Kinematic Concepts
Kinematic Concepts tests the geometry of the robot and the workholding, and how that geometry constrains a weld path. Items are practical: why a path is jerky near the edge of the envelope, why coordinated positioner motion matters, and why repeatability is not accuracy.
Axes and degrees of freedom
A typical arc welding robot has six axes: base rotation, shoulder, elbow, and three wrist axes. Six degrees of freedom allow arbitrary position and orientation of the TCP. Fewer than six constrains orientation, which is why a five-axis machine needs a positioner to present the joint.
Reach, payload, and singularity
- The work envelope is the set of points the wrist can reach. A path near the boundary forces full extension, where accuracy drops and the wrist has little room to hold an angle.
- Payload includes the torch, collision sensor, cable dress, and wire in the conduit. Overloading shows up as positional error and overheating.
- Wrist singularity occurs when wrist axes line up, typically as the fifth axis approaches zero. Joint speeds rise sharply and the path can deviate. Reposition the part or reorient the torch.
Frames and coordinated motion
The kinematic chain runs from the robot base frame through each joint to the tool frame, with a user or work object frame defining the part. A positioner can be an external axis: coordinated motion integrates it into the path so TCP speed stays constant relative to the part, while independent motion does not.
Accuracy and repeatability
Repeatability is the spread of repeated returns to the same taught point, usually a fraction of a millimeter on a good robot. Accuracy is how close the robot gets to a commanded coordinate. A robot can be very repeatable and still off by a millimeter, which is why TCP and frame calibration are separate from teaching.
Worked example
A circular weld at the top of a tall fixture runs fast and jerky, and the bead narrows on one side. The path passes near full extension with the wrist close to a singularity, so the controller trades joint speed for path following. The fix is to lower the fixture or rotate the positioner so the same circle sits nearer the center of the envelope with the wrist away from alignment.