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S5-95F
9.1.4
PID Algorithm (OB 251)
A PID algorithm is integrated in the operating system of the S5-95F. OB251 helps you use this
algorithm to meet your needs.
Before calling up OB251, you must first open a data block called the controller DB. It contains the
controller parameters and other controller specific data. The PID algorithm must be invoked peri-
odically and generates the manipulated variable. The more closely the scan time is maintained, the
more accurately the controller fulfills its task. The control parameters specified in the controller DB
must be adapted to the scan time.
You should always call OB251 from the time OB (OB13). You can set time OBs at a call up interval
ranging between 10 ms and 655350 ms. The PID algorithm requires no more than 1.7 ms to
process.
OB13
Time-controlled
execution
C
DB N
JU
OB 251
.
.
.
.
.
BE
The continuous-action controller is designed for controlled systems such as those present in
process engineering for controlling pressure, temperature, or flow rate.
The "R" variable sets the proportional component of the PID controller.
If proportional action is required, most controller designs use the value R = 1.
The individual Proportional action, Integral action, and Derivative action components can be
deactivated via their parameters (R, TI, and TD) by presetting the pertinent data words to zero.
This enables you to implement all required controller structures without difficulty, e.g., PI, PD, or PID
controllers.
You can forward the system deviation XW or, using the XZ input, any disturbance variable or the
inverted actual value X to the derivative action element. Specify a negative K value for a reverse
acting controller.
When the correction information (dY or Y) is at a limit, the integral action component is automatically
deactivated in order not to impair the dynamic response of the controller.
The switch settings in the block diagram are implemented by setting the respective bits in control
word "STEU".
EWA 4NEB 812 6210-02
OB251
PID
algorithm
Figure 9-1. Calling Up the OB251 PID Algorithm
Blocks and Their Functions
DBN
Controller
data block
DW
1
.
.
.
.
.
.
DW
49
9-3
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