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

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Philipp Smilgies il 22 Gen 2018
Risposto: Rajiv Singh il 23 Set 2022
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
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soeren graabaek il 16 Set 2022
Hey,
Did you find a solutuion?
Best regard
Søren

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### Risposte (1)

Rajiv Singh il 23 Set 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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