Feedback

Feedback  describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or occurrences of the same (i.e. same defined) event / phenomenon (or the continuation / development of the original 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 that 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. In systems containing an input and output, feeding back part of the output so as to increase the input is positive feedback (regeneration); feeding back part of the output in such a way as to partially oppose the input is negative feedback (degeneration).

Generally, 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 provides 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 that are, in essence, a rapid escape from the initial state. 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 momentum 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 that 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


When feedback acts in response to an event/phenomenon, it can influence the input signal in one of two ways:
 * 1) An in-phase feedback signal, where a positive-going wave on the input leads to a positive-going change on the output, will amplify the input signal, leading to more modification. This is known as positive feedback.
 * 2) A feedback signal which is inverted, where a positive-going change on the input leads to a negative-going change on the output, will dampen the effect of the input signal, leading to less modification. This is known as negative feedback.
 * Note that an increase or decrease of the feedback signal here refers to the magnitude relative to the input signal's absolute value, without regard to the polarity or sign of the feedback signal. For example if the input signal changes by 100 then a change in feedback signal value from +5 to +10 is a positive feedback. If the input signal changes by -100 then a change in the feedback signal from -5 to -10 is also a positive feedback.

Positive feedback tends to increase the event that caused it, such as in a nuclear chain-reaction. It is also known as a self-reinforcing loop. An event influenced by positive feedback can 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 tends to reduce the input signal that caused it, is also known as a self-correcting or balancing loop. 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 negative feedback signal, resulting in over-correction, wherein the signal becomes a positive feedback.

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. Moreover, when used technically, negative feedback leads to stability that is, in general, considered good, whereas positive feedback can lead to unstable and explosive situations that are considered bad. Thus, when used colloquially, these terms imply the opposite desirability to that when used technically.

Negative feedback was applied by Harold Stephen Black to electrical amplifiers in 1927, but he could not get his idea patented until 1937. Arturo Rosenblueth, a Mexican researcher and physician, co-authored a seminal 1943 paper Behavior, Purpose and Teleology 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. This kind of feedback is studied in cybernetics and control theory.

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).

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 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.

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.

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. 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.

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.

In 1745, the windmill was improved with 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.

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 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 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". 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 ensure 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 microphone will come out of the speaker. Since the microphone is in front of the speaker, the original sound (now coming from the speaker) goes back into the microphone. This happens over and over, getting louder each time. This process produces the squeal.


 * Electronic feedback loops: They 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 from an electronic device 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. Electronic feedback loops take two forms: negative feedback loops and positive feedback loops.


 * Negative feedback loops: They exist 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: They occur 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.

Software engineering and computing systems
Feedback loops provide generic mechanisms for controlling the running, maintenance, and evolution of software and computing systems. 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. 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 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.

Social sciences
A feedback loop to control human behaviour involves four distinct stages. Firstly - Evidence. A behaviour must be measured, captured, and data stored. Secondly - 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. Thirdly - Consequence. The information must illuminate one or more paths ahead. Fourthly - 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
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 - 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.

Education
Young students will often look up to instructors as experts in the field and take to heart 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 educational reinforcement. Some general types of reinforcement that can be used in many types of student assessment are:

(Adapted from Flemming and Levie. )

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).

Government
Examples of feedback in government are:
 * Elections
 * Mass media
 * Revolution
 * Curfews
 * Surveys

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. 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.

Examples of feedback in organizations:
 * Financial audit
 * Performance appraisal
 * Shareholders' meeting
 * Marketing research
 * 360-degree feedback
 * Walkout
 * Lockout