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2. Introduction to the Principles of Feedback


2.3.4 A First Indication of Trade-offs

On simulating the performance of the above control loop for K=1 (see Figure 2.5), we find that the smaller controller gain (K=1) results in a slower response to a change in the mould-level set-point. On the other hand, the larger controller gain (K=5) results in a faster response but also increases the effects of measurement noise as seen by the less steady level control and by the significantly more aggressive valve movements.


  
Figure 2.5: A first indication of trade-offs: Increased responsiveness to set-point changes also increases sensitivity to measurement noise and actuator wear.
A first indication of trade-offs: Increased responsiveness to set-point changes also increases sensitivity to measurement noise and actuator wear.

Thus, the performance requirements derived in subsection § 2.3.1 appear to be in conflict with each other, at least to some degree.

At this point, a control engineer who does not have a systematic background in control-system design would have a difficult time in assessing whether this conflict is merely a consequence of having such a simple controller or whether it is fundamental. How much effort should be spent in finding a good value for K? Should one choose a more complex controller? Should one spend more effort in modeling the mould-level process?

The remainder of the book is devoted to developing systematic answers to these and other related questions.