Siemens SIMATIC S7-1200 - Page 643

Technology instructions
10.2 PID control
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
643
PWM limits
Actuators that are controlled with the software PWM function of the PID_Temp may need to
be protected from too short pulse durations (for example, a thyristor relay needs to be turned
on for more than 20 ms before it can react at all); you assign a minimum on time. The
actuator can also neglect short impulses and therefore corrupt the control quality. A
minimum off time can be necessary (for example, to prevent overheating).
To show up the PWM limits view, you must open the functional view in the Technology
objects (TO) configuration and select “PWM limits” from the “Advanced settings” node in the
navigation tree.
If you open the “PWM limits” view in the functional view and activate monitoring (“glasses”
button), all controls show the online monitor value from TO-DB with orange background color
and multi-value control, and you can edit the values (if configuration conditions are fulfilled;
refer to the table below).
Setting
TO-DB parameter
Data
type
Value range
Description
Minimum
on
time (heat-
ing)
1,2
"Config.Output.Heat.
MinimumOnTime"
Real 100000.0
>= "Config.Output.
Heat.
MinimumOnTime
>= 0.0
A pulse at Out-
putHeat_PWM" is
never shorter than this
value.
Minimum
off
time (heat-
ing)
1,2
"Config.Output.Heat.
MinimumOffTime"
Real 100000.0
>= "Config.Output.
Heat.
MinimumOffTime
>= 0.0
A break at Out-
putHeat_PWM is nev-
er shorter than this
value.