feedback

Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence the same event/phenomenon in the present or future. When an event is part of a chain of cause-and-effect that forms a circuit or loop, then the event is said to "feed back" into itself.

Feedback is also a synonym for:

• Feedback signal; the information about the initial event that is the basis for subsequent modification of the event.
• Feedback loop; the causal path that leads from the initial generation of the feedback signal to the subsequent modification of the event.
• Audio feedback; the special kind of positive feedback which occurs when a loop exists between an audio input and output.

Overview

Feedback is a mechanism, process or signal that is looped back to control a system within itself. Such a loop is called a feedback loop. Intuitively many systems have an obvious input and output; feeding back part of the output so as to increase the input is positive feedback; feeding back part of the output in such a way as to partially oppose the input is negative feedback.

In more general terms, a control system has input from an external signal source and output to an external load; this defines a natural sense (or direction) or path of propagation of signal; the feedforward sense or path describes the signal propagation from input to output; feedback describes signal propagation in the reverse sense. When a sample of the output of the system is fed back, in the reverse sense, by a distinct feedback path into the interior of the system, to contribute to the input of one of its internal feedforward components, especially an active device or a substance that is consumed in an irreversible reaction, it is called the "feedback". The propagation of the signal around the feedback loop takes a finite time because it is causal.

The natural sense of feedforward is defined chemically by some irreversible reaction, or electronically by an active circuit element that has access to an auxiliary power supply, so as to be able to provide power gain to amplify the signal as it propagates from input to output. For example, an amplifier can use power from its controlled power reservoir, such as its battery, to provide power gain to amplify the signal; but the reverse is not possible: the signal cannot provide power to re-charge the battery of the amplifier.

Feedforward, feedback and regulation are self related. The feedforward carries the signal from source to load.

Negative feedback helps to maintain stability in a system in spite of external changes. It is related to homeostasis. For example, in a population of foxes (predators) and rabbits (prey), an increase in the number of foxes will cause a reduction in the number of rabbits; the smaller rabbit population will sustain fewer foxes, and the fox population will fall back. In an electronic amplifier feeding back a negative copy of the output to the input will tend to cancel distortion, making the output a more accurate replica of the input signal.

Positive feedback amplifies possibilities of divergences (evolution, change of goals); it is the condition to change, evolution, growth; it gives the system the ability to access new points of equilibrium.

For example, in an organism, most positive feedback provide for fast autoexcitation of elements of endocrine and nervous systems (in particular, in stress responses conditions) and are believed to play a key role in morphogenesis, growth, and development of organs, all processes which are in essence a rapid escape from the initial state.^{[citation needed]} Homeostasis is especially visible in the nervous and endocrine systems when considered at organism level. Chemical potential energy for irreversible reactions or electrical potential energy for irreversible cell-membrane current powers the feedforward sense of the process. However, in the case of morphogenesis, feedback may only be enough to explain the increase in momemtum of the system, and may not be sufficient in itself to account for the movement or direction of its parts.

When a public-address system is used with a microphone to amplify speech, the output from a random sound at the microphone may produce sound at a loudspeaker which reaches the microphone such as to reinforce and amplify the original signal (positive feedback), building up to a howl (of frequency dependent upon the acoustics of the hall). A similar process is used deliberately to produce oscillating electrical signals.

Feedback is distinctly different from reinforcement that occurs in learning, or in conditioned reflexes. Feedback combines immediately with the immediate input signal to drive the responsive power gain element, without changing the basic responsiveness of the system to future signals. Reinforcement changes the basic responsiveness of the system to future signals, without combining with the immediate input signal. Reinforcement is a permanent change in the responsiveness of the system to all future signals. Feedback is only transient, being limited by the duration of the immediate signal.

Types of feedback

Figure 1: Ideal feedback model. The feedback is negative if B < 0

When feedback modifies an event/phenomenon, the modification will subsequently influence the feedback signal in one of three ways:

• 1 - the feedback signal increases, leading to more modification. This is known as positive feedback.
• 2 - the feedback signal decreases, leading to less modification. This is known as negative feedback.
• 3 - the feedback signal does not change, indicating the phenomenon is in equilibrium.
• Note that an increase or decrease of the feedback signal here refers to the magnitude of the signal's absolute value, without regard to the polarity or sign of the signal. For example a change in signal value from +5 to +10 or from -3 to -6 are both considered to be increasing.

Positive feedback, which seeks to increase the event that caused it, as in a nuclear chain-reaction, is also known as a self-reinforcing loop.^[1] An event influenced by positive feedback will increase or decrease its output/activation until it hits a limiting constraint. Such a constraint may be destructive, as in thermal runaway or a nuclear chain reaction. Self-reinforcing loops can be a smaller part of a larger balancing loop, especially in biological systems such as regulatory circuits.

Negative feedback, which seeks to reduce the feedback signal that caused it, is also known as a self-correcting or balancing loop.^[1] Such loops tend to be goal-seeking, as in a thermostat which compares actual temperature with desired temperature and seeks to reduce the difference. Balancing loops are sometimes prone to hunting: an oscillation caused by an excessive or delayed feedback signal, resulting in over-correction.

The terms negative and positive feedback can be used loosely or colloquially to describe or imply criticism and praise, respectively. This may lead to confusion with the more technically accurate terms positive and negative reinforcement, which refer to something that changes the likelihood of a future behaviour.

Negative feedback was applied by Harold Stephen Black to electrical amplifiers in 1927, but he could not get his idea patented until 1937.^[2] Arturo Rosenblueth, a Mexican researcher and physician, co-authored a seminal 1943 paper Behavior, Purpose and Teleology^[3] that, according to Norbert Wiener (another co-author of the paper), set the basis for the new science cybernetics. Rosenblueth proposed that behaviour controlled by negative feedback, whether in animal, human or machine, was a determinative, directive principle in nature and human creations.^{[citation needed]}. This kind of feedback is studied in cybernetics and control theory.

Applications

In biology

In bio 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.

Biological systems contain many types of regulatory circuits, both positive and negative. As in other contexts, positive and negative don't imply consequences of the feedback have good or bad final effect. A negative feedback loop is one that tends to slow down a process, while 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).

Any self-regulating natural process involves feedback and/or is prone to hunting. A well known example in ecology is the oscillation of the population of snowshoe hares due to predation from lynxes.

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 ovaraian or testicular axis is largely controlled by positive and negative feedback, much of which is still unknown.

In 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.

In 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.

The most common general-purpose controller using a control-loop feedback mechanism is a proportional-integral-derivative (PID) controller. Each term of the PID controller copes with time. The proportional term handles the present state of the system, the integral term handles its past, and the derivative or slope term tries to predict and handle the future^{[citation needed]}.

In economics and finance

A system prone to hunting (oscillating) is the stock market, which has both positive and negative feedback mechanisms. This is due to cognitive and emotional factors belonging to the field of behavioural finance. For example,

• When stocks are rising (a bull market), the belief that further rises are probable gives investors an incentive to buy (positive feedback, 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).
• Once the market begins to fall regularly (a bear market), some investors may expect further losing days and refrain from buying (positive feedback), but others may buy because stocks become more and more of a bargain (negative feedback).

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.

In the 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.^[4]

In education

Young students will often look up to instructors as experts in the field and take to hear most of the things instructors say. Thus, it is believed that spending a fair amount of time and effort thinking about how to respond to students may be a worthwhile time investment. Sometimes the term "feedback" is used loosely or carelessly to refer to what is more accurately called reinforcement. Here are some general types of reinforcement that can be used in many types of student assessment:

(Adapted from Flemming and Levie.^[5])

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)^[6]

In electronic engineering

The main applications of feedback in electronics are in the designs of amplifiers, oscillators, and logic circuit elements.

The processing and control of feedback is engineered into many electronic devices and may also be embedded in other technologies.

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 results in unintended positive feedback, causing the output to oscillate or hunt^[7] Oscillation is usually an unwanted consequence of system behaviour.

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 insure stability often involves frequency compensation, one method of compensation being pole splitting.

The high-pitched squeal that sometimes occurs in audio systems, PA systems and rock music is known as audio feedback. If a microphone is in front of a speaker that it is connected to, the noise put into the mic will come out of the speaker. Since the mic is in front of the speaker, the original sound (now coming from the speaker) goes back into the mic. This happens over and over, getting louder each time. This process produces the squeal.

In government

Examples of feedback in government are:

• Elections
• Mass media
• Revolution
• Curfews
• Surveys

In 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 windmill was enhanced in 1745 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 speed of rotation 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.

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.

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

In email administration

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.

Examples of feedback in organizations:

• Financial audit
• Performance appraisal
• Shareholders' meetings
• Marketing research
• 360-degree feedback
• Walkouts
• Lockouts

See also

Look up feedback in Wiktionary, the free dictionary.

• Audio feedback
• Cybernetics
• Feed-forward
• Interaction
• Low-key feedback
• Negative feedback amplifier
• Optical feedback
• Perverse incentive
• Resonance
• Stability criterion
• Tactile
• Unintended consequence

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.
• Korotayev A., Malkov A., Khaltourina D. Introduction to Social Macrodynamics: Secular Cycles and Millennial Trends. Moscow: URSS, 2006. 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.