Dear Colleagues,

As those of you in the industrial automation business would know – in a negative feedback loop, the system responds in the opposite direction to a stimulus, thus providing overall stability or equilibrium. Positive feedback on the other hand isn’t always so useful and creates instability and one thus has to be careful about applying it.

A Flow Loop
For example, in a flow loop, the controller is set for a target flow rate. If the flow rate is detected to be above this target set point rate, the controller will send a stimulus to a valve to close slightly to reduce the flow to meet the target rate. The reverse happens when the flow rate is above the target rate and the valve is opened to increase the flow rate. A good example of negative feedback doing the right thing.

Equilibrium is Good
This negative feedback should operate everywhere to keep everyone happy and in equilibrium. Rapid population growth (of humans or animals) may result in overconsumption of the food supply; which then leads to a decrease in food and thus a reduction in the population, which then leads to an increase in food supply, which then results in a growing population. Eventually some equilibrium condition is reached.

Positive Feedback means decrease in Equilibrium
Positive feedback on the other hand is where the system responds in the same direction as the stimulus, thus resulting in a decrease in equilibrium and some resultant unhappiness. A small disturbance on a system can result in huge movements of output.

An example of positive feedback can be seen in the environment when an alien species is introduced (e.g. the cane toad or rabbit to Australia). This tends to gobble up the food supply of the native species; which then reduces allowing the alien species to expand rapidly and move further afield.

Thermal runaway in semi conductors is another example of positive feedback where increasing heat causing more current to flow in the chip thus causing more heat at the junction. Eventually the semiconductor is destroyed. Probably the most common example (to my mind) is positive feedback with the howling or squealing sound produced by audio feedback in public address systems. The microphone picks up sounds, amplifies it and sends it through the speakers again.

Technically speaking
Positive feedback occurs when one has positive loop gain around a feedback loop. Positive feedback is in phase with the input so that this makes the resultant output larger. This causes instability and increasing oscillations from equilibrium (set point).

Sometimes Positive Feedback is Good
Although most engineering systems rely on negative feedback to successfully operate and to be sustainable there are occasional examples of positive feedback loops. Such as when momentum is required. But they are unusual and need to be carefully handled.
Other good examples of positive feedback include: contractions in child birth, blood clotting and lactation. Bad examples are of some chemical reactions which release heat and which speed up at higher temperatures – these can accelerate very quickly and lead to an explosion.

Suggestion
Look at the systems around you and assess which are positive and negative feedback loops. This will increase your understanding of their operation.

Ken Blanchard remarked:   Feedback is the breakfast of champions.

Thanks to Wikipedia for some interesting reading.

Yours in engineering learning,

Steve