S5-100U Integrated Blocks and Their Functions
At instant t
k
, manipulated variable Y
k
is computed as follows:
m=k
Y
k
= dY
m
m=0
Initializing the PID Algorithm
OB251's interface to its environment is the controller DB. All data needed to compute the next
manipulated variable value is stored in this DB. Each controller has its own controller data block.
The controller-specific data are initialized in a data block that must comprise at least 49 data words.
The CPU goes to STOP with a transfer error (TRAF) if no DB is open or if the DB is too short.
!
Caution
Make sure that the right controller DB is open before calling control algorithm OB251.
Table 9-8. Structure of the Controller DB
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1 It is possible to have larger gains, if sudden incremental changes to the system deviation are small
enough. This is the reason you have to divide larger deviations into smaller ones such as adding
the setpoint via a ramp function.
Data Word Name Comments
1K
Proportional coefficient (-32 768 to + 32 767) for controllers
without a derivative-action element
Proportional coefficient (- 1500 to +1500) for controllers with a
derivative-action element
1
K is greater than zero when the control is direct acting, and less
than zero when the control is reverse acting; the specified value is
multiplied by a factor of 0.001.
3R
R parameter (- 32 768 to + 32 767) for controllers without
aderivative-action element
R parameter (- 1500 to + 1500) for controllers with a derivative-
action element
1
Normally 1 for controllers with P element; the specified value is
multiplied with a factor of 0.0001
5 TI Constant TI (0 to 9999)
TI=
The specified value is multiplied by a factor of 0.001
3 Constant TD (0 to 999)
TD=
9 W Setpoint (- 2047 to +2047)
11 STEU Control word (bit pattern)
12 YH Value for Manual operation (- 2047 to +2047)
14 BGOG Upper limit value (- 2047 to +2047)
16 BGUG Lower limit value (- 2047 to +2047)
Sampling interval TA
Integral-action time
Derivative-action time TV
Sampling interval TA
EWA 4NEB 812 6120-02
9-19
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