What does this mean (found in user manual)

xArm/850 Troubleshooting
1

Hi Support,

These bullets are from the XArm 6 User Manual 1.2.2 page 60.
Could somebody at UFactory please clarify what they mean?


● Repeat the same set of Cartesian commands. When no joint command is used, the posture of the arm will change slightly. If it continues to accumulate, it may cause an error. In motion planning, a joint command correction is needed.

[Andre says:] What is this saying? Do you mean that if only XYZ type commands are used, then errors will accumulate? Does this apply only to where the robot goes when you tell it to move or does it also apply to where it says it is when you ask for its location?

What exactly needs to be done to keep the robot from accumulating errors?

Please answer these questions! This may be the cause of repeatability issues we (and some of your other customers) are seeing!

● When going to the same position, the rotation process of the
joint motion, Cartesian motion, and the robotic arm are
different.

[Andre says:] I do not understand this sentence. Could you please clarify and provide a more detailed explanation?

● In a certain posture of the robotic arm, it may not be able to
take the Cartesian linear motion. In this case, the rotation
angle of the joint needs to be readjusted.

[Andre says:] What is this saying? Do you mean that sometimes the robot can’t solve a Cartesian motion path and the user must specify servo angles explicitly to get it unstuck? Please clarify? _

● If the x/y/z cannot move simultaneously with the Cartesian
linear motion, please decompose the position.

[Andre says:] I do not understand this sentence. Could you please clarify?

Thank you for your attention!

Andre Sant’Anna
Director, R&D
Applied Anodize, Inc.

2
  1. Due to the physical design of current version of xArm, the Inverse-Kinematic (IK) solutions (used for Cartesian planning) are performed in numerical order, there are no closed-form solutions to keep robot posture in each execution exactly the same when executing Cartesian commands in a loop.

You may find the joint configurations changes little by little while keeping the consistency of end-effector trajectory during the looped execution. If leave it for some time, chances are certain joint may go beyond its operation range and trigger an error. In this case, user may have to insert a joint-space command before executing critical Cartesian operations to eliminate the chance of inconsistency, since IK will find solutions in the neighborhood of its starting joint configuration.

  1. The intention of this sentence is to say the trajectory of Cartesian execution and Joint space execution to the same target is different.

  2. This means if you find there are no solution to a Cartesian command (straight-line trajectory) at some configuration, you have to adjust the starting position and retry.

4.This is a previous limitation that no longer exist in current version.

Thanks for the feedback. We will modify our instructions and make it more clear to the user.

3

Thanks!