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I am studying the forward kinematics for robot manipulators, and I am focusing on the Denavit Hartenberg convenction for assigning frames to the various joints of the manipulator.

The problem is that I am often confused, and so it is easy to make mistakes for me.

I have understood the concept of the D-H convenction and how it assigns frames, but when I start having more complex manipulators to analyze, for example spacial manipulators with both prismatic and revoulute joints , I start making confusion and assigning frames wrongly.

So, my question is:

Is there a way to assign frames being sure (or at least reducing a lot) the chances of making mistakes?

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First of all, I have to share with you this link:

https://www.youtube.com/watch?v=rA9tm0gTln8

I like to think of a few tricks with D-H params. First, the Z-axis is the axis of rotation/translation. Then, theta rotates along the Z-axis to align with the start of the next D-H frame. You then translate along the Z axis such that the point is tangent to the next frame's Z-X plane. Then, the A term moves along the CURRENT X-axis to the start of the next frame. Finally, you rotate about the X-Axis to align the Z-axis with the rotation/translation axis of the next frame.

Summary:

  1. Rotate about Z-axis (theta)
  2. Translate along Z-axis (z)
  3. Translate along X-axis (a)
  4. Reorient Z-Axis about X-axis (alpha)

Note

The starting X-axis can be anywhere. You will want to choose a convenient orientation such that the math is easy later. If there is a world frame you are trying to align to, that can help you think of a start. Remember, you do NOT use the Y-axis for anything.

Does this make sense?

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