A major goal of the Space Station era is to reduce reliance on support from ground based experts. The development of software programs using expert systems technology is one means of reaching this goal without requiring crew members to become intimately familiar with the many complex spacecraft subsystems. Development of an expert systems program requires a validation of the software with actual flight hardware. By combining accurate hardware and software modelling techniques with a modular systems approach to expert systems development, the validation of these software programs can be successfully completed with minimum risk and effort. The TIMES Expert System (TES) is an application that monitors and evaluates real time data to perform fault detection and fault isolation tasks as they would otherwise be carried out by a knowledgeable designer. The development process and primary features of TES, a modular systems approach, and the lessons learned are discussed

Cacace, Ralph

England, Brenda

English

NASA Technical Reports Server

TES - A Modular Systems Approach to Expert _ystem
Development for Real-Time Space Applications
Ralph Cacace
Brenda England
UNITEDTECHNOLOGIESCORPORATION
Hamilton Standard Division
One Hamilton Road
Windsor Locks, CT 06096
Abstract
A major goal of the Space Station Era is to reduce reliance on support
from ground based experts. The development of software programs using
expert systems technology is one means of reaching this goal without
requiring crew members to become intimately familiar with the many
complex spacecraft subsystems. Development of an expert systems program
requires a validation of the software with actual flight hardware. By
combining accurate hardware and software modelling techniques with a
modular systems approach to expert systems development, the vali_lation of
these software programs can be successfully completed with minimum risk
and effort. The TIMES Expert System (TES) is an application that mon tors
and evaluates real-time data to perform fault detection and fault isolation
tasks as they would otherwise be carried out by a knowledgeable designer.
This paper discusses (1) the development process and primary features of
TES, (2) a modular systems approach and (3) lessons learned.
Introduction
The Thermoelectric Integrated Membrane Evaporation Subsystem
(TIMES) is a spacecraft life support system designed and produced by
Hamilton Standard. The TIMES Expert System (TES) is a prototype developed
as a first step toward capturing in-house diagnostic expertise on this
subsystem. The primary goals of the prototype development effort were to
(1) investigate expert systems as space-based tools for reducing reliance on
ground based support and (2) explore a modular systems approach to
developing real-time expert systems. To address these goas, TES was
developed as a fault detection and fault isolation expert system to monitor
real-time TIMES data. The TIMES Expert System was created under
NASA/Johnson Space Flight Center contract number NAS 9-17209.
What is TIMES? _ ]_TIMES is a spacecraft °°
waste water processor that IE_ E_11¢
employs vacuum distillation,
thermoelectric heat pumping, / / "
and membrane separation to ° ]' __ I' o,,o,,'----7,
reclaim high quahty product J _ I =. I _ IRS23'_ ._o°,.o,,......
water [2]. A system TIMES _ / _ i .; IMS.,SS3B,[ _ .....,. ______,
controller contains the [_
electrical and software logic
necessary for process F,,u,e|: Present TIMES Deliver, £onfiguralion _ r] _ _] /
5£
https://ntrs.nasa.gov/search.jsp?R=19880014813 2020-03-20T06:54:21+00:00Z
control, instrumentation and critical fault detection. In addition to data
provided to a MS-1553B bus, sensor readings and performance calculations
are made available by the controller via RS-232C for use by data checkout
and recording systems. An electrical driver box standardizes all signals
between the hardware and the controller. The TIMES, along with numerous
other life support subsystems, is operated and visually monitored from )acent al Display and Control Console (DCC) via MS-1553B bus (see Figure 1 .
Primary TES Features
The TIMES Expert System was developed using Knowledge Engineering
Environment (KEE) and Common Lisp on a Symbolics computer.
Approximately 45 potential subsystem and sensor problems have been
addressed as both independent and concurrent failures. The TIMES controller
relies on threshold violations to provide limited error detection and will
shut the subsystem down if a critical problem is suspected. TES enhances
controller capabilities by operating as an early warning system, providing
additional fault tolerance and acting as an extended problem explanation tool
for the astronaut.
Combining the graphics features of KEE and Common Lisp has given TES
versatile display capabilities. Some examples are dynamic flows, valve
positions, and sensor readings presented on an operational schematic of the
TIMES. In addition to these features, the operator also has the ability to
customize the monitoring environment by replacing some or all numerical
displays with dials and to view real-time historical trend plots of expected
and actual values for all sensor and performance data.
A Modular System Design
When real-time prototype hardware output results in inputs to an expert
system, the following key issues must be addressed:
1. Limited availability of system hardware for expert system
testing and verification due to:
* A limited number of each prototype system is
usually developed and available.
* Prototype systems are often dedicated to
rigorous test schedules and numerous design changes.
* System testing often has priority over expert
system testing.
* It is costly and difficult to move hardware from one
site to another for demonstration or testing
purposes.
2. Actual insertion of faults into prototype hardware is
unrealistic in many cases because:
* Altering a system component to introduce a fault
can be costly and sometimes hazardous.
* Many system sensors and components are not
physically accessible for alteration.
These issues may be addressed by taking a modular approach to system
design. The modular approach involves the development of an accurate
software simulation of the hardware (See Figure 2). The closer the model is
to duplicating the hardware's data output in terms of content, format and
protocol, the more valuable it will be whe n addressing the above issues.
I concept Modularity has been appliedto the TES progr m by using a
thermodynamic model of TIMES as
I the expert system test dataDesign source. The TIMES model was
developed using Quick BASIC on anIBM PC. The RS-232C output from
[ Simulation I Hardware II this model realistically duplicates
IDevelopmentl IDevel°pmentl the R9-232C test data output from
j1 the TIMES controller in content,
i Expert I / format and protocol. This data
system I / supplies TES with the sensor,
Testing IS control and performance
and ! _ information it needs to monitor
Verification the TIMES (model).
Figure 2: Modular Real Time Testing
The TIMES model can be operated as a stand alone system or in
conjunction with the TIMES Expert System. The model's graphics interface
allow the user to visually monitor simulated normal TIMES operation, or to
insert faults and observe the effects on subsystem operation independent of
the TIMES Expert System. This
allows for independent model
verification using TIMES data and _BMr'C Symbo_ics
makes the model itself a valuable f
tool in understanding detailed [f-__-)o[ I
TIMES performance and fault II  odo,Iol j TES
characteristics. The model, with _,_----, j_,-tos-23_
its interface (Figure 3,_ [_-_--_ l,,'_) /
effectwely replaces the TIME
hardware and the DCC (Figure 1)('_°_*_°*_
• for testing and demonstration of F_ure3:T,eTIMESE Sys,¢m
TES.
The modular approach taken in the TES design has helped to reduce
development costs byproviding a more flexible and accessible testing and
verfication platform. For example, TIMES operates on an approximate 24
hour cycle and fault symptoms often vary. as a function of the cycle duration.
To test the TES fault detection capabdities and insure that cyclic data is
being handled successfully, testing must be conducted at numerous cycle
points. Tests must be repeated a number of times because expert system
evaluation is an iterative process [4] . These test constraints would demand
valuable subsystem time and may require costly repair or replacement of
components damaged during testing or additional hardware to simulate
failures. By using a software model as the test bed, an open circuit in a
thermoelectric module or a separator motor magnetic drive decoupling can be
generated repeatedly. Simulation rates can be adjusted to decrease test
time. In these ways, the TIMES model facilitates testing without damaging
subsystem hardware or interrupting hardware testing.
61
A Modular Diagnostic Approach
Remaining consistent with a modular svstems annroach TES's
diagnostic content is also structured modularly: Applica._ion oi verbal
reporting techniques [3] during the formalization phase [4] of TESs
development revealed a distinctly two level diagnostic approach used by the
experts. High level performance parameters provide the first clues to a
subsystem performance problem. Detailed diagnostics are only employed
when such a problem is detected. Because it is the function of the TIMES to
efficiently reclaim high quality product water, there are three primary
indicators of degraded performance: (1) low water production rate, (2) poor
roduct water quality, and (3)high power consumption. The first two areas
ave been addressed by the TES prototype. This approach has been captured
in a modular fashion modelled after that of the experts allowing for rapid
expert system performance, and facilitating future expansion to include
other systems [1], [5].
A front end processor in TES
monitors TIMES performance H_g,LevelHealthParameterMonitor
parameters using trend analysis to RS-232 Front End[..._ Health _-_1
operation, the only other
functions performed by TES are the Control--'-*l [Kn°wle_ge]--_ Rule I
storing of trend data, real-time Data .--_[ I Ba,e I-I Ba,e I
display update and periodic sensor Figure 4, TES Functional Modularity
checkout to flag drifts or
inconsistent readings. Only when subsystem performance appears to have
deteriorated does TES perform detailed diagnosis to isolate the problem and
warn the astronaut before shutdown thresholds are exceeded. (Figure 4)
The rules in the TIMES expert system have been divided into four major
categories: (1) General Health rules, (2) Water Production Rate rues, (3)
Water Quality rules and (4) Sensor Health rules. Division of the rule base
into problem areas has further enabled rapid fault diagnosis. For instance,
if a low water production rate were detected, TES would reason over rule
types (1) and (2) drawing on the current knowledge base and trend
information to diagnose the problem. A separate Sensor Health rule base has
increased TES credibility by flagging unreliable sensors so that they are not
relied on in future reasoning, and by differentiating between sensor drifts or
failures and actual subsystem health problems (i.e. high temperature).
Conclosions
Using the TIMES model as an input to the TIMES Expert System provides
a flexible and portable means of addressing the availability, cost, and time
issues associated with developing an expert system to monitor real-time
hardware data. It also increases TES's d_agnostic credibility by separating
fault insertion and fault detection. The operator can observe TIMES sensor
readings and performance indicators on the model to reach his own
conclusions, while the expert system provides the expert diagnosis.
TES's, functional modularity, patterned after the expert's own
diagnostic approach, increases its monitoring efficiency without limiting
62
future growth. The front end processor makes TES an efficient, high level
health monitor until detailed anomaly analysis is needed. The front end
processor also supplies TES (and hence the operator) with a trend history of
all sensors and performance parameters. Detailed trend analysis on this data
supplies the TES reasoning system with the information that the expert uses
by either visually examining trend plots or performing detailed analysis ofh_storical data.
Independent sensor monitoring increases TES's diagnostic capability .by
flagging unreliable sensors thus excluding them from use _n future reasoning
and by distinguishing between sensor drift or failure and actual performance
problems.
Future TES expansion areas could include: (1) further development of
the front end processor to address more complex data characteristics, (2) a
detailed feasibility study that would address the modular system design to
multiple systems, (3) connection to TIMES hardware, and (4) further
investigation of multiple faults.
References
[1] A. K. Coiling Jr., T. A. Nalette, et.al., "Development Status of Regenerable
Solid Amine CO2 Control Systems," SAE Technical Paper Series,
Proceedings of the 15 Intersoc_ety Conf on Environmental Systems, San
Francisco, CA, July 15-17, 1985.
[2] G. F. Dehner and D. F. Price, "Thermoelectric Integrated Membrane
Evaporation subsystem Testing," SAE Technical Paper Series, Proceedings
of the 17th Intl. Conf. on Environmental Systems, Seattle, WA, July
13-15, 1987.
[3] A. H. Silva and D. C. Regan, "Using Cognitive Psychology Techniques for
Knowledge Acquisition," IEEE Trans. on Systems, Man and Cybernetics,
Atlanta, GA, October 1986.
[4] M. Stefik, J. Aikins et. al., "Basic Concepts for Building Expert Systems,"
in F. Hayes-roth, D. Waterman and D. Lenat (eds), Building Expert Systems,
Addison-Wesley Publishing, Reeding, MA, 1983.
[5] C. E. Verostko and R. K. Forsythe, "A Study of Sabatier Reactor Operation
in Zero 'G' ," SAE Technical Paper Series, Proceedings of the 14th
Intersociety Conference on Environmental Systems, San Diego, CA, July
16-19, 1984.
KEE is a registered trademark of Intellicorp Inc.
Quick Basic is a registered trademark of Microsoft Inc.
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TES: A modular systems approach to expert system development for real-time space applications

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