ABB CL Series, Applications Manual

Page: 213 / 428

Wiring with the display system
210
1SVC 440 795 M1100
The differential component in the PID controller
The differential component Y
D
is proportional to the change
in the control difference. So as to avoid step changes or
jumps in the manipulated variable caused by the differential
behaviour when the setpoint is changed, the change of the
actual value (the process variable) is calculated and not the
change in the control difference. This is shown in the
following equation:
Y
D
( t ) = K
p
x
T
v
/T
c
x
(X
i
( t –1) – X
i
( t ) )
K
p
= Proportional gain
T
c
= Scan time
T
v
= Differential time of the control system
(also called the rate time)
X
i
(t) = Actual value with scan time t
X
i
( t –1) = Actual value with scan time t – 1
Scan time T
c
Scan time T
c
determines the duration of the interval in which
the function block is called by the operating system for
processing. The value range is between 0 and 6553.5 s.
If the value 0 is set, the cycle time of the device is the pause
time between the function block calls.
Manual mode of the PID controller
A value must be present at the
>MV
input in order to set the
manipulated variable directly. If the coil DC…SE is activated,
the value at
>MV
is transferred directly as manipulated
variable
QV>.
This value is present for as long as the
DC…SE coil is activated or the value at the
>MV
input is
changed. The controller algorithm is re-activated when
DC..SE is reset.
h The device cycle time varies according to the length of the
program. With a scan time of 0 s, this may cause an
irregular control response.
h Use the Set cycle time function block ( a page 245) in
order to keep the cycle time of the device constant.