2011/4/19 andy pugh <[email protected]>: > > I have found the same thing, and I am wondering if it is inherent in > the nature of the DH parameters that they are defined relative to an > arbitrary initial alignment, and that means that the first joint can > not be prismatic in X. > (This is just a guess)
>From my readings about DH parameters, I understand that in prismatic joints the D is the joint variable. D is measured along Z axis of joint's coordinate system. So for joint 0 to be prismatic in X, I just need to rotate it's coordinate frame so that joint's Z matches base's X. It is done with Theta and Alpha parameters: 1) Theta rotates around "old Z" - that is base's Z. So I should rotate it by 90 degrees so that "new X" point along "old Y"; 2) Alpha rotates along "new X" - so I should rotate by the same 90 degrees in the same direction so that "new Z" matches "old X" And so I would have prismatic joint, moving along base's X axis. This is, how it _should_ work. But it just won't. I used systematic approach - I set alpha and theta to zero and then tested, how are alligned arm's coordinates against base coordinates. X and Z are switched. Then I started changing alpha values to see, which direction and how are the arm's coordinate frame rotated, then I did the same with theta. Then I tried all the combinations of alpha and theta in range from minus PI to plus PI and I came to conclusion that, since _both_ X and Z are swapped, it is not possible to swap them back by two rotations - at least three are needed. And this is, where I do not understand, why... That does not work as the theory explains. > There is a spurious conversion from linear mm to radians. You need to > move the joint 360mm to move Y or Z by pi mm. > line 444 in genserkins looks like one likely culprit. Line 521 converts them back to degrees. But line 528 is still in radians, I just do not understand, what does line 528 do. Viesturs ------------------------------------------------------------------------------ Benefiting from Server Virtualization: Beyond Initial Workload Consolidation -- Increasing the use of server virtualization is a top priority.Virtualization can reduce costs, simplify management, and improve application availability and disaster protection. Learn more about boosting the value of server virtualization. http://p.sf.net/sfu/vmware-sfdev2dev _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
