Why is the automatic control system a closed loop with negative feedback?

For any simple automatic control system, no matter how different its block diagram is on the surface, its components will always form a closed loop in the signal transmission relationship. Any one of these signals, as long as it goes along the arrow direction, will eventually return to the starting point of the origin after several links. Therefore, the automatic control system is a closed-loop system.

Look at the "block diagram of automatic control system" again. The output parameter of the system is the adjusted parameter, but after it passes through the measuring element and transmitter, it returns to the input end of the system and is compared with the given value. This practice of returning the output signal of the system (or link) to the input directly or through some links is called feedback. From the "block diagram of automatic control system", we can also see that there is a negative sign "-"next to the feedback signal Z, and a "+"next to the given value X (which can also be omitted). Here, sum and negative mean that when comparing, X is taken as a positive value and Z is taken as a negative value, that is, the deviation signal e=x-z to the regulator. Because the feedback signal z in the block diagram of automatic control system takes a negative value, it is called negative feedback, which can weaken the original signal. If the feedback signal takes a positive value instead of a negative value, and the feedback signal strengthens the original signal, it is called positive feedback. In this case, the plus sign "+"should be used next to the feedback signal z in the block diagram, and the deviation e=x+z at this time. Negative feedback is used in automatic control system. Because only negative feedback can make the adjusted parameter y rise due to interference, the feedback signal z will rise, and the deviation signal e going to the regulator will decrease after comparison. At this time, the regulator will send a signal to change the opening of the regulating valve in a negative direction, so that the adjusted parameter will fall back to a given value. In this way, the purpose of adjustment is achieved. If the form of positive feedback is adopted, the adjustment can not only overcome the influence of interference, but also add fuel to the flames, that is, when the adjusted parameters are disturbed, Z will also rise. The action direction of the regulating valve is to further increase the adjusted parameters, and as long as there is a slight deviation, the adjustment will make the deviation bigger and bigger until the adjusted parameters exceed the safe range and destroy production. Therefore, the regulation system must not adopt positive feedback alone.