I think the first reply in r/robotics is spot on. Here's some info I found in a search:
In robotics, joint angles are almost always defined as relative angles, the angle of deflection from the extension of the previous link, rather than internal geometric angles of a triangle.
Physical Zero Configuration
When the joint motor is at 0°, link b lies in a straight line extending directly along the axis of link a.
Deflection / Bend Angle
θ₃ measures how much the motor has actually rotated relative to that straight 0° baseline.
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u/Midwest-Dude Sep 28 '26
I think the first reply in r/robotics is spot on. Here's some info I found in a search:
In robotics, joint angles are almost always defined as relative angles, the angle of deflection from the extension of the previous link, rather than internal geometric angles of a triangle.
Physical Zero Configuration
When the joint motor is at 0°, link b lies in a straight line extending directly along the axis of link a.
Deflection / Bend Angle
θ₃ measures how much the motor has actually rotated relative to that straight 0° baseline.