MATLAB: Can’t use compare for greybox model

discretegrey-boxgreyboxmodelmodellingnonlinearsimulinksimulink toolboxstate-spacestatespace

I used nlgreyest to estimate parameters in my system. However, although it was able to estimate the parameter, I cannot use the compare tool to plot a graph of the estimation, as it takes too long (1hr+). Is there another way to plot the greybox model?

Best Answer

You might want to do it manually by accessing the details from estimation report such as parameter values, initial conditions, and fit ( https://www.mathworks.com/help/ident/ug/estimation-report.html#bup2q9z-1 ). However, I suspect the manual process would be any faster than compare()

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MATLAB: Greybox linear model error

Hi Imisi, In your code below

function [A,B,C,D] = batteryeq_lin(T1,T2,C1,C2,Cnom,R0,Ts)
A=[-1/T1 0 0;0 -1/T2 0;0 0 0]; %(V1 V2 SOC)
B=[1/C1;1/C2;-1/Cnom];
C=[-1 -1 -1];
D=R0;

C is defined as a row vector, which implies that the state-space model has only 1 output, as opposed to 3 outputs created with iddata. This is causing a mismatch and results in an error. Essentially C*x + D returns a scalar according to the equations you have specified.

MATLAB: How to force tfest to estimate the process with “only real poles”

TFEST cannot guarantee real poles.

If you can work with <=3 poles and <=1 zero, try PROCEST. This is a process model estimator.
Otherwise, consider a grey-box formulation where your parameters are the real poles. See for example:

https://www.mathworks.com/help/ident/ug/estimate-complex-conjugate-poles-and-zeros-using-grey-box-model.html

The downside to using grey-box approach is that you will need to initialize the parameter values manually; the software will not do that for you. You can run TFEST and use the poles of the resulting model to may be take a guess at the initial real pole values for grey-box estimation.

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MATLAB: Greybox linear model error

MATLAB: How to force tfest to estimate the process with “only real poles”

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