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Meeting	ACGS Committee Meeting 100 - Cocoa Beach - October 2007
Agenda Location	5 SUBCOMMITTEE D--DYNAMICS, COMPUTATIONS AND ANALYSIS 5.2 Real-Time Stability Margins in Flight Test
Title	Real-Time Stability Margins in Flight Test
Presenter	Dave Ward
Affiliation	Barron Associates
Available Downloads*	presentation
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Abstract	As the complexity of flight controllers grows so does the cost associated with verification and validation (V&V). Current-generation controllers are reaching levels of complexity that push the envelopes of existing V&V approaches, and further increases in controller complexity are required to provide the operational capabilities desired for next generation systems. Without improved approaches, there is little hope for affordable V&V of next-generation intelligent systems, and thus little hope that they can be safely fielded. Barron Associates believes that successful V&V of advanced control approaches will require a combination of a variety of tools to address all aspects of the control-law design and testing process, as well as methods for monitoring the behavior of high-risk components in operational settings. The current work addresses the safety of the flight test program for advanced control laws. Barron Associates, under a NASA Langley SBIR contract, is developing a software tool that estimates stability margins in real-time during a flight test, and compares these to a priori predictions to validate the design model. The tool uses worst-case analysis techniques to develop off-line estimates of worst-case stability margins at various flight conditions based on a prior uncertain linear models corresponding to those flight conditions. On-line, “broken-loop” transfer functions are estimated using recursive, non-parametric system identification techniques, and margin estimates are derived from these transfer function estimates. In addition to providing efficient on-line computations, the selected estimation approach provides a quantification of the uncertainty in the estimate. This uncertainty in the transfer function estimate provides an indication of when sufficient data has been collected to draw useful conclusions. The team is also developing approaches for online fitting of simplified uncertainty models to observed data, and on-line worst-case margins can be computed from these models. The user is alerted to potential safety problems when on-line margins degrade below a minimum acceptable bound, and when margins degrade beyond off-line worst-case estimates, which indicates an invalid design model. Ultimately, the tool will provide safety estimates for the current test condition as well as planned future test points. The software tool is implemented using Matlab and Java, with Matlab used for backend computations and Java used to interface to a real-time data stream and to provide a graphical user interface.