Hello,
I have a 6dof robot gp20hl. When activating the system and throughout the operation of the robot, a message appears at the bottom of the teach pendant screen: There is an axis needing ground checking. Please perform brake line ground checking.
This message did not appear ~3 months ago.
Do you know what it is and how to get it out?
Thank you 
The "Brake line ground check" procedure and what do to in this case is described in the manual of your controller.
The controller thinks at one time a brake lead is shorted. In at least Management security level go to ROBOT then BRAKE LINE GROUND CHECK. Once you go through the procedure, as long as the judgement is ok, the message will go away.
