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This lecture is from Process Control course. Some key points for this lecture are: Feedback Controllers, Error Signal, Set Point, Measured Value, Controlled Variable, Equivalent Signal, Transmitter, Laplace, Controller Output, Bias Value
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where
= set point (3-15 psig; 4-20mA; 1-5V)
= measured value of controlled variable
(or equivalent signal from transmitter)
(3-15 psig; 4-20mA; 1-5V)
Define
0;
e t R t B t
R t
B t
e t e t
= −
′ = −
;
0
Take Laplace Transform
R t R t R B t B t B
R B e t R t B t
E s R s B s
′ ′ = − = −
′ ′ ′ = − ⇒ = −
′ ′ ′ = −
Reverse or Direct Acting
Controller
as input increases"
as input decreases"
PB is the error (% of the range of controlled
variable) required to move the output from
its lowest to its highest value.
100%
c
PB
K
≡
Example 3: Liquid Level Control
Chapter 8
Question: Type of controller action?
p =9 psig
10 psig
in order to make 170 gpm
o
p
q
=
=
at steady state
i o
q = q
( ) ( )
( ) ( ) ( )
where 0
1 0
c c
c
p t p K e t K
K e e R B
= + <
∞ → ∴ ∞ = − ∞ <
Chapter 8
INTEGRAL CONTROL ACTION
( )
0
: reset time (or integral time) - adjustable
(1) If ( ) 0, then ( ) varies continuously with time.
(2) If ( ) 0 for *, then
t
I
I
I
p t p e d
P s
E s s
e t p t
e t t t
p t
η η
τ
τ
τ
∫
0
( ) constant
t
s
I
p p e η d η
τ
∫
Reset time is the time that the integral mode repeats the
action of proportional mode.
I
t *