A Master-slave Approach to Aircraft Engine Bleed Flow Sharing Control50

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1100 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 13, NO. 6, NOVEMBER 2005

A Master–Slave Approach to Aircraft Engine Bleed Flow Sharing Control

Guangjun Liu, Guozhong Bao, Chun Ho Lam, and Jin Jiang

Abstract—A master–slave control strategy is developed for flow

sharing control of multiple engine systems, applicable to aircraftand industrial plants. Under the proposed control strategy, one of airflow channels is designated as the master control channel, andits pressure is regulated using pressure sensor measurement. Theremaining airflow channels are treated as the slave channels. Theair mass flow is also measured in the master channel, and the air-flow sensor output of the master channel is utilized to slave theother channels, which are flow-controlled. The proposed strategyavoids simultaneous pressure control of multiple channels and con-trols the flow sharing error directly with a feedback loop. The the-oretical aspect of the proposed scheme has been analyzed using therelative gain array (RGA) technique, and the effectiveness of theproposed control method has been confirmed by results of anal-ysis, computer simulations and experiments.

Index Terms—Airflow sharing control, control systems, engines,feedback, master–slave control.

I. INTRODUCTION

THE development of turbine engines has led not only to

advances in thrust generation, but also to the evolution

of aircraft pneumatic system design, especially for transport

type aircraft. A modern turbine engine is an excellent source of

high-pressure and high-temperature air that can be “bled” from

the compressors, and then be used for various purposes on the

aircraft. These include, for example, air-conditioning/heating,

wing and engine anti-ice protection, as well as windscreen

demisting and rain dispersal. Most of commercial and military

aircraft utilizing turbine engine propulsion units are powered by

two or more turbine engines. It has been recognized for some

time that in order to operate a multiengine aircraft more effi-

ciently, it is desirable to extract bleed air from all of the engines

equally, especially when advanced high performance engines

are installed on the aircraft. Bleed flow sharing among multiple

engines has been attempted in the past based on pressure

control of each channel, but with limited success at relatively

low steady-state accuracy and slow dynamic responses due

to strong dynamic coupling among the channels. The overall

system accuracy and efficiency have to be sacrificed to maintain

acceptable stability margin of the control system. Bruun [1]

Manuscript received August 31, 2004; revised May 3, 2005. Manuscriptreceived in final form July 25, 2005. Recommended by Associate Ed-itor A. Stefanopoulou. This work was supported by the Natural Sciences andEngineering Research Council of Canada (NSERC) and Honeywell Enginesand Systems.

G. Liu and G. Bao are with the Department of Aerospace Engineering,Ryerson University,Toronto, ON M5B 2K3 Canada (e-mail: [email protected]).

C. H. Lam is with the Honeywell Engines & Systems, Mississauga, ON L5L3S6 Canada.

J. Jiang is with the Department of Electrical and Computer Engineering,University of Western Ontario, London, ON N6A 5B9 Canada (e-mail:

[email protected]).Digital Object Identifier 10.1109/TCST.2005.857406

discloses a bleed airflow sharing technique for a two engine

system that uses a venturi and a pressure sensor to estimate the

bleed flow in each engine bleed flow path. The difference in

the two flow signals is then conditioned to drive the pressure

regulator in each engine bleed flow path. Another two US

patents [2] issued to Benson disclose a bleed flow control

method for each engine using a pressure regulator upstream of

a heat exchanger (HX). Since the bleed air pressure drop across

the HX is a function of the flow rate, the pressure drop is used

as the feedback signal to control the flow rate. Despite all these

efforts, balancing bleed flow extraction in conventional systems

has not been entirely satisfactory. The engine that is required to

supply more bleed air will tend to have higher stress and may

have to be overhauled or replaced sooner.

In this brief, a master–slave control strategy is proposed,

mathematically analyzed and experimentally tested for engine

bleed flow sharing control system design. In the proposed

strategy, one of the channels is selected as the master channel,

and the pressure at the inlet of the downstream systems re-

ceiving the bleed air is controlled to achieve a desirable inlet

pressure range. To slave the remaining airflow control channels,

the mass flow rate of the master channel is also measured, and

this measured airflow rate is then utilized as the airflow set point

for the slave channels, which are flow-controlled. In addition,

this control strategy enables the flow to be equalized for each

engine without the need of the knowledge on the total flow

demand from the downstream systems where the bleed airflow

is used. Hence, this control approach is self-contained and

can work independently of the environmental control systems

(ECS) or other load demands and controllers, which is often

an essential requirement in aircraft systems development. The

master–slave control strategy can be tailored for a two, three,

four or more engine bleed air systems (BAS) [3]. The analysis

based on relative gain array (RGA), numerical simulation and

test results have confirmed the effectiveness of the proposed

method.

The concept of master–slave control strategy has been

utilized in various areas, such as power electronics. Hur and

Nam [4] proposed a robust load-sharing control scheme for

parallel-connected pulsewidth modulation converters and a

speed and tension control for a bridle roll system in a steel

mill. Rajagopalan et al. [5] developed a master–slave current

sharing control strategy and applied it to a paralleled dc/dc

converter system where an explicit current sharing mechanism

is required to ensure proper operation. More applications of the

master–slave control concept can be found in [6]–[8].

In the proposed master–slave control system architecture, the

flow sharing is directly controlled, rather than indirectly by con-

trolling the pressure errors in all channels, which is a more

1063-6536/$20.00 © 2005 IEEE



IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 13, NO. 6, NOVEMBER 2005 1101

Fig. 1. Illustrative diagram of a two-engine BAS.

stringent control problem. In this sense, the proposed control

strategy is conceptually similar to the cross-coupling control of

coordinated contour following in machining operations [9], [10]

and more recently for mobile robot control [11], where the most

significant error is directly controlled to improve accuracy and

robustness over individual drive loop control.

The rest of the brief is organized as follows. In Section II,

an engine bleed flow sharing system is described, and the dy-

namic model is presented. The proposed control method and

analysis are presented in Section III. The proposed method has

been examined by simulation and experiments, and the results

are presented in Section IV and Section V. Concluding remarks

are given in Section VI.

II. ENGINE BLEED FLOW SHARING SYSTEM

AND ITS DYNAMIC MODEL

A. Engine Bleed Flow Sharing System

A simplified illustrative diagram of a two-engine BAS underconsideration is shown in Fig. 1. To regulate the engine bleed

pressure, and to provide over temperature and over pressure pro-

tection, the bleed air from each engine passes through a pressure

regulator and shutoff valve first. The valve regulates the down-

stream pressure before the bleed air passes through a precooler

HX. In this work, we assume that the channels are of identical

physical construction, and each channel is instrumented with a

pressure transducer and a flow sensor downstream the HX. The

bleed air flows with lower pressures and temperatures from both

channels are merged into one stream to feed the downstream

ECS or other pneumatic loads.

In Fig. 1, , and denote flow rate in lb/min, absolute

pressure in psia and temperature in degree Rankine (Fahren-

heit), respectively. represents volume in in . , and

are pneumatic pressure regulators with inner feedback loops.

and are primary pressure regulating valves.

B. Linearized Model of the Two-Engine BAS

For the example of the two-engine transport airplane BAS

under consideration, a transfer function block diagram is de-

rived from linearized component models and is shown in Fig. 2.

The systeminputs and are the inputsto the pressurereg-

ulators and , as indicated in Fig. 1, and , , and

are the valve angular displacements and valve opening areasof pressure regulators and , respectively. The transfer

Fig. 2. Block diagram representation of the two-engine BAS.

TABLE IPARAMETER VALUES OF THE TRANSFER FUNCTIONS IN FIG. 2

function representations of the system in Fig. 2 have the fol-

lowing forms:

where is the universal gas constant, 639.6 in .

For the system under consideration, the time constants of

the pressure regulators are 0.1 s and 0.5 s. The

system parameters are 3600 in , in ,

, pipe diameters , .

The steady-state operating conditions are: ,

, ,

, ,

. With a mass flow rate of 150 lb/min

for each channel, the parameters in Fig. 2 are calculated and as

shown in Table I.

C. State–Space Model

By defining the following state variables.

Channel #1 pressure .


State variable from , no particular physical

meaning.channel #2 mass airflow rate .





State variable from , no particular physical

meaning.

Node pressure .

The following state–space representation is obtained from the

transfer function block diagram of Fig. 2:

(1)

where the state vector

and the system matrices, shown in the equation at bottom of the

page. The control input vector is

The system output vector for the conventional pressure con-

trol is

(2)

where the output matrix

The system output vector for the master–slave control is

(3)

where the output matrix

The eigenvalues of the open-loop system matrix are

, , , , ,

, and . Since all the eigenvalues have neg-ative real parts, the open-loop system is stable.

III. CONTROL SYSTEM DESIGN

The objective of the control system for the engine bleed

flow sharing control is to maintain the pressures of all channels

within a predefined range while equalizing the bleed air flow

among all channels.

Fig. 3. Master–slave control strategy for an engine bleed flow sharing system.

A conventional control strategy for this two-channel system

is to control the pressures of both channels by two separate

single-loop controllers, which determine the driving signals of both pressure regulating valves in order to maintain the pres-

sures and within a prespecified range. With such a con-

trol strategy, and have to be maintained at almost the

same magnitude all the time in order to achieve flow sharing.

Since the pneumatic impedances in the system are small, a small

disturbance in the engine bleed pressure could cause a large flow

difference between the two channels.

The proposed master–slave control strategy is illustrated

in Fig. 3. With the proposed master–slave engine bleed flow

sharing control architecture, any one of the bleed air channels

can be chosen as the master channel, and all the remaining

channels are treated as slave channels. As shown in Fig. 3,

the master channel is pressure controlled, i.e., the pressure at

the inlet of the downstream system receiving the bleed air is

controlled to be within a desirable inlet pressure range. The

pressure reference of the master channel is set to the same value

as that in the conventional pressure control and is determined

by the system design requirements. The mass flow rate of the

master channel is also measured, which then serves as the set

point for the flow-controlled slave channels. The master–slave




control strategy avoids simultaneous pressure control of mul-

tiple channels and directly controls the flow sharing error with

a feedback loop. In addition, since the set point for the flow

rate of the slave channels is automatically determined by the

master channel flow rate, which is governed by the downstream

loads, this control strategy enables the flow balance without

requiring the knowledge of the total flow demand from thesystem where the bleed airflow is used. Hence, this control

approach is self-contained and can work independently of the

load demands and any other downstream controllers.

Compared with independent pressure control, the proposed

master–slave control strategy turns the slave channels from pres-

sure control to flow control. The flow sharing is directly con-

trolled, rather than indirectly by controlling the pressure in both

channels. In order to implement such a flow sharing strategy, the

flow sensor measurement of the master channel is used as the

flow control command signal for the slave channels. Thus, the

slave channel controllers do not directly respond to the pressure

changes in the slave channels but only after the master channel

reacts and provides a flow reference signal to the slave chan-

nels. Note that the pressure and the flow in the master channel

are still affected by the responses from the slave channels due

to the pneumatic coupling through the channel connections.

The proposed master–slave control strategy maintains a de-

centralized architecture as in the independent pressure control

strategy, which is often preferred for simplicity and ease of fault

isolation for aircraft systems control. Engine BAS protection is a

safety critical issue as high temperature and high pressure bleed

air leakage can cause catastrophic damage if unchecked. Hence,

sensors are normally installed for bleed leak detection, which

can then trigger the shutoff of engine bleed supply if a leak is

detected.The master–slave architecture, as compared with conven-

tional independent pressure control, requires flow sensors, such

as the commonly used thermal mass flow sensor in aircraft

ECS. For the master channel, both flow sensor and pressure

sensor are required. For the slave channels, even though only

flow sensor is required in normal operations, it is highly de-

sirable to install both flow sensor and pressure sensor so that

any channel can serve as a master channel following a system

reconfiguration.

For the two-engine BAS model described in Section II, and

without any loss of generality, channel #1 is designated as the

master channel, and channel #2 is therefore the slave channel.Assuming that the proportional controllers and are used

in the master and the slave channels, respectively, and the con-

troller output signals and are determined as

(4)

(5)

In the state feedback form, (4) and (5) can be written as

(6)

where is the state feedback gain matrix.

Fig. 4. Trajectory of the dominant pole as the controller gain increases.

For the master–slave control strategy, the state feedback gain

matrix is

(7)

In the conventional pressure control strategy, the controller

output signals and can be written as

(8)

(9)

The corresponding state feedback gain matrix would be

(10)

Since the master channel consists of a single pressure control

loop in both strategies, this controller can be tuned indepen-dently first. Assuming that the gain of the master controller is

set to unity, the influence of the magnitude of the slave channel

control gain on the system performance can be examined.

Fig. 4 shows the trajectory of the dominant pole of the

closed-loop system as the slave channel control gain

is increased. It can be seen that the dominant pole of the

closed-loop system moves away from the origin as increases

in the master–slave control strategy. While in the conventional

pressure control strategy, the location of the dominant pole of

the closed-loop system stays close to the origin despite of the

control gain changes. This indicates that the dynamic response

of the closed-loop system will be faster in the master–slave

control than that in the conventional pressure control when a

higher controller gain is selected.

To investigate the loop interactions further, the analysis is car-

ried out using the RGA technique for the conventional pressure

control and the master–slave control strategies. From the system

model matrices , and , the steady-state gain matrix

(SSGM) for the pressure control is obtained as

SSGM

and for the master–slave control SSGM is obtained using ,

and as

SSGM




Fig. 5. Step change of 10 psi in the master channel pressure reference (PIcontrol).

Then the RGA for the pressure control is calculated as

RGA

and for the master–slave control

RGA

The pressure control RGA matrix shows significant steady-

state interactions between the two channels. Since only the diag-

onal elements are positive, the preferred control pairs are using

to control and to control . The RGA matrix di-

agonal element value is 18.335 (about 25.3 db changes), indi-

cating that the pressure control open-loop gain is small when

the other pressure control loop is closed. Consequently, it is

tempting to use a large control gain to achieve pressure con-

trol. However, if one of the pressure control loops is either open

or fails, the chosen control channel gain could be too high and

cause stability problem. Hence, the control gains of both pres-

sure channels need to be de-tuned to achieve better control in-tegrity just in case of some component failures. Also, the sum

Fig. 6. Step change of 10 psi in the engine #1 bleed pressure (PI control).

of the absolute value of these elements is large (71.34), sug-

gesting that the pressure control system is likely to be ill con-

ditioned. All these indicate that it is more complicated to apply

single-input–single-output (SISO) design techniques to achieve

desirable dynamic performance and multi-input–multi-output

(MIMO) control is likely required to achieve satisfactory pres-

sure control performance.

For the master–slave approach, all the RGA matrix elements

are about 0.5. In this case, the open-loop gain characteristics

of each control channel increases by about 100% (about 6 db

change) when the other control channel is closed. The control

interaction is therefore significantly less when compared with

the aforementioned pressure control approach. Also, the sum

of the absolute value of all RGA elements is 2, suggesting that

the master slave control system is not ill conditioned, and the

decentralized SISO control approach for the pressure and flow

difference is possible.

IV. SIMULATION RESULTS

Simulations are performed using Matlab/Simulink to inves-

tigate the effectiveness of the proposed master–slave controlstrategy based on the state–space model of the two-engine BAS.




Fig. 7. Test rig for flow sharing control studies.

TABLE II

MAIN COMPONENTS IN THE TEST RIG

When proportional only control is used, there exists steady-state

error in both the master channel pressure control and the flow

sharing control, which can be removed when an integral con-

trol is introduced. Figs. 5 and 6 show the system step responses.

The PI controller parameters are set to for the master

channel controller and for the slave channel con-

troller, and the integral time is set to 0.5 s for both controllers.

The sensor dynamics has been ignored in the simulations.

A quick dip in the initial flow response of the slave channelhas been observed from the results of the simulation. To confirm

that this is a nonminimum phase (NMP) behavior by analysis,

the zeros of the transfer function from the pressure reference

of the master channel to the flow rate of the slave channel are

obtained using the system model in Section II-C as

Zeros

There is a pair of complex zeros with a positive real part in thetransfer function, which causes the NMP behavior in the system.

Fig. 8. PI control experiment: step change of 5 psi in the master channel

pressure reference.

V. EXPERIMENTAL RESULTS

The proposed master–slave flow sharing control strategy is

also tested experimentally in laboratory settings. The test rig

developed in our laboratory is shown in Fig. 7, which consists

of four parallel channels. In addition to an electrically actuated

control valve, each channel is equipped with a thermal mass flow

sensor, two pressure transducers, and several manual valves for

reconfiguring the system.

A compressor charges two high-pressure tanks to a maximum

pressure of 100 psig. An electrical control valve is installed be-

tween the tanks and the test rig to control the air pressure at the

inlet of the test rig. The inlet air pressure can be kept between

20–30 psig for approximately 400 s when the mass flow rate is

controlled at about 10 lb/min. Table II lists the main components

of the test rig.

Fig. 8 shows the experimental results of the master–slave

control, where a PI controller is used for both the master and

slave channels. The controller parameters are set to ,

2 s for the master channel controller and ,

10 s for the slave channel controller.

The saw-teeth shape in the pressure responses and flow re-sponses are due to the upstream pressure fluctuation, caused




mainly by the control valve sensitivity. The slow pressure and

flow responses are partly due to the slow flow sensor dynamics,

which has a time constant of about 10 s. However, from the

experimental results, it can be seen that the mass flow rate of

the slave channel follows that of the master channel satisfacto-

rily. In addition, although not being controlled directly, the slave

channel pressure is approximately the same as that of the masterchannel. When PI controllers are used, the steady-state errors

are nearly eliminated in both the pressures and flow rates, and

excellent flow sharing control has been achieved.

NMP behavior is also noticed in the flow response of the slave

channel as shown in Fig. 8, which is consistent with the analysis

and simulation results for the aircraft BAS model in Section IV.

VI. CONCLUDING REMARKS

In this brief, a master–slave control strategy for multi-engine

BAS flow sharing control is proposed. Based on a linearized

state–space model of a two-engine BAS, analysis and simula-

tion results have confirmed that the proposed control scheme canreduce the interaction effect among the channels in comparison

with the conventional individual pressure control. Experimental

results with an airflow sharing control test rig have also demon-

strated the effectiveness of the proposed master–slave control

strategy.

REFERENCES

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no. 5 155991, 1991.

[2] P. A. Benson, “Aircraft Engine Bleed Air Flow Balancing Technique,”US Patent Nos. 4 765131 and 4 765131, 1988.

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A Master-slave Approach to Aircraft Engine Bleed Flow Sharing Control50