PD¶
The library util.lib provides the following PD controller 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 |
| 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. |
| 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). |
Y_OFFSET, Y_MIN und 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 timede/ dt and stores these values internally.
The output, i.e. the manipulated variable Y is calculated as follows:
Y = KP (D + TV dD/dt) + Y_OFFSET whereby D = SET_POINT-ACTUAL
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.