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S5-100U
Initialization and Call Up of the PID Controller in a STEP 5 Program
Several different PID controllers can be implemented by calling up OB251 repeatedly. A data block
must be initialized prior to each OB251 call up. These DBs serve as data interface between the
controllers and the user.
Note
Important controller data are stored in the high-order byte of control word DW11 (DL11).
Therefore make sure that only T DR 11/SU D11.0 to D11.7 or RU D 11.0 to D11.7
operations are used to modify user-specific bits in the control word.
Selecting the Sampling Interval
In order to be able to use the known analog method of consideration for digital control loops, do not
select a sampling interval that is too large.
Experience has shown that a TA sampling interval of approximately 1/10 of the time constant
T
* produces a control result comparable to the equivalent analog result. Dominant system
RK, dom
time constant T
determines the step response of the closed control loop.
RK, dom
TA = 1/10
T
•
RK, dom
In order to ensure the constancy of the sampling interval, OB251 must always be called up in the
service routine for time interrupts (OB13).
x
T
RK,dom
x
d
*
T
= dominant system time constant of the closed control loop
RK, dom
EWA 4NEB 812 6120-02
T
A
Figure 9-8. Principle of Interval Sampling
Integrated Blocks and Their Functions
x
=
t
=
T
=
A
T
=
RK,dom
w
=
x
=
d
w
t
Control variable
Time
Sampling interval
Dominant system
time constant of
the closed control
loop
Reference
variable / Setpoint
Control deviation
9-21
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