1
$\begingroup$

I would like to get a simple estimation of the parameters for a humidity chamber system. The humidity chamber has one constant inflow ($k_1$) and and outflow which depends on the humidity of a neighbor humidity chamber (diffusion, modeled as $k_2(RH_{ext} - RH)$. $RH$ is the humidity of the chamber, $RH_{ext}$ is the humidity of the external chamber. I have the following system description (first order): $$ \frac{dRH}{dt} = k_1 + k_2(RH_{ext} - RH) $$

Now, I have measurement data for $RH$ (output) and $RH_{ext}$ (input). $k_1$ could be seen as a second (constant) input. I would like to estimate the parameters $k_1$ and $k_2$ in a simple manner (maybe least-square based) using my (discrete) measurement data in Matlab, but I don't know how I could proceed. Use the system identification toolbox? Other ways?

$\endgroup$
3
  • $\begingroup$ Hi Fabian, welcome to engineering.SE. Our site supports Latex style equation typesetting; I've edited your post to use it. $\endgroup$ Jun 9, 2015 at 12:30
  • $\begingroup$ What type of measurement data do you have? Is it $RH$ vs. time with all other parameters constant? If so, you can simply integrate the equation and do a 3 parameter curve fit ($k_1$, $k_2$, and the integration constant). $\endgroup$ Jun 9, 2015 at 12:33
  • $\begingroup$ Hi Chris, thanks very much for the edit and your answer. $RH_{ext}$ isn't a constant as well. Your approach seems clever, but I'm not so sure about how to do this in practice. You would simply try to fit $RH(t)$ to $f(t)=k_1 t + k2\left(\int RH_{ext} - \int RH \right)+c$, by using the numerical integrations $\int RH_{ext}$ and $\int RH$ ? $\endgroup$
    – donald
    Jun 9, 2015 at 13:02

1 Answer 1

1
$\begingroup$

It sounds like you have MATLAB, so it's a simple linear fit:

  • From your vector $RH(t)$, compute $\frac{d RH}{dt}$ by numerical differentiation
  • Create a matrix $A$ with ones on the first column and the values of $RH_{ext}(t) - RH(t)$ on the second column
  • Create a matrix $B$ with the values of $\frac{d RH}{dt}$ (use 0 as the initial value)
  • Your result vector is given $A$ \ $B$

    dRH = [0; diff(RH)./diff(t)];

    A = [ones(size(RH)) RH_ext-RH];

    k = A\dRH; % k(1) is k1, k(2) is k2

$\endgroup$
2
  • $\begingroup$ Just to emphasize the point; $A\setminus B$ uses Matlab's left division operator which is used to solve a system of linear equations described in matrix notation. $\endgroup$ Jun 9, 2015 at 16:56
  • $\begingroup$ @ChrisMueller yes, that's right. Thanks for the clarification $\endgroup$
    – am304
    Jun 9, 2015 at 17:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.