/* key dimensions
joint[0] = Entire arm rotates around a vertical axis at its inner end
which is attached to the earth. A value of zero means the
inner arm is pointing along the X axis.
D1 = Vertical distance from the ground plane to the center of the inner
arm.
D2 = Horizontal distance between joint[0] axis and joint[1] axis, ie.
the length of the inner arm.
joint[1] = Outer arm rotates around a vertical axis at its inner end
which is attached to the outer end of the inner arm. A
value of zero means the outer arm is parallel to the
inner arm (and extending outward).
D3 = Vertical distance from the center of the inner arm to the center
of the outer arm. May be positive or negative depending
on the structure of the robot.
joint[2] = End effector slides along a vertical axis at the outer end
of the outer arm. A value of zero means the end effector
is at the same height as the center of the outer arm, and
positive values mean downward movement.
D4 = Horizontal distance between joint[1] axis and joint[2] axis, ie.
the length of the outer arm
joint[3] = End effector rotates around the same vertical axis that it
slides along. A value of zero means that the tooltip (if
offset from the axis) is pointing in the same direction
as the centerline of the outer arm.
D5 = Vertical distance from the end effector to the tooltip. Positive
means the tooltip is lower than the end effector, and is
the normal case.
D6 = Horizontal distance from the centerline of the end effector (and
the joints 2 and 3 axis) and the tooltip. Zero means the
tooltip is on the centerline. Non-zero values should be
positive, if negative they introduce a 180 degree offset
on the value of joint[3].
*/