When an output of a process is taken and fed back to the input of a process it causes changes to the output. This changed output is then fed back to the input and so on. This basic idea has myriads of applications, in nature, in science, and in real life.
Feedback can be positive or negative. If it is positive, it adds to the input, which increases the output, which is then fed back to the input, which increases it still more, and we have a runaway increase. This is what causes the howl that occurs when the output from a microphone amplifier is accidentally fed back to the microphone.
Negative feedback subtracts from the input, which can result in a reduction of the output of the process. It won’t necessarily result in NO output however, as the amount of feedback is reduced as a result of the output being reduced, and therefore the output may drop to a fixed value. There are relatively complex equations which govern feedback behaviour which I’m not going to go into here.
Of course the input and output must be related for feedback to be possible. Electrical circuits are a classic example, of course where the input and output are both voltages, and in the case of a cruise control system, the speed of the car is converted to a signal (which may be a voltage, I’d guess) and the feedback is via a signal applied to the fuel control system, which again could be a voltage.
Feedback is inevitably delayed with respect to the inputs. In any real system the input takes time to be fed back, and sometimes this interferes with the intended operation of the feedback loop. It can cause swings in the size of the output, and the system state oscillates.
This is how electronic oscillators are designed to work, but in control systems such oscillations are unwanted and could be destructive. One way to deal with this is to “damp” the circuit, which effectively slows the feedback so that the system state moves more slowly towards the desired state rather than attempting to jump directly to it. Such damping helps reduces overshoot where the momentum of the raw feedback would cause the output to go past the required value.
The input and output together with the feedback form a feedback loop. Feedback loops can be found everywhere, in mechanical and electrical systems, in climate systems and biological systems.
One interesting question is whether or not there is a long term feedback loop that will react to global warming to reduce the effects after a while. If so, would the feedback be more detrimental to the human race than global warming itself.
Such feedback could be something like increased storms and disappearance of seasonal rain that will eventually finish off the human race, perhaps. According to the Gaia hypothesis the Earth is a dynamical system that help to maintain life on Earth. If that is true, it may be broken by global warming, or it may react against global warming in ways which may not yet be apparent.
Systems may have more complex feedback going on than a single simple positive/negative. A process may have several independent positive and negative feedback loops operating at the same time. The various loops may be connected in complex ways and the behaviour may be impossible to accurately predict.
A biological example is the case of the rabbits and the foxes. The population of the rabbits depends on many things – how many bunnies there are, the extent of their food supply, the maturity of the average bunny – how many are mature enough to be able to produce more bunnies. Similarly such factors apply to Basil Brush and his cohorts.
If the rabbits food is plentiful, then they will breed, well, like rabbits and the population will rise. This provides an increased food supply for the foxes and their population increases. Eventually the rabbits manage to increase to the stage where the food becomes limited and the population stops increasing.
Alternatively the increase in the fox population may grow faster than the rabbit population. The foxes kill more rabbits than the rabbits can replace and the rabbit population crashes. The foxes then starve to death as the rabbits start to recover. There are various opinions as to the exact mechanism is concerned, but there is no doubt that boom and bust cycles are seen in the predator/prey relationship, and there is no doubt that feedback cycles are involved somehow.
It is often said that negative feedback acts to return the system to equilibrium. While this may be true in the short term, any such equilibrium is temporary, and as the rabbits and foxes example shows, it is more likely that a system will only temporarily return to the equilibrium and often a system will pass through equilibrium many times as it oscillates too and fro.
In fact, in most cases the “equilibrium position” will rarely be occupied by the system for any length of time. The typical system that oscillates about an “equilibrium position” is a pendulum. A pendulum is travelling its fastest when it passes the lowest point of its arc. The “feedback” in this case is provided by gravity of course.
Feedback also describes the missives and reports sent to an organisation about its services. The organisation may have sought such feedback by distributing questionnaires, by links on a web site, or maybe by word of mouth. Respondents have the opportunity to provide both positive and negative feedback depending on their experience with the organisation.
Such research and feedback is called “market research” and has seen organisations change their stance on some topics. McDonald’s Corporation has banned plastic food containers (in 1990) and plastic drink containers (in 2013) as a result of feedback from environmental lobby groups.
Politicians also get feedback from the voters in the form of opinion polls and surveys. It would be a brave politician (perhaps a soon to be former politician) who ignores the opinion polls. Such a politician would be looking for a fresh job after the next election.
(I believe that I am now all caught up on the posts that I missed. Yeah!)
Me on the net 