<title>Friction model of the 2.5-mts SDSS telescope</title>

Abstract

The 2.5mts telescope designed for the Sloan Digital Sky Survey is a mechancial structure that presents five degree- of-freedom. Azimuth, altitude and the instrument rotator axis are fitted with servo controls. The low frequency dynamic are dominated by the bearing friction. Several mathematical models have been presented in the literature to include its effect into the dynamic model of mechanical structures. The model employed in this paper includes consideration of the Striebeck effect, dynamic behavior at very low velocities and the pre-sliding at near zero- velocity. Result of the parameter estimation of the friction model of the three principle axes are presented as well as the behavior of the structure when different torque stimuli are applied. The mathematical model used to include the friction phenomena into the telescope dynamic model is simple. It does a good job of describing the friction over a wide range of velocities but particularly at or below sidereal rate. It is a straight forward process to determine the parameters and, in simulations, does not require large amounts of computer time.