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negative feedback
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{{for|use of criticism and punishment to modify behavior|performance appraisal|reinforcement}}Image:Ideal feedback model.svg|thumb | right | A simple negative feedback system descriptive, for example, of some electronic amplifiers. The feedback is negative if the loop gainloop gainNegative feedback (or balancing feedback) occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by other disturbances.Whereas positive feedback tends to lead to instability via exponential growth, oscillation or chaotic behavior, negative feedback generally promotes stability. Negative feedback tends to promote a settling to equilibrium, and reduces the effects of perturbations. Negative feedback loops in which just the right amount of correction is applied with optimum timing can be very stable, accurate, and responsive.Negative feedback is widely used in mechanical and electronic engineering, and also within living organisms,BOOK, Introduction to cybernetics, Chapter 12: The error-controlled regulator, W. Ross Ashby,weblink 219–243, 1957, Chapman & Hall Ltd.; Internet (1999), BOOK, Ecology, §6.1 Homeostasis depends upon negative feedback,weblink 92, 9780716728290, 2000, Macmillan, Robert E. Ricklefs, Gary Leon Miller,
and can be seen in many other fields from chemistry and economics to physical systems such as the climate. General negative feedback systems are studied in control systems engineering.
(File:Negative Feedback Gif.gif|thumb|Blood glucose levels are maintained at a constant level in the body by a negative feedback mechanism. When the blood glucose level is too high, the pancreas secretes insulin and when the level is too low, the pancreas then secretes glucagon. The flat line shown represents the homeostatic set point. The sinusoidal line represents the blood glucose level.)

Examples

  • Mercury thermostats (circa 1600) using expansion and contraction of columns of mercury in response to temperature changes were used in negative feedback systems to control vents in furnaces, maintaining a steady internal temperature.
  • In the invisible hand of the market metaphor of economic theory (1776), reactions to price movements provide a feedback mechanism to match supply and demand.
  • In centrifugal governors (1788), negative feedback is used to maintain a near-constant speed of an engine, irrespective of the load or fuel-supply conditions.
  • In a steering engine (1866), power assistance is applied to the rudder with a feedback loop, to maintain the direction set by the steersman.
  • In servomechanisms, the speed or position of an output, as determined by a sensor, is compared to a set value, and any error is reduced by negative feedback to the input.
  • In audio amplifiers, negative feedback reduces distortion, minimises the effect of manufacturing variations in component parameters, and compensates for changes in characteristics due to temperature change.
  • In analog computing feedback around operational amplifiers is used to generate mathematical functions such as addition, subtraction, integration, differentiation, logarithm, and antilog functions.
  • In a phase locked loop (1932) feedback is used to maintain a generated alternating waveform in a constant phase to a reference signal. In many implementations the generated waveform is the output, but when used as a demodulator in an FM radio receiver, the error feedback voltage serves as the demodulated output signal. If there is a frequency divider between the generated waveform and the phase comparator, the device acts as a frequency multiplier.
  • In organisms, feedback enables various measures (e.g. body temperature, or blood sugar level) to be maintained within a desired range by homeostatic processes.

History

Negative feedback as a control technique may be seen in the refinements of the water clock introduced by Ktesibios of Alexandria in the 3rd century BCE. Self-regulating mechanisms have existed since antiquity, and were used to maintain a constant level in the reservoirs of water clocks as early as 200 BCE.JOURNAL, Breedveld, Peter C, 2004, Port-based modeling of mechatronic systems, Mathematics and Computers in Simulation, 66, 2, 99–128, 10.1016/j.matcom.2003.11.002, File:Centrifugal governor.png|thumb|The fly-ball governor is an early example of negative feedback.]]Negative feedback was implemented in the 17th Century. Cornelius Drebbel had built thermostatically-controlled incubators and ovens in the early 1600s,WEB,weblink Tierie, Gerrit. Cornelis Drebbel. Amsterdam: HJ Paris, 1932., 2013-05-03, and centrifugal governors were used to regulate the distance and pressure between millstones in windmills.{{citation|last=Hills|first=Richard L|title=Power From the Wind|publisher=Cambridge University Press|year=1996}}
James Watt patented a form of governor in 1788 to control the speed of his steam engine, and James Clerk Maxwell in 1868 described "component motions" associated with these governors that lead to a decrease in a disturbance or the amplitude of an oscillation.
JOURNAL, Maxwell, James Clerk, On Governors,weblink Proceedings of the Royal Society of London, 16, 1868, 270–283, Wikimedia, 10.1098/rspl.1867.0055, The term "feedback" was well established by the 1920s, in reference to a means of boosting the gain of an electronic amplifier.BOOK, David A. Mindell, Between Human and Machine : Feedback, Control, and Computing before Cybernetics, 2002, Johns Hopkins University Press, Baltimore, MD, USA{e(tau)},{dtau} + T_dfrac{d}{dt}e(t)right)where
T_i is the integral time T_d is the derivative time

Negative feedback amplifier

The negative feedback amplifier was invented by Harold Stephen Black at Bell Laboratories in 1927, and granted a patent in 1937 (US Patent 2,102,671 "a continuation of application Serial No. 298,155, filed August 8, 1928 ...").JOURNAL, James E Brittain, Electrical engineering hall of fame: Harold S Black, Proceedings of the IEEE, February 2011, 2, 99, 351–353,weblink 10.1109/jproc.2010.2090997, JOURNAL, CA Desoer, In Memoriam: Harold Stephen Black, IEEE Transactions on Automatic Control, AC-29, 8, August 1984,weblink 10.1109/tac.1984.1103645,
"The patent is 52 pages long plus 35 pages of figures. The first 43 pages amount to a small treatise on feedback amplifiers!"
There are many advantages to feedback in amplifiers.BOOK, Santiram Kal, Basic electronics: Devices, circuits and its fundamentals, §6.3 Advantages of negative feedback amplifiers, 193 ff,weblink 9788120319523, 2009, PHI Learning Pvt. Ltd,
In design, the type of feedback and amount of feedback are carefully selected to weigh and optimize these various benefits.
Though negative feedback has many advantages, amplifiers with feedback can oscillate. See the article on step response. They may even exhibit instability. Harry Nyquist of Bell Laboratories proposed the Nyquist stability criterion and the Nyquist plot that identify stable feedback systems, including amplifiers and control systems.File:Negative feedback amplifier with disturbance.png|200px|thumb|Negative feedback amplifier with external disturbance.BOOK, Marc Thomson, Intuitive Analog Circuit Design, Figure 11-4: Classical single input, single output control loop,weblink 9780080478753, 2006, Newnes, The figure shows a simplified block diagram of a negative feedback amplifier.The feedback sets the overall (closed-loop) amplifier gain at a value:
frac{O}{I} =frac {A} { 1+beta A } approx frac {1}{beta} ,
where the approximate value assumes βA >> 1. This expression shows that a gain greater than one requires β < 1. Because the approximate gain 1/β is independent of the open-loop gain A, the feedback is said to 'desensitize' the closed-loop gain to variations in A (for example, due to manufacturing variations between units, or temperature effects upon components), provided only that the gain A is sufficiently large.BOOK,weblink §6.3.1 Gain stability, Santiram Kal, Basic Electronics: Devices, Circuits, and IT Fundamentals, 9788120319523, 2009, PHI Learning Pvt. Ltd, 193–194,
In this context, the factor (1+βA) is often called the 'desensitivity factor',
Marc T Thompson, p. 309BOOK, The Design of CMOS Radio Frequency Circuits, Thomas H Lee, 447, 2nd, Cambridge University Press, 2004,weblink 9780521835398,
and in the broader context of feedback effects that include other matters like electrical impedance and bandwidth, the 'improvement factor'.
BOOK, Electronic Circuits: Analysis simulation and design, Norbert A Malik, 671, Improvement Factor,weblink 9780023749100, 1995, Prentice Hall, If the disturbance D is included, the amplifier output becomes:
O =frac {AI} { 1+beta A } +frac {D}{1+ beta A} ,
which shows that the feedback reduces the effect of the disturbance by the 'improvement factor' (1+β A). The disturbance D might arise from fluctuations in the amplifier output due to noise and nonlinearity (distortion) within this amplifier, or from other noise sources such as power supplies.BOOK, Basic Electronics: Devices, Circuits and IT fundamentals, Santiram Kal,weblink 194, §6.3.2 Noise Reduction, BOOK, Linear Control Systems: For Punjab Technical University, SK Bhattacharya, §5.3.3 Effect of feedback on disturbance signal,weblink 9788131759523, The difference signal I–βO at the amplifier input is sometimes called the "error signal".BOOK,weblink 642, Muhammad Rashid, Microelectronic Circuits: Analysis & Design, 9780495667728, Cengage Learning, 2nd, 2010,
According to the diagram, the error signal is:


text{Error signal} = I - beta O = I left ( 1-beta frac{O}{I} right ) =frac {I} {1 + beta A} - frac { beta D} {1+beta A} .
From this expression, it can be seen that a large 'improvement factor' (or a large loop gain βA) tends to keep this error signal small.Although the diagram illustrates the principles of the negative feedback amplifier, modeling a real amplifier as a unilateral forward amplification block and a unilateral feedback block has significant limitations.BOOK, Circuit Analysis and Feedback Amplifier Theory, Wai-Kai Chen, Chapter 13: General feedback theory,weblink [In a practical amplifier] the forward path may not be strictly unilateral, the feedback path is usually bilateral, and the input and output coupling networks are often complicated., 13–1, 9781420037272, 2005, CRC Press,
For methods of analysis that do not make these idealizations, see the article Negative feedback amplifier.

Operational amplifier circuits

(File:Feedback op-amp voltage amplifier.png|thumb|200px|A feedback voltage amplifier using an op amp with finite gain but infinite input impedances and zero output impedance.See, for example, Figure 1.4, p. 7 Ideal op amp model in BOOK, Design with operational amplifiers and analog integrated circuits, 3rd, Sergio Franco,weblink McGraw-Hill, 2002, 978-0078028168, or BOOK, Fundamentals of Circuits and Filters, Wai-Kai Chen, David G Nair, Sergio B Franco, Figure 16.2: The four possible op-amp configurations, 16–2, 9781420058888, 2009, CRC Press, The Circuits and Filters Handbook, 3rd,weblink )The operational amplifier was originally developed as a building block for the construction of analog computers, but is now used almost universally in all kinds of applications including audio equipment and control systems.Operational amplifier circuits typically employ negative feedback to get a predictable transfer function. Since the open-loop gain of an op-amp is extremely large, a small differential input signal would drive the output of the amplifier to one rail or the other in the absence of negative feedback. A simple example of the use of feedback is the op-amp voltage amplifier shown in the figure.The idealized model of an operational amplifier assumes that the gain is infinite, the input impedance is infinite, output resistance is zero, and input offset currents and voltages are zero. Such an ideal amplifier draws no current from the resistor divider.BOOK, The Design of High Performance Mechatronics, G. Schitter, A. Rankers, 499,weblink §6.3.4 Linear amplifiers with operational amplifiers, 9781614993681, 2014, IOS Press,
Ignoring dynamics (transient effects and propagation delay), the infinite gain of the ideal op-amp means this feedback circuit drives the voltage difference between the two op-amp inputs to zero. Consequently, the voltage gain of the circuit in the diagram, assuming an ideal op amp, is the reciprocal of feedback voltage division ratio β:
V_{text{out}} = frac{ R_{text{1}} + R_{text{2}} }{ R_{text{1}} } V_{text{in}}! = frac{1}{beta} V_{text{in}} ,.
A real op-amp has a high but finite gain A at low frequencies, decreasing gradually at higher frequencies. In addition, it exhibits a finite input impedance and a non-zero output impedance. Although practical op-amps are not ideal, the model of an ideal op-amp often suffices to understand circuit operation at low enough frequencies. As discussed in the previous section, the feedback circuit stabilizes the closed-loop gain and desensitizes the output to fluctuations generated inside the amplifier itself.BOOK, Op Amp Applications Handbook, Walter G Jung, Noise gain (NG), 12 ff, 9780750678445, 2005, Newnes,weblink

Mechanical engineering

{{See also|Control systems|Control engineering}}File:Ballcock.svg|thumb|The ballcockballcockAn example of the use of negative feedback control is the ballcock control of water level (see diagram). In modern engineering, negative feedback loops are found in fuel injection systems and carburettors. Similar control mechanisms are used in heating and cooling systems, such as those involving air conditioners, refrigerators, or freezers.

Biology and chemistry

thumb|250pc|Control of endocrine hormones by negative feedback.Some biological systems exhibit negative feedback such as the baroreflex in blood pressure regulation and erythropoiesis. Many biological process (e.g., in the human anatomy) use negative feedback. Examples of this are numerous, from the regulating of body temperature, to the regulating of blood glucose levels. The disruption of feedback loops can lead to undesirable results: in the case of blood glucose levels, if negative feedback fails, the glucose levels in the blood may begin to rise dramatically, thus resulting in diabetes.For hormone secretion regulated by the negative feedback loop: when gland X releases hormone X, this stimulates target cells to release hormone Y. When there is an excess of hormone Y, gland X "senses" this and inhibits its release of hormone X. As shown in the figure, most endocrinehormones are controlled by a physiologic negative feedback inhibition loop, such as the glucocorticoids secreted by the adrenal cortex. The hypothalamus secretes corticotropin-releasing hormone (CRH), which directs the anterior pituitary gland to secrete adrenocorticotropic hormone (ACTH). In turn, ACTH directs the adrenal cortex to secrete glucocorticoids, such as cortisol. Glucocorticoids not only perform their respective functions throughout the body but also negatively affect the release of further stimulating secretions of both the hypothalamus and the pituitary gland, effectively reducing the output of glucocorticoids once a sufficient amount has been released.Raven, PH; Johnson, GB. Biology, Fifth Edition, Boston: Hill Companies, Inc. 1999. page 1058.

Self-organization

Self-organization is the capability of certain systems "of organizing their own behavior or structure".BOOK,weblink William R. Uttal, Psychomythics: Sources of Artifacts and Misconceptions in Scientific Psychology, 9781135623722, Psychology Press, 2014, 95 ff,
There are many possible factors contributing to this capacity, and most often positive feedback is identified as a possible contributor. However, negative feedback also can play a role.
BOOK, Self-organization in biological systems, Scott Camazine, Jean-Louis Deneubourg, Nigel R Franks, James Sneyd, Guy Theraulaz, Eric Bonabeau, Chapter 2: How self-organization works, 15 ff, 9780691116242, 2003, Princeton University Press,weblink

Economics

In economics, automatic stabilisers are government programs that are intended to work as negative feedback to dampen fluctuations in real GDP.Mainstream economics asserts that the market pricing mechanism operates to match supply and demand, because mismatches between them feed back into the decision-making of suppliers and demanders of goods, altering prices and thereby reducing any discrepancy. However Norbert Wiener wrote in 1948:
"There is a belief current in many countries and elevated to the rank of an official article of faith in the United States that free competition is itself a homeostatic process... Unfortunately the evidence, such as it is, is against this simple-minded theory."(Cybernetics: Or Control and Communication in the Animal and the Machine) p.158
The notion of economic equilibrium being maintained in this fashion by market forces has also been questioned by numerous heterodox economists such as financierGeorge SorosGoeroge Soros, The Alchemy of Finance and leading ecological economist and steady-state theoristHerman Daly, who was with the World Bank in 1988-1994.Herman Daly, Steady State Economics

See also

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References

{{Reflist|30em}}

External links

  • WEB, Physiological Homeostasis,weblink biology online: answers to your biology questions, Biology-Online.org,
{{Systems science}}

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isbn=9780801868955, Friis and Jensen described this action as "positive feedback" and made passing mention of a contrasting "negative feed-back action" in 1924.Friis, H.T., and A.G. Jensen. "High Frequency Amplifiers" Bell System Technical Journal 3 (April 1924):181-205.Harold Stephen Black came up with the idea of using negative feedback in electronic amplifiers in 1927, submitted a patent application in 1928, and detailed its use in his paper of 1934, where he defined negative feedback as a type of coupling that reduced the gain of the amplifier, in the process greatly increasing its stability and bandwidth.JOURNAL
, Black
, H.S.
, Stabilized Feedback Amplifiers
, Bell System Tech. J.
, 13
, 1
, 1–18
, American Telephone & Telegraph
,
, January 1934
,weblink
,
, 10.1002/j.1538-7305.1934.tb00652.x
,
, January 2, 2013,
BOOK, A history of control engineering 1930-1955, Chapter 3: The electronic negative feedback amplifier, 70 ff, Stuart Bennett, 9780863412806, 1993, Institution of Electrical Engineers,weblink Karl Küpfmüller published papers on a negative-feedback-based automatic gain control system and a feedback system stability criterion in 1928.JOURNAL
, C. Bissell
, Karl Kupfmuller, 1928: an early time-domain, closed-loop, stability criterion
, IEEE Control Systems
, 2006
, 115–116, 126
,weblink
, Nyquist and Bode built on Black’s work to develop a theory of amplifier stability.Early researchers in the area of cybernetics subsequently generalized the idea of negative feedback to cover any goal-seeking or purposeful behavior.Rosenblueth, Arturo, Norbert Wiener, and Julian Bigelow. "Behavior, purpose and teleology." Philosophy of science 10.1 (1943): 18-24.Cybernetics pioneer Norbert Wiener helped to formalize the concepts of feedback control, defining feedback in general as "the chain of the transmission and return of information",Norbert Wiener (Cybernetics: Or Control and Communication in the Animal and the Machine). Cambridge, Massachusetts: The Technology Press; New York: John Wiley & Sons, Inc., 1948. and negative feedback as the case when:}}While the view of feedback as any "circularity of action" helped to keep the theory simple and consistent, Ashby pointed out that, while it may clash with definitions that require a "materially evident" connection, "the exact definition of feedback is nowhere important". Ashby pointed out the limitations of the concept of "feedback":
}}
To reduce confusion, later authors have suggested alternative terms such as degenerative,Hermann A Haus and Richard B. Adler, Circuit Theory of Linear Noisy Networks, MIT Press, 1959
self-correcting,
BOOK, Peter M. Senge, The Fifth Discipline: The Art and Practice of the Learning Organization, 1990, Doubleday, New York, 0-385-26094-6, 424,
balancing,
BOOK, Science and Policy in Natural Resource Management: Understanding System Complexity, 9781139458603, 2006, Helen E. Allison, Richard J. Hobbs,weblink Cambridge University Press, Balancing or negative feedback counteracts and opposes change, 205,
or discrepancy-reducing
Charles S. Carver, Michael F. Scheier: On the Self-Regulation of Behavior Cambridge University Press, 2001
in place of "negative".

Overview

missing image!
- 105 Negative Feedback Loops.jpg -
Feedback loops in the human body
In many physical and biological systems, qualitatively different influences can oppose each other. For example, in biochemistry, one set of chemicals drives the system in a given direction, whereas another set of chemicals drives it in an opposing direction. If one or both of these opposing influences are non-linear, equilibrium point(s) result.In biology, this process (in general, biochemical) is often referred to as homeostasis; whereas in mechanics, the more common term is equilibrium.In engineering, mathematics and the physical, and biological sciences, common terms for the points around which the system gravitates include: attractors, stable states, eigenstates/eigenfunctions, equilibrium points, and setpoints.In control theory, negative refers to the sign of the multiplier in mathematical models for feedback. In delta notation, −Δoutput is added to or mixed into the input. In multivariate systems, vectors help to illustrate how several influences can both partially complement and partially oppose each other.Some authors, in particular with respect to modelling business systems, use negative to refer to the reduction in difference between the desired and actual behavior of a system.JOURNAL, Arkalgud Ramaprasad,weblink On The Definition of Feedback, Behavioral Science, 28, 1, 4–13, 1983, 10.1002/bs.3830280103, John D.Sterman, Business Dynamics: Systems Thinking and Modeling for a Complex World McGraw Hill/Irwin, 2000. {{ISBN|9780072389159}}
In a psychology context, on the other hand, negative refers to the valence of the feedback – attractive versus aversive, or praise versus criticism.JOURNAL, Herold, David M., Greller, Martin M., 1977, Research Notes. Feedback: The Definition of a Construct, Academy of Management Journal, 20, 1, 142–147, 10.2307/255468,
In contrast, positive feedback is feedback in which the system responds so as to increase the magnitude of any particular perturbation, resulting in amplification of the original signal instead of stabilization. Any system in which there is positive feedback together with a gain greater than one will result in a runaway situation. Both positive and negative feedback require a feedback loop to operate.However, negative feedback systems can still be subject to oscillations. This is caused by the slight delays around any loop. Due to these delays the feedback signal of some frequencies can arrive one half cycle later which will have a similar effect to positive feedback and these frequencies can reinforce themselves and grow over time. This problem is often dealt with by attenuating or changing the phase of the problematic frequencies. Unless the system naturally has sufficient damping, many negative feedback systems have low pass filters or dampers fitted.

Some specific implementations

There are a large number of different examples of negative feedback and some are discussed below.

Error-controlled regulation

{{See also|Control engineering|Homeostasis|Allostasis}}(File:Regulator with feedback.png|thumb|200px|A regulator R adjusts the input to a system T so the monitored essential variables E are held to set-point values S that result in the desired system output despite disturbances D.BOOK, Process Dynamics and Control, Sudheer S Bhagade, Govind Das Nageshwar,weblink 9788120344051, 2011, PHI Learning Pvt. Ltd, 6, 9, )One use of feedback is to make a system (say T) self-regulating to minimize the effect of a disturbance (say D). Using a negative feedback loop, a measurement of some variable (for example, a process variable, say E) is subtracted from a required value (the 'set point') to estimate an operational error in system status, which is then used by a regulator (say R) to reduce the gap between the measurement and the required value.BOOK, Charles H. Wilts, Principles of Feedback Control,weblink Addison-Wesley Pub. Co, 1960, 1, In a simple feedback system a specific physical quantity is being controlled, and control is brought about by making an actual comparison of this quantity with its desired value and utilizing the difference to reduce the error observed. Such a system is self-correcting in the sense that any deviations from the desired performance are used to produce corrective action., BOOK, Process Control: Concepts Dynamics And Applications, SK Singh,weblink 9788120336780, 2010, PHI Learning Pvt. Ltd, 222,
The regulator modifies the input to the system T according to its interpretation of the error in the status of the system. This error may be introduced by a variety of possible disturbances or 'upsets', some slow and some rapid.
For example, input and load disturbances. See BOOK, A Real-Time Approach to Process Control, William Y. Svrcek, Donald P. Mahoney, Brent R. Young,weblink 57, 9781118684733, 2013, 3rd, John Wiley & Sons,
The regulation in such systems can range from a simple 'on-off' control to a more complex processing of the error signal.
WEB, Types of feedback control,weblink University of Exeter: Physics and astronomy, Feedback and temperature control, Charles D H Williams, 2014-06-08, It may be noted that the physical form of the signals in the system may change from point to point. So, for example, a change in weather may cause a disturbance to the heat input to a house (as an example of the system T) that is monitored by a thermometer as a change in temperature (as an example of an 'essential variable' E), converted by the thermostat (a 'comparator') into an electrical error in status compared to the 'set point' S, and subsequently used by the regulator (containing a 'controller' that commands gas control valves and an ignitor) ultimately to change the heat provided by a furnace (an 'effector') to counter the initial weather-related disturbance in heat input to the house.Error controlled regulation is typically carried out using a Proportional-Integral-Derivative Controller (PID controller). The regulator signal is derived from a weighted sum of the error signal, integral of the error signal, and derivative of the error signal. The weights of the respective components depend on the application.JOURNAL, Bechhoefer, John, Feedback for Physicists: A Tutorial Essay On Control, Reviews of Modern Physics, 77, 3, 783–835, APS Physics,weblink 10.1103/revmodphys.77.783, 10.1.1.124.7043, Mathematically, the regulator signal is given by:
mathrm{MV(t)}=K_pleft(,{e(t)} + frac{1}{T_i}int_{0}^