User manual

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.