Jeffrey T. Csank and Thomas J. Stueber

Glenn Research Center, Cleveland, Ohio

Advanced Shock Position Control for Mode

Transition in a Turbine Based Combined

Cycle Engine Inlet Model

NASA/TM—2013-216515

May 2013
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Jeffrey T. Csank and Thomas J. Stueber

Glenn Research Center, Cleveland, Ohio

Advanced Shock Position Control for Mode

Transition in a Turbine Based Combined

Cycle Engine Inlet Model

NASA/TM—2013-216515

May 2013

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The work presented in this paper was funded by the NASA Fundamental Aeronautics Program Hypersonics project. The 

authors thank Dr. James L. Pittman, NASA Langley Research Center (LaRC), Hypersonics project principal investigator and

Mr. Donald I Soloway, NASA Ames Research Center (ARC), Hypersonics project associate principle investigator for controls, 

for their support and leadership.

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

This work was sponsored by the Fundamental Aeronautics Program 

at the NASA Glenn Research Center.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2013-216515 1 

Advanced Shock Position Control for Mode Transition in a

Turbine Based Combined Cycle Engine Inlet Model

Jeffrey T. Csank and Thomas J. Stueber

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135 

Abstract

A dual flow-path inlet system is being tested to evaluate methodologies for a Turbine Based 

Combined Cycle (TBCC) propulsion system to perform a controlled inlet mode transition. Prior to 

experimental testing, simulation models are used to test, debug, and validate potential control algorithms. 

One simulation package being used for testing is the High Mach Transient Engine Cycle Code simulation, 

known as HiTECC. This paper discusses the closed loop control system, which utilizes a shock location 

sensor to improve inlet performance and operability. Even though the shock location feedback has a 

coarse resolution, the feedback allows for a reduction in steady state error and, in some cases, better 

performance than with previous proposed pressure ratio based methods. This paper demonstrates the 

design and benefit with the implementation of a proportional-integral controller, an H-Infinity based 

controller, and a disturbance observer based controller. 

Nomenclature

ADRC Active Disturbance Rejection Control 

AIP Aerodynamic Interface Plane

CCE Combined Cycle Engine 

CCE-LIMX Combined Cycle Engine Large-Scale Inlet for Mode Transition Experiments 

DMSJ Dual Mode Scram Jet

HiTECC High Mach Transient Engine Cycle Code

HSFP High-Speed Flow-Path

LSFP Low-Speed Flow-Path

PD Proportional-Derivative control

PI Proportional-Integral control 

RALV Reusable Air Breathing Launch Vehicles

TBCC Turbine Based Combined Cycle 

1.0 Introduction

The National Aeronautics and Space Administration (NASA) is interested in technology leading to 

routine airline-type access to space through the use of Reusable Air Breathing Launch Vehicles (RALV).

To achieve this, the Fundamental Aeronautics Program’s Hypersonics project was assigned to investigate 

a Turbine Based Combined Cycle (TBCC) propulsion system for horizontal takeoff and acceleration to 

hypersonic speeds (Ref. 1). The TBCC propulsion system features a split-flow inlet capable of directing 

airflow to either or both a turbine engine and dual mode scramjet (DMSJ). One major TBCC objective is 

the transitioning of thrust production from the turbine engine to the DMSJ, known as mode transition. 

The technical challenge being addressed in this work is to transition airflow between low speed and high 

speed flow paths while maintaining inlet performance goals that deliver a required air-flow with tolerable 

distortion at the turbine engine face, without unstarting the inlet.
NASA/TM—2013-216515 2 

Figure 1.—Diagram of the Combined Cycle Engine Large-scale Inlet for Mode Transition Experiments (CCE-LIMX) 

in its designed Mach 4.0 configuration with prominent features that pertain to this work identified with labels.

An experimental TBCC system has been designed, built, and mounted in the NASA Glenn Research 

Center’s 10- by 10 Foot Supersonic Wind Tunnel. This inlet is known as the Combined Cycle Engine 

Large-scale Inlet Model for Mode Transition Experiments (CCE-LIMX) (Ref. 2). A diagram of the CCELIMX is shown in Figure 1, as it is mounted in the wind tunnel in the Mach 4 design configuration, with 

air flow from left to right. As illustrated in Figure 1, air captured by the inlet can be diverted to either an 

over-mounted low-speed flow-path (LSFP) or under-mounted high-speed flow-path (HSFP) via the 

splitter. The CCE-LIMX has two cold-pipes with conical mass-flow plugs, which are mounted 

downstream of the diffuser in the LSFP and downstream of the isolator in the HSFP. The cold-pipe and 

plug systems are currently used to test the controlled mode transition concept and will be replaced by 

engines in later phases of testing. Each mass-flow plug can be linearly moved fore or aft, towards or away 

from cold-pipe close off, to affect the flow path backpressure. For the LSFP, this will simulate the effects 

from the turbine engine. With wind tunnel air flowing supersonic, using the LSFP mass-flow plug to

decrease the cold-pipe exit area will increase backpressure and the static pressure in the subsonic diffuser, 

moving the normal shock upstream. This course of events is similar to the effect of the turbine engine 

decreasing power. The LSFP also contains bypass doors, located at the aft end of the diffuser, which can 

be opened to increase the effective exit area and decrease the diffuser static pressure. A started inlet has 

the mass-flow plug for the LSFP and bypass doors properly positioned such that the normal shock is aft 

of the minimum flow path cross sectional area. The location in the LSFP of the minimum cross sectional 

area is known as the aerodynamic throat and it is located upstream of the diffuser volume. For this work, 

a started and well controlled inlet will prevent the normal shock from moving upstream of the 

aerodynamic throat—an unstarted condition. 

Airflow disturbances from upstream atmospheric pressure changes or a downstream pressure increase 

in the diffuser, resulting from the operation of the turbine engine, may cause the shock to naturally move 

forward of the throat and unstart the inlet. To prevent inlet unstart, the bypass doors can be actively 

controlled to regulate the static pressure in the diffuser. 

When using the diffuser static pressure as a control variable, it is typically normalized with respect to 

free stream total pressure. Let the term defined as the ratio of the diffuser static pressure to the free stream 

total pressure be identified as the diffuser pressure ratio. Maximizing this pressure ratio is the main metric 

of inlet performance. Further opening the bypass doors will continuously decrease the back pressure, and 

pressure ratio, to maintain the normal shock downstream of the throat and keep the inlet started.
NASA/TM—2013-216515 3 

The following are three concerns when operating the bypass doors: First, as the bypass doors open, 

the normal shock will move further downstream which will result in intolerable distortion at the engine 

face. Second, opening of the bypass doors will increase the amount of captured air mass that will be 

vented from the diffuser and back to free stream, essentially increasing the vehicle drag. Third, inlet 

performance increases as the normal shock moves upstream to the aerodynamic throat, so long as it does

not unstart. Therefore, the challenge is to regulate the bypass doors sufficiently to prevent the inlet from 

unstarting while maximizing inlet performance and minimizing the amount of vented air mass from the 

diffuser. 

The use of bypass doors as an actuator to regulate the pressure ratio and maintain the normal shock in 

supersonic inlets has been previously shown to be a successful technique (Refs. 3 to 5). Other work has 

attempted to replace the pressure ratio measurement and use the shock position sensor to provide a more 

direct measurement of the position of the normal shock (Refs. 4 and 5). The shock position sensor 

employs an array of pressure measurements located axially down the inlet path in the area of the 

aerodynamic throat. The location of the normal shock is determined by analyzing the output of the array 

of sensors to detect an abrupt pressure change due to the shock. Previous work (Refs. 4 and 5) showed 

that when using the shock position sensor, the disturbed normal shock movement could be attenuated, but 

only within the resolution of the sensor spacing. 

The goal of this work is to develop control algorithms to actively regulate the bypass doors and 

prevent the inlet from unstarting at the highest possible performance. To reach this goal, potential control 

algorithms will be implemented and studied using a computation simulation tool that was designed and 

developed to simulate this type of propulsion system. The simulation tool is known as the High Mach 

Transient Engine Cycle Code (HiTECC). HiTECC (Ref. 6) is a combined cycle engine (CCE) simulation 

tool featuring turbine and DMSJ propulsion system models, nozzle models, a heat transfer model, fuel 

flow models, hydraulics models, etc. The HiTECC tools consist of the following four building blocks, a 

propulsion system (Ref. 7), a thermal management and fuel system model (Ref. 8), hydraulics and 

kinematics model (Ref. 9), and finally a control system. The original HiTECC model has been modified 

to represent the CCE-LIMX (Ref. 10) as it is configured and mounted in the wind tunnel. The 

modifications included replacing the propulsion system models with a cold pipe and mass-flow plug 

systems and adding a bypass door at the aft end of the diffuser, as was done experimentally.

With the updated HiTECC model, a closed loop control system has been designed to maintain the 

normal shock position and prevent inlet unstart in the presence of external disturbances using the pressure 

ratio as the feedback control variable (Ref. 11). A block diagram of this closed loop control system that 

regulates the bypass door open area is shown in Figure 2. This closed loop control system consists of a 

setpoint function, summing junction, controller, plant, and a feedback path. The setpoint function is used 

to calculate a desired pressure ratio, r. The desired pressure ratio is determined from a calculation using 

free stream conditions and splitter angle as factors. The output of the summing junction is the error signal 

(e) that is a calculated difference between the desired pressure ratio and the plant pressure ratio (y). The 

controller block (K) represents where the control algorithm will reside. The plant (G), made up of the 

bypass door and diffuser dynamics, will calculate the feedback signal (y) that is applied to the summing 

junction. In this system, the error and the desired control action are inversely related. To correct this issue, 

the error signal is multiplied by –1 as shown in Figure 2. If the pressure ratio feedback signal is greater

Figure 2.—Closed loop control system using the pressure ratio as the control variable, with an error (e) signal applied 

to the controller (K), which produces a bypass door open area command (u) applied to the plant (G) to produce the 

desired diffuser pressure ratio output (y). 

Setpoint

Function -1 Controller,K

Bypass 

Door Diffuser

Plant, G

Splitter 

Angle

r e u y
NASA/TM—2013-216515 4 

than the setpoint, then the normal shock is further upstream than intended. Increasing the door command 

will open the bypass doors to decrease the back pressure, ideally returning the feedback signal to the 

desired level.

With some knowledge of the relationship between the pressure ratio and shock position at various 

splitter angles, a setpoint function can be designed to apply values to the summing junction that will keep 

the normal shock safely downstream from the throat. The safe shock position is one that will position the 

normal shock a little further downstream from the throat than what is necessary to insure that expected 

fluctuations in diffuser pressure do not unstart the inlet. The potential range of shock movement upstream 

from the desired shock position that will not result in an unstart is referred to as the operability margin, or 

stability margin. The goal of the closed loop control system, is to increase inlet performance by allowing 

a decreased operability margin without compromising the ability to keep the inlet started. The following 

three control algorithms were designed to be inserted into the controller block in Figure 2 to meet this 

goal: (Ref. 11) a proportional-integral (PI) controller, H-Infinity based mixed synthesis controller, and 

disturbance observer based active disturbance rejection control (ADRC) controller. These closed loop 

controls were computationally demonstrated with the HiTECC tool to be capable of preventing inlet 

unstarts during a mode transition even while pressure disturbances were being added to the feedback 

signal. However, these strategies include two major factors which may devalue the capability of the 

overall closed loop system. The first factor considers the relationship between measured pressure ratio 

and the ensuing calculated shock position. These calculations have a coarse resolution, thus, adding 

uncertainty into the closed loop system. The second factor pertains to the actual closing of the splitter. 

Once activated, the splitter movement is anticipated to close off the LSFP in a continuous negative ramp 

profile. As the splitter closes, the pressure ratio setpoint will follow the splitter position and it will also 

decrease. Therefore, if the splitter angle command follows a ramp input pattern, the setpoint function 

input signal to the closed loop system will also be a ramp pattern. This will result in a steady state error 

during the mode transition while the splitter is moving. 

This paper focuses on modifying the control systems developed in Reference 11 by incorporating a 

shock position sensor described above and utilizing its output to determine values to be applied to the 

controller feedback variable. In this work, two types of feedback will be used; first is a continuous signal 

that measures the position of the normal shock. This is a theoretical sensor with unlimited resolution that 

can easily be implemented in the HiTECC computational simulation. The second is the limited resolution

shock position sensor which can only report the shock position at the first pressure tap location 

downstream of the actual shock location, similar to what has been used in previous work (Refs. 4 and 5). 

Section 2.0 discusses modification of the control system to directly accept the shock position feedback 

signal as the feedback variable. Simulation results and comparison of the same control methods presented 

in Reference 11 are shown in Section 3.0. 

2.0 Shock Position Control With Fixed-Resolution

Shock Position Measurement

With the addition of a shock position sensor being used as the feedback variable for the control 

system of Figure 2, the desired setpoint will be a constant value for all splitter angles. Therefore, the new 

control system is a modified version of that illustrated in Figure 2 with the exclusion of the setpoint 

function. Since the goal is to place the shock in a desired position regardless of the control variable, the 

addition of a shock position sensor should decrease the uncertainty and result in better performance than 

using a related parameter such as the pressure ratio. 

Similar to the approach taken in Reference 11, a linear model of the plant was developed to aid in the 

control design process. The linear models were developed empirically using the HiTECC tool. The 

approach taken was to apply a sine-sweep perturbation input signal, starting from a frequency of 0.01 to 

50 Hz, to the simulated bypass door actuator and populate a database of bypass door open area and shock 

position verses time. A step perturbation signal was also applied to the bypass door actuator with the 
NASA/TM—2013-216515 5 

same amplitude as the sine sweep signal. Next, the sine sweep database was reduced, using the MATLAB

arx command (provided in the MATLAB System Identification Toolbox), to generate linear models of 

the bypass door open area to shock position. This process yielded many linear model candidates with 

varying order. The linear models were tested against the step response data using the MATLAB compare

function to identify an appropriate model to move forward with. This process netted the following single 

input single output transfer function that correlates the bypass door open area as the input signal to shock 

position as the output signal: 

( ) ( )( )

( )( )( 1843007.11904.6712.12 )

4.1000 1664006.50519.56

2

2

++ ++

+ +− =

ssss

sss tf s SL (1)

The subscript SL for the transfer function indicates tfSL is a sensitivity function of the shock location 

measurement to the open area of the bypass doors. 

The tfSL transfer function contains right half plane zeroes. Cancellation of right half zeroes is difficult 

since direct cancellation would require right half plane poles in the controller, which would make the 

controller transfer function unstable. One method to deal with right half plane zeros is to limit the 

controller bandwidth to a frequency below the natural frequency of the right half plane zero (Ref. 12). 

Since the frequency of the right half plane zeroes are quite large, 408 radians per second, limiting the

control bandwidth to less than this may be sufficient to avoid the effect of the right half plane zero. 

Bode plots of the plant transfer function in Equation (1), which uses the shock location as the 

controlled variable, are compared to the plant transfer functions given in Reference 11, which uses the 

pressure ratio as the controlled variable in Figure 3. Assuming that the desired bandwidth and response is 

below 10 radians per second, these plots show that the shock location plant has more gain and a higher 

resonant frequency than using the pressure ratio plant; however, the pressure ratio plant has a higher roll 

off. There is a constant gain offset for all frequencies below 10 radians per second as shown in all of the 

magnitude plots. In addition, if there is little concern for frequencies above 10 radians per second, then 

the difference in roll off is of little to no concern. The only remaining concern is the difference in the 

resonant frequency and the difference is relatively small.

Figure 3.—Bode plot comparing the plant transfer 

function using the shock location (SL) and pressure 

ratio (PR) as the feedback variable with the shock 

located 173 in and 176 in downstream from the tip of 

the inlet system pre-compression forebody plate. 

10-1 100101102

103 -90

0

90

180

270

360

Phase (deg)

Frequency (rad/sec)

-60

-40

-20

0

20

40

Magnitude (dB)

SL 173

SL 176

PR 173

PR 176
NASA/TM—2013-216515 6 

The simulated shock location sensor employed for this paper was designed to be representative of the 

shock position sensor in Reference 4. The shock position sensor is implemented as a series of pressure 

taps aft of the throat. Signal measurements from these sensors were applied to an algorithm that 

calculated the approximate location of the shock, to the nearest downstream pressure tap. The resolution 

of this sensor was limited to the spacing between each pressure tap. However, the HiTECC simulation 

will give an exact location for the normal shock. To simulate this sensor in HiTECC, an additional step 

was necessary to capture the loss in measured position resolution. An additional look-up table was added 

to the feedback path in the HiTECC simulation to output the higher position (downstream) value when 

the normal shock is between two adjacent pressure taps. This implementation approach in HiTECC 

assumed that there is a pressure tap measurement every inch starting 1 in. upstream from the throat. 

3.0 Results and Comparison

This section discusses the implementation of all three closed loop controllers that were designed 

based on the transfer function identified in Equation (1) to represent the plant with a measured shock 

location being the controlled variable. The following three controllers being considered are the same three 

identified in Reference 11: a PI controller, an H-Infinity based mixed synthesis controller, and an active 

disturbance rejection controller (ADRC). A simulation of these controllers was incorporated into the 

modified version of HiTECC, as done in previous work (Refs. 10 and 11). 

For the mode transition simulations, the splitter will start rotating towards LSFP close-off, a negative 

angular rotation from the Mach 4.0 design configuration, 0.5 sec after the simulation starts. The splitter 

will linearly rotate –7.5° in the 0.5 to 2.0 sec simulation window, as shown in Figure 4, which will be 

used for all simulations in this paper. While the splitter is rotating, the mass flow plug will linearly move

0.25 in. towards cold-pipe close-off to simulate the powering down of the turbine. This span is the critical 

period. Beyond –7.5°, the bypass doors will be controlled to a fully opened position to maintain 

supersonic airflow through the throat. This paper assumes fuel flow to the turbine will be terminated, 

which allows the engine to windmill, when the splitter reaches –7.5° and supersonic flow through the 

inlet is acceptable. However, unstarting is still not acceptable. While simulating mode transition with the 

HiTECC tool, commands to change the bypass door open area were added, as shown in Figure 5, to create 

a disturbance in the diffuser sensors. These disturbances were added to the simulation to determine the 

susceptibility of the inlet to unstarts while performing a mode transition. These disturbances involved 

displacements in a bypass door position from fully open to fully closed or vice versa with a frequency 

response of 20 Hz. The bypass door used to provide the disturbance is referred to as the disturbance door. 

Prior to applying the initial disturbance, which was an act of closing the disturbance door, the disturbance 

door was positioned to full open and the exit area of the diffuser was decreased to account for the 

additional open area of the disturbance door. When decreasing the disturbance door open area, the 

diffuser pressure will increase which may lead to an unstart. While increasing the disturbance door open 

0 0.5 1 1.5 2 2.5 3

-8

-6

-4

-2

0

Splitter Angle, degrees

Time, s

0 0.5 1 1.5 2 2.5 3

0

1

2

3

4

Area, sq. in

Time, s

Figure 4.—Splitter command for mode Figure 5.—Disturbance input profile.

transition in the HiTECC simulation.
NASA/TM—2013-216515 7 

Figure 6.—The bypass door area and shock location using a PI controller and a continuous shock location 

measurement (left plot) and with a limited resolution shock location feedback signal (right plots).

area, the diffuser pressure will decrease and the normal shock will move safely away from unstart but 

towards an inlet operating condition of lower performance. As illustrated in Figure 5, the simulation

initiated with the disturbance door open area being set at 3.49 in.2

 and the mass flow plug was positioned

to place the normal shock at a desired location. When the simulation time reached 0.7 sec, the disturbance 

door open area rapidly decreased to zero. Then, at simulation time 1.25 sec, the disturbance door opened 

again to 3.49 in.2

. Finally, at 2.3 sec, the disturbance door was closed again. Throughout the simulation, 

another computational bypass door was used to reject the pressure disturbances in the diffuser created by 

the disturbance door. This door, referred to as the bypass door, received position commands from the 

controller. The control signal, u, regulates the open area of the bypass door. For these simulations, 

moving the disturbance door from full open to full closed, without using an active controller to regulate 

the bypass door, will surely unstart the inlet.

3.1 Proportional-Integral Control

The first control method chosen to implement is the PI controller, where the controller output is: 

( ) ( ) ( ) ∫ += dttektektu ip (2)

where e(t) is the error (r(t)-y(t)), and the scalars kp and ki are the PI controller gains. The PI controller 

gains were set to provide the quickest response time with no overshoot, using trial and error PI tuning 

methods (Ref. 13). For tuning, the continuous feedback measurement (without the limited resolution) was 

used and the same controller was applied with the limited resolution feedback for an additional 

comparison. This method proved to provide good results and decreased the tuning effort. The PI 

controller was chosen to be the baseline for comparison against other controller methodologies because it 

is the most widely used closed loop controller and because of its intuitive tuning process. The response of 

the bypass door open area and the normal shock position while using the PI controller, with gains of 

kp = 1.7 and ki = 28 with a continuous shock position measurement for feedback is shown in the left plots 

of Figure 6. The right plots in Figure 6 are from a similar simulation, except the feedback signal for the 

controller was a discrete shock location signal with a resolution of 1 in. For these simulations, the furthest 

upstream location where the normal shock may reside is at the 161 in. point. If the shock moves upstream 

of the 161 in. point, the inlet will unstart. This unstart threshold is identified in the lower plots in Figure 6 

with a black dotted line. As illustrated in these lower plots in Figure 6, with disturbances applied to the 

diffuser, while using the PI controller the normal shock remained safely away from unstart. For the 
NASA/TM—2013-216515 8 

simulations represented in the left plots, some ringing was observed with the bypass door; however, this 

ringing was considered acceptable. For the results represented in the right plots, high frequency spikes, or 

chatter, was recorded during the mode transition due to the limited resolution of the feedback. The limited

resolution of the feedback resulted in additional integration, which leads to the high frequency chatter 

when the error was small. Furthermore, the right side plots include a steady state error at the end of the 

mode transition due to the limited resolution of the feedback signal. 

3.2 Mixed Synthesis Control

The second closed loop controller considered was an H-Infinity mixed synthesis controller. The 

mixed synthesis controller is an H-Infinity optimal controller that can be designed using the Matlab 

mixsyn command from the Robust Toolbox from MathWorks Inc. The mixsyn command computes a state 

space control law that minimizes the H-Infinity norm for the weighted mixed sensitivity control problem: 





















=

TW

RW

SW

Tyu

3

2

1

(3)

where W1, W2, and W3 are user defined weighting functions. One method for choosing W1 is to have: 

( )

Aws

w

W

B

M B s

+

+

1 = (4)

where M is the value of the weighting function W1 at very high frequencies, A is the value of the 

weighting function W1 at steady state, and wB is the desired bandwidth (10 radians per second). These 

values can be used as a starting point for determining the weighting function to obtain the desired control 

performance. Increasing wB will increase the overall gain of the system and make the response more 

aggressive. The weighting function W2 defines the shape of the control input and can be defined as a filter 

to reduce high frequency spikes from the controller. The weighting function W3 is typically chosen to be 

of small magnitude outside the control bandwidth and large inside the control bandwidth which is 

opposite of W1. 

For this application, after using trial and error tuning methods, the mixed sensitivity weights have 

been tuned to be: 

( ) 016.0

000088.0

16050/

1 2 = 3 = + π

π+ = W W

s

s W (5)

Defining W2 as a scalar allows for very fast actuation, which maybe unachievable in actual hardware. In 

addition, this simulation does not include actuation rate limiters which may allow for high performance 

than can be achieved in the physical system. Applying these values to the mixsyn function resulted in the 

following closed loop controller transfer function:

( )( )

( )( )( 0002765.019.5010322.4 )

12.1204.676.250487

5 ++×+

++ =

s ss

ss KMS (6) 
NASA/TM—2013-216515 9 

Figure 7.—The bypass door area and shock location response to the mixed synthesis controller with a 

continuous shock location feedback signal (left plot) and limited resolution shock location feedback 

signal (right plot).

The response of the inlet using the Mixed Synthesis controller Equation (6) with a continuous shock 

position signal for feedback is shown in the Figure 7 left plots. The difference between the left and right 

plots in Figure 7 is the same difference between the left and right plots in Figure 6. The performance of 

the system when using the feedback signal with the limited resolution, right plots, resulted in a high 

frequency spike response at the start of the mode transition. This spike in the actuator resulted in a 

quicker shock position response and overshoot at approximately 0.6 sec (both directions). Note that the 

minimal open area was 0.0 in.2 and the negative area was clipped to 0.0 before being applied to the 

simulation. Using the mixed synthesis controller, the shock did move closer to the unstart position than 

with the PI controller; hence, the mixed synthesis controller would require a larger operability margin 

than with the PI controller. 

3.3 Active Disturbance Rejection Control

The final controller is a disturbance observer type controller known as the active disturbance rejection 

controller (ADRC) (Refs. 14 to 16). The ADRC approach includes a controller and an observer. Details 

regarding the implementation of the ADRC controller can be found in References 14 to 16. 

The ADRC control configuration being considered in this work includes the following three tuning 

parameters: The DC plant gain, b0, the controller bandwidth, wc, and the observer bandwidth wo. The 

controller bandwidth is tuned to provide the quickest response time with smallest overshoot. Increasing 

the controller bandwidth makes the control system response more aggressive. In most applications, the 

observer bandwidth is set to 10 times the controller bandwidth, but can be tuned manually if needed 

(Refs. 14 and 15). Usually increasing the observer bandwidth increases the disturbance rejection 

capability, but sometimes this bandwidth is needed to be limited to filter out high frequency dynamics. 

Even though the diffuser model of Equation (1) is not a second order plant, the ADRC controller can still 

be implemented. 

The inlet responses with a controlled mode transition using ADRC, with both the continuous and 

discrete shock location measurements, are shown in Figure 8. Through trial and error tuning efforts, the 

gains for the ADRC controller were found to be b0 = 585, wc = 251.33 rad/sec, and wo = 2513 rad/sec. 

The simulation results, when employing the ADRC controller for the process is illustrated in Figure 7. 

The difference between the left and right plots is the same difference as discussed in the previous two 

sections. Again, with the discrete feedback signal, the control bypass door response becomes excessively 

oscillatory.

0 0.5 1 1.5 2 2.5 3 -10

-5

0

5

10

Time, s

Bypass Area, sq in

0 0.5 1 1.5 2 2.5 3 160

165

170

Time, s

Shock Position, in

Feedback

Setpoint

0 0.5 1 1.5 2 2.5 3 -10

-5

0

5

10

Time, s

Bypass Area, sq in

0 0.5 1 1.5 2 2.5 3 160

165

170

Time, s

Shock Position, in

Feedback

Setpoint
NASA/TM—2013-216515 10

Figure 8.—The bypass door area and shock location response to the Active Disturbance Rejection Control 

(ADRC) controller for a continuous shock location feedback signal (left plot) and limited resolution 

feedback signal (right plot).

3.4 Controller Comparison

Three different closed loop controllers have been designed to control the shock position using a 

continuous shock position feedback signal and a limited resolution shock position feedback signal. There 

are three closed loop controllers compared in this section: a PI controller, subsection 3.1 and results 

shown in Figure 6; a mixed synthesis controller, subsection 3.2 and results shown in Figure 7; and 

ADRC, subsection 3.3 and results shown in Figure 8. Comparing the results with the continuous feedback 

signal, left hand plots in Figures 6 to 8, the ADRC controller was able to keep the shock furthest 

downstream of the throat, just slightly better than the PI controller. The PI controller however was able to 

recover better from the disturbance and return the shock to the setpoint (167 in.) when the disturbance 

door was closed between a simulation time of 0.7 and 1.2 sec. The mixed synthesis controller, which 

performed the worst in terms of disturbance rejection and recovery, had the smoothest control signal and 

least amount of ringing in the control signal. All three controllers were capable of eliminating steady state 

error during mode transition; something that could not be done in the previous work with HiTECC 

(Ref. 11). 

The results are also compared using the simulation results with the limited resolution feedback signal 

(1 in.), shown in the right half plots of Figures 6 to 8. With the limited resolution feedback signal, the PI 

controller clearly provided the best results in terms of disturbance rejection. The mixed synthesis 

controller and ADRC controllers allowed the shock to move upstream of the 165 point mark and much 

closer to inlet unstart compared to the PI controller. By limiting the resolution of the feedback, the 

response of all three systems becomes slightly oscillatory with the ADRC controller being considerably 

the worst.

This paper shows that with a continuous shock position feedback signal, a controller can be designed 

using a bypass gate to prevent inlet unstart. The shock position can be more accurately regulated using the 

continuous shock position feedback signal than what was capable with the previous techniques that used a 

pressure ratio signal as the feedback signal. However the ability to prevent unstart and disturbance 

rejection capability degrade as the resolution of the feedback signal decreases. Limiting the resolution of 

the feedback signal increases the uncertainty and weakens the advantage of this method over previous 

work using a pressure ratio feedback signal, especially when considering the amount of sensors required 

to obtain a limited resolution feedback signal. With a pressure tap at every inch starting at the throat and 

with a resolution of 1 in., 10 sensors would be required for a diffuser (and throat) of 10 in., whereas using 

the pressure ratio would only require 1 sensor. 

0 0.5 1 1.5 2 2.5 3 -2

0

2

4

6

Time, s

Bypass Area, sq in

0 0.5 1 1.5 2 2.5 3 160

165

170

Time, s

Shock Position, in

Feedback

Setpoint

0 0.5 1 1.5 2 2.5 3 -5

0

5

10

Time, s

Bypass Area, sq in

0 0.5 1 1.5 2 2.5 3 160

165

170

Time, s

Shock Position, in

Feedback

Setpoint
NASA/TM—2013-216515 11

4.0 Conclusion

This paper provides alternative closed loop controller designs to prevent an inlet from unstarting 

during mode transition for Turbine Based Combined Cycle (TBCC) propulsion systems. These controllers 

employed both shock position measurement sensors and shock position estimator sensors as the controller 

feedback. All three closed loop controllers were implemented in the High Mach Transient Engine Cycle 

Code (HiTECC) simulation designed to simulate the combined cycle engine large-scale inlet for mode 

transition experiments (CCE-LIMX). Previous work demonstrated that with an approximate (estimated)

relationship between shock position location and diffuser pressure ratio, a closed loop system can be 

designed to regulate shock position location by actually controlling the diffuser pressure ratio. In this 

paper, the control system is modified to use simulated direct measurement of shock position location via a 

theoretical, high-resolution, sensor as well as a limited-resolution sensor that utilizes multiple pressure 

sensors located axially in the inlet throat. Using the ideal (high resolution) shock location measurement all 

three control systems eliminated the steady state error during mode transition. Additionally, all three 

proposed controllers, Proportional-Integral, H-Infinity based Mixed Synthesis Controller, and the Active 

Disturbance Rejection Controller were able to prevent inlet unstart even in the presence of large pressure 

disturbances. However, when the feedback signal was modified to a limited resolution, the performance 

of the three controllers decreased, mainly due to high frequency oscillations in the feedback signal. 

Although a determination of the lower bound of resolution for direct shock position measurement was not 

achieved this report clearly indicates that having a sensor that directly measures shock position can result 

in better system performance, i.e., higher pressure ratio and a tighter shock position control margin. 

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NASA/TM—2013-216515 12

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