Page 92 - Practical Control Engineering a Guide for Engineers, Managers, and Practitioners

Basic Co1cepts in Precess A1alysls 67

S (Set point)

U (Controller output/
process input valve position)

U (Process input)
+ L...----~
L...----N (Disturbances and sensor noise)

FaauRE 3-12 Block diagram for a controlled system subjected to
disturbances and sensor noise.

The error is corrupted by the noise N

For the time being, ignore the set point

Solve forE

E(1 + GPG c)= -N

E 1
(3-43)
N = 1+GpGc

During the development of E/N the set point was removed
because it is assumed constant at zero. More will be made of f./N
when the frequency domain is introduced in the next chapter.
3·3·8 Zeros and Poles
This section will repeatedly refer to Eq. (3-40) which is

Y= gks+gl sc
2
-rs +(1+ gk)s+ gl s