Ob 251 Pid Algorithm - Siemens SIMATIC S5 Manual

S5-115F Manual
6.2

OB 251 PID Algorithm

The operating sytem of the CPU 942F has a PID algorithm which the user can apply to his needs
with the help of the OB 251 organization block.
Before calling OB 251, you must open a data block (PID DB) which contains the PID parameters and
other controller-specific data. The PID algorithm is called in a specific time grid (sampling interval)
and generates a manipulated variable. The greater the accuracy of the sampling interval, the more
accurately the controller can perform its tasks. The PID control parameters specified in the PID DB
must be adapted to the sampling interval.
Time-controlled processing is generally implemented with the time-interrupt OB (OB 13).
Organization block OB 13 can be programmed in call intervals of 10 ms to 10 min. Execution of the
PID control algorithm can take up to 1.7 ms.
0
1
STEU BIT 5
W
X
XW
0
°
1
XZ
STEU BIT 1
K R TI TD
Legend:
K = Proportional action 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
W = Setpoint
XW = Control deviation
The continuous-action controller is designed for controlled systems involving pressure, tem-
perature, or flow rate control.
The "R" parameter sets the proportional-action component of the PID controller.
If proportional action is required, most controller designs use the value R=1.
EWA 4NEB 811 6149-02
Z YH (STEU BIT 3 to 0)
dYH(STEU BIT 3 to 1)
1 0
1
°
°
Auto
STEU BIT 0
PID
algorithm
STEU BIT 3
STEU
Figure 6-4. Block Diagram of the PID Controller
STEU
Y
dY
YH
dYH
BGOG
BGUG
X
Z
XZ
YA
dYA
OG
Y
0
°
1
UG
dY
BGOG BGUG
= Control word
= Manipulated variable
= Correcting increment
= Value for manual input
= Correcting increment for manual input
= Upper limit
= Lower limit
= Actual value input
= Disturbance variable
= Substitute variable for control deviation
= Controller output:
Manipulated variable limited
= Controller output:
Correcting increment limited
Blocks
YA (STEU BIT 3 to 0)
dYA (STEU BIT 3 to 1)
6-15