Abstract: The performance of on Alt-Az telescope depend strongly on its operating conditions. During pointing the telescope can move at a relatively high velocity, and the system can tolerate trajectory position errors higher than during tracking. On the contrary, during tracking Alt-Az telescopes generally move slower but still in a large dynamic range. In this case the position errors must be as close to zero as possible. Furthermore the two pointing and tracking phases are executed in sequence without a well defined switch phase between them. A fixed structure controller cannot optimize the telescope performance in terms of error amount in the whole requested dynamic range. On the contrary a digital controller has the ability to modify its structure and its parameter values during operations, according to the instantaneous error values, system status, system actual speed and system characterization. This paper analyzes the problem of tracking errors and the solutions adopted in a case study. !4
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