Detailed description
Original instructions
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Structure of the controller
Structure of the three-point controller:
Dependency of outputs Y_POS and Y_NEG on the variable Y:
If...
Then...
Y = 1
Y_POS = 1
Y_NEG = 0
Y = 0
Y_POS = 0
Y_NEG = 0
Y = -1
Y_POS = 0
Y_NEG = 1
Principle of the three-point controller
The actual three-point controller will have two dynamic feedbacks (PT1 elements) added. By appropriately selecting the time constant of these feedback elements, the three-point controller exhibits a dynamic behavior corresponding to that of a PID controller.
Principle of the three-point controller
The parameter GAIN must > be 0
NOTE: Entries for XF_MAN (percentages from -100% to 100%) must be in the range -100 to 100 inclusive!
Internal feedback paths
The function block has a parameter set for the internal feedback paths consisting of the feedback gain GAIN and the feedback time constants LAG_NEG and LAG_POS.
The following table provides more exact information about it:
Feedback
LAG_NEG
LAG_POS
3-Point-Behavior (without feedback)
= 0
= 0
negative feedback
> 0
= 0
negative + positive feedback
> 0
> LAG_NEG
Warning, regeneration (neg. feedback with LAG_POS)
= 0
> 0
Warning, regeneration (pos. feedback switched off)
> LAG_POS
> 0
Dead zone
The DB parameter fixes the operate point for the outputs Y_POS and Y_NEG. If the effective switch value ERR_EFF is positive and is greater than DB, then the output Y_POS will switch from "0" to "1". If the effective switch value ERR_EFF is negative and is greater than DB, then the output Y_NEG will switch from "0" to "1". The parameter DB is typically set to 1% of the maximum control range (max. SP - PV).
NOTE: The amount is evaluated from the DB dead zone!
Hysteresis
The parameter HYS indicates the switching hysteresis, i.e. the value which the effective switch value ERR_EFF outgoing from control point DB must be reduced by, before the output Y_POS (Y_NEG) is reset to "0". The connection between Y_POS and Y_NEG depending on the effective switch value ERR_EFF and the parameters DB and HYS is illustrated in the image "Principle of the three-point controller". The parameter HYS is typically set to 0.5% of the maximum control range (max. SP - PV).
NOTE: The amount is evaluated from the HYS hysterisis!
Operating modes
There are three operating modes, which are selected via the inputs MAN and HALT:
Operating mode
MAN
HALT
Meaning
Automatic
0
0
The function block will be handled as described above.
Manual mode
1
0 or 1
The outputs Y_POS and Y_NEG are set to the values YMAN_POS and YMAN_NEG. A priority logic - Y_NEG is dominant over Y_POS - prevents both outputs being simultaneously set.
xf1 and xf2 are calculated using the following formula:
Halt
0
1
The outputs Y_POS and Y_NEG are held at their last respective values. xf1 and xf2 are set to GAIN * Y.