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In order to achieve high performance control for servo mechanisms with electrical dynamics and unmeasurable states, an observer-based adaptive robust controller (ARC) is developed via dynamic surface control (DSC) technique. To represent electrical dynamics, a third-order model is used to describe the servo mechanism. However, the third-order model brings some difficulties to observer construction and recursive controller design. To solve this problem, we first transform the model into a particular form suitable for observer design, and then construct a parameterized observer to estimate the unmeasurable states. The state estimation is based on the output and its derivatives, which can be acquired by an output differential observer. Subsequently, an observer-based ARC can be developed through DSC technique, with which the problem of "explosion of complexity" caused by backstepping method in the traditional ARC design can be overcome. A stability analysis is given, showing that our control law can guarantee uniformly ultimate boundedness of the solution of the closed-loop system, and make the tracking error arbitrarily small. This scheme is implemented on a precision two-axis turntable. Experimental results are presented to illustrate the effectiveness and the achievable control performance of the proposed scheme.