Can anyone explain why I am getting an error here? This was a code given to my by my teacher for a project, but it will not work for me. It is for a linkage analysis.

I attached the full code below that is giving me an error. It is for a machine dynamics project, and was a

code given to us by the professor in the class. I am unable to get a hold of them, so thought I would post here.

The error is included as well for context.

*Note: The inputs for the code, in case that is relevant, are:

G = 34 deg
R = 3.2
f = 53.5845
h = 21.3672
beta = 155 deg
gamma = 145 deg
the2_start = 225 deg

There is a seperate code attached to this where the inputs are entered.

I am not having issues with that code so it did not seem relevant.

Thanks

ERROR:

Error using atan2

Inputs must be real.

Error in fivebar_position (line 60)

the51(i)=atan2(BB-g1(i)*sin(gammar),AA-g1(i)*cos(gammar));

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

FULL CODE:

function [the2, the3, the4, trans1, Px, Py, g2, the52, trans2]=fivebar_position(a,b,c,d,P,delta3,the2_start,f,beta,h,gamma)

deg2rad=pi/180; % convert degree to radian
rad2deg=180/pi; % convert radian to degree
K1=d/a; % Eq. (4.8a)
K2=d/c; % Eq. (4.8a)
K3=(a^2-b^2+c^2+d^2)/(2*a*c); % Eq. (4.8a)
K4=d/b; % Eq. (4.11b)
K5=(c^2-d^2-a^2-b^2)/(2*a*b); % Eq. (4.11b)
the2=239*deg2rad; % generate an array for theta2
n=length(the2); % length of the array

for i=1:n
  A=cos(the2(i))-K1-K2*cos(the2(i))+K3;
  B=-2*sin(the2(i));
  C=K1-(K2+1)*cos(the2(i))+K3;
  D=cos(the2(i))-K1+K4*cos(the2(i))+K5;
  E=-2*sin(the2(i));
  F=K1+(K4-1)*cos(the2(i))+K5;
  
  disc=sqrt(B^2-4*A*C);
  temp=2*atan2(-B-disc,2*A); % for open configuration
  if (temp <= 0) % make theta4 positve
     the4(i)=2*pi+temp;
  else
     the4(i)=temp;
  end
  
  disc=sqrt(E^2-4*D*F);
  temp=2*atan2(-E-disc,2*D); % for open configuration
  if (temp <= 0) % make theta3 positive
     the3(i)=2*pi+temp; 
  else
     the3(i)=temp;
  end
    
  temp=abs(the3(i)-the4(i)); % transmission angles 
  if (temp > 1.5*pi)
      trans1(i)=2*pi-temp;
  elseif (temp > pi)
      trans1(i)=temp-pi;
  elseif (temp > pi/2)
     trans1(i)=pi-temp;      
  else
     trans1(i)=temp;      
  end
  
  Px(i)=a*cos(the2(i))+P*cos(the3(i)+delta3); % coupler point 
  Py(i)=a*sin(the2(i))+P*sin(the3(i)+delta3); % x and y 
end

for i=1:n
    AA=a*cos(the2(i))+P*cos(the3(i)+delta3)-d-h*cos(beta);
    BB=a*sin(the2(i))+P*sin(the3(i)+delta3)-h*sin(beta);
    C1=-2*(AA*cos(gamma)+BB*sin(gamma));
    C2=AA^2+BB^2-f^2;
    dis=sqrt(C1^2-4*C2);
    g1(i)=(-C1+sqrt(C1^2-4*C2))/2;
    g2(i)=(-C1-sqrt(C1^2-4*C2))/2;
    the51(i)=atan2(BB-g1(i)*sin(gamma),AA-g1(i)*cos(gamma));
    the52(i)=atan2(BB-g2(i)*sin(gamma),AA-g2(i)*cos(gamma));
    if the51(i) < 0
        the51(i)=the51(i)+2*pi;
    end
    if the52(i) < 0
        the52(i)=the52(i)+2*pi;
    end
    trans2(i)=pi/2-(the52(i)-gamma);
    if trans2(i) > pi/2
        trans2(i)=pi-trans2(i);
    end
end

Answer 1

[Sam Chak]

Hi @Ben

If the error message says that the inputs of atan2() must be real, then one or both of the inputs must be complex. What in the world of MATLAB functions could possibly produce a complex number? My mathematical instincts suggest that the usual suspect is the square root function!

sqrt(-4)

ans = 0.0000 + 2.0000i

[Ben]

Thank you for the response. I missed a negtive sign in front of an angle