THE SYSTEMS CONCEPT REDEFINED 



301 



Since the corrective order must operate in a direction opposite to the 

 measurement of error, the principle is one of negative feedback. For instance, 

 if a factory's production occurs at a rate larger than the rate of sale, product 

 soon piles up: the amount of product, measured against some economically 

 sound inventory, increases. The difference, A, increases. Fed back into the 

 production line, this information (A) is used to cause a decrease in the rate 

 of production, so that the excess inventory will decrease toward zero. Again, 

 in cholesterol synthesis, the rate is controlled by enzyme-catalyzed proc- 

 esses in which there exists inhibition by a reaction product. Thus, as the 

 cholesterol concentration gets larger, more of it absorbs on the enzyme, and 

 the over-all rate of synthesis slows down because of the inhibition. Hence 

 there can be general feedback to control the over-all process, or there can be 

 particular feedbacks to control small parts of it (Figure 11-2 (b) ) . 



As a whole, the human body obtains feedback from the five sensory organs 

 plus a number of other internal detectors such as the organ of balance in the 

 inner ear and the temperature controller at the base of the brain. Man's 

 thermostat, in the hypothalamus at the base of the brain, was recently 

 appreciated for the first time. The trimmer, or fine controller, is the 

 cerebellum. 



The human body has the physical properties of a zero-seeking servo- 

 mechanism — a device which sets for itself a goal, attempts to achieve that 

 goal, then measures the error in the achievement before it feeds this informa- 

 tion back negatively through a control amplifier so that the error is can- 

 celled. The system diagram in its barest essentials of general feedback is 

 given in (a) of Figure 11-2, while (b) illustrates the case oi particular feed- 

 backs. 



The feedback and the amplification of the error by the control ampli- 

 fier, are both critical if satisfactory control is to be achieved — as we can see 

 from Figure 11-3. The broken line denotes the task and the solid lines the 



the task 



overshoot 



TIME 



(a) 



(b) 



Figure 11-3. The System Diagram, II. (a) Hunting, overshoot, and "dead-beat" approach 

 to the task; (b) Operating process and negative feedback. 



