Siemens PID Configurations..how?

Hi all my expert friends...I'm still new with Siemens PLC and I am using the Siemens PLC model S7-300 (CPU315-2 DP). I am facing some programming issues as follows:

1) The PID configuration - The PID action is not correct. Its action is at only one way even when the PV > SP or PV < SP. What's the most proper method to configure the PID and where can i access to such a thing? I'm using the FB41 CONT_C function block.

Wow, that's the way! "I'm still new with Siemens..." but already I know enough to tell them their "...PID action is not correct". Hmmm...

Can you provide any clear description of what you're doing with this block in software, what you're expecting it to do, and what it's doing instead? To say "its action is at only one way" is not much more descriptive than saying 'it's broke'.

The most proper way to configure it is the way that gives you the control characteristics your process requires. What is the calling path to FB41 in your code? What have you specified as the CYCLE parameter? etc. etc. More info = better help.

Regards

Ken.

My expection for the PID controls..

Thanks for your response.

In fact,I want to use the PID to control the Process Parameter("HMI_ANL".LT075.SCALED_INPUT) so that to match as close as possible to the Set Point ("HMI_PID".LV075.SETPOINT).This Process Parameter is physically affected by the PID Output Parameter ("HMI_PID".LV075.OUTPUT) in an Reverse-action relation, which means, Output Parameter increase, Process parameter will decrease.

Thus, in this control, I expect the PID will calculate the alogorithm based on the Set Point input and give suitable value (either to increase or to decrease) at the Output Parameter so as can control the Process Parameter to match the Set Point.

However, when I changed the Process Parameter in 2 ways (either to be > Set Point OR to be < Set Point), the PID seems to react only at one way, which is only increase the value of the Output Parameter.This is a fail control to the Process Parameter.

My STL code looks as below. But, in fact I use LAD to write the program.Please comment furhter about it...Anything I lack of in the configurations?Or anything??
Thanks......

A "HMI_PID".LV075.MANUAL_ON
= L 46.1
BLD 103
A "ALW_ON"
= L 46.2
BLD 103
A "ALW_ON"
= L 46.3
BLD 103
A "ALW_ON"
= L 46.4
BLD 103
A "ALW_OFF"
= L 46.7
BLD 103
A(
L "HMI_ANL".LT075.SCALED_INPUT
T #LT075W
SET
SAVE
CLR
A BR
)
JNB _002
CALL "CONT_C" , "PID_LV075"
COM_RST :=
MAN_ON :=L46.1
PVPER_ON:=L46.2
P_SEL :=L46.3
I_SEL :=L46.4
INT_HOLD:=
I_ITL_ON:=
D_SEL :=L46.7
CYCLE :=
SP_INT :="HMI_PID".LV075.SETPOINT
PV_IN :=#LT075W
PV_PER :=
MAN :="HMI_PID".LV075.MANUAL_OUTPUT
GAIN :="HMI_PID".LV075.GAIN
TI :="HMI_PID".LV075.INTEGRAL
TD :="HMI_PID".LV075.DERIVATIVE
TM_LAG :=
DEADB_W :=
LMN_HLM :="HMI_PID".LV075.UPPPER_CLAMP
LMN_LLM :="HMI_PID".LV075.LOWER_CLAMP
PV_FAC :=
PV_OFF :=
LMN_FAC :=1.000000e+000
LMN_OFF :=0.000000e+000
I_ITLVAL:=0.000000e+000
DISV :=
LMN :="HMI_PID".LV075.OUTPUT
LMN_PER :=
QLMN_HLM:=
QLMN_LLM:=
LMN_P :=
LMN_I :=
LMN_D :=
PV :=
ER :=
_002: NOP 0

One immediate issue is the topic of a 'reverse-acting' loop. This Function Block FB41 uses the GAIN parameter to influence this aspect. A positive value for GAIN will result in direct-acting; a negative value will produce a reverse-acting loop. Leave the magnitude of the GAIN the same - just invert the sign, so for example +2.5 becomes -2.5

I'm a bit confused regarding some of the use of the "ALW_ON" signal - I assume this is an Always On signal generated elsewhere?. This appears to set L46.2 true, and then L46.2 is applied to PVPER_ON. So far, so good. But you haven't specified an actual parameter for PV_PER, you've used PV_IN instead. So the PVPER_ON should tell the loop to use the unscaled value at the PV_PER input, but you've applied an actual parameter to the scaled PV_IN instead.

Regards

Ken.