Can anybody help figure out HD parameters for the case where two links with a revolute joint are in the same plane, thus that the variable angle is 0, but the twist is not 0. This is a simple drawing. I think that x-axis that is perpendicular to both z-axis, points away and goes through the intercection of z-axis. The link length is 0, the twist is a and the offset is d. Whould it be correct? Thanks.
You can use dummy transformations when the DH parameterization cannot get you from one axis to the other. Think of it as two succesive DH transformations where the first one does not have a moving joint. It is just there to get to the other joint the the second DH transformation
to complete the answer with reference:
If your previous Z axis is not intersecting the next X axis and/or they are not perpendicular, you will not be able to use one DH transformation matrix from one coordiante system to the other