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Function for linearization of nonlinear system of equations
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- V =F(rp,x1,x2,theta(t), psi(t), phi(t),wx(t),wy(t) ) is a non linear equation and the objective is to minimize output vector V for the given optimization variables rp, x1 and x2. Can anyone recommend me a method to linearize it using matlab ?
10 Comments
Alan Stevens
on 12 Nov 2020
Why do you want to linearize it? The function fminsearch can work with nonlinear equations.
HN
on 12 Nov 2020
Edited: HN
on 16 Nov 2020
Alan Stevens , rp,x1,x2 are bounded. So, I wanted to use constrained optimization. I couldn't implement fmincon because V is a vector. I wanted to get the minimum of V(1),V(2),V(3) simultaneously. Can I still use fminsearch ? (rp,x1,x2) = (rp,xi_2,xi_3).
Below is the function I wanted to minimize . Thank you.
function V = Opt_QN(rp,xi_2,xi_3)
L=1;
z=0.7071068;
ts=1/2;
t=0:ts:1;
for k=1:length(t)
xi_2=beta(k);
rp=radius(k);
for j=1:length(t)
xi_1=0;
xi_2=alpha(j);
xi_3=beta(j);
th(j)=-0.2*cos(2*pi*t(j));
psi(k)=0.2*sin(2*pi*t(k));
phi(j)=atan2(sin(psi(k))*sin(th(j)),(cos(psi(k))+cos(th(j))));
R=Rot('y',th(j))*Rot('x',psi(k))*Rot('z',phi(j));
x(k)=1/2*rp*(-cos(phi(j))*cos(psi(k))+cos(phi(j))*cos(th(j))+sin(phi(j))*sin(psi(k))*sin(th(j)));
y(k)=-rp*cos(psi(k))*sin(phi(j));
zc(k)=z;
delta(k)=sqrt(x(k)^2+y(k)^2)
p=[x(k);y(k);zc(k)];
end
end
end
Alan Stevens
on 12 Nov 2020
You pass rp, xi_2 and xi_3 to the function, but almst immediately reassign them, so the input values do nothing!
Your output is a vector, so what is your minimization measure?
If you have access to the Optimisation toolbox you will have more options than fminsearch.
HN
on 12 Nov 2020
Edited: HN
on 12 Nov 2020
Alan Stevens, I tried to use fmincon but I can't trust the result. It is also difficult to find global minimum and corresponding optimum parameters. Moreover, the computation time is quite long. Did I do something wrong in the double four loop shown below to call the function ? Yes, I have the toolbox.
%main program
clc
close all
%% UB LB setting
x2L = deg2rad(90);
x2U = deg2rad(150);
x3U = deg2rad(-90);
x3L = deg2rad(-150);
x2=x2L:(x2U-x2L)/ts:x2U;
x3=x3L:(x3U-x3L)/ts:x3U;
A = [];
b = [];
Aeq = [];
beq = [];
%%
LB = [deg2rad(90), deg2rad(-150)]; % 120 =2.0944 , 150 =-2.6180
UB= [deg2rad(150), deg2rad(-90)];
options = optimoptions('fmincon','Algorithm','interior-point','SpecifyObjectiveGradient',true,'SpecifyConstraintGradient',true,'Display' ,'iter');
options = optimoptions(options,'SpecifyObjectiveGradient',false,'SpecifyConstraintGradient',false);
% %% Plot
[X,Y] = meshgrid(x2,x3);
for idx =1:length(x2)
for jdx=1:length(x3)
x12 = [x1(idx) , x2(jdx)];
fun = @(x)PRSopt_QN(x)
[V,S]=fun(x12);
[Vec,Fval,exitflag,output,lambda,grad,hessian] = fmincon(fun,x12,[],[],[],[],LB,UB, [],options);
Min(idx,jdx)=Fval;
[xs,index]= sort(Min)
result = Min(sort(index(1:x1)));
Opt(:,idx)=Vec ;
Average(idx,jdx)=V;
end
end
Mini=min(Min,[],'all');
figure,
contour3(X,Y,Average*100,'color','blue','ShowText','on');
title('Function evaluation')
hLeg=legend({''},'FontSize',12);
set(hLeg,'Interpreter','latex');
box off;
xlabel('\xi_1')
ylabel('\xi_2')
zlabel('mm/s')
figure,contour3(X,Y,Min,'color','blue','ShowText','on');
title('Function evaluation')
hLeg=legend({''},'FontSize',12);
set(hLeg,'Interpreter','latex');
box off;
xlabel('\xi_1')
ylabel('\xi_2')
zlabel('mm/s')
figure, surf(X,Y,Min)
figure, surf(X,Y,Average*100)
hLeg=legend({''},'FontSize',12);
set(hLeg,'Interpreter','latex');
box off;
title('Optimized linear velocity ')
xlabel('\xi_1')
ylabel('\xi_2')
zlabel('mm/s')
% [vout, fout] = fmincon(fun,x);
HN
on 12 Nov 2020
Alan Stevens, the output V elements are hetrogeneous physical models. So, I am confused how can I set minimization measure. My plan was to minimize each elements separately.
Thank you
HN
on 12 Nov 2020
Edited: HN
on 12 Nov 2020
John D'Errico , I am just afraid if my assignment/difinition of input variables is not suitable to fmincon. Would you please share me your rough assesment on the following lines of calling the fucntion ? Thank you
% %% Plot
x2L = deg2rad(90);
x2U = deg2rad(150);
x3U = deg2rad(-90);
x3L = deg2rad(-150);
x2=x2L:(x2U-x2L)/ts:x2U;
x3=x3L:(x3U-x3L)/ts:x3U;
[X,Y] = meshgrid(x2,x3);
for idx =1:length(x2)
for jdx=1:length(x3)
x12 = [x1(idx) , x2(jdx)];
fun = @(x)PRSopt_QN(x)
[V,S]=fun(x12);
[Vec,Fval,exitflag,output,lambda,grad,hessian] = fmincon(fun,x12,[],[],[],[],LB,UB, [],options);
Min(idx,jdx)=Fval;
[xs,index]= sort(Min)
result = Min(sort(index(1:x1)));
Opt(:,idx)=Vec ;
Average(idx,jdx)=V;
end
end
Walter iacomacci
on 16 Nov 2020
Have you tried making the Block diagram of that differential equation with Simulink?
If you have a model in Simulink there is a pretty useful command called linmod which is easy to use, the thing is that it will linearize the model for you in terms of a State-space model (A,B,C,D) matrices which I dont know if works for you.
HN
on 16 Nov 2020
Walter iacomacci Thank you,
I don't have model in simulink. I hope I can convert the program above easily. right ?
Walter iacomacci
on 16 Nov 2020
If you have the actual differential equation you can recreate it easily in Simulink. Here is an example:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/415378/image.png)
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/415383/image.png)
Which is a Differential Equation System of a Circuit, the Block diagram looks like this with Vin=30
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/415388/image.png)
Ignore the gain with 1/4 going to VL it is just a relation to obtain the values in time for a voltage in the inductor.
If you have only 1 differential equation is much easier. Let me know if you need help.
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