A Fault Detection, Isolation and Recovery (FDIR) system needs some internal concept of how the monitored system operates. This “operating concept” needs mathematical logic to represent the cause and effect relationship between constituent components of the system, health and status of those components, and the resulting health and status of the overall system. Models constructed with commercial simulation systems such as MATLAB and Simulink may ignore individual component contribution to system operation. Instead, the modeler may naturally focus on overall system parameters with lumped values representing the individual component characteristics. The resulting simulations faithfully calculate overall system operation, but may miss anomalies that occur because of plugged, stuck, or otherwise inoperable individual components.

This innovation is a spreadsheet written for Microsoft Excel with equations that simulate cryogenic propellant loading system operating characteristics. The spreadsheet takes commands from an operator to open and close valves, change pump speeds, and change pressure set-points. The spreadsheet continuously calculates the operating state of the propellant loading system including measurements such as pressures, flow rates, pump speeds, and tank levels. The simulation is component- oriented, containing comprehensive mathematical descriptions of valves of different types and sizes, pneumatic supply system pumps, filters, and tanks. An “antagonist” operator may introduce faults in certain of the components. The spreadsheet simulates the effects of the introduced faults.

The software simulates operation of the Simulated Rapid Propellant Loading (SRPL) System hardware at the KSC Cryogenics Laboratory. The Excel spreadsheet uses Active Control buttons and sliders drawn in Excel that allow valves to be opened and closed and pumps to be controlled at various speeds. Other Active Control sliders allow the antagonist operator to introduce faults in all the components that make up the physics model of the SRPL.

The Excel spreadsheet has a unique update feature that calculates the pressures, temperatures, and flows in the physics model of the SRPL. The update feature calculates all these values, then waits one second and recalculates based on updated tank levels, ullage quantities, and bulk liquid temperatures. Thus, the spreadsheet is capable of continuously updating the liquid levels integrating flow rates and heat flux in the system in real time.

Equations can be translated into C code and used in the embedded processor. The results calculated by the C code can be validated easily by comparing calculated values to the output of the spreadsheet.

This work was done by Charles Goodrich, Rebecca Oostdyk, Mark Lewis, and Christopher Forney of Kennedy Space Center. KSC-13631

NASA Tech Briefs Magazine

This article first appeared in the March, 2014 issue of NASA Tech Briefs Magazine.

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