Inverse Kinematics - Algebraic approach - 6 DOF robotic arm, ERROR: Atan2: Inputs must be real.

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Uede Max 2017-12-11

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编辑： Uede Max 2017-12-11

Hey guys, read a lot about this error but I guess the root cause of this issue is the calculation itself. And I don't know how to move forward...

I am calculating theta1, theta3 and then theta2: a2=10; a3=8.5; d3=5; d4=5.5;

 %
%
%Theta1
theta1=atan2(py,px)-atan2(d3,sqrt(px*px +py*py-d3*d3));
K=(px*px+py*py+pz*pz-a2*a2-a3*a3-d3*d3-d4*d4)/(2*a2);
 %
%
%Theta3
theta3=atan2(a3,d4)-atan2(K,-sqrt(a3*a3+d4*d4-K*K));

By using my variables (DH parameters), all positive values in that case, i'm getting the error: Error using atan2: inputs must be real.

This data will lead for a solution for K=995.0281 Changing the inputs to 1, 0.85, 0.5, 0.55 does not make any difference, although: K=1.0063e+04

I am new to the world of MATLAB and assistance is appreciated! Cheers

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Birdman 2017-12-11

What do you expect theta3 to be?

Uede Max 2017-12-11

Theta1=0°, Theta3=45°

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Birdman 2017-12-11

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https://ww2.mathworks.cn/matlabcentral/answers/372221-inverse-kinematics-algebraic-approach-6-dof-robotic-arm-error-atan2-inputs-must-be-real#answer_295645

编辑：Birdman 2017-12-11

在 MATLAB Online 中打开

The problem arises from the statement

sqrt(a3*a3+d4*d4-K*K)

because the argument is negative which is

-6.5598e+06

and which will end up a complex number.

Therefore, you might want to look at that part again. Maybe there is some kind of miscalculation.

If you change theta3 calculation to the following, you get pi/4.

theta3=atan2(a3,d4)-atan2(K,sqrt(abs(a3*a3+d4*d4-K*K)));

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Uede Max 2017-12-11

编辑：Uede Max 2017-12-11

Thanks! Formula seems to be alright, though. Too bad, need to re-think the approach.

thank you!

Birdman 2017-12-11

You are welcome. Maybe you need to reconsider the forward kinematic model.

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