Siemens SIMATIC S5 Manual page 220

S5-115F Manual
Important
!
The data words not listed in the table are used by the PID algorithm as auxiliary words.
Initializing and calling the PID controller in the STEP 5 program
Different PID controllers can be implemented with multiple OB 251 calls. Data transfer via a
relevant data block must be ensured before each call. These DBs are the data interface between
the controllers and the user.
Note
The important controller information is stored in the high byte of control word DW 11
(DL 11). It is therefore important to note that when influencing the user bits in the
control word, write accesses are only made with T DR 11/SU D 11.0 to D 11.7 or with
RU D 11.0 to D 11.7.
Selecting the sampling interval
The sampling time in digital control loops must not be too long (cf. analog control loops).
Experience has shown that a sampling interval TA of approximately 1/10 of the time constant
T * leads to a controller result comparable with the analog case. The time constant T
RK, dom
determines the jump response of the closed control circuit.
•
TA = 1/10 T
RK, dom
Note
To guarantee a constant sampling interval, OB 251 must be called in the time OB
(OB 13).
* T =dominant time constant of the closed control loop
RK, dom
EWA 4NEB 811 6149-02
Blocks
RK, dom
6-19