Siemens SIMATIC S5-95F - Page 263

S5-95F Blocks and Their Functions
Selecting the Sampling Interval
In order to be able to use the known analog method of consideration for digital control loops too, 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
RK, dom*
produces a control result comparable to the equivalent analog result. Dominant system
time constant T
RK, dom
determines the step response of the closed control loop.
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).
Figure 9-3. Principle of Interval Sampling
x
d
t
x
w
a a a a
a
a a
a
a
a a
a
a
a a
a
a a a a
T
A
T
RK,dom
x = Control variable
t = Time
T
A
= Sampling interval
T
RK,dom
= Dominant system
time constant of
the closed control
loop
w = System error /
Setpoint
x
d
= Control deviation
*T
RK, dom
=dominant system time constant of the closed control loop
EWA 4NEB 812 6210-02 9-9