Page 30 - Practical Control Engineering a Guide for Engineers, Managers, and Practitioners

Qualitative Concepts in Control Engineering 5

y

F,niCt?ts and toilets
fiGURE 1-6 A feedforward controller.

U (Controller output/proCL'SS input)
Controller

Process Y (Proct'ss output)

0 (DisturbancL's) __ ___,
...J...._ ___
fiGURE 1-7 Feedforward controller block diagram.

1-3 Process Disturbances
Referring back to Fig 1-5, the tank on the hotel roof, let's spend some
time discussing the impact of the faucets, the toilet flushings, and the
drain \'alve on the tank level. First, consider the response of the tank
level to a step change in the drain valve position. That is, we sud-
denly crank the drain \'alve from its initial constant position to a new,
say more open, position and hold it there indefinitely. Figure 1-8
shows the response This kind of a disturbance is considered dctcr-
lllillistic because one would usually know the exact time and amount
of the val\'e adjustment.