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Troubleshooting Path Offset Errors near Robot Workspace Limit
Troubleshooting Path Offset Errors near Robot Workspace Limit

Encountering Path Offset and Acceleration warnings while Weaving

Gaurav Shetty avatar
Written by Gaurav Shetty
Updated over a week ago

Robot Workspace

The workspace of the UR robot is spherical. The diagrams below show the “Recommended Reach” and the “Maximum Working Area” of the robot arm. The area directly above or below the robot base joint does have restrictions on the movement of the arm.

Outer Workspace Limit

The robot can move the TCP to any point within the recommended reach space with multiple orientations, depending on the limitations of the accessories on the arm such as a weld torch.

When the robot is programmed to move beyond the recommended range (shown in blue) but still within the maximum work area, there are restrictions to the motion. The addition of a weld torch moves the TCP further out and hence, the maximum work area is reduced.

Performing path offsetting motion such as weaving in the region beyond the recommended range will lead to unpredictable movements and acceleration errors.

Inner Workspace Limit

It is highly recommended to avoid turning on path offsets (weaving) in the column directly above and below the robot base (represented in yellow in the animation below).

Rapid linear motion, like weaving in the space near the cylinder may cause unpredictable movements and acceleration errors, as mentioned above. This is because a relatively slow tool speed requires a very high speed of rotation of the base joint, making some tool movements unachievable or unsafe.

Where are Linear Movements used?

Linear movements are used in path-critical application such as following a weld path, where the robot is required follow a path with precision and accuracy. The cobot uses Linear movements while performing an approach to the start of the weld, performing a weld, and while departing from the end of a weld.

Where are Joint Angle Movements used?

The robot uses "Joint Motion" while performing air moves and while moving to/from the "home position". While using Joint Motion, also called Point-to-Point motion, the robot takes the quickest path path between the two points. The motion is predictable but it's not linear.

NOTE: The additions of “air moves” within the Inner Workspace Limits and beyond the Outer Recommended Range are safe since the Joint angle moves do not require kinematic conversions and hence are not affected by Singularity Errors.

What is Robot Singularity?

Robot Singularity is a condition in which certain end-effector positions relative to the other robot joints leads to errors in the motion control algorithm causing irregular behaviors such as large accelerations and reduced mobility.

Wrist Alignment Singularity

The shoulder joint, elbow joint and wrist 1 joints all rotate in the same plane on UR robots, as shown by the arrows numbered 1, 2 and 3 in the animation below.

But when we also align the movement of wrist joint 2 (labeled 4 in the animation) with this same plane, we encounter the singularity condition which limits the range of movements of the robot, regardless of the area of the workspace.

As mentioned previously, “air moves” are not affected by the Singularity condition since it uses “joint angles movement”. But the “welding path” uses linear motion which restricts motion under certain conditions explained above.

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