For large telescopes, management of axis friction presents a significant challenge. In some cases, this is
avoided or minimized in the design stage by employing hydrostatic bearings. However, the main axis servo
systems of many large telescopes must cope with bearing or wheel friction. This friction affects or limits
servo control performance in several ways. The most obvious is the stick-slip limit cycle that is characteristic
of trying to hold position with an integrating control system in the presence of friction. If it is not taken
into account, friction also introduces effects into the state estimation in model-based controllers.
There are several standard approaches to friction compensation. These include dithering (introducing a
noise signal to the drive motors), direct Coulomb friction compensation (sending an additional torque based
on the rate command), and adaptive techniques based on monitoring of the final drive velocities.
In this work, we experimentally compare different friction compensation approaches on the static positioning
performance of the Large Millimeter Telescope/Gran Telescopio Milimetrico (LMT). Single and
double integrator systems are investigated, as well as direct Coulomb friction compensation.