Siemens SIMATIC S5-100U System Manual page 253

S5-100U
At instant t , manipulated variable Y
k
m=k
Y = dY
k 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.
Data Word Name
1 K
3 R
5 TI
3
9 W
11 STEU
12 YH
14 BGOG
16 BGUG
* 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.
** The factor 0.001 is an approximate value. The exact value of the factor is 1/1024 or 0.000976.
EWA 4NEB 812 6120-02b
is computed as follows:
k
Table 9-8. Structure of the Controller DB
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*
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**.
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*
Normally 1 for controllers with P element; the specified value is
multiplied with a factor of 0.001**
Constant TI (0 to 9999)
Sampling interval TA
TI=
Integral-action time
The specified value is multiplied by a factor of 0.001
Constant TD (0 to 999)
Derivative-action time TV
TD=
Sampling interval TA
Setpoint (- 2047 to +2047)
Control word (bit pattern)
Value for Manual operation (- 2047 to +2047)
Upper limit value (- 2047 to +2047)
Lower limit value (- 2047 to +2047)
Integrated Blocks and Their Functions
Comments
9-19