Mathematics and dynamical systems (black) within a continuously colored environment is plotted by repeatedly feeding back values through a simple equation and recording the points on the imaginary plane that fail to diverge.|alt= 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.
Physics Physical systems present feedback through the mutual interactions of their parts. Feedback is also relevant for the regulation of experimental conditions, noise reduction, and signal control. The thermodynamics of feedback-controlled systems has intrigued physicist since the
Maxwell's demon, with recent advances on the consequences for entropy reduction and performance increase.
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 changes in internal and external environments. A change of some of the environmental conditions may also require a change of that range 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 a 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. In an injured or infected tissue, inflammatory mediators elicit feedback responses in cells, which alter gene expression and change the groups of molecules expressed and secreted, including molecules that induce diverse cells to cooperate and restore tissue structure and 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. 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
François 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. In
zymology, feedback serves as regulation of activity of an enzyme by its direct or downstream 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.
Climate science s) or inhibit (
negative feedbacks) warming. 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, and so forth. 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: 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 the current rate of change.
Education For feedback in the educational context, see
corrective feedback.
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,
Tom 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. 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 File:Ideal feedback model.svg|thumb|The simplest form of a feedback amplifier can be represented by the
ideal block diagram made up of unilateral elements.|280px|right The use of feedback is widespread in the design of
electronic components such as
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 over-correction, causing the output to
oscillate or "hunt". While often an unwanted consequence of system behaviour, this effect is used deliberately in electronic oscillators.
Harry Nyquist at
Bell Labs derived the
Nyquist stability criterion for determining the stability of feedback systems. An easier method, but less general, is to use
Bode plots developed by
Hendrik Bode to determine the
gain margin and phase margin. Design to ensure stability often involves
frequency compensation to control the location of the
poles of the amplifier. 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: • astable circuits, which act as oscillators • monostable circuits, which can be pushed into a state, and will return to the stable state after some time • bistable circuits, which have two stable states that the circuit can be switched between
Negative feedback Negative feedback occurs when the fed-back output signal has a relative phase of 180° with respect to the input signal (upside down). This situation is sometimes referred to as being
out of phase, but that term is also used to indicate other phase separations, as in "90° out of phase". Negative feedback can be used to correct output errors or to desensitize a system to unwanted fluctuations. 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 Positive feedback occurs 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. This fact is very widely used in digital electronics to make
bistable circuits for volatile storage of information. The loud squeals that sometimes occur in
sound reinforcement,
public address systems, and
rock music are known as
audio feedback. If a microphone is in front of a loudspeaker that it is connected to, the sound that the microphone picks up comes out of the speaker, and is picked up by the microphone and re-amplified. If the
loop gain is sufficient, howling or squealing at the maximum power of the amplifier is possible.
Loop gain Loop gain is the sum of the
gain, expressed as a ratio or in
decibels, around a feedback loop. In a feedback loop, the output of a device, process or plant is sampled and applied to alter the input, to better control the output. The loop gain, along with the related concept of loop
phase shift, determines the behavior of the device, and particularly whether the output is
stable, or unstable, which can result in
oscillation. The importance of loop gain as a parameter for characterizing electronic feedback amplifiers was first recognized by
Heinrich Barkhausen in 1921, and was developed further by
Hendrik Wade Bode and
Harry Nyquist at
Bell Labs in the 1930s. The input signal is applied to the amplifier with
open-loop gain A and amplified. The output of the amplifier is applied to a feedback network with gain
B, and subtracted from the input to the amplifier. The loop gain is the product of all gains in the loop. In the diagram shown, the loop gain is the product of the gains of the amplifier and the feedback network,
−AB. The minus sign is because the feedback signal is subtracted from the input. The gains
A and
B, and therefore the loop gain, generally vary with the
frequency of the input signal, and so are usually expressed as functions of the
angular frequency ω in
radians per second. It is often displayed as a graph with the horizontal axis frequency
ω and the vertical axis gain. In amplifiers, the loop gain is the difference between the open-loop gain curve and the closed-loop gain curve (actually, the 1/B curve) on a dB scale.
Oscillator An
electronic oscillator is an
electronic circuit that produces a periodic,
oscillating electronic signal, often a
sine wave or a
square wave. Oscillators convert
direct current (DC) from a power supply to an
alternating current signal. They are widely used in many electronic devices. Common examples of signals generated by oscillators include signals broadcast by
radio and
television transmitters, clock signals that regulate computers and
quartz clocks, and the sounds produced by electronic beepers and
video games.
Latches and flip-flops using
D-type flip flops A latch or a
flip-flop is a
circuit that has two stable states and can be used to store state information. They typically constructed using feedback that crosses over between two arms of the circuit, to provide the circuit with a state. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in
sequential logic. Latches and flip-flops are fundamental building blocks of
digital electronics systems used in computers, communications, and many other types of systems. Latches and flip-flops are used as data storage elements. Such data storage can be used for storage of
state, and such a circuit is described as
sequential logic. When used in a
finite-state machine, the output and next state depend not only on its current input, but also on its current state (and hence, previous inputs). It can also be used for counting of pulses, and for synchronizing variably-timed input signals to some reference timing signal. Flip-flops can be either simple (transparent or opaque) or
clocked (synchronous or edge-triggered). Although the term flip-flop has historically referred generically to both simple and clocked circuits, in modern usage it is common to reserve the term
flip-flop exclusively for discussing clocked circuits; the simple ones are commonly called
latches. Using this terminology, a latch is level-sensitive, whereas a flip-flop is edge-sensitive. That is, when a latch is enabled, it becomes transparent, while a flip flop's output only changes on a single type (positive going or negative going) of clock edge.
Software 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 have been successfully applied to computing systems. In particular, they have been applied to the development of products such as
IBM Db2 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.
Software development User interface design Feedback is also a useful design principle for designing
user interfaces.
Video feedback Video feedback is the
video equivalent of
acoustic feedback. It involves a loop between a
video camera input and a video output, e.g., a
television screen or
monitor. Aiming the camera at the display produces a complex video image based on the feedback.
Human resource management ==See also==