fsolve giving error that solution is not finite and real. When I test using vpasolve() I get a solution, then I input the same solution and get same error of not finite/real.

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Gordon 2022-6-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1739510-fsolve-giving-error-that-solution-is-not-finite-and-real-when-i-test-using-vpasolve-i-get-a-solut

编辑： Torsten 2022-6-13

c1 = 0.5176;
c2 = 116;
c3 = .4;
c4 = 0.4;
c5 = 5;
c6 =21;
c7 = .08;
c8 = .035;
syms lambda
for k = 1:600
    theta = pitch.Data(k);
    %cp(k) = c1*(c2/lambda - c3*theta -c5)*exp(c6/lambda);
    cp(k) = c1*(c6*lambda + (-c4 - c3*(2.5 + theta) + c2*(1/(lambda + ...
        c7*(2.5 + theta)) - c8/(1 + (2.5 + theta)^3)))/exp(c5*(1/(lambda + ...
        c7*(2.5 + theta)) - c8/(1 + (2.5 + theta)^3))));
    eqn(k) = cp(k)/(2*lambda^3) == 1e7*(ta_kf.Data(k))/(rho*pi*N^5*wr_kf.Data(k)^2);
    tsr(k) = vpasolve(eqn(k),lambda);%
    tsr_check(k) = fzero(@(lambda)cp(k),[-1 0]);
end

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Gordon 2022-6-13

Note: I actually gave the input as -1 as an initial guess. The actual solution via vpasolve() is -.6....

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Answer 1

Torsten 2022-6-13

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https://ww2.mathworks.cn/matlabcentral/answers/1739510-fsolve-giving-error-that-solution-is-not-finite-and-real-when-i-test-using-vpasolve-i-get-a-solut#answer_984735

在 MATLAB Online 中打开

c1 = 0.5176;
c2 = 116;
c3 = .4;
c4 = 0.4;
c5 = 5;
c6 =21;
c7 = .08;
c8 = .035;
syms lambda
for k = 1:600
    theta = pitch.Data(k);
    %cp(k) = c1*(c2/lambda - c3*theta -c5)*exp(c6/lambda);
    cp(k) = c1*(c6*lambda + (-c4 - c3*(2.5 + theta) + c2*(1/(lambda + ...
            c7*(2.5 + theta)) - c8/(1 + (2.5 + theta)^3)))/exp(c5*(1/(lambda + ...
            c7*(2.5 + theta)) - c8/(1 + (2.5 + theta)^3))));
    eqn(k) = cp(k)/(2*lambda^3) == 1e7*(ta_kf.Data(k))/(rho*pi*N^5*wr_kf.Data(k)^2);
    tsr(k) = vpasolve(eqn(k),lambda);%
    expr = cp(k)/(2*lambda^3) - 1e7*(ta_kf.Data(k))/(rho*pi*N^5*wr_kf.Data(k)^2);
    fun = matlabFunction(expr,'Vars',lambda);
    tsr_check(k) = fzero(fun,[-1 0]);
end

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Torsten 2022-6-13

@Gordon comment moved here:

fsolve() is not a good enough solver in this situation because of the rate of change of the data. Therefore, vpasolve() needed to be used. The reason for trying to implement fsolve is because simulink does not allow vpasolve() a solution therefore is to use code.extrinsic() to implement function including vpasolve().

Torsten 2022-6-13

编辑：Torsten 2022-6-13

在 MATLAB Online 中打开

If the solution for index k is "near" to the solution of index k-1, it is usually a good idea to take the solution of step k-1 as initial guess for the solution of index k. Something like

c1 = 0.5176;
c2 = 116;
c3 = .4;
c4 = 0.4;
c5 = 5;
c6 =21;
c7 = .08;
c8 = .035;
tsr_guess = 1.0;
syms lambda
for k = 1:600
    theta = pitch.Data(k);
    %cp(k) = c1*(c2/lambda - c3*theta -c5)*exp(c6/lambda);
    cp(k) = c1*(c6*lambda + (-c4 - c3*(2.5 + theta) + c2*(1/(lambda + ...
            c7*(2.5 + theta)) - c8/(1 + (2.5 + theta)^3)))/exp(c5*(1/(lambda + ...
            c7*(2.5 + theta)) - c8/(1 + (2.5 + theta)^3))));
    eqn(k) = cp(k)/(2*lambda^3) == 1e7*(ta_kf.Data(k))/(rho*pi*N^5*wr_kf.Data(k)^2);
    tsr(k) = vpasolve(eqn(k),lambda);%
    expr = cp(k)/(2*lambda^3) - 1e7*(ta_kf.Data(k))/(rho*pi*N^5*wr_kf.Data(k)^2);
    fun = matlabFunction(expr,'Vars',lambda);
    tsr_check(k) = fsolve(fun,tsr_guess);
    tsr_guess = tsr_check(k);
end