PID

The library util.lib provides the following PID controller function block:

Unlike the PD controller, this function block contains a further REAL input TN for the readjusting time in sec (e.g. “0.5” for 500 msec).

Inputs of the function block

Variable	Data type	Description
ACTUAL	REAL	Current value of the controlled variable
SET_POINT	REAL	Desired value, command variable
KP	REAL	Proportionality coefficient, unity gain of the P-part
TN	REAL	Reset time, reciprocal unity gain of the I-part; given in seconds, e.g. “0.5” for 500 msec
TV	REAL	Derivative action time, unity gain of the D-part  in seconds, e.g. “0.5” for 500 msec
Y_MANUAL	REAL	Defines output value Y in case of MANUAL = TRUE
Y_OFFSET	REAL	Offset for the manipulated variable Y
Y_MIN, Y_MAX	REAL	Lower resp. upper limit for the manipulated variable Y. If Y exceeds these limits, output LIMITS_ACTIVE will be set to TRUE and Y will be kept within the prescribed range. This control will only work if Y_MIN<Y_MAX.
MANUAL	BOOL	If TRUE, manual operation will be active, i.e. the manipulated value will be defined by Y_MANUAL.
RESET	BOOL	TRUE resets the controller; during reinitialization Y = Y_OFFSET.

Outputs of the function block

Variable	Data type	Description
Y	REAL	Manipulated value, calculated by the function block (see below)
LIMITS_ACTIVE	BOOL	TRUE indicates that Y has exceeded the given limits (Y_MIN, Y_MAX).
OVERFLOW	BOOL	TRUE indicates an overflow (see below)

Y_OFFSET, Y_MIN und Y_MAX serve for transformation of the manipulated variable within a prescribed range.

MANUAL can be used to switch to manual operation; RESET can be used to re-initialize the controller..

In normal operation (MANUAL = RESET = LIMITS_ACTIVE = FALSE) the controller calculates the controller error e as difference from SET_POINT ““ ACTUAL, generates the derivation with respect to time de/dt and stores these values internally.

The output, i.e. the manipulated variable Y unlike the PD controller contains an additional integral part and is calculated as follows:

Y = KP (D + 1/TN òedt + TV dD /dt) + Y_OFFSET

So besides the P-part also the current change of the controller error (D-part) and the history of the controller error (I-part) influence the manipulated variable.

The PID controller can be easily converted to a PI-controller by setting TV=0.

Because of the additional integral part, an overflow can come about by incorrect parameterization of the controller, if the integral of the error D becomes to great. Therefore for the sake of safety a BOOLean output called OVERFLOW is present, which in this case would have the value TRUE. This only will happen if the control system is instable due to incorrect parameterization. At the same time, the controller will be suspended and will only be activated again by re-initialization.