Simatic 505 SoftShop
9 - PID Loops
••••
167
Essentially, the SmarTune utility creates a disturbance by initiating a step
increase of the PID output. Process variable samples are collected as this
increase in output precipitates a change in the process variable. When the
sample period is complete, the data collection is analyzed for time lag,
gradient, overshoot, steady state error, and oscillation. Using a frequency
analysis method, the optimum PID gain values are determined. You can
choose to accept the newly calculated gain values, or keep the present
PID gain settings.
The SmarTune variable parameters are listed and described in this
section. Start Variable is the only variable that must be specified. It
names a discrete variable used to activate a SmarTune session. The others
may be null.
Variable parameters provide the coupling between a PLC program and
SmarTune. If only Start Variable is specified, no program coupling is
needed; a session begins when Start Variable becomes true and ends with
a loop changing back to its previous mode and SP. Since coupling is done
with variables, any program type may be used to monitor and control
SmarTune (relay ladder logic, SFPGM, or SFSUB).
Table 9-3 lists the variable parameters used by SmarTune. The following
paragraphs describe the parameters.
Table 9-3 Variable Parameters
Name
Type
Allowable Variable
Types
Start Variable
discrete
X Y C WX WY V
Abort Variable
discrete
X Y C WX WY V
Ack Variable
discrete
Y C WY V
SmarTune Restart
discrete
X Y C WX WY V
Status Variable
word
WY V
PIN Variable
word
WX WY V
Previous Mode
Word
V
Previous SP
Real
V
Previous Output
Word
V
Previous Gain
Real
V
Previous Reset
Real
V
Previous Rate
Real
V
Calculated Gain
Real
V
Calculated Reset
Real
V
Calculated Rate
Real
V
Start Variable, Abort Variable, Ack Variable
These three discrete variables allow easy activation/deactivation via an
RLL program, as shown in Figure 9-10.
