Denavit-hartenberg parameters sign 6-DOF

Question

Trying to understand how does the Denavit-Hartenberg work.
While I understood how is the table constructed, I however don't understand how are the signs for the orientation parameters (theta, joint angle and alpha, twist link) determined.

1. Is there any rule that I'm missing, that determines whether the value is going to be negative or positive?
2. Why is the joint angle of the last link 6 has the value of 180 even though there's no transformation between the end effector and the rotational joint?

Answer 1

First thing first- you can consider any direction as positive or negative, but need to keep it constant while solving the problem and even while programming robot.

In almost all cases of 6-DOF manipulator, the co-ordinate system of joint 1 is aligned with the universal coordinate system.

Theta- its joint parameter, it signifies angle between 2 x-axis about the z-axis.

Alpha- its link parameter, it signifies twist of 2 z-axis about x-axis. Consider one link and try to visualize the coordinate system at each end of the link.

The answer to question-2 is- you can either write 0 or 180 degree, but remember it while programming for setting home position of the robot and while calibration of joints.

Hope this helps.