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At this point I kept working with the package. I wanted to manage multiple EPOS in a CAN net by managing them through accessing to one of them via USB. The following message was kept being printed periodically as I mentioned in my question:

Write processed successfully, but number of bytes written is 0: Operation now in progress
Write processed successfully, but number of bytes written is 0: Operation now in progress
Write processed successfully, but number of bytes written is 0: Operation now in progress

I could launch the controller but I lost control of the EPOS after sometime by not know reason. I did not know if the problem was due to the limited handling package capability of EPOS or if I was not defining rigth my URDF or if there was some kind of bug in the implementation of the controllers of the hardware abstraction of ros_control (not so much reliable documentation explaining in detail its behavior).

The level of documentation of this package is quite poor from my point of view. A lot of explanations on the ROS Code API is missing and it gets really messy to look debug sometimes. It is a great idea but it gives the sensation that the work was left have done (as realease version was published as 0.0.3).

The USB connection works but when trying to make it work in the configuration I was testing the USB falls making it impossible to communicate with the EPOS unless the code is relaunch. Then you have to pray for the system to be able to clear errors on the first try.

I debugged my code, my URDF and I was not understanding the reasons behind this. Also tried in Windows LabView 2015 and the system did not lost control, the problem of the implementation of this package is that depends on dll libraries which are not supported in Linux based OS so I could not (yet) working with the linux library implementation of epos control.

It seems that the USB communication is not suitable or robust enough so I would recommend to change to RS232 (google the differences on how the communication is done in each case). At least now it looks that I am not experiencing the USB connection fall but I will update with my results in future.

Hope I can save you some time that this problem has taken from me.