Siemens SIMATIC S5-115U User Manual page 411

S5-115U Manual
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 must have 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 has been opened or if the DB is too
short.
Caution
!
Make sure that the right controller DB has been invoked before calling control
algorithm OB251.
Data Word Name
1 K
3 R
5 TI
7 TD
9 W
11 STEU
12 YH
14 BGOG
16 BGUG
1 Larger gains are possible if abrupt changes to the system deviation are sufficiently small. Large changes of the system
deviation should therefore be divided up into small changes; e.g. by feeding the setpoint via a ramp function.
The factor 0.001 is an approximate value. The precise value for the factor is 1/1024 or 0.000976.
EWA 4NEB 811 6130-02b
is computed as follows:
k
Table 11-11. Format of the Controller DB
Proportional coefficient (- 32 768 to+32 767) for controllers without D component
Proportional coefficient (- 1500 to+1500) for controllers with D component
K is greater than zero when the control direction is positive, and less than zero
when the control direction is negative; the specified value is multiplied with the
factor 0.001.
R parameter (- 32 768 to+32 767) for controllers without D component
R parameter (-1500 to+1500) for controllers with D component
Normally 1 for controllers with P component: the specified value is multiplied
with the factor 0.001.
Constant TI (0 to 9999)
Sampling interval TA
TI=
Integral-action time TN
The specified value is multiplied with 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 limiting value (- 2047 to +2047)
Lower limiting value (- 2047 to +2047)
Comments
1
Integral Blocks
1
11-37