TCP missing target position and trajectory by up to 8mm

Technical Questions
1

Hello everybody,

I recently started to work with a UR5e and as first steps i created some simple exmaple programms in order to get used to the different types of movements.
As our inteded application is relevant to constant speed a more or less precise follow up of the intended trajectory I then decided to measure to UR’s path during an example trajectory by means of a Laser Tracker in order to get to know the capabilities of the system.
I used the movep command with a blend radius of 2mm to have more or less constant speed. I programmed the hole movement in one movep node and added waypoints for all linear and circular movements in thís node.
I expected the robot to miss the waypoints by the blending radius and then always come back to the tintended trajectory, but i was very suprised to see the actual results. The intended trajectory is never reached and position differences of up to 8 mm were observable.
I would like to know what i could have done wrong when programming the trajectory? Is movep the wrong command?

For completness, I added the URscript file of the programm and a screenshot of the path masurement to this post.

pathTest.script (8.9 KB)

Positionning_Error_UR5e_20210205_ISO9823
Positionning_Error_UR5e_20210205_ISO98231332×935 59.6 KB

2

Hi, I think we need some more data to evaluate what you are observing. Please provide urp file and a description of your setup.

Typical issues if you want to measure the robot with a 3D tracker are:

  • Coordinates of the robot and tracker are not aligned. Even when you use steering pins in the base and tool of the robot, small angle errors there lead to significant positional differences at the TCP in the 3D traker’s perspective
  • Remember to set the TCP to the point you are tracking
  • If the robot has been disassembled (e.g. for joint replacement), the robot must be calibrated

An error of 8mm should not occur in a situation like yours.