Siemens SIMATIC S5-115U, User Manual

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S5-115U Manual Integral Blocks
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.
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YH (STEU BIT 3 at 0)
dYH (STEU BIT 3 at 1)
Auto
1
Y
dY
°
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STEU BIT 1
1
°
Z
XW
UG
OG
0
0
1
°
1
0
PID
algorithm
°
W
X
XZ
dYA (STEU BIT 3
at 1)
YA (STEU BIT 3
at 0)
BGOG BGUG
K R TI TD
0
STEU BIT 3
STEU BIT 5
STEU BIT 0
STEU
a a a
a a a
a a a
a a a
a a a
1
Figure 11-3. Block Diagram of the PID Controller
Legend:
K = Proportional coefficient
K>0 positive control direction
K<0 negative control direction
Y = Manipulated variable
dY = Correcting increment
YH = Value for manual input
R = R parameter (usually 1000) dYH = Correcting increment for manual input
TA = Sampling interval BGOG = Upper limiting value
TN = Integral-action time BGUG = Lower limiting value
TV = Derivative-action time X = Actual value
TI = TA/TN Z = Disturbance variable
TD = TV/TA XZ = Substitute variable for control deviation
STEU = Control word YA = Controller output: manipulated variable
limited
W = Setpoint dYA = Controller output: correcting increment
limited
XW = Controller difference
EWA 4NEB 811 6130-02b
11-33