The Thermal Expert System (TEXSYS) computer program exerts real-time control over a complicated thermal-regulatory system that includes evaporators, condensers, a pump, valves, and sensors. TEXSYS observes differences between actual and expected conditions and analyzes differences to determine whether a given condition signifies a malfunction in a component or at the system level. It then takes corrective action (e.g., it commands the opening or closing of a valve).

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The Symbolic Control Hierarchy Extends from the hardware layer to the expert system at the top. The hardware is a thermal regulatory system (Carnot refrigerator)
A knowledge base of engineering expertise on the particular thermal-regulatory system is contained in an expert-system computer program called "core TEXSYS." TEXSYS was developed by adding core TEXSYS to conventional software for acquisition of data and for control, forming a hierarchical symbolic controller. The architecture of TEXSYS is layered, with the expert system at the top, the controlled hardware and conventional controlling hardware and software at the lowest two levels, and an intermediate layer that integrates the expert system with the lower levels (see figure).

The thermal regulatory system to be controlled includes a thermal bus that functions as a Carnot refrigerator, using anhydrous ammonia as the working fluid. A heat-acquisition branch absorbs heat from an external source, changing the working fluid from liquid to mixed liquid and vapor. The mixture is separated in a heat-transport branch by a centrifugal pitot pump, which sends vapor to condensers in a heat-rejection branch and liquid back to the heat-acquisition branch. A regulating valve on the downstream vapor line maintains a constant set-point pressure (and constant temperature, if the vapor is saturated), much like a relief valve. Once in normal operation, the thermal bus tends to balance itself, requiring control of the valve setting, pump power, and occasionally, the set point.

Real-time control requires response times of tens of seconds — 15 seconds during startup. To ensure fast response, data to be processed by TEXSYS are filtered so that only data on significant changes are entered; steady or slowly changing data, which would take an inordinate amount of time to consider, are eliminated. TEXSYS can identify all of the 7 known system-level faults and the 10 of 34 component-level faults that were chosen by thermal engineers as most interesting or representative.

So that TEXSYS can accept changes in hardware, a library of behaviors of generic components (how valves, pipes, and pumps function) was incorporated and separated from information on the behavior of the specific thermal bus. When thermal bus hardware changes, a new mathematical model is created in TEXSYS by choosing components from this library and connecting them as in the schematic diagram of the hardware. New data from the intermediate or integration layer of TEXSYS are placed into the model at sensor locations, then processed both by active values (across connections) and by rules (across components). The insertion of a datum at a given location in the model may then result in a chain of inferences about the behavior of the system.

This work was done by W. Erickson, B. J. Glass, R. Owens, and M. S. Rudokas of Ames Research Center, R. Levinson of Recom Software, Inc., and J. Nienart of Sterling Software. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Information Sciences category.

Inquiries concerning rights for the commercial use of this invention should be addressed to

the Patent Counsel
Ames Research Center; (650) 604-5104.

Refer to ARC-13166.