MATLAB: Error using atan2 Inputs must be real.

atan2input must be real

Error using atan2
Inputs must be real.
how can i fix this?
clc;
clear all;
a1 = 150; alpha1 = pi/2; d1 = 470;
a2 = 600; alpha2 = 0; d2 = 0;
a3 = 120; alpha3 = pi/2; d3 = 720; 
syms c1 c2 c23 c3 c4 c5 c6 s1 c1 s2 s23 s3 s4 s5 s6 R03 R36 th1 th2 th3 th4 th5 th6 Xc Yc Zc R06 ox oz oy d6 r d D r11 r12 r13 r21 r22 r23 r31 r32 r33
c1 = cos(th1)
c2 = cos(th2)
c3 = cos(th3)
s1 = sin(th1)
s2 = sin(th2)
s3 = sin(th3)
c23 = cos(th2+th3)
s23 = sin(th2+th3)
d = a1
d6 = 50
%assume d6 = 5
R = [1 0 0 500;
    0 1 0 100;
    0 0 1 1500;
    0 0 0 1];
r11 = R(1,1)
r12 = R(1,2)
r13 = R(1,3)
r21 = R(2,1)
r22 = R(2,2)
r23 = R(2,3)
r31 = R(3,1)
r32 = R(3,2)
r33 = R(3,3)
ox = R(1,4)
oy = R(2,4)
oz = R(3,4)
Xc = ox - d6*R(1,3)
Yc = oy - d6*R(2,3)
Zc = oz - d6*R(3,3)
D = (Xc^2+Yc^2-d^2+(Zc-d1)^2-a2^2-a3^2)/(2*a2*a3)
%inverse kinematic 
th1 = atan2(Xc,Yc)
th2 = atan2(sqrt(Xc^2+Yc^2-d^2),Zc-d1)-atan2(a2+a3*c3,a3*s3)
th3 = atan2 (D,sqrt((1-d^2)))
th4 = atan2(c1*c23*r13+s1*c23*r23+s23r33,-c1*s23*r13-s1*s23*r23+c23*r33)
th5 = atan2(s1*r13-c1*r23,sqrt(1-(s1*r13-c1*r23)^2))
th6 = atan2(-s1*r11+c1*r21, s1*r12-c1*r22)

Best Answer

It gives the value of th4, but the output is in the symbolic format. Convert it into a floating-point value by using double(). Change the line like this
th4 = double(atan2(c1*c23*r13+s1*c23*r23+s23*r33,-c1*s23*r13-s1*s23*r23+c23*r33))
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