of success in an organization, working with other people, a person should learn how to accept any kind of feedback, analyze it in the most positive manner possible, and use it to further impact future decision making.[37]

Sterman (2000, p 14) makes the point that the use of the term feedback in organizations can sometimes be misleading.

In common parlance the term "feedback" has come to serve as a euphemism for criticizing others, as in "the boss gave me feedback on my presentation." This use of feedback is not what we mean in system dynamics. Further, "positive feedback" does not mean "praise" and "negative feedback" does not mean "criticism". Positive feedback denotes a self-reinforcing process, and negative feedback denotes a self-correcting one. ... Telling someone your opinion does not constitute feedback unless they act on your suggestions and thus lead you to revise your view.[10]

Examples of feedback in organizations:

See also

References

Further reading

  • Katie Salen and Eric Zimmerman. Rules of Play. MIT Press. 2004. ISBN 0-262-24045-9. Chapter 18: Games as Cybernetic Systems.
  • ISBN 5-484-00559-0
  • Dijk, E., Cremer, D.D., Mulder, L.B., and Stouten, J. "How Do We React to Feedback in Social Dilemmas?" In Biel, Eek, Garling & Gustafsson, (eds.), New Issues and Paradigms in Research on Social Dilemmas, New York: Springer, 2008.
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