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 |
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|
KP |
REAL |
Proportionality constant P |
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|
TV |
REAL |
Rate time, derivative time D [sec]. If set to 0, than it works a P controller |
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|
Y_MANUAL |
REAL |
Y is set to this value as long as MANUAL = TRUE |
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|
Y_OFFSET |
REAL |
Offset for manipulated variable |
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|
Y_MIN |
REAL |
Minimum value for manipulated variable. If no limitation is desired, it must be be set to 0 |
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|
Y_MAX |
REAL |
Maximum value for manipulated variable. If no limitation is desired, it must be be set to 0 |
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|
MANUAL |
BOOL |
TRUE: Manual: Y is not influenced by controller FALSE: Controller determines Y |
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|
RESET |
BOOL |
TRUE: Set Y output to Y_OFFSET |
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|
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 |
