Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
The invention relates to a control system for a gas turbine
engine and more particularly to a method and apparatus for failure
correction in such control systerns.
Gas turbine engine control systems particularly for use
in aircraft generally include electrical sensOrs which measure
various engine parameters for control oE the engine. Such sensors
have typically measured gas generator and fan rotational velocities
and engine operalting pressures and temperatures. These, in turn,
are used to control the position of the actuators which vary engine
controlled parameters, typically fuel flow and geometry. In
conventional engine control systems ~he control pararneter sensors
and sensors indicating the value of controlled parameters are
connected by electrical cables to the electronic cornputation sections
of the engine control system. The electronic computation section
may be the analog in which dedicated circuits perEorm continuous
computations or it may he digital in which computations are performed
sequentially on a time-sharing basis, In either case, the computational
unit of the control using the sensed values for control and controlled
parameters and known engine behavior characteristics computes values
of output signals to modify the controlled parameters in order to
maintain a desired level of engine performance. The electrical outputs
from the computation section are amplified, conditioned and then
input to the various electrohydraulic and electromechanical actuators
which position the electric~lly controlled fuel and hydraulic valves
which alter the controlled parameters. These in turn modify engine
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operation which change the measured control parameters, i. e.
engine rotational velocity, pressures, and temperatur2s, thereby
completing closed loop control of the engine.
A major problem associated with such prior art control
systerns is that loss of tne proper input from one or more sensors
may cause loss of control of the corresponding engine pararneter
resulting in major degradation of engine performance. Such
signal loss may be dub to any nurnber of factors including failure
of thLe sensor, failure of the interconnection between the sensor
and control or failure of any of the processing circuitry for such
s ignals .
Object of the Invention
.~
Accordingly, it is the primary object of this invention to provide
a control system for gas turbine engines which prevents major
degradat~on of engine performance in the event of loss of the
proper signal from the engine control parameter sensors.
It is a further object of this invention to provide a control
system which transmits an indication to the engine operator when
the failure of an engine sensor has occurred.
SummLary of the Invention
-
These and other objects of the invention have been achieved in
the preferred embodiment of the invention wherein a system for
controlling the gas turbine engine is irnproved by providing a small
digital computer between the input sensors which measure engine
control parameters and the computational unit which cornputes values
of output signals to the actuators which control engine controlled
parameters such as fuel flow, and geometry. The digital computer
includes a stored program which models the engine being controlled
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and which is continually updated by the signals received from
the control parameter sensors and signals received from sensors
for the controlled pararneters. The computer in turn uses these
inputs, the model and stored data items representing est ima te s
of the internal status o~ the engine to generate output signals for
transmittal to the control computational unit. These signals represent
the models estimate of the actual value oE the control parameter sensor
signals input to the computer and are continuously updated by received
control and controlled parameter sensor signals. The computer
continuously compares each control parameter sensor input with
the corresponding output to the control computationaL uni-t and in the
event a difference exceeding s-tored or calculated tolerances is detected
that con~rol parameter sensor is inhibited from updating the engine n-lodel.
However, the loss OI a control parameter sensor input does not
significantly degrade engine performance~ since the computer continues
to provide output signals corresponding to the inhibited input sensor
signal. This outpu1; represents the computers best estimate of the value
of the inhibited input signal which is calculated using the stored engine
model updated by the remaining uninhibited control parameter input
signals and by the controlled parameter input signals. An indication
that a particular control parameter sensor input signal has been
interrupted may be provided to the engine operator through an engine
operator interface circuit, The control system of this invention thus
permits continued operation OI the engine control without major
performance degradation, even though one or more control parameter
sensor signal.s is not properly received.
131~V-659
Further, because the model uses the interaction ~etween the
various control parameters to compute the value oî a s gnal
parameterJ spurious error or small inaccuracies in measured
parameters is nninimi~ed such that when no sensors are inhibited
the value of th~ control parameters input from the computer are
more accurate than those input directly from the individual control
parameter sensors.
When used with a control system incorporating a digital
computer, the system may be integrated into the control withoul; the
need of separate computer components. The system may also be
added to an existing analog or other electrical control dS an
interconnection between control parameter engine sensors and
the control computational unit.
Brief Description OI the Drawings
_ . . _ . . . .
The invention may be better understood by reading the following
description of the preferred embodiment in conjunction with the
accompanying drawings wherein:
Figure 1 is a schematic diagram in block format of the control
system of this invention,
Figure 2 is a computer ~low diagram of a control program for the
control system of this invention.
Description of the Preferred Embodiment
:E~eferring to Figure 1 there is shown a control system for a gas
turbine engine 10.
Gas turbine engine 10 may be of any type known in the art
including a turbojet, turboprop, turboshaft) or high or low bypass
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turbofan. All such engines generally include a plurality of
sensors 12 used to measure the values of the parameters used to
control the various engine controlled parameters such as engine
fuel flow and geometry in order to malntain a desired level of
engine performance. A plurality of sensors 13 is also provided
to measure the value of the controlled parameters.
The analog signals from the sensors 12 and 13 are
input to signal processing amplifiers and analog to digital
converters 16 which amplify the signals to a standard DC analog
voltage range and thereafter convert these analog signals to a
digital format. The digital control signals are then input to
a digital computer 18 through the input/output unit 20 thereof.
Digital computer 20 is of the type well-known in the art and
comprises a machine having a cycle time fast enough to provide
real time operation. It further includes an input/output unit 20
for external interface, an arithmetic unit 22 for performing
arithmetic calculations and a program memory 24 for storing
programs, information and data. In accordance with this inven-
tion, the program memory 24 includes an engine model which
simulates the performance of the gas turbine engine 10. Such
models are well known in the gas tur~ine engine art and generally
comprise a set of mathematical equations defining the inter-
relationship between the engine control parameters, typically
the pressures and temperatures of the various engine components,
as a function of the value of controlled parameters, typically
engine fuel flow and geometry (e~g. inlet and e~haust area)
and the internal status of the engine. Thus, ~or any given set
of controlled parameter conditions, the model will provide
calculated values for engine control parameters and vice
versa.
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The program memory 24 also includes an operational
program to control data processiny. Referring -to Figure 2,
therein is shown a flow chart for the operational program.
After an initialization in which initial values for both control
and controlled parameters are preset into the computer, the
operational program continually receives signals from the
controlled parameter sensors 13 which are input to the engine
model. The engine model uses the sensed controlled parameter
values plus stored data representing estimates of the internal
status of the engine to compute corresponding values for the
control parameters. These calculated control parameter values
are then inpu-t to the control computational unit which uses
them and the controlled parameter values from the sensors 13
to calculate signals for positioning the actuators 14. The
calculated control parameter values are also compared with the
control parameter values received from the engine sensors 12
and their respective difference calculated. If these differences
exceed a tolerance, which may either be stored or calculated
as a function of other sensed engine control or operational
parametexs, the control program takes action (as will he described
below~ to prevent the out of tolerance signal inputs from
further updating the model. The operational program may also
output a signal to the engine operator indicating that an out
of tolerance sensor signal has been received and that the sensor
has been inhibited from updating the engine model.
The engine model is updated by changing the stored
data representing estimates of the internal status of the
engine as a function of the difference between the calculated
control parameter values and the corresponding control
parameter values received from the engine sensors 12
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Thus, for each control parameter difference computed,
a corresponding change is made to each stored data item re-
presenting an estimate of the internal status of the engine
which is affected by that control parameter. If the control
parameter has been inhibited then no corresponding changes
are made to the data items affected by that control parameter.
Further, for greater accuracy of response, the magnitude of
the change to each stored data item representing an
estimate of the internal status of the englne may be
weighted so as to proportional to the degree of control
the responsible sensed control parameter has over that data
item as compared with the degree of control the remaining
control parameters have over that data item.
The calculated values for the control parameters
are transmitted by the computer to the control computational
unit 26. The control computational unit 26 is of the type
well-known in the art ~hich may be digital or analog. If
analog, a digital to analog converter (not shown) must be
provided between the computer 18 and computational unit 26.
The computational unit in the manner well-known in the gas
turbine control art uses signals received from the engine
control parameter sensors r controlled parameter sensors
and known engine behavior characteris-tics, which are generally
stored as analog algorithms or digital data r to compute values
of output signals to position the engine electromechanical
and electrohyclraulic actuators 14 in order to maintain a desired
engine performance level. Suitable amplifiers and digital to
analoy converters 28 are provided intermediate the control compu-
tational unit and actuators 14, in order to condition and/or con-
vert the output signals from the control computational unit to
the standard voltage range required to operate actuators 14.
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The signal processing amplifiers and analog to digital converters
~, 16, -~t~ , control computational uni-t 26, signal processing
amplifiers and digital to analog converters 28 are a11 commercially
available devices well-known in the art, the details of which do not
form a part of this invention.
In this manner, the controi system of thts invention permits
continued operation of the engine controi without major performance
degradation even though one or more control parameters sensors
inputs are lost.
Various changes could be made to the digital control system as
shown in :Figures 1 and 2 without departing from the sc.~pe of this
invention. Thus, if this system i9 used with a digital control the
logic described herein may be programmed and integrated therein
without the need of separate computer components. Alternatively,
15 this system may be applied following a sensor failure in a control system
in which direct sensor input to the control computational unit is used.
In this case, the failed sensor signal will be supplied by the engine
model,
