Explanation
Negative 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 or
oscillation, 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 in the most
timely manner can be very stable, accurate, and
responsive.
In place of the adjective "Negative" before feedback loop,
alternative terms are used, such as: degenerative,
self-inhibiting, self-correcting, self-dampening, balancing,
discrepancy-reducing, or centripetal.
Negative feedback is widely used in mechanical and electronic
engineering, but it also occurs naturally within living
organisms, 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.
In organisms, feedback enables various measures (eg body
temperature, or blood sugar level) to be maintained within
precise desired ranges by homeostatic processes. 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.
Figure: Both starting points eventuate in equilibrium, during a negative feedback loop.
Early researchers in the area of cybernetics subsequently
generalized the idea of negative feedback to cover any
goal-seeking or purposeful behavior. All purposeful behavior may
be considered to require negative feed-back. If a goal is to be
attained, some signals from the goal are necessary at some time
to direct the behavior.
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". Wiener
defind negative feedback as the case when: "The information fed
back to the control center tends to oppose the departure of the
controlled from the controlling quantity...".
Operant conditioning
Confusion arose after BF Skinner introduced the terms positive
and negative reinforcement, both of which can be considered
negative feedback mechanisms in the sense that they try to
minimize deviations from the desired behavior.
In a similar context, Herold and Greller used the term "negative"
to refer to the valence of the feedback: that is, cases where a
subject receives an evaluation with an unpleasant emotional
connotation.
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.
In a psychology context, on the other hand, negative refers to
the valence of the feedback – attractive versus aversive, or
praise versus criticism.
Negative versus Positive
Negative feedback is feedback in which the system responds so as
to decrease the magnitude of any particular perturbation, leading
to dampening of the original signal, resulting in stabilization
of the process.
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 (see: Positive Feedback
Loop).
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, some 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.
Fields of application
Control systems
Examples of the use of negative feedback to control its system
are: thermostat control, the phase-locked loop oscillator, the
ballcock control of water level, and temperature regulation in
animals.
WC
The ballcock or float valve uses negative feedback to control the
water level in a cistern of a toilette.
Figure: the ballcock control of water level via negative feedback.
Thermostat
A simple and practical example is a thermostat. When the
temperature in a heated room reaches a certain upper limit, the
room heating is switched off so that the temperature begins to
fall. When the temperature drops to a lower limit, the heating is
switched on again. Provided the limits are close to each other, a
steady room temperature is maintained. Similar control mechanisms
are used in cooling systems, such as an air conditioner, a
refrigerator, or a freezer.
Biology and chemistry
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 human
physiology) 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 below, most endocrine hormones 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.
Figure: Stress hormone Cortisol dampens its own creation indirectly.
Self-organization
Self-organization is the capability of certain systems "of
organizing their own behavior or structure". 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.
Economics
In economics, automatic stabilisers are government programs that
are intended to work as negative feedback to dampen fluctuations
in real GDP. Free market economic theorists claim that the
pricing mechanism operated to match supply and demand. 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."
External sources
http://en.wikipedia.org/wiki/Negative_feedback
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