Feedback
From Wikinfo
- See also Feedback according to Wikipedia and feedback (disambiguation)
Feedback is a basic process whereby some proportion of the output signal of a system is fed back to the input, in order to change the dynamic behaviour of the system.
Feedback may be negative, thus tending to reduce output, or positive, thus increasing output. Systems which include feedback are prone to hunting, which is oscillation of output resulting from improperly tuned inputs of first positive then negative feedback.
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Feedback in engineering
Feedback is designed into many electronic and other technical devices.
In engineering control theory, feedback is a process in which a signal generated from the output of a system is applied as an input to the same system.
The most common general-purpose controller is a proportional-integral-derivative controller. Each term of the PID controller copes with time. The proportional term handles the present state of the system, the integral term handles its past, and the derivative or slope term tries to predict and handle the future.
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. This scheme can fail if the input changes faster than the system can respond to it. When this happens, the negative feedback signal begins to act as positive feedback, causing the output to oscillate or hunt. Positive feedback is usually an unwanted consequence of system behaviour.
A well-known example of runaway positive feedback in electronic systems is called "howl" or "howl-round". This occurs in public address systems when sound from the loudspeakers reaches the microphone, is amplified by the system, and is then fed back into the system at even higher volume. The frequency of the howl is the reciprocal of the transit time of the signal around the system.
With mechanical devices, hunting can be severe enough to destroy the device.
Feedback in nature
Feedback occurs [[nature|naturally, as does hunting. A well known example is the oscillation of the population of snowshoe hares due to predation from lynxes.
Another natural system prone to hunting is the stock market, which has both positive and negative feedback mechanisms. For example, when stocks are rising (a bull market), the belief that further rises are probable gives investors an incentive to buy (positive feedback); but the increased price of the shares, and the knowledge that there must be a peak after which the market will fall, deter buyers (negative feedback). Once the market begins to fall regularly (a bear market), some investors may expect further losing days and refrain from buying (positive feedback), but others may buy because stocks become more and more of a bargain (negative feedback).
Any self-regulating natural process involves feedback and is prone to hunting.
Homeostasis of biological systems is possible due to feedback mechanisms: it is especially visible in the nervous and endocrine systems.
Financial feedback
Feedback operates in financial market, as the effect of optimism and pessimism. See this New York Times article where fear of an adverse feedback loop is reported.
Feedback in business
Feedback is sought when doing marketing and public relations analyses of a business or its methods, and is gathered through surveys, focus groups and other methods in order to gauge customer reaction and opinions about a product, service, procedure or event.
References
- Adapted from the Wikipedia article, "Feedback" http://www.wikipedia.org/w/wiki.phtml?title=Feedback&oldid=444223 November 7, 2002
