PD (FB)

FUNCTION_BLOCK PD

Represents a PD controller

A PD controller continuously calculates an error value e(t) as the difference between a desired set point and a measured process variable. The PD controller applies a correction based on proportional and derivative terms (sometimes denoted P and D respectively) which give their name to the controller type.

Y_OFFSET, Y_MIN and Y_MAX are used for the transformation of the manipulated variable within a prescribed range. MANUAL can be used to switch on and off manual operation. RESET serves to reset the controller. In normal operation ( MANUAL = RESET = LIMITS_ACTIVE = FALSE) the controller calculates the controller error e as difference SET_POINT – ACTUAL, generates the derivation with respect to time and stores these values internally.

The output, that is the manipulated variable Y, is calculated as follows:

So besides the P-part also the current change of the controller error (D-part) influences the manipulated variable. Additionally Y is limited on a range prescribed by Y_MIN and Y_MAX. If Y exceeds these limits, LIMITS_ACTIVE will get TRUE. If no limitation of the manipulated variable is desired, Y_MIN and Y_MAX have to be set to 0. As long as MANUAL=TRUE, Y_MANUAL will be written to Y. A P controller can be easily created by setting TV=0.

For more information see: PID.

InOut:

Scope Name Type Initial Comment Input ACTUAL REAL   Actual value, process variable SET_POINT REAL   Specified value, set point KP REAL   Proportionality constant P TV REAL   Rate time, derivative time D [sec]. If set to 0, than it works a P controller Y_MANUAL REAL   Y is set to this value as long as MANUAL = TRUE Y_OFFSET REAL   Offset for manipulated variable Y_MIN REAL   Minimum value for manipulated variable. If no limitation is desired, it must be be set to 0 Y_MAX REAL   Maximum value for manipulated variable. If no limitation is desired, it must be be set to 0 MANUAL BOOL   TRUE: Manual: Y is not influenced by controller FALSE: Controller determines Y RESET BOOL   TRUE: Set Y output to Y_OFFSET Output Y REAL   Manipulated variable, set value LIMITS_ACTIVE BOOL FALSE TRUE: Y has exceeded the given limits Y_MIN, Y_MAX and is limited to these values