Page 33 - Practical Control Engineering a Guide for Engineers, Managers, and Practitioners

8 Chapter One

is not ahvays clear how the algorithm should react to the measured
disturbance-causing event. Often, each feedforward control algorithm
is a special custom application. Finally, if perchance, the feedforward
control algorithm acts mistakenly on a perceived disturbance-causing
event it can actually generate a more severe disturbance.
The feedback controller cannot anticipate the disturbance. It can
only react "after the damage has been done." If the disturbance is
relatively constant there may be a good chance that the feedback con-
troller can slowly compensate for it and perhaps even remove it. As
we will show in the next couple of pages, there are some disturbances
that simply should be left alone. The feedback controller can tell how
well it has been done and it can often react appropriately. Unlike the
case with feedforward control algorithms, there are a few well-
known, easily applied feedback control algorithms that, under appro-
priate conditions can deal quite effectively with disturbances.

Question 1-1 Can a set point change be considered as a disturbance? If so, could
it be used to easily test a feedback controller?
Answer Yes, to both questions Changing a set-point is a repeatable test for
e\"aluating the tuning of a feedback controller

1-5 Combining Feedforward and Feedback Controllers
Figure 1-11 shows how feedforward and feedback controllers can be
combined for our hotel example and Fig. 1-12 shows an abstraction of

Faucets and toilets
FIGURE 1-11 A feedforward/feedback controller.