EFFICIENT SAMPLING RATES FOR DIGITAL CONTROLLERS AND POINTING SYSTEMS WITH DIGITAL SOLID-STATE DETECTORS

Applications of digital control have grown enormously with the development of powerful, inexpensive microprocessors. The lower cost of the limited speed and precision models of microprocessors encourage low sampling rates for digital controls. This research examines the design methods available for low sampling rates and applies the various concepts of an aircraft autopilot and a telescope pointing system.;We compare the fraction of available computer time a digital autopilot requires using the preceding optimal and classical designs. This fraction is the product of controller sampling rate and control algorithm execution time. The lower sampling rate of the optimal design reduces demand on available computer time 38 percent even though estimating the system states increases the algorithm execution time 22 percent. Expressing the optimal compensation in a computationally efficient canonical form helps to minimize the alogrithm execution time and enhance the computational benefit of the low sampling rate.;In another application of slow sampling design, rate selection for pointing systems using digital solid-state detectors was examined. Our example is the Shuttle Infrared Telescope Facility (SIRTF), which uses a digital charge-coupled-device star tracker to update its gyro inertial reference. An error model shows that low sampling rates decrease the tracker's accuracy by allowing it to integrate light from the target longer between measurements. At the same time, more frequent updates improve the pointing control's correction of gyro drift errors. This trade-off between low measurement noise and fast dynamic response causes one sampling rate to minimize the pointing error.;A unique method for digital-control design at low sampling rates was conceived that basically relies on optimal control procedures. It has the very desirable properties that the design can be carried out on the continuous system and can be easily transformed to a digital controller with excellent fidelity. A stability-augmentation system for a 737 aircraft designed using this method has excellent dynamic response at sampling rates only two or three times the system bandwidth. In contrast, simpler classical designs with lead compensation and no compensation of sampling effects require sampling rates two to seventeen(17) times higher for similar response.