# Timeline for DH parameters solution for forward kinematics

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Aug 5 '17 at 6:51 comment @Chuck, thank you for the tip, didn't know about the lego leagues :), The quadrilateral in my question is a crossed/ complex quadrilateral, in this case the sum of 4 internal angles you mentioned does not sum up to 360.. what about the angle at the crossing ?
Aug 4 '17 at 20:02 comment @Roboticsnerd - If you're interested, try finding First Lego League competitions in your area. You can volunteer, mentor, judge, etc. Super rewarding to help out the kids :)
Aug 4 '17 at 18:47 comment all good chuck, I will try to figure out the motor angle part, thank you so much, hopefully one day I would be in a position to help others like you helped me <3
Aug 4 '17 at 18:31 comment @Roboticsnerd - Sorry again, you're correct. Deleted my comment :-[
Aug 4 '17 at 16:10 comment @Chuck, I think it would be too much to ask but could you please make a drawing of the alignment of axis of θ41 and the motor. I can see that the x axes of θ12 and the motor joint align but not θ41.. Thank you for your time and help :))
Aug 4 '17 at 1:15 comment Nice details @Chuck
Aug 3 '17 at 21:37 comment I made a small edit in the end of the question, please take a look.
Aug 3 '17 at 20:26 comment I assumed that it was something mis-drawn and assumed the motor joint as $J_1$ in the drawing; you can kind of see that in the first drawing in my answer. The motor can't mount on link $L_{12}$ and still be able to twist the "blue dot." It also can't mount on the box and twist the blue dot if there really is a joint $J_1$ that attaches $L_{12}$ to the box, because then you have the motor and $J_1$ attaching link $L_{12}$ to the box, which would fix $L_{12}$ and prevent it from rotating. This would probably be clearer if you could build a cardboard mockup or similar.
Aug 3 '17 at 20:23 comment @Roboticsnerd - Yeah, those were a couple typos I noticed re-reading earlier. The diagram isn't especially clear, which is how I botched the $J_4$ equation. And you're right, what I should have said was 2 equations and 3 unknowns. I'm not sure what you mean about the motor angle - the motor needs a means for counter-torque. If you refer back to your Page 3 diagram in the question, you have a $J_1$, which appears to be "pinned" to the box. You also have a blue dot, which looks like maybe the motor input? But they can't both be pinned to the box. How would the blue dot actuate?
Aug 3 '17 at 20:05 comment I solved based on your method, all good, just 2 corrections, The internal angle of J4 would be Thera34 - (Theta-41 - pi) and when you find the couple equations, you mentioned "2 equations and 2 unknows" shouldn't it be 2 equations and 3 unknowns ? we know 1 angle and all 4 link lenghts. After all set and done, I can only measure the motor angle from the encoder. How can I find Theta41 or ThetaJ1 from the motor angle? any ideas? The link L12 connects the motor shaft with the joint J2. I have spent the whole day thinking about this but no solution till now..
Aug 2 '17 at 5:02 comment saved :) , thank you for your insight and time. I will use your solution as a way point and make my own : )
Aug 1 '17 at 14:35 history edited
Aug 1 '17 at 13:33 comment @Roboticsnerd Could this thread be left open for any future questions regarding this topic? No. First, this isn't a thread (this site isn't a forum), it's a question. Second, if you have more questions, just ask new questions! If you think the question is related, just link to this question. You can get a small URL for this question by clicking the "share" button under the question. You can ask as many questions as you want.