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S5-95F
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
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 6210-02
T
A
Figure 9-3.
Principle of Interval Sampling
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
= System error /
Setpoint
= Control deviation
9-9
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