Detailed description
Original instructions
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Structure of the controller
Structure of the two-point controller:
Principle of the two-point controller
The actual two-point controller will have two dynamic feedback paths (PT1 elements) added. By appropriately choosing the time constant of these feedback elements, the two-point controller exhibits a dynamic behavior corresponding to that of a PID controller.
Principle of the two-point controller:
The selected feedback gain K must be greater than zero!
Entries for XF_MAN (percentages from 0 to 100%) must be in the range 0 to 100 inclusive!
Internal feedback paths
The feedback parameter set, consisting of the feedback gain K and the feedback time constants LAG_NEG and LAG_POS, allows a universal employment of the two-point controller.
The following table provides more exact information about it:
Feedback
LAG_NEG
LAG_POS
2-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
Hysteresis
The parameter DB indicates the switching-hysteresis, i.e. the value which the effective switch value ERR_EFF outgoing from control point DB/2 must be reduced by, before the output Y is reset to "0". The dependence of the output Y depending of the effective switch value ERR_EFF and the Parameter DB, becomes clear in the illustration "Principle of the two-point controller". The parameter DB is typically set to 1% of the maximum control range [max. (SP - PV)].
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 output Y is set to the YMAN value.
xf1 and xf2 are calculated using the following formula:
Halt
0
1
The output Y is held at its last value. xf1 and xf2 are set to GAIN * Y.