MATLAB: Greybox nonlniear estimation model

batteryestimationgreyboxidentificationnonlinearnonlinearestimationparameterestimationstatessystem

I am trying to use the nonlinear greybox model to estimate the parameter R0 in my state space model (attached). However, I have an extra variable Voc which is a column vector and varies depending on the time. How can I write this in my function under modelfile name (ie nlm = idnlgrey('batteryeq',order,parameters,initial_states,Ts)? I get an error saying "Subscript indices must either be real positive integers or logicals." with my current code:
function [dx,y] = batteryeq(t,x,u,T1,T2,C1,C2,R0,OCV,varargin)
% Output equation.
y = [-x(1);-x(2)]-R0*u+OCV(t); % output equation
% State equations.
dx = [-1/T1+(1/C1)*u;-1/T2+(1/C2)*u];
end
Thank you.

Best Answer

Hi Imisi,
If you already know the dependence of OCV on time, you can definitely pass it as a parameter. However, as time is a floating value, you'll need to "index" into OCV with the corresponding index of time, rather than the actual decimal value of time. (OCV(t) in your code)
For example, let's say OCV = 100X1 where OCV defines the Open-circuit voltage for time = 1 to 100 seconds. Given that you know the value of current time in the function as variable, t, you can compute the OCV at the time using linear interpolation:
interp1(1:100,OCV,t);
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