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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.