The results of the plots does not match the actual ones

Hello everyone,

I am trying to plot a simple kinematics function. I did it first in Excel and the plots were corrects however when I transfered problem into Matlab, I got different results. Below are the function, true plots from excel and results from the code:

The matlab code and the plots are shown below:

L_BC = 430; % mm

L_CA = 900; % mm

L_AA_prime = 100; % mm

theta_Anchor_range = deg2rad(linspace(127.2, 187.46, 1000));

L_AB = sqrt((L_AA_prime^2)+(L_BC^2)+(L_CA^2)+(2*L_BC*(L_CA*sin(theta_Anchor_range+pi)))-L_AA_prime*cos(pi+theta_Anchor_range));

L_Stroke = L_AB - 580;

figure;

plot(rad2deg(theta_Anchor_range), L_Stroke);

xlabel('Theta Anchor (degrees)');

ylabel('L_Stroke (mm)');

title('Variation of L_AB with Theta BC');

grid on;

theta_AB = atan((L_CA+(L_BC*sin(pi+theta_Anchor_range))./(L_BC*cos(pi+theta_Anchor_range)-L_AA_prime)));

figure;

plot(rad2deg(theta_Anchor_range), rad2deg(theta_AB));

xlabel('Theta BC (degrees)');

ylabel('Theta AB (deg)');

The variation of length have the same phenomena however it does not start from zero.

Your support is highly appreciated.

Answer 1

[Star Strider]

There may have been a problem converting degrees to radian measure, however that is not required in MATLAB. Use trigonometric functions with the ‘d’ (sind, cosd, atan2d) to use degrees without having to convert them. Additionally, I use atan2d here, rather than atand. It seems to give the appropriate results.

Try this —

L_BC = 430;  % mm
L_CA = 900;  % mm
L_AA_prime = 100;  % mm
theta_Anchor_range = linspace(127.2, 187.46, 1000); 
L_AB = sqrt((L_AA_prime^2)+(L_BC^2)+(L_CA^2)+(2*L_BC*(L_CA*sind(theta_Anchor_range+180)))-L_AA_prime*cosd(theta_Anchor_range));
L_Stroke = L_AB - 580;
figure;
plot(theta_Anchor_range, L_Stroke);
xlabel('Theta Anchor (degrees)');
ylabel('L_Stroke (mm)');
title('Variation of L_AB with Theta BC');
grid on;

theta_AB = atan2d((L_CA+L_BC*sind(180+theta_Anchor_range)),(L_BC*cosd(180+theta_Anchor_range)-L_AA_prime));
figure;
plot(theta_Anchor_range, theta_AB);
grid
xlabel('Theta BC (degrees)');
ylabel('Theta AB (deg)');

% axis('equal')

EDIT — Corrected typographical errors.

.

[Abdulrahman Alhassan]

Thank you this code seems working fine. However, I still cannot figure out why there is around 50mm jumb in the length. The excel code starts from 0 and matlab code starts from 43mm.

[Star Strider]

My pleasure!

I checked your code and it appears to match the image (as far as I can determine), so I cannot account for whatever discrepancy you may be seeing between MATLAB and Excel. There appears to be a constant offset in the top plot, because the upper limit in the MATLAB plot is higher than in the posted image, as well. Please check your MATLAB code and the data you are providing to your MATLAB code. Something is apparently different between it and what you’re providing to Excel.

[Abdulrahman Alhassan]

Will do that.

Thank you for your efforts.

[Star Strider]

As always, my pleasure!