Hello Hassan;
We are refering to the GAIN parameter (proportional gain). Setting this to a negative value will reverse the action of the PID block to the manipulated value (LMNPER).
From Siemens' website:
Reversing the direction of control action of the controller "CONT_C" (SFB41/FB41)
QUESTION:
How can I reverse the direction of control action of a controller, e.g. the "Continue_Control" (SFB41/FB41)?
ANSWER:
You can reverse the direction of control action of a controller by entering a negative value for the gain at the "GAIN" input of the controller.
More information is available in the Online Help for STEP 7.
Keywords:
Controller block, Continue_Control
Entry ID:8718842 Date:07/09/2003
Setting the LMNPER parameter at 100.0 (REAL value), in maual mode, will force your output to 100%. In auto mode, it will have no effect, as the PID block will calculate the output required to minimize the error.
Setting the GAIN to a negative value will produce a negative LMNPER value, which will cause your valve to close when the error increases.
Hope this helps,
Daniel Chartier