  The servo control algorithms of the TNG, developed in the nineties, have been
working for more than 20 years with no major updates. The original hardware was
based on a VME-bus based platform running a real time operating system, a
rather popular choice for similar applications at the time. Recently, the
obsolescence of the hardware and the lack of spares pushed the observatory
towards a complete replacement of the electronics that is now being implemented
in steps, respecting the basic requirement of never stopping the observatory
night operations. Within the framework of this major hardware work, we are
taking the opportunity to review and update the existing control schemes. This
servo control update, crucial for the telescope performance, envisages a new
study from scratch of the controlled plant, including a re-identification of
the main axes transfer functions and a re-design of the control filters in the
two nested position and speed loops. The ongoing work is described, including
preliminary results in the case study of the azimuth axis and our plans for
possible further improvements.
