A real-time system for validating sensor health has been developed
for the reusable launch vehicle (RLV) program. This system, which
is part of the propulsion checkout and control system (PCCS),
was designed for use in an integrated propulsion technology demonstrator
testbed built by Rockwell International and located at the NASA
Marshall Space Flight Center. Work on the sensor health validation
system, a result of an industry-NASA partnership, was completed
at the NASA Lewis Research Center, then delivered to Marshall
for integration and testing.
The sensor validation software performs three basic functions:
it identifies failed sensors, it provides reconstructed signals
for failed sensors, and it identifies off-nominal system transient
behavior that cannot be attributed to a failed sensor. The code
is initiated by host software before the start of a propulsion
system test, and it is called by the host program every control
cycle. The output is posted to global memory for use by other
PCCS modules. Output includes a list indicating the status of
each sensor (i.e., failed, healthy, or reconstructed) and a list
of features that are not due to a sensor failure. If a sensor
failure is found, the system modifies that sensor's data array
by substituting a reconstructed signal, when possible, for use
by other PCCS modules.
To determine whether a sensor has failed, the software first scans
for hard failures by checking its range and rate of change against
predetermined limits. The data are then processed to find any
features such as drifts, shifts in level, peaks and spikes, and
disagreement between redundant channels. Any discrepancies found
among redundant sensors during steady-state system operation are
compared with features found in the data. This arbitration is
performed by an expert system encoded by using an expert system
shell. Any detected features that are not associated with a failed
sensor are checked against planned system events, such as valve
changes. Features that cannot be explained by a malfunctioning
sensor or known system event are reported as transient behavior.
This information is then used by other PCCS modules to determine
the health of the remaining system components.
Review of test data from initial integration testing verified operation in real time. Tests were performed for both hard and soft sensor failures, and the sensor validation system was shown to work properly.
Previous articleLast updated April 30, 1997
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