LPI – Loop performance index

Block SymbolLicensing group: ADVANCED

Function Description
The LPI (Loop Performance Index) functional block is designed to evaluate the quality of feedback control by influencing the signal value before it is fed into the controller and measures the system’s response. This block is useful for analyzing and identifying the behavior of the control loop in real-time.

The process variable pv of the control loop is connected to the upv input and from the ypv output to the controller. The setpoint sp of the control loop is fed to the usp input. The block is activated by the RUN signal only in the automatic mode of the controller when it is desired to perform identification of the control loop.

Upon activation (RUN=1), the LPI block injects a sinusoidal signal into the process variable with a defined amplitude ad and frequency fd, allowing the measurement of the system’s response. The output signal is further processed by a BandPass filter and Fourier transform to determine the average signal amplitude. The resulting performance index Ip is calculated based on the ratio between the set parameters and the measured amplitude, providing a quantitative evaluation of the control system’s disturbance suppression.

The output Ip reflects how effectively the control system suppresses disturbances in the defined frequency band fa. A value of Ip=1 indicates that the system suppresses disturbances in accordance with expectations; values higher than 1 indicate better performance; lower values indicate poorer control loop settings.

This block propagates the signal quality. More information can be found in the 1.4 section.

Input

upv	Input process variable	Double (F64)
usp	Input setpoint variable	Double (F64)
RUN	Enable execution	Bool

Parameter

ms	Sensitivity function upper limit $↓$ 1.00001 $↑$ 1000.0 $⊙$ 2.0	Double (F64)
fa	Available bandwidth $↓$ 1e-10 $↑$ 1e+10 $⊙$ 10.0	Double (F64)
fd	Excitation/measured frequency $↓$ 1e-10 $↑$ 1e+10 $⊙$ 1.0	Double (F64)
ad	Excitation amplitude $⊙$ 0.01	Double (F64)
nper	Window size (number of periods of fd) $↓$ 1 $⊙$ 4	Long (I32)
ifrunit	Frequency units $⊙$ 1	Long (I32)
	1 .... Hz 2 .... rad/s
xi	Filter damping ratio $↓$ 0.001 $↑$ 100.0 $⊙$ 1.0	Double (F64)
nmax	Allocated size of array $↓$ 10 $↑$ 10000000 $⊙$ 256	Long (I32)

Output

ypv	Output process variable	Double (F64)
amp	Signal amplitude after filtering	Double (F64)
Ip	Control loop performance index	Double (F64)
E	Error indicator	Bool
iE	Error code	Error

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