Unexpected safety limit violation detected from xbot

[IoannisDadiotis]

I noticed that xbot in some cases detects safety limit violation for the derivative of the position reference although I only send effort reference (to close the gripper) with the filters activated at safe cutoff. The error in the xbot2-core terminal is:

[err ][rt_main] [dagana_2_claw_joint] safety limit violation detected in delta tx (0.5948931602151871  --->  0.5927931296170821): d(pos_ref)/dt exceeded: |-2.100030| > 2.000000;

This may be because internally the position reference tracks the actual position.

To reproduce the issue:

1. load the gui plugin in xbot-gui shown below:
   
2. Send 0.0 position to open the gripper.
3. Send -10.0 torque to close the gripper. The gripper starts closing until at some point this error is thrown from xbot2.

[IoannisDadiotis]

@alaurenzi