How can I set nonlinear constraints in a nonlinear grey-box model?

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Hi,
I´m estimating pramaters of a nonlinear state-space model with the System Identification Toolbox and the function nlgreyest.
Is it possible to add nonlinear constraints to the estimation?
For example:
One equation of my state-space model is
dx(1) = K1*x(1) + K2*x(2) + K3*u(1);
And now I want to add a nonlinear constraint like this to the estimation.
0.5 = K1 / (2*sqrt(K2));
Can someone help me solve this problem?
Thanks, Philipp

Answers (1)

Rajiv Singh
Rajiv Singh on 23 Sep 2022
Linear or nonlinear constraints are not supported in nlgreyest directly. Depending upon the problem, you can sometimes get away with a re-parameterization, and.or simple min/max bounds. For example in the example you have given, you can eliminate K1 by replacing it with sqrt(K2), and a lower bound K2>0.

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