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Negative feedback and Positive feedback. This scheme can fail if the input changes faster than the system can respond to it. 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.
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Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The notion of cause-and-effect has to be handled carefully when applied to feedback systems:.
Simple causal reasoning about a feedback system is difficult because the first system influences the second and second system influences the first, leading to a circular argument. This makes reasoning based upon cause and effect tricky, and it is necessary to analyze the system as a whole. 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.
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 s,   and in , Nobel laureate Karl Ferdinand Braun used the term "feed-back" as a noun to refer to undesired coupling between components of an electronic circuit.
By the end of , 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. Over the years there has been some dispute as to the best definition of feedback.
According to Ashby , 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. 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.
Focusing on uses in management theory, Ramaprasad defines feedback generally as " There are two types of feedback: As an example of negative feedback, the diagram might represent a cruise control system in a car, for example, 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 departure of the speed as measured by the speedometer from the target speed set point. This measured error is interpreted by the controller 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 changing road grade to reduce the error in speed, minimizing the road disturbance.
The terms "positive" and "negative" were first applied to feedback prior to WWII. The idea of positive feedback was already current in the s with the introduction of the regenerative circuit. Positive feed-back increases the gain of the amplifier, negative feed-back reduces it. 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.
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 that lead to a decrease of the same.
The terms positive and negative feedback are defined in different ways within different disciplines. The two definitions may cause confusion, such as when an incentive reward is used to boost poor performance narrow a gap. 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 put it:. Quantitative feedback tells us how much and how many.
Qualitative feedback tells us how good, bad or indifferent. While simple systems can sometimes be described as one or the other type, many systems with feedback loops cannot be so easily designated as simply positive or negative, and this is especially true when multiple loops are present. 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.
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. Some systems with feedback can have very complex behaviors such as 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. By using feedback properties, the behavior of a system can be altered to meet the needs of an application; systems can be made stable, responsive or held constant.
It is shown that dynamical systems with a feedback experience an adaptation to the edge of chaos. 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 that the feedback causes good or bad effects. 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. Normal tissue integrity is preserved by feedback interactions between diverse cell types mediated by adhesion molecules and secreted molecules that act as mediators; failure of key feedback mechanisms in cancer disrupts tissue function.
This type of feedback is important because it enables coordination of immune responses and recovery from infections and injuries. During cancer, key elements of this feedback fail. This disrupts tissue function and immunity. Mechanisms of feedback were first elucidated in bacteria, where a nutrient elicits changes in some of their metabolic functions. Repressor see Lac repressor and activator proteins are used to create genetic operons , which were identified by Francois Jacob and Jacques Monod in as feedback loops.
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. 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.
The hypothalamic—pituitary—adrenal 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 people who blush easily. When they realize that they are blushing, they become even more embarrassed, which leads to further blushing, and so on.
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. 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". 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: 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 built thermostats c to control the temperature of chicken incubators and chemical furnaces.
In , the windmill was improved by blacksmith Edmund Lee, who added a fantail to keep the face of the windmill pointing into the wind. In , 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. The use of the centrifugal governor by James Watt in 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 The Great Eastern was one of the largest steamships of its time and employed a steam powered rudder with feedback mechanism designed in by John McFarlane Gray. Joseph Farcot coined the word servo in 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 Nicolas Minorsky published a theoretical analysis of automatic ship steering in 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. The use of feedback is widespread in the design of electronic amplifiers, oscillators, and stateful logic circuit elements such as flip-flops and counters.
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 or reducing the influence of 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 overcorrection, causing the output to oscillate or "hunt". 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. When two or more amplifiers are cross-coupled using positive feedback, complex behaviors can be created.
These multivibrators are widely used and include:.