This paper presents a new observer-based sub-optimal digital tracker for the continuous-time system with input and state time delays. We directly convert the continuous-time input-state delayed system into an equivalent discrete-time input-state delayed model and its extended discrete-time delay-free model. In addition, we directly discretize the linear quadratic performance index specified in the continuous-time domain into an equivalent decoupled discrete-time performance index using the newly developed extended delay-free model. As a consequence, the well-developed discrete-time optimal control theory for discrete-time delay-free system can be applied to determine the optimal digital tracker for the continuous-time input-state delayed system. When the states of the continuous-time input-state delayed system are not available for measurements, we develope a sub-optimal digital observer for the original continuous-time input-state delayed system using the state-matching digital redesign technique and the digital-to-analog model conversion technique. As a result, the proposed observer-based linear quadratic digital tracker is able to make the output of the digitally controlled continuous-time input-state delayed system sub-optimally track the desired reference signals. An illustrative example is presented to demonstrate the effectiveness of the proposed design methodology.
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