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                          robust control. It can be shown that if the additive uncertainty is
                          smaller than a certain calculable quantity, then a conceptual control
                          system could stabilize the model structure and could also stabilize
                          the real structure. This criterion can be incorporated into an iterative
                          design procedure. In this procedure, each controller in a sequence of
                          controllers for the model structure would be designed to perform bet-
                          ter than the previous one did, until the condition for robust capability
                          was violated. Once the violation occurred, one could accept the pen-
                          ultimate design (if its performances were satisfactory) or continue the
                          design process by increasing a robustness weighting (if available). In
                          principle, convergence of this iterative process guarantees a control
                          design that provides high performance for the model structure while
                          guaranteeing robustness of stability to all perturbations of the struc-
                          ture within the additive uncertainty.



                     6.19  Optoelectronic Sensor Tracking Targets
                             on a Structure
                          The location and exact position of a target can be accurately sensed
                          through optoelectronic sensors for tracking a retroreflective target on
                          a structure. An optoelectronic system simultaneously measures the
                          positions of as many as 50 retroreflective targets within 35° of view
                          with an accuracy of 0.1 mm. The system repeats the measurement 10
                          times per second. The system provides an unambiguous indication of
                          the distance to each target that is not more than 75 m away from its
                          sensor module. The system is called  spatial high-accuracy position-
                          encoding sensor (SHAPES).
                             SHAPES fills current needs in the areas of system identification
                          and control of large flexible structures, such as large space- and
                          ground-based antennas and elements of earth-orbiting observational
                          platforms. It is also well-suited to applications in rendezvous and
                          docking systems. Ground-based applications include boresight deter-
                          mination and precise pointing of 70-m deep-space-network antennas.
                             SHAPES illuminates the retroreflective targets by means of a set
                          of lasers in its sensor module. In a typical application (Fig. 6.21) a
                          laser diode illuminates each target with 30-ps pulses at a repetition
                          rate of 100 MHz. Light reflected from the target is focused by a lens
                          and passed through a beam splitter to form images on a charge-
                          coupled device (CCD) and on the photocathode of a streak tube. The
                          angular position of the target is determined simply from the position
                          of its reflection on the charge-coupled device.
                             The measurement of the distance to the target is based on the
                          round-trip time of the optical pulses. The round-trip distance can be
                          measured in terms of the difference between the phase of the train of
                          return pulses incident on the photocathode and the phase of a ref-
                          erence sine wave that drives the deflection plate of the streak tube.
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