Control Design for a Generic Commercial Aircraft Engine

TL;DR: In this paper, the authors describe the control algorithms and control design process for a generic commercial aircraft engine simulation of a 40,000 lb thrust class, two spool, high bypass ratio turbofan engine.

Abstract: This paper describes the control algorithms and control design process for a generic commercial aircraft engine simulation of a 40,000 lb thrust class, two spool, high bypass ratio turbofan engine. The aircraft engine is a complex nonlinear system designed to operate over an extreme range of environmental conditions, at temperatures from approximately -60 to 120+ F, and at altitudes from below sea level to 40,000 ft, posing multiple control design constraints. The objective of this paper is to provide the reader an overview of the control design process, design considerations, and justifications as to why the particular architecture and limits have been chosen. The controller architecture contains a gain-scheduled Proportional Integral controller along with logic to protect the aircraft engine from exceeding any limits. Simulation results illustrate that the closed loop system meets the Federal Aviation Administration s thrust response requirements

Summary

Nomenclature

• Thus the engine controller logic can be divided into two parts: power management and protection logic.
• The difference between the setpoint and the feedback produces an error, which is the input to the setpoint controller.
• Performance requirements are different for small throttle transients and steady-state operation than for large transients.

3.1 Controlled Variables

• For aircraft engines, the ideal controlled variable would be thrust, since the throttle input corresponds to a demanded thrust level.
• Thrust is not measurable during flight but is proportional to the airflow through the engine, allowing other engine outputs to be used to control thrust indirectly .
• The Engine Pressure Ratio (EPR), which is the low pressure turbine discharge pressure divided by the inlet pressure (P2), correlates well with the airflow through the engine and thus is a good variable to use to regulate thrust (Ref. 3).
• Other options for the control variables are fan speed (Nf) or core speed (Nc), both of which are shown in Figure 2.
• Fan speed correlates better with thrust than core speed does, since the fan handles all of the airflow that produces thrust, both bypass flow and core flow, while the core speed only varies with the airflow through the core.

3.2 Setpoints

• The design goal of the setpoint specification is to produce thrust as a linear function of throttle position at any environmental condition, as well as produce the requested thrust (such as take-off thrust, flight idle, max power, cruise power, etc.) independent of the environmental condition (Ref. 3).
• The actual setpoint value is determined from the throttle position and is in terms of either EPR or Nf.
• The required take-off thrust is different at Sea Level than at 5,000 ft; regardless, when the pilot moves the throttle to the take-off thrust position, the corresponding setpoint must produce the required thrust for take-off at that environmental condition.
• The thrust profile at other environmental conditions is developed by scaling the SLS thrust profile at different environmental conditions.
• The setpoints for C-MAPSS40k were designed by adjusting the fuel flow input into the open-loop engine until the net thrust produced by the open loop C-MAPSS40k engine matched the desired thrust for the particular throttle setting.

3.3 Control Requirements

• The power management controller requirements may be stated in the frequency domain in terms of gain margin, phase margin, and bandwidth.
• All three of these characteristics can be determined by generating a Bode plot of the loop gain.
• The loop gain is the product of the controller transfer function and the open loop engine transfer function (including sensor and actuator dynamics) at a specified flight condition; the specific details of this process can be found in any introductory linear systems textbook such as Reference 7 1.
• From Reference , the gain margin should be greater than 6dB and the phase margin should be greater than 45°.
• For C-MAPSS40k, the controller was designed to achieve high bandwidth with a constraint of producing a gain margin of at least 6 dB, a phase margin greater than 45°, and a critically damped closed-loop response.

3.4 Setpoint Controller

• A Proportional-Integral (PI) controller is used and the gains (Kp and Ki, respectively) are scheduled based on altitude and Mach number.
• There is an additional gain before the integrator (FB) that is scheduled based on the altitude and the power level ; this aids in producing a critically damped response at different power levels.
• The EPR controller features a low-pass filter for the setpoint error.
• This filter serves to remove the high frequency components of the error signal, which are not found in the fan speed error signal.
• The output of the controller, Wf Reg, is the controller’s desired fuel flow rate.

3.5 Gain Scheduling

• The aircraft engine has to operate over a wide range of environmental conditions.
• One way of handling this complexity is to use gain scheduling, which takes advantage of interpolation.
• During operation, the breakpoint values are used to determine the value of the controller gains at each time step by interpolating between the nearest defined subsystems.
• For large throttle transients, the power management controller only regulates the controlled variable, while additional logic, or limiters, are used to protect critical engine variables from exceeding physical bounds and to ensure safe operation (Ref. 3).
• The main difference here is that there is no feedback gain (FB) and the PI gains are constant, not scheduled.

4.1 Maximum Speed Limiters

• The maximum speed limiters protect both the fan and core shafts from excessive speed that could cause a disk to burst or blade failure.
• The core shaft may exceed its limit when the fan shaft is driven into over-speed, the fuel metering valve fails open, or a speed sensor fails, resulting in excessive fuel flow to the engine (Ref. 2).
• 2 Combustor Pressure Limiters (Ps3 Minimum and Maximum) Additionally, the lower limit is adjusted based on the engine operational inputs, such as the customer bleed, additional power extraction, and whether the aircraft is on the ground or in flight.

4.3 Acceleration Limiter

• The purpose of the acceleration limiter is to prevent high pressure compressor stall during quick accelerations or large changes in thrust demand.
• For C-MAPSS40k controller design, an acceleration schedule was selected to allow even a NASA/TM—2010-216811 9 deteriorated engine to meet the necessary performance requirements while still ensuring an acceptable stall margin (Ref. 2).
• Each limit controller determines the fuel flow necessary to drive the engine to its individual limit without overshoot, or maintain the limit.
• Next, the outputs of the min limiters are compared.
• This error is continuously integrated by the individual controller’s integral term, and the integral increases in magnitude since, when the controller is not active, the variable does not reach its setpoint.

Summary

• This paper provides the reader an explanation of the design process and choices made in the development of the baseline controller for C-MAPSS40k.
• The C-MAPSS40k controller contains an Engine Pressure Ratio and Fan Speed setpoint controller as well as a rotor speed limiter, a combustor pressure limiter, an acceleration schedule, and a ratio unit limiter.
• The control architecture applies a MIN-MAX strategy to determine which control signal is sent to the engine’s fuel metering valve.
• This paper also shows that the C-MAPSS40k simulation with the baseline controller passes the Federal Aviation Regulation on thrust response.
• While all results are specific to the C-MAPSS40k engine, this architecture and control design procedure are applicable to any commercial aircraft engine.

Figures

Figure 5.—Locations of the Breakpoints Chosen for the EPR Setpoint Controller (blue “x”s) and the C-MAPSS40k Flight Envelope (magenta line).

Figure 1.—Diagram of an Aircraft Engine Control System.

Figure 4.—C-MAPSS40k Setpoint Controller Structure.

Figure 9.—C-MAPSS40k Response to FAR 33.73b.

Figure 7.—C-MAPSS40k MIN-MAX Controller Architecture, showing the integration of the Protection Logic Controller (Max Limiters and Min Limiters) and the Power Management Controller.

Figure 6.—The Acceleration Schedule Implemented in C-MAPSS40k.

Figure 2.—Diagram of the C-MAPSS40k Engine.

Figure 11.—Minimum Combustor Pressure Limiter (Ps3 Min).

Figure 10.—Max Fan Speed Controller (Nf_max).

Figure 8.—C-MAPSS40k Response to FAR 33.73a.

Figure 3.—Net Thrust and EPR Setpoint Curve for C-MAPSS40k at SLS Condition. 40 45 50 55 60 65 70 75 80

Figure 12.—Acceleration Schedule Limiter.

Figure 13.—Ratio Unit Limiter (Wf/Ps3).

Full text

Jeffrey Csank
N&R Engineering and Management Services, Cleveland, Ohio
Ryan D. May
ASRC Aerospace Corporation, Cleveland, Ohio
Jonathan S. Litt and Ten-Huei Guo
Glenn Research Center, Cleveland, Ohio
Control Design for a Generic Commercial
Aircraft Engine
NASA/TM—2010-216811
October 2010
AIAA–2010–6629

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Jeffrey Csank
N&R Engineering and Management Services, Cleveland, Ohio
Ryan D. May
ASRC Aerospace Corporation, Cleveland, Ohio
Jonathan S. Litt and Ten-Huei Guo
Glenn Research Center, Cleveland, Ohio
Control Design for a Generic Commercial
Aircraft Engine
NASA/TM—2010-216811
October 2010
AIAA–2010–6629
National Aeronautics and
Space Administration
Glenn Research Center
Cleveland, Ohio 44135
Prepared for the
46th Joint Propulsion Conference and Exhibit
cosponsored by the AIAA, ASME, SAE, and ASEE
Nashville, Tennessee, July 25–28, 2010

Acknowledgments
The authors would like to thank Juan Marcos and James Fuller at Pratt & Whitney, who reviewed the baseline controller architecture
and provided guidance to ensure that a realistic simulation was developed. Our thanks also go to Diana Drury of ASRC Aerospace,
Corp., who handled the version control system for C-MAPSS40k. Finally, our thanks go to the NASA Aviation Safety Program’s
Integrated Resilient Aircraft Control Project for funding this work.
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Level of Review: This material has been technically reviewed by technical management.

NASA/TM—2010-216811 1
Control Design for a Generic Commercial Aircraft Engine
Jeffrey Csank
N&R Engineering and Management Services
Cleveland, Ohio 44130
Ryan D. May
ASRC Aerospace Corporation
Cleveland, Ohio 44135
Jonathan S. Litt and Ten-Huei Guo
National Aeronautics and Space Administration
Glenn Research Center
Cleveland, Ohio 44135
Abstract
This paper describes the control algorithms and control design process for a generic commercial
aircraft engine simulation of a 40,000 lb thrust class, two spool, high bypass ratio turbofan engine. The
aircraft engine is a complex nonlinear system designed to operate over an extreme range of environmental
conditions, at temperatures from approximately –60 to 120+ °F, and at altitudes from below sea level to
40,000 ft, posing multiple control design constraints. The objective of this paper is to provide the reader
an overview of the control design process, design considerations, and justifications as to why the
particular architecture and limits have been chosen. The controller architecture contains a gain-scheduled
Proportional Integral controller along with logic to protect the aircraft engine from exceeding any limits.
Simulation results illustrate that the closed loop system meets the Federal Aviation Administration’s
thrust response requirements.
Nomenclature
Alt Altitude (ft)
EPR Engine Pressure Ratio
FAA Federal Aviation Administration
FB Integral gain multiplier based on current power level
HPC High Pressure Compressor
IFB Integral Feedback Gain
IWUP Integral Wind-Up Protection
Ki Integral Gain
Kp Proportional Gain
LPC Low Pressure Compressor
LPT Low Pressure Turbine
MAX Maximum function
MIN Minimum function
Mn Mach Number
N Rotor Speed (either core of fan)
Ndot Rotor Acceleration
Nc Core Speed
Nf Fan Speed
PI Proportional plus Integral (control)
Ps3 High Pressure Compressor Discharge Static Pressure (psi)

Frequently asked questions

Q1. What are the contributions in this paper?

This paper describes the control algorithms and control design process for a generic commercial aircraft engine simulation of a 40,000 lb thrust class, two spool, high bypass ratio turbofan engine. The objective of this paper is to provide the reader an overview of the control design process, design considerations, and justifications as to why the particular architecture and limits have been chosen. 

Q2. What is the thrust profile of an open loop engine?

The desired SLS thrust profile, defined by steady-state values of thrust versus throttle position, is created by determining the full power thrust, take-off thrust, cruise thrust, flight idle thrust, and an appropriate ground idle thrust at SLS conditions. 

Q3. What is the ideal control variable for a small engine?

For aircraft engines, the ideal controlled variable would be thrust, since the throttle input corresponds to a demanded thrust level. 

Q4. What is the purpose of the acceleration limiter?

The purpose of the acceleration limiter is to prevent high pressure compressor stall during quick accelerations or large changes in thrust demand. 

Q5. What is the power management controller requirement for the SLS?

The power management controller requirements may be stated in the frequency domain in terms of gain margin, phase margin, and bandwidth. 

Q6. What is the main challenge in aircraft engine control design?

A major challenge in aircraft engine control design is that the engine must operate reliably over an extended range of environmental conditions, defined by altitude, Mach number, and temperature. 

Q7. Why is it desirable to have an integral term in the controller?

It is desirable to have integral action in the controller since the presence of an integral term eliminates steady state error in the controlled variable. 

Q8. What is the thrust profile at other environmental conditions?

The thrust profile at other environmental conditions is developed by scaling the SLS thrust profile at different environmental conditions. 

Q9. What is the thrust profile for the C-MAPSS40k?

The setpoints for C-MAPSS40k were designed by adjusting the fuel flow input into the open-loop engine until the net thrust produced by the open loop C-MAPSS40k engine matched the desired thrust for the particular throttle setting. 

Q10. Why is the MIN-MAX structure designed to drive the fan speed to Nf Max?

For each individual controller in the MIN-MAX structure, an error is produced that the control law tries to drive to zero; for example the Nf Max controller is designed to drive the fan speed to Nf Max. 

Q11. What is the PI gain of the C-MAPSS40k?

When the engine is at flight condition 1, shown in Figure 5, the PI gains at breakpoints A, B, C, and D are used by a linear interpolation algorithm to determine the PI gain that should be used by the controller at flight condition 1. 

Q12. Why is the error continuously integrated by the individual controller’s integral term?

This error is continuously integrated by the individual controller’s integral term, and the integral increases in magnitude since, when the controller is not active, the variable does not reach its setpoint. 

Q13. What is the low-pass filter for the setpoint error?

This filter serves to remove the high frequency components of the error signal, which are not found in the fan speed error signal. 

Q14. Why is the power management controller added to the MIN selector?

adding the power management controller to the MIN selector will allow the output to be compared to both the max and min limiters and satisfy both sets of constraints. 

Q15. Why is the PI controller used to reduce the effect of the integral term?

Because of this, Integral Wind-Up Protection (IWUP) is used to reduce the effect of the integral term in each non-selected PI controller. 

Q16. Why is the error between the desired and actual variable nonzero?

only one controller may be active at any time, and for each of those not selected by the MIN-MAX strategy, the error between the desired and actual variable value will remain nonzero.