Meaning of variable K:

The actual three-point controller will have a dynamic feedback path (PT1-element) added. By appropriately choosing the time constants TI and T_PROC of these feedback elements, the three-point controller exhibits a dynamic behavior corresponding to that of a PID controller.

The parameter GAIN must > be 0

The DB parameter fixes the operate point for the outputs Y_POS and Y_NEG. If the effective switching value ERR_EFF = SP - PV - XR is positive and is greater than DB, then the Y_POS output switches 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)).

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)).

Should the time constant TI = 0 or the proportional action coefficient GAIN ≤ 0 (configuration error), the block will still continue to operate. The functions feedback path is disabled however, so that the block operates as a conventional three-point switch.

If the time constant T_PROC = 0 (configuration error), the block will still continue to operate. In this case T_PROC is set to a preset value of T_PROC = 60s (60 000 msec).