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S5-115U Manual
11.2.3 OB251 PID Control Algorithm
The operating systems of the central processing units have an integral PID control algorithm
which you can use for your own purposes with the help of organization block OB251.
Before calling OB251, a data block (PID controller DB) containing the controller parameters and
other controller-specific data must be opened. The PID control algorithm is called periodically
(sampling interval) and generates the manipulated variable. The more closely the sampling
interval is observed, the more accurately can the controller fulfill its appointed task. The control
parameters specified in the controller DB must be matched to the sampling interval. Typically,
timed interrupts are serviced by a time block (OB10 to OB13).
Timed-interrupt OBs can be called at intervals between 10 msec. and 10 minutes. The maximum
execution time of the PID control algorithm is 2 msec.
1
STEU BIT 5
W
X
XW
0
°
1
XZ
STEU BIT 1
Legend:
K
= Proportional coefficient
K>0 positive control direction
K<0 negative control direction
R
= R parameter (usually 1000)
TA
= Sampling interval
TN
= Integral-action time
TV
= Derivative-action time
TI
= TA/TN
TD
= TV/TA
STEU = Control word
W
= Setpoint
XW
= Controller difference
EWA 4NEB 811 6130-02b
Z
0
1
0
1
°
°
Auto
PID
algorithm
K
R TI
TD
STEU
Figure 11-3. Block Diagram of the PID Controller
Y
dY
YH
dYH
BGOG = Upper limiting value
BGUG = Lower limiting value
X
Z
XZ
YA
dYA
YH (STEU BIT 3 at 0)
dYH (STEU BIT 3 at 1)
STEU BIT 0
OG
dY
1
°
0
UG
Y
STEU BIT 3
BGOG BGUG
= Manipulated variable
= Correcting increment
= Value for manual input
= Correcting increment for manual input
= Actual value
= Disturbance variable
= Substitute variable for control deviation
= Controller output: manipulated variable
limited
= Controller output: correcting increment
limited
Integral Blocks
dYA (STEU BIT 3
at 1)
YA
(STEU BIT 3
at 0)
11-33
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