The activator is a decision maker. It evaluates alternative courses of corrective action in light of the significance of the deviations transmitted by the comparator. Once the system has been determined to be out of control, the benefits of bringing it under control are compared with the estimated cost of implementing the proposed corrective actions. Corrective actions can include examining the accuracy of the detector and of the comparator, reevaluating the feasibility of the goal being pursued, or investigating the optimal combination of the inputs to the system-that is, the efficiency of the process employed by the operating system. In other words, the output of the activating system can be a corrective action aimed at investigating the controllability of the operating system and/or the controllability of the controller itself.
The particular behavior pattern exhibited by a control system is dependent on the sensitivity and accuracy of the feedback loop (detector, comparator, and activator) as well as on the time required to transmit the error message from the detector to the activator. Oversensitive feedback control systems may contribute as much to instability as do inert and sluggish ones.
Time delay is the most important contributor to instability in social systems such as business enterprises and governments. Although information technology has considerably accelerated the transmission of information from the detector to the activator and has expedited the comparison and evaluation of information inside the comparator, still the impact of a corrective action on the control object's behavior is felt only after a considerable time lag.
Continuous oscillations of the kind shown in the upper right-hand portion of Figure 16.1 are the result of two characteristics of feedback systems: (1) the length of the time delays and (2) the size of the feedback effect. This particular graph illustrates the behavior of a system controlled by a feedback system characterized by a one-half-cycle time delay. When there is that large a time delay, the impact of the corrective action designed to counteract a deviation is felt at a time when this deviation is of a considerably different magnitude, although it has the same direction. This time lag causes the system to overcorrect. In Figure 16.1 , a deviation of magnitude SA is detected at time t1. At time t2 , new imputs are added to bring the output back to the standard, S. The impact of this corrective action on the system's output is not felt until t3' By that time the system's actual output is at point B. At time t4 the detector senses this new deviation. But because of the one-half-cycle time lag, the system's actual output has oscillated above the upper limits to point H by the time new corrective action is initiated. This new corrective action, aimed at bringing the output back to the standard, will be felt at time t6.
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