Detailed description
Original instructions
About us
Structure of the controller
Structure of the three-point controller:
Hence:
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 2 dynamic feedback paths (PT1-elements) added. Through appropriate selection of the time constants of the feedback-element, the three-point controller maintains dynamic behavior that corresponds to the behavior of a PID controller.
Feedback
The function block has a parameter sequence for the internal feedback paths, comprised of the feedback gain and the reset time constant 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
The parameter gain must be > 0
The amount will be resolved from the Hysterisis hys and the dead zone db!
For xf_man (meaning -100 to 100%) values between -100 and 100 are to be entered!
Dead zone
The parameter db sets the connection 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 smaller than db, then the output Y_NEG will switch from "0" to "1". The value of the db parameter is typically set to 1% of the maximum control area (max. SP - PV).
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, before the output Y_POS (Y_NEG) is reset to "0". The connection between Y_POS and Y_NEG depending on effective switch value ERR_EFF and the parameters db and hys is illustrated in the image Principle of the three-point controller. The value of the hys parameter is typically set to 0.5% of the maximum control area (max. SP - PV).
Operating modes
There are three operating mode selectable through the man and halt parameter inputs:
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. In this case, the built in priority logic - Y_NEG is dominant over Y_POS, which prohibits both outputs from being set simultaneously.
xf1 and xf2 are calculated using the following formula:
xf1 = xf_man * gain /100
xf2 = xf_man * gain /100
Halt
0
1
In Halt mode, both outputs Y_POS and Y_NEG will be held at the last value.
xf1 and xf2 are set to gain * Y.