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I am working on a forward kinematics problem where I have already calculated the relevant transformation between the fixed base frame and the end effector frame of the robotic arm using Homogeneous Transformation Matrix. The convention I am following is

$$ T^{n-1}_{n} = R(z, \theta_n) \cdot Trans(z, d_n) \cdot Trans(x, a_n) \cdot R(x, \alpha_n) $$

My issue starts here. The orientation of the end effector as per the urdf of the arm is different from above calculation. So, I have to make some changes to my transformation, i.e., $T^{0}_{7}$ in order to orient it in the same direction as per the urdf.

In order to do that, I have to rotate $T^{0}_{7}$ by 180 about Z-axis, followed by 90 about Y-axis.

My code:

(using Eigen C++ library for matrix multiplication)

T0_7 = T0_1*T1_2*T2_3*T3_4*T4_5*T5_6*T6_7;

cout << T0_7 << endl;

T_Z180 <<  -1, 0, 0, 0,
            0, -1, 0, 0,
            0, 0, 1, 0,
            0, 0, 0, 1;
T_Y90 << 0, 0, -1, 0,
            0, 1, 0, 0,
            1, 0, 0, 0,
            0, 0, 0, 1;

cout << T0_7*T_Z180*T_Y90 << endl;

My question is - Am I doing final transformation correctly in the above code?

I would really appreciate some help with this.

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1 Answer 1

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Your transformation matrix for y and z are correct, but the order of multiplication is important in intrinsic rotations. Here you are first rotating about z axis at 180, so pre-multiply the transformation matrix with your end effector matrix

T1 =T_Z180*T0_7

and now do transformation about the y axis:

Tout = T_Y90*T1

or you can always directly pre-multuply them in order:

Tout = T_Y90*T_Z180*T0_7
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  • $\begingroup$ Thanks for your answer. I should have pre-multiply it, since it was intrinsic rotation. $\endgroup$ Jan 16 at 11:52

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