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