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 \(\frac{\delta e}{\delta t}\) and stores these values internally.

The output, that is the manipulated variable Y, is calculated as follows: \(Y = KP \cdot (e + TV \frac{\delta e}{\delta t}) + Y_{OFFSET}\)

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
Output	`LIMITS_ACTIVE`	`BOOL`	FALSE	`TRUE`: `Y` has exceeded the given limits `Y_MIN`, `Y_MAX` and is limited to these values