MATLAB: Solve Function returning empy values

solve

I can't figure out why the values returned for each variable is empty. I have 5 equations and 5 unkowns. Where am i going wrong?

Best Answer

We cannot copy and paste code from images of the code in order to test it.
It is a categorical error to use solve() in any equation that containst floating point coefficients. solve() is for finding exact solutions, but floating point numbers are never exact. You want an exact solution, but the gravity you used is not exact: standard g is 9.80665, and g on Earth is measured to be between 9.76392 m/s^2 and 9.83366 m/s^2 https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/grl.50838 . Exactly where on Earth you are making the calculation for will make a difference to the exact answer -- and exact answers are the only ones you should be using solve() for.
If your parameters are only approximate, you should be using the approximate solver, vpasolve()

Related Solutions

MATLAB: How to use a Struct defined in previous calculation

The easiest way is to store the structs arrays in a cell array and then later reuse them. For example

L1=6.243;
L2=2.0;
l1=2.1215;
theta=[1:1:90]
e_array = {}; % <--- initialize empty cell array
for i=1:90
    syms F5 F6x F6y
    eqn9=([c.F4x])+([d.F3x])==(F5.*cosd(theta(i)))+F6x+([d.Fp].*cosd(beta))
    eqn10=([c.F4y])+([d.F3y])+(F5.*sind(theta(i)))+F6y==([d.Fp].*cosd(beta))
    eqn11=(F5.*cosd(theta(i))*l1)+(F5.*sind(theta(i))*l1)+([d.F3y]*l1)+([d.Fp].*cosd(beta)*L1)==([d.F3x]*(L1-l1))+([c.F4x]*L1)
    % eqn12=(F5.*cosd(theta)*(L1-l1))+(F6x*L1)==(F5.*sind(theta)*l1)+([d.F3x]*l1)+([d.F3y]*l1)
    for K = 1 : length(eqn9)
      e(K) = (vpasolve([eqn9(K),eqn10(K),eqn11(K)], [F5,F6x,F6y]))
    end
    e_array{i} = e; % <--- store them for later use
    plot(theta,[e.F5],'r')
    hold on 
end

Then use them as follow

gamma=theta-hoeksup
syms F7x F7y
for i=1:numel(e_array)
    eqn12=([e_array{i}.F5].*cosd(theta))+(Fp2.*cosd(gamma))==F7x
    eqn13=([e_array{i}.F5].*sind(theta))+(Fp2.*sind(gamma))==F7y
    % do other calculations
end

MATLAB: Finding angle of rotations from a given unit vector to rotate a given vector using those angle to align with the previous unit vector

The following might be what you're looking for. It uses my AxelRot utility from the File Exchange

https://www.mathworks.com/matlabcentral/fileexchange/30864-3d-rotation-about-shifted-axis?focused=5191309&tab=function

Or, if you only want theta, you could modify the code to return only that, which then wouldn't require AxelRot.

function [M,theta, Nrm]=vecrot(vstart,vend)
%Find rotation carrying one vector toward another about their common perpendicular
%axis.
%           
%IN:         
%            


%    vstart: Initial vector
%      vend: Final vector
%            
%OUT:        
%            
%    M:  homogeneous 4x4 rotation matrix carrying vstart to vend in a
%        rotation about the axis Nrm=cross(vstart,vend)
%    theta:  the rotation angle in degrees
%    Nrm: the rotation axis
  vstart=vstart(:)/norm(vstart);
  vend=vend(:)/norm(vend);
 
  Nrm=cross(vstart,vend);
   
  b=vend.'*vstart;
  
  theta = atan2d(sqrt(1-b^2),b);
   M=AxelRot(theta,Nrm,[]);

Best Answer

Related Solutions

MATLAB: How to use a Struct defined in previous calculation

MATLAB: Finding angle of rotations from a given unit vector to rotate a given vector using those angle to align with the previous unit vector

Related Question