Positive and negative in this sense refer to loop gains greater than or less than zero, and do not imply any value judgements as to the desirability of the outcomes or effects. Positive feedback reinforces and negative feedback moderates the original process. Such systems can give rich qualitative behaviors, but whether the feedback is instantaneously positive or negative in sign has an extremely important influence on the results. The feedback from the outcome to the originating process can be direct, or it can be via other state variables. For example, when part of an electronic output signal returns to the input, and is in phase with it, the system gain is increased. Positive feedback enhances or amplifies an effect by it having an influence on the process which gave rise to it. The exponential form of the graph illustrates the positive feedback mechanism. The process gets faster and faster until the blood vessel wall is completely sealed and the positive feedback loop has ended. As more platelets gather, more chemicals are released that speed up the process. The damaged blood vessel wall releases chemicals that initiate the formation of a blood clot through platelet congregation. Platelet clotting demonstrates positive feedback. A familiar example of positive feedback is the loud squealing or howling sound produced by audio feedback in public address systems: the microphone picks up sound from its own loudspeakers, amplifies it, and sends it through the speakers again. Positive feedback in economic systems can cause boom-then-bust cycles. Out of control, it can cause bridges to collapse. Positive feedback in mechanical design causes tipping-point, or 'over-centre', mechanisms to snap into position, for example in switches and locking pliers. Positive feedback in chemical reactions can increase the rate of reactions, and in some cases can lead to explosions. On the other hand, thermal runaway is a type of positive feedback that can destroy semiconductor junctions. Positive feedback is used in digital electronics to force voltages away from intermediate voltages into '0' and '1' states. Positive feedback may be controlled by signals in the system being filtered, damped, or limited, or it can be cancelled or reduced by adding negative feedback. System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system latched into a new stable state. When the loop gain is positive and above 1, there will typically be exponential growth, increasing oscillations, chaotic behavior or other divergences from equilibrium. Positive feedback tends to cause system instability. That is, positive feedback is in phase with the input, in the sense that it adds to make the input larger. Mathematically, positive feedback is defined as a positive loop gain around a closed loop of cause and effect. Both concepts play an important role in science and engineering, including biology, chemistry, and cybernetics. In contrast, a system in which the results of a change act to reduce or counteract it has negative feedback. That is, A produces more of B which in turn produces more of A. That is, the effects of a perturbation on a system include an increase in the magnitude of the perturbation. Positive feedback ( exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. The above photo is of the UK Northern Rock 2007 bank run. In sociology a network effect can quickly create the positive feedback of a bank run. Alarm or panic can sometimes be spread by positive feedback among a herd of animals to cause a stampede. Feedback loop that increases an initial small effect Causal loop diagram that depicts the causes of a stampede as a positive feedback loop.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |