Advanced PI Controller

Advanced PI Controller

Full Name

Lib.Mervis.v1_0.AdvancedPiController

Required Project References

Lib.Mervis

Description

This auxiliary block is used to implement a PI controller with the ability to track a control variable.

Usage in ST

program test
    var
        pi: lib.mervis.v1_0.AdvancedPiController := (Xp := ..., Ti := ..., Min := ..., Max := ...);
    end_var

    pi(Input := ..., SetPoint := ..., Enable := ..., TrackingEnable := ..., TrackingValue := ..., Output => ...);
end_program

Inputs

Name	Data Type	Allowed Range	Retain	Required	Meaning
Input	real		No	Yes	Actual value
SetPoint	real		No	Yes	Desired value
Enable	bool		No	No	Function enable, if false the output is 0
TrackingEnable	bool		No	No	Enable tracking mode
TrackingValue	real		No	No	Tracked value
Min	real	⇐ 0	No	No	Minimum output value
Max	real	>= 0	No	No	Maximum output value
Xp	real	> 0	No	No	Proportional gain
Ti	real	> 0	No	No	Integral time constant
TrackingDontIncludePPart	bool		No	No	Determines the method of calculating the new control action in tracking mode - see description below
TrackingThreshold	real		No	No	Value for converting the input variable to the control action size (e.g., how many percent of output corresponds to one degree Celsius)

Outputs

Name	Data Type	Meaning
Output	real	Output
TrackingCalculatedThreshold	real	Value for converting the input variable to the control action size (e.g., how many percent of output corresponds to one degree Celsius)
InvalidParameters	bool	Indicates invalid controller settings (Xp, Ti, Min, or Max out of allowed range)

Block Methods

Name	Meaning
Adjust	Used to adjust the integral component so the output reaches the maximum value
AdjustToValue	Used to adjust the integral component so the output reaches the desired value
Restart	Used to reset internal values and restart the calculation

Detailed Function Description

The functional block implements a PI controller with the ability to operate in tracking mode, where the control variable can be obtained from another controller. The following formulas are used for output calculation:

Control deviation

e = SetPoint - Input

Proportional component

PPart = (Max - Min) / Xp * e

Integral component (every scan cycle)

IPart = IPart + (Max - Min) / (Xp * Ti) * e * ΔT

Output

Output = PPart + IPart

The controller includes algorithms to limit the size of the integral component in case of saturation - Anti-windup

Tracking Mode

When the TrackingEnable input is set to true, the controller's behavior changes. In each calculation cycle, the TrackingValue is used as the basis for calculating the new control action. The usage method varies according to the setting of the TrackingDontIncludePPart parameter:

true. The controller adjusts its I-part to exactly match the TrackingValue - considering e = 0 for the calculation.
false. The controller adjusts its I-part so that the controller output exactly matches the TrackingValue - including the current values of the SetPoint and Input inputs in the calculation.

Then, the new control action (including the current P-part) is calculated and set to the Output. For easier use with the TrackingPiSelector block, the TrackingThreshold input and TrackingCalculatedThreshold output are provided to determine the hysteresis for selecting the controller whose output will be used.

Usage Notes

For use in heating control, connect the controller as follows
- SetPoint = Desired value
- Input = Measured value
For use in cooling control, connect the controller as follows
- SetPoint = Measured value
- Input = Desired value
! Due to its definition and implementation of anti-windup, the controller does not support “non-zero” mode, meaning the range Min..Max does not include a zero value - this is also indicated by the InvalidParameters output

Table of Contents