Apparatus for Sensor Failure Detection and Correction in a Gas Turbine Engine Control System

1. Field of the Invention

This invention relates to gas turbine engine control systems and more particularly to an apparatus in a gas turbine control system for detecting the failure of an engine parameter sensor and taking corrective action.

2. Description of the Prior Art

Gas turbine engine control systems, particularly for use in controlling aircraft engines, generally include a number of sensors which measure various engine parameters. These sensors measure performance parameters, typically gas generator and fan rotational velocities and engine operating pressures and temperatures as well as controlled parameters, typically fuel flow and variable geometry positions. The signals developed by such sensors are used to control actuators such as of the electrohydraulic and electromechanical type which vary the position of the controlled parameters, to control the operation of the engine. In a conventional engine control system the engine parameter sensors are connected directly to a control computational unit within the engine control system. The control computational unit may be analog in which dedicated circuits perform continuous computations or it may be digital in which computations are performed sequentially on a time-sharing basis. In either case, the control computational unit using directly the sensed values for both the engine performance parameters and the controlled parameters and known engine behavior characteristics computes signals to modify the controlled parameters in order to maintain a selected level of engine performance. The signals from the control computation unit are amplified, conditioned and then transmitted to the actuators which alter the controlled parameters.

One potential problem associated with such prior art control systems is that the loss of a signal representative of the true value of the parameter from one or more engine performance parameter sensors or controlled parameter sensors may cause a loss of control of one or more of the controlled parameters resulting in a loss of control of engine performance. Such a signal loss may be due to any number of factors including the failure or malfunctioning of the sensor itself, the failure or malfunctioning of the interconnection between the sensor and the control computational unit or the failure of malfunctioning of any of the processing circuitry for the sensor signals. All such failures, abnormal operations, malfunctions, etc. sometimes are referred to herein as failure or failures.

Accordingly, it is the primary object of this invention to provide a control system for a gas turbine engine which prevents loss of control of engine performance in the event of the loss of a true signal from an engine parameter sensor.

It is a further object of this invention to provide such a control system which transmits an indication to the engine operator when the failure or malfunctioning of an engine parameter sensor or its associated circuitry has occurred.

These and other objects, which will hereinafter become apparent, are accomplished by the improved control system of the present invention which includes means for sensing actual engine performance and controlled parameters, control computational means, actuator means and further includes failure indication and corrective action means for receiving signals from the sensor means and from the control computational means and for generating signals representing real time estimates of the engine performance parameters and the controlled parameters while the engine is in operation. The estimate signals are transmitted to the control computational means in lieu of the actual engine performance and controlled parameter sensor signals. The control computational means utilizes the received estimate signals to generate control signals which are transmitted to the actuator means to modify the engine controlled parameters. The control system thus maintains a selected level of engine performance despite the failure or abnormal operation of one or more engine parameter sensors or associated circuitry.

In a more specific form of the present invention, the improved control system employs a continuously updated engine model to simulate engine performance and to generate signals which represent estimates of the various engine parameters for the particular engine operation. The model is comprised of control system logic hardware whose performance is analogous to a set of mathematical equations which define the interrelationships between the various engine performance parameters, typically rotational velocities, pressures and temperatures of the various components, as a function of the various controlled parameters, typically fuel flow and geometry. The model generated estimate signals are compared with the corresponding actual engine parameter sensor signals and difference signals are produced. As long as the difference signals are within specific tolerance limits, they are utilized to update the model. If one or more of the difference signals exceeds the tolerance limits, the particular signal or signals are inhibited from updating the model and a sensor failure indication may be provided to the engine operator. The model is then updated by the remaining uninhibited difference signals.

FIG. 1 is a schematic block diagram of the preferred embodiment of the improved control system of the present invention.

FIG. 2 is an operational flow diagram of a portion of the control system of FIG. 1.

Referring to FIG. 1, there is shown a block diagram of a control system shown generally at 10 for a gas turbine engine. Shown in dashed lines in FIG. 1 is one form of the failure indication and corrective action means of the improved control system of the present invention. The gas turbine engine may be of any type known in the art, for example including a turbojet, turboprop, turboshaft or turbofan, and a detailed description of its operation is not necessary for purposes of describing the present invention.

The control system 10 includes engine performance parameter sensor means 12, such as a plurality of sensors for measuring the actual values of the engine performance parameters. The performance parameters include the pressures and temperatures of various engine components as well as velocities of various parts of the engine. The sensed performance parameter values are utilized by the control system 10 to control various engine controlled mechanisms 14, typically fuel flow and engine geometry, in order to maintain a selected level of engine performance.

In a typical prior art gas turbine engine control system, such as the one shown in solid lines in FIG. 1, signals from both the engine performance parameter sensors 12 and controlled parameter sensor means 16 are transmitted to a control computational means or unit 26. The control computational unit 26, which may be of the analog or digital type, utilizes the sensor signals to generate control signals which are transmitted to actuator means 28 such as of the electromechanical and electrohydraulic type. The actuators in turn operate to modify the controlled mechanisms in order to maintain a selected level of engine performance.

Although, under normal operating conditions, such a control system is adequate for controlling a gas turbine engine, the failure or malfunctioning of an engine parameter sensor or its associated transmitting or receiving circuitry could result in a loss of control of one or more controlled parameters and a corresponding degradation in engine performance.

The present invention, in one form, operates to overcome the effects of sensor failure or malfunction by interposing a digital computer shown generally, in dashed lines, as 18 between the output of the engine parameter sensors 12 and 16 and the control computational unit 26. The digital computer 18 includes an input/output unit 20 for external interface, an arithmetic unit 22 for performing arithmetic calculations and a memory 24 for storing programs and data. The computer hardware may be of a type well known in the art and comprises a machine having a cycle time which is fast enough to provide real time operation.

Output signals from the engine parameter sensor means 12 and 16 are transmitted to the computer input/output unit 20 in lieu of being sent directly to the control computational unit 26. Depending upon the type of sensors and digital computer being employed, it may be necessary to employ signal processing amplifiers and analog to digital converters (neither of which are shown) in order to condition the sensor output signals to be compatible with the input requirements of the digital computer. Such amplifiers and converters may be co-located with or form a part of either the sensors or the computer input/output unit 20 or may be individually located at a convenient place therebetween. The amplifiers and converters may be of any suitable type well known in the art and a detailed description of their operation is not necessary for purposes of describing the present invention.

The computer memory 24 comprises known hardware which includes an engine model program which simulates the performance of the gas turbine engine. The model is comprised of a set of mathematical equations which define the interrelationships between the various engine performance parameters as a function of the various engine controlled parameters. The equations utilized to construct the model of this particular embodiment of the invention are shown below. It was necessary to introduce a number of constants (K1-K67) for purposes of scaling, unit correction and to achieve the best possible mathematical fit for certain non-linear functions. ##EQU1## wherein: W2=fan airflow

P1=fan inlet pressure

T1=fan inlet temperature

NL=low pressure rotor RPM

P15=fan discharge pressure

K1, K2, K3=constants ##EQU2## wherein: T15=fan discharge temperature

K4, K5, K6=constants

wherein:

T21=core inlet temperature

K7, K8=constants ##EQU3## wherein: P21=core inlet pressure

NH=high pressure rotor RPM

K9, K10, K11, K12, K13=constants

K9=K13 ##EQU4## wherein: W21=compressor inlet airflow

P3=compressor discharge pressure

fZ.beta.=deviation of stator position from design schedule

K14, K15, K16=constants ##EQU5## wherein: T3=compressor discharge temperature

K17, K18, K19=constants

wherein:

W3=combustor inlet airflow

K20=constant ##EQU6## wherein: T4=combustor discharge temperature

Wf=fuel flow

K21, K22, K23, K24=constants

wherein:

W4=combustor discharge gas flow ##EQU7## wherein: P4=high pressure turbine inlet pressure

K25=constant

wherein:

P3=compressor discharge pressure

K26=constant

wherein:

Wf=fuel flow

ZWf=fuel valve position

K27, K28, K29=constants

wherein:

W15=bypass duct airflow ##EQU8## wherein: PS56=static pressure at turbine discharge

K30, K31=constants

wherein:

W56=turbine discharge gas flow ##EQU9## wherein: P56=total pressure at turbine discharge

K32=constant ##EQU10## wherein: P42=turbine interstage pressure

K33 to K41=constants

K36=K41 ##EQU11## wherein: T42=turbine interstage temperature

K42, K43=constants ##EQU12## wherein: T56=low pressure turbine discharge temperature

K44, K45, K46=constants

wherein:

W8=gas flow leaving mixer

W2=fan airflow

Wf=fuel flow ##EQU13## wherein: T8=mixer discharge temperature

K47, K48, K49=constants ##EQU14## wherein: P8=nozzle pressure

PAMB=ambient static pressure

A8=nozzle effective area

K50, K51=constants ##EQU15## wherein: PS56X=mixer inlet static pressure

K52, K53=constants ##EQU16## wherein: PW1=power to fan

K54, K55=constants

wherein:

PW25=power to compressor

K56=constant ##EQU17## wherein: PW4=power from high pressure turbine

K57, K58, K59, K60=constants ##EQU18## wherein: PW48=power from low pressure turbine

K61, K62, K63, K64=constants ##EQU19## wherein: (dNL/dt)=rate of change of low pressure rotor RPM

K65, K66=constants ##EQU20## wherein: (dNH/dt)=rate of change of high pressure rotor RPM

K67=constant

In addition to the above-described exemplary equations, a section of the computer memory 24 is provided with stored data items from sensor means 12 and 16 which represent the current, real time internal status of the engine. As will be discussed below, these stored data items are retrieved and processed to develop estimate signals which correspond to the appropriate actual engine performance and actual engine controlled parameters. After an initial start-up period during which values for both the engine performance parameters and the engine controlled parameters are preset into the engine model, the model continuously receives signals from the engine parameter sensors in means 12 and 16 as well as the control signals from the control computational unit 26. The engine model utilizes these signals in conjunction with the mathematical equations and the data items to calculate signals representing estimates of the engine parameters sensor signals. The estimate signals thus provided are then transmitted to the control computational unit 26 in lieu of the actual engine parameter sensor signals. The control computational unit 26 utilizes the estimate signals to generate control signals which are transmitted to the actuators 28.

A computer printout representative of a fortran language computer program which implements the engine model is shown below. The right hand marginal notations indicate the portions of the program in which the above-identified equations are actually implemented. Some of the equations are identified as being implemented in several places within the program due to the use of iterative calculations and subcalculations and in some cases, the subcalculations are identified as being utilized in several equations as indicated. ##SPC1##

In addition to the above-described model, in one form of the present invention the computer memory 24 includes a logic program for both model updating and failure detection and indication.

Referring now to the operational flow diagram of FIG. 2, model updating is accomplished by first comparing each calculated engine parameter estimate signal with the corresponding actual engine parameter sensor signal. Each resulting difference signal is then utilized to update the engine model by changing all of the stored data items which are affected by the engine parameter which corresponds to the difference signal. In order to provide improved accuracy, the magnitude of the change produced by a particular difference signal to each stored data item is weighted so as to be proportional to the degree of control that the engine parameter corresponding to the difference signal has over the particular data item as compared with the degree of control that the other engine parameters have over that data item.

In order to determine whether a particular engine parameter sensor in means 12 and 16 or its associated transmission and interconnection circuitry is functioning properly, each difference signal is compared with a set of specific tolerance limits. The tolerance limits may be predetermined stored values or may be continuously calculated as a function of current engine operational characteristics. If a particular difference signal exceeds the tolerance limits either by being greater than an upper limit or by being less than a lower limit, then the corresponding engine parameter signal being received by computer input/output unit 20 is deemed to be improper. Such an improper signal may be the result of a number of possible occurrences including a complete or partial failure of the particular sensor, a complete or partial failure of the signal processing circuitry associated with the particular sensor, or a complete or partial failure of the interconnecting circuitry between the particular sensor and the computer input/output unit 20. For those difference signals which exceed the tolerance limits, corrective action is taken to insure that a significant degradation of engine performance does not occur. Such corrective action includes immediately inhibiting the out-of-tolerance difference signals from being utilized to update the engine model. The model is updated, however, by the remaining uninhibited difference signals as described above and continues to generate estimate signals for all of the engine parameters.

It should be noted that although a particular difference signal has been inhibited from updating the engine model, the engine parameter sensor from which the difference signal was derived, has not been inhibited and continues to provide a signal to the computer input/output unit 20. Thus, if an improper sensor signal resulted from a transient or some other temporary condition and a proper signal is subsequently received, the difference signal associated with the particular sensor is no longer inhibited and is again utilized to update the engine model.

In addition to inhibiting an out-of-tolerance difference signal from updating the model, an indication that a particular engine performance parameter sensor is providing an improper signal may be provided to the engine operator. Such an indication may take the form of a light or any other suitable indicator (not shown) which is known in the art.

A computer printout representative of a fortran language computer program which implements the above-discussed logic program is shown below. ##SPC2##

From the foregoing description it can be seen that the present invention provides an improved gas turbine engine control system which maintains engine operation without any significant degradation even though one or more engine parameter sensors are providing improper signals. This represents a significant improvement over prior art control systems in which the loss of a single sensor signal could lead to a drastic reduction in engine performance and perhaps necessitate completely shutting down the engine.

It will be obvious to one skilled in the art that changes can be made to the above-described invention without departing from the broad inventive concepts thereof. For example, although the invention has been illustrated in connection with add-on hardware for use with an existing control system, when used with an existing control system which already employs a digital computer, the invention could be programmed into the existing computer without the addition of separate computer hardware. Also, although illustrated as employing a digital computer 18, the invention may be realized by employing a specialized assembly of analog electronic and/or hydromechanical elements of the type widely applied in the control of gas turbine engines. Alternatively, this invention could be employed in a system in which the sensor signals are transmitted directly to the control computational unit unless an improper signal is detected by the invention, in which case the engine model would supply an estimate signal only for that sensor.

It is to be understood, therefore, that this invention is not limited to the particular embodiment disclosed, but it is intended to cover all modifications which are within the spirit and scope of the invention as claimed.