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Hello Robotics Stack Exchange. I am trying to put frames to this robot so I can make forward and inverse kinematics. However, when I use the DH parameters I do not get the same results as roboDK (a robot visualisation program).

Here is said robot. enter image description here

Here are my reasoning for putting the frames there, showing my reasoning I believe will show my mistakes to those more knowledgable than me hopefully.

-1: First i identify the joint axis. The second joint axis is displaced 25 mm to the side according to the data sheet so that's why its moved in the illustration.

0: Then I put a base frame. The base frame I chose to be exactly as the first frame since i want zeros in my data sheet to give me fewer variables when I need to make inverse kinematics later on.

1: Then I put frame one in. I put the z direction around the axis of rotation. Then I put the frame one next to axis 2 because i want the common normal. For that they have to next to each other so it can be a line with 90 degrees. When they are next to each other x_1 is pointing to the common perpendicular.

2: Frame 2 is there because the next link is parallel with it. Now z_2 is pointing into the page and x_2 is pointing towards the common perpendicular.

3: Same logic as in 2.

4: Frame four physically is at joint 4 but the origin is at frame 5. That is because frame 4 intersects frame 5 at the z axis. X_5 points toward the common normal.

5: Now z_5 and z_6 do intersect but we chose that it has to be where frame 4 is because they already intersect. So the origin is at the same place as 5.

6: Z follows the axis of rotation. The x_6 axis looks down because that's what the roboDK said it looks like. We want our calculations to match with that program and the last frame is also arbitrary because it does not have any link after to compare after it.

This was my reasoning. I hope someone can spot my mistake in my reasoning because I am sure I can do the DH-parameters from the frame description so the problems must be with the frames themselves. Thank you.

The data is here if it is needed: https://www.kuka.com/-/media/kuka-downloads/imported/6b77eecacfe542d3b736af377562ecaa/0000210361_en.pdf

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  • $\begingroup$ Does roboDK claim to be using a DH convention? $\endgroup$
    – RLH
    Apr 26, 2020 at 0:51
  • $\begingroup$ No but the location of the end effector should be the same. If it uses or doesn't use DH wouldn't matter in my view. $\endgroup$
    – Hamzalihi
    Apr 26, 2020 at 8:06

1 Answer 1

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I have studied manipulators kinematic by Craig book "Intro to robotics". Based on this book you must consider frame 0 and 1 co-incident. Look at this much similar example in this book: enter image description here enter image description here

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  • $\begingroup$ But what about the distance from the floor to frame one? $\endgroup$
    – Hamzalihi
    Apr 27, 2020 at 9:09
  • $\begingroup$ As you know, when you do fwd/inv kin the tool frame defines relative to 0 frame. You can simply consider a station frame S at the robot base. You know the relation between frame S and 0, so simply do a transformation. $\endgroup$
    – hosh0425
    Apr 28, 2020 at 6:33

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