GB2226423A

GB2226423A - Control system for a gas turbine engine

Info

Publication number: GB2226423A
Application number: GB8918920A
Authority: GB
Inventors: Michael Solomon Idelchik; R Daniel Robinson Gilmore
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1988-12-21
Filing date: 1989-08-18
Publication date: 1990-06-27
Also published as: GB8918920D0; JPH0367028A; DE3926705A1; FR2640689A1

Description

-1 CONTROL SYSTEM FOR A GAS TURBINE ENGINE The invention described herein

was made in the performance of work under NASA Contract No. NAS3-22752 and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 (72 Stat.

435; 42 U.S.C. 2457) of the United States of America.

This invention relates to a control system for a gas turbine engine.

BACKGROUND OF THE INVENTION modern gas turbine engines have numerous systems for controlling a wide range of parameters to achieve the most efficient engine operation. Some of these parameters which are controlled include fuel flow, fan speed, fan pitch, and fan exhaust nozzle areas. Recently, full authority digital electronic systems have been used to provide increased system control over a plurality of previously uncoordinated control loops. However, %hen these electronic systems are utilized in a gas turbine engine it is desirable to have a backup control system which will assure continued operation in the event of an electrical failure or malfunctions of portions of the primary control system. Typically, these backup systems consist of either an additional hydro-mechanical unit or an electronic system, both of which control the engine only after the primary control system has failed. Typically, failure detection in the primary control system is self monitoring by the use of a built-in-test (BIT) system which is able to verify proper operation of typically 95% of the control system by checking values, ranges, sensors and performing sample tests throughout the control system, such as the random access memory (RAM), analog to digital converters, input buses, the microprocessor, and voltage values in the power supply. After the BIT system detects a failure, the primary control system takes itself off-line and the backup control system is activated. However, since the electronic control system must also constantly be monitoring and controlling engine operation, the BIT system must time share with the control system's computer controller. Consequently, the system takes an extended period of time to verify proper operation of the entire system and therefore an undesirable delay may occur before a failure is detected. Further, after a failure in the primary control system is detected, delays may occur before the backup control system becomes fully operational. Finally, since a typical BIT system detects 95 percent of all failures, five percent of the failures are undetected and consequently the primary control system will not take itself off-line during these undetected failure modes. The inability to detect some failures, the delays in detecting failures, or the delays in bringing the backup system in full operation may result in either interruptions or instabilities in engine control which may result in undesirable engine performance such as overspeed, stall or flameout of the engine. Therefore, it would 1 be desirable to have a control system which is tolerant of delays in detecting failures and is also tolerant of undetected failures.

SUMMARY OF THE INVENTION

A control system for a gas turbine engine has both a first and second channel which simultaneously control the engine. The first channel receives a set of inputs and processes the inputs to form a first rate signal. The second channel also receives the same set of inputs and processes the inputs to form a second rate signal. The first and second rate signal are coupled to a means for combining the signals and the output is coupled to a common integrator which activates an actuator of the engine.

The invention also includes an apparatus for controlling a gas turbine engine comprising first and second means for receiving an engine control signal and first and second means for generating first and second monitor signals. A first means for generating a difference signal representative of the difference between the control signal and the first monitor signal is coupled to the first receiving means. A first means for processing the first difference signal to form a first rate signal is coupled to the first difference means. A second means for generating a second difference signal representative of the difference between the control signal and the second monitor signal is coupled to the second receiving means. A second means for processing the second difference signal to form a second rate signal is coupled to the second difference means. A means for combining the output of the first and second processing means and a common integrator and actuator is coupled to the output of the combining means.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a schematic diagram of one embodiment of the invention.

Figure 2 is a schematic diagram of an integral controller which is another embodiment of the invention.

Figure 3 is a schematic diagram of a proportional controller of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 1, the present invention is shown generally as a control system 10 for a gas turbine engine. It should be understood that the control system is equally applicable with any gas turbine engine, such as a turboshaft, turbojet, or a turbofan engine.

The control system 10 comprises first and second control loops or channels, 12 and 14 respectively, which both simultaneously monitor and control engine operation. The first control loop 12 comprises a first receiving means 20 which receives a set of inputs 22. The inputs are processed to form a first error signal 24 representative of a desired change in engine operation. The first receiving means 20 is coupled to a first means 26 for generating a difference signal 28 representative of the difference between the first error signa7l 24 and a first feedback signal 30 which corresponds to actuator operation.

- The difference signal 28 of the first difference means 26 is preferably coupled to first dynamics 32 for processing the signal to the desired level and the signal output from the first dynamics 32 is a first rate signal 34 indicating the rate of change desired of the controlled parameter. The output of the first dynamics 32 is coupled to a means for combining the signals 36, a common integrator 38, and an actuator means 40. The first control loop 12 is then completed by a first means 42 for generating a first monitor signal 44 which is representative of actuator conditions. The first monitor signal 44 is preferably connected to a first feedback processor 46 which,generates the first feedback signal 3M and the first feedback signal 30 is coupled to the first difference means 26. The second control loop 14 is comprised of similar components as that of the first control loop 12, such that a second receiving means 50 receives the same set of inputs 22 as the first receiving means and processes those inputs in a similar manner to form a second error signal 54. It should be understood that the set of inputs 22 received by the second receiving means 50 is preferably generated by utilizing separate sensors, monitors and processors from that used to form the set of inputs 22 received by the first receiving means 20. Thus, variations'may occur between the set of inputs 22 received by the first and second receiving means 20 and 50 respectively, due to tolerance variations or component failures, such as a failed sensor. The second receiving means 50 is coupled to a second means 56 for generating a difference signal 58 representative of the difference between the second error signal 54 and a second feedback signal 60 which cortesponds to actuator operation. The second means 56 for generating the -difference signal 58 is preferably coupled a second set of dynamics 62 for processing the signal to the desired level and the signal output from the second dynamics 62 is a second rate signal 64 indicating the rate of change desired of the controlled parameter. The second rate signal 64 is coupled to the means for combining the signals 36, the common integrator 38, and the actuator means 40. The second control loop is completed by a second means 72 for generating a second monitor signal 74 which is coupled to a second feedback processor 76 which generates the feedback signal 60 which is coupled to the second difference means 56. Thus it is to be understood that the signals from each control loop are coinbined prior to the common integrator 38.

The first and second receiving means, 20 and 50 respectively, may be any means for receiving either an electrical or mechanical signal, however, preferably the receiving means is an input port of a digital electronic control (DEC) system adapted to receive electronic control signals. The first and second means for generating a difference signal, 26 and 56 respectively, may be any means for generating a signal representing the difference between the two inputs. Typically the difference signal is generated by the DEC computer. Further the first and second set of dynamics, 32 and 62 respectively, are also preferably implemented through the DEC as is well known to those skilled in gas turbine engine controllers. Additionally, if the first and second feedback processors, 46 and 76 respectively, are utilized, this processing is also preferably conducted by the DEC system.

The means for combining the signals 36 preferably comprises a means for either averaging or summing the first and second rate signals, 34 and 64 respectively. The common integrator may be either electronically implemented or preferably is implemented by hardware. It should be understood that the means for combining the signals 36, the common integrator 38, and the actuator 40 may be either combined by a single component or, alternatively they may be separate components. For example, as shown in Z 1 Fig. 2 in an integral fuel flow control system, a dual coil torque motor 60 may serve as both a summer or an averager, depending on coil configuration, for both the fi"rst and the second rate signals. The dual coil torque motor 60 may be coupled to a pilot and metering valve 62 which way serve as both the common integrator and the actuator. The pilot and metering valve 62 then controls actual fuel flow. Preferably the pilot and metering valve has the following transfer function.L i 0 K S wherein K is a multiplier and s is the complex frequency variable. Further, in the integral control system the first and second feedback processors, 46 and 76 respectively, preferably have the following 15 transfer function:

Ks (T1s + 1) wherein K is a multiplier, s is the complex frequency variable, and Tj is a time constant as described by Laplacian algebra which is used to describe the control systems response to vdrious inputs. The controllers may operate with a rate feedback as in the integral controller, or alternatively, the first and second set of dynamics, 32 and 62 respectively, may process the signals to be rate signals.

The first and second means for generating the monitor signal, 42 and 72 respectively, are typically individual engine sensors which may be of any well known type so as to provide an appropriate indication of the particular engine parameter being measured. In the case of a fuel flow control system, the sensors are.preferably a pair of linear differential transformers (LDVT) which measures the metering valve position. As shown in Fig. 3, a proportional fuel flow control system provides a proportional signal outputs for the first and second moni7tor signals 44 and 74 respectively. These porportional outputs are then input into the first and second means for generating a difference signal, 26 and 56 respectively. It is desirable that the DEC system utilize the input from the position sensors and by the use of an algorithm determine the rate of change of the metering valve. However, it should be understood that in both the integral and proportional controllers the common integrator receives a combined rate input.

In operation, both the first and second receiving means, 20 and 40 respectively, receive at least one control signal representative of the desired level of engine operation. Both the first and second control loops 12 and 14 respectively, then generate difference signals which are separately processed in each control loop. Each control loop is processed to form a rate signal and the rate signals are combined and the combined signal is input into the common integrator which activates the actuator of the engine. Thus, each control loop simultaneosly controls engine operation. It is to be understood that by the term, simultaneously, each loop is actively monitoring and controlling engine operation. However, since the controller is typically microprocessor based, some delays wil 1 occur between the updating and control changes of each loop due the normal clocking sequence of microprocessor operations. In the case of a fuel flow control system, the signal from each channel is applied to two separate coils of a dual coil torque -g- motor. The torque motor then controls the pilot and metering valve which controls fuel flow. The position of the valve is monitored and two corresponding signals are separately coupled back into each of the two control loops by inputting the signals into the DEC where it may be processed by implementing a transfer function and the resultant then is applied to its own difference means, thereby providing closed loop control. Thus, each control loop generates its own command and translates the command into the proper current, which is then applied to separate coils of the torque motor which acts as a summer for both control loops. In this system a single integrator may be utilized and thereby provides for natural sharing between two control loops. Thus, when a failure should occur in one control loop the remaining control loop will automatically compensate for any incorrect control signals applied to the dual coil torque motor. The control system of the present invention is tolerant not only of delays in detecting failures or bringing backup systems into full operation but the system is also tolerant of faults which are not detected by the Built-In-Test system. Should a fault occur within one control loop the other loop will provide control and compensate- for the defective system rather than providing unstable or no control over the gas turbine engine. Thus, the present system is fault tolerant and allows for greater time in detecting failures and provides for control over the engine during this period and the period during which a backup system is made operational if one is utilized. The first and second channels 12,14 both continually monitor and control the operation of the engine.

CUIMS l. A control system for a gas turbine engine comprising: a first channel which receives a set of inputs and processes the inputs to form a first rate signal; a second channel which simultaneously receives and processes said same set of inputs, and produces a second rate signal; and a means for combining said first and second rate signals and the output of said combining means is coupled to a common integrator and said integrator is coupled to an actuator of said engine.

2. The control system of claim 1, wherein said common integrator and said actuator are combined.

3. The control system of claim 1, wherein said common integrator comprises an electronically implemented integrator.

Claims

4. The control system of Claim 1, 2 or 3 wherein said first and second

rate signals are combined by summing the signals.

5. The control system of Claim 1, 2 or 3 wherein said first and second rate signals are combined by averaging the signals.

6. The control system of Claim 1, 2 or 3 wherein said means for combining the signals comprises a dual coil torque motor.

7. An apparatus for controlling a gas turbine engine, comprising: a first means for receiving.at least one engine control signal representative of desired engine operation; a means for generating a first monitor signal representative of actual engine condition; 0 1 1 a means for generating a first difference signal output representative of the difference between said first engine control signal and said first monitor signal, said first difference generating means being coupled to said first receiving means and said first monitor signal generating means',. a means for processing said first difference signal to form a first rate signal output, said means for processing said first difference signal is coupled to the output of said first difference generating means; a second means for receiving said control signal; a means for generating a second monitor signal representative of actual engine condition; a means for generating a second difference signal output representative of the difference between said control signal and said second monitor signal, said first difference means being coupled to said second receiving means and said second monitor signal generating means; a means for processing said second difference signal to form a second rate signal output, said means for processing said second difference signal is coupled to the output of said second difference signal generating means; a means for combining the output of said first and second difference signal processing means; and a common integrator and an actuator which is coupled to the output of said combining means.

8. The apparatus of claim 7, wherein said means for combining the output of said first and second processing means comprises a dual coil torque motor.

9. The apparatus of claim 7 or 8 wherein said conmn integrator and actuator comprise a fuel flow valve.

10. The apparatus of claim 7, 8 or 9 wherMn said means for generating first monitor signal and said means for generating a second monito r signal are each position sensors for a fuel flow metering valve.

11. The apparatus of Claim 7,8,9 or 10 further comprising a first feedback processor which processes the first monitor signal prior to the signal being input into the first difference means, and a second feedback processor which processes the second monitor signal prior to the signal being input into the second difference means.

12. The apparatus of claim 11, wherein said first and second feedback processors each generate a rate signal from said first and second monitor signals.

13. A control system substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

1, 2 or 14. A gas turbine engine comprising a control system according to any preceding claim.

Published 1990 at The Patent lee. State House. 66711-lighijolborr.-Lop. donWC1R4TP Further copies inky be obtainedfrom The Pazent Office Sales Branch. St Mary Cray. Orpington. Kent BE5 3RD- PrintEd by Mu:t.,,:ex techruques ltd. St Maiy Cray. Ken'. Cen 1 87