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
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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 .
Channel #1 pressure .
State variable from , no particular physical
meaning.channel #2 mass airflow rate .
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Channel #2 pressure .
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
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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
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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.
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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
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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.
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