A.J. Keane, S.J. Elliott, M.J. Brennan and E. Rogers
Computational Engineering & Design Centre, Faculty of Engineering and Applied Science,
University of Southampton, Highfield, Southampton, SO17 1BJ, U.K.
RESEARCH TEAM
This work will be supervised by Prof. A.J. Keane who has considerable expertise
in the field of structural dynamics having worked on Statistical Energy Analysis
methods for many years[1]. He has also worked on optimization since
completing his M.Sc. in this field and has studied design related problems for
nearly 20 years. This has included further studies in the concept design of
warships[2], the use of modern stochastic design methods in structural design[3],
as well as studies of the fundamentals under-pinning these methods[4]. As a
result of this work a large package of software[5] has been developed and is now
in use with a number of UK companies. Work in this area has been funded by
organizations as diverse as the Defence Research Agency, Cable and Wireless,
Glaxo Wellcome, British Aerospace and the EPSRC (grants GR/J06856 for £117k
and GR/L04733 for £215k). Based on this experience, model satellite booms
have been designed using SEA and design optimization methods and then built
and tested, showing the same dramatically improved vibration isolation predicted
by theory to be present in practice. The present project therefore represents a
logical extension to this work drawing on the expertise of other staff at
Southampton University.
The work will take place in the Computational Engineering and Design Centre
(CEDC) at Southampton, which is a faculty-wide activity, and the facilities of the
Signal Processing and Control Group of the Institute of Sound and Vibration
Research. The CEDC, which Prof. Keane directs, is dedicated to investigating
the uses of high performance computing (HPC) in engineering and has access to
several HPC facilities that have been recently installed at Southampton. Other
projects within the centre are focusing on CFD, FE, control theory and related
areas and will be able to give significant support to the project. The Signal
Processing and Control Group houses experimental facilities dedicated to the
study of active noise control systems and is led by Prof. S.J. Elliott. Other
projects within the group deal with the active control of vibration transmission in
helicopter struts, funded by the EC in collaboration with Westland Helicopters,
the use of distributed sensors and actuators for active vibration control of
structures and the active control of sound radiation funded by the DTI, and the
control algorithms and actuator technologies required for active mounts, funded
by the EC and DRA Farnborough. Recent work on adaptive controllers is being
funded by EPSRC (grants GR/K11857 for £104k and GR/L62979 for £161k).
The experimental aspects of the study will be carried out under the supervision of
Prof. S. J. Elliott and Dr. M. J. Brennan. Prof. Elliott's main research interests lie
in the interaction between physical systems and the signal processing or control
methods used to analyse or control them[6]. Of particular interest is the active
control of sound and vibration, and research in this area has resulted in full-scale
demonstrations of active control systems on aircraft, cars and helicopters[7]. This
has involved the real-time implementation of various adaptive control algorithms,
as well as an analytic evaluation of their potential performance at the error
sensors and of the effect this will have at other points on the distributed system
under control[8]. Dr. Brennan has extensive experience of the practical
implementation and testing of active vibration control systems. He has been
particularly involved in the design and evaluation of novel sensors and actuators
for active control systems in pipes, plates and struts using both piezoelectric and
magnetostrictive technology[9, 10].
Assistance on the theoretical active control aspects of this project be provided by
Dr E. Rogers. Dr. Rogers is Reader in Control Systems and a member of the ISIS
group in the Department of Electronics and Computer Science. His major
research interests lie in the general area of control systems theory and
design[11, 12]. He is the author/co-author of 3 research monographs and more
than 120 refereed papers, the editor of the International Journal of Control, and
joint editor of the Taylor and Francis research book series in Systems and Control.
COLLABORATORS
The project will take place in collaboration with staff from various divisions of
Matra Marconi Space plc. Matra Marconi Space (MMS) is a joint venture
company 51% owned by Matra Hachette of France and 49% owned by GEC of
the United Kingdom. MMS combines the complementary expertise of three
former companies, each having over 25 years of experience on major space
programmes:
(1) Matra Espace, now MMS-France,
(2) Marconi Space Systems, MMS-UK since 1991
(3) British Aerospace Space Systems Ltd., part of MMS-UK since 1994
Matra Marconi Space is a major international space company and is Europe's
largest satellite manufacturer. Since the early 1960's the company has been a
major contributor to missions aimed at furthering Man's knowledge and
understanding of the Earth, the Solar System and the Universe. The companies
spacecraft engineering achievements span more than 50 national, European, and
international projects and has contributed significantly to the activities of
agencies such as ESA, NASA, CNES and EUMETSAT.
MMS operates as a number of directorates each specialising in a particular aspect
of the space business. The project will take place in collaboration with staff from
the Directorate of Science and Radar (DSR) at Bristol and Portsmouth. The work
will also involve consultation with staff from the equivalent directorate in MMS-
France.
MMS staff will supply data on the structural requirements for the systems being
studied as well as typical noise levels and spectra and other launch and flight
data. They will attend progress meetings and evaluate the work being carried out
and the software being developed.
BACKGROUND
Introduction
Vibration and noise control problems arise in many engineering projects. These
problems are most severe when light weight structures, such as those found in the
automobile and aerospace sectors, are used. Perhaps the most challenging
vibration control issues arise in the design of space missions that involve
satellites with highly sensitive instrumentation packages. To function correctly
these packages must be supported on structures where the vibration levels have
been reduced to extremely low levels (micro vibrations). This need becomes most
severe when the instruments concerned form the individual sensors of a multi
sensor interferometric telescope or synthetic aperture radar. In such cases there is
a need to support instruments spaced tens of metres apart using structural booms,
with the relative motions between their ends being restricted to microns over
wide ranges of excitation frequency[13]. A number of design approaches have
been proposed to try to meet these demanding requirements but it is still not clear
how best to proceed in this field[14]. The current project is concerned with the
development of a vibration control system for use where there are demanding
noise and vibration control targets. It is based on a novel design approach
combining several existing methods which it is believed will provide a real
breakthrough for problems of this class. Essentially, the idea is to combine active
vibration control using robust control techniques, drawn from the work of the
Control community, together with booms geometrically optimized for passive
vibration isolation, based on methods developed by the Structural Dynamics
community; the whole being designed using evolutionary algorithms, coming
from the Computer Science community. This process will be carried out so as to
allow for the dynamics of the active system when carrying out the passive
isolation optimization. The combined evolutionary design process will be carried
out off-line to locate sensors & actuators and to define the geometric
configuration needed for passive noise rejection. Following construction of the
system, the models used during the design phase will then form the basis for the
H2/Hinf (or related) on-line robust active control system.
Of course, mixed active/passive noise control is not a new idea[15] nor is the use
of evolutionary methods in control system design[16]. We believe, however, that
there are a number of new ideas in this project. First, we do not know of any
other work mixing all three of these ideas together in one system. Secondly, the
kind of passive vibration control to be used is based on reflecting rather than
absorbing energy; this is an idea that is very much in its infancy and one that was
originally proposed by those supervising this research. Thirdly, we aim to use the
two different noise control methods over different, but adjacent, ranges of the
frequency spectrum. Lastly, we intend to build a working prototype system
whereas almost all previous work in this area has been essentially computational
in nature. The project is thus highly inter-disciplinary in nature and much of its
novelty stems from its drawing on the results of three separate research
communities to design and build a complex but practical, integrated engineering
system.
Active noise control methods are now well established but their high frequency
capabilities are fundamentally limited by the signal processing, actuator and
sensor requirements of such systems[17]. Passive methods using geometric
optimization are less widely used but, with increasing computational power, they
are of growing interest and, moreover, are directly applicable to frequencies
above those amenable to active control[18]. (Work in both these areas has
recently been funded by the EPSRC and this project seeks to build on and exploit
the successes of that work.) The combination of these methods should allow a
wide spectrum of noise control problems to be dealt with without the weight and
bulk penalties associated with traditional added damping approaches for
problems of this kind. It is, of course, difficult to make exact predictions of the
likely vibration reductions that such an approach will give. However, earlier
work on model scales suggest that 10-20dB reduction in vibrational energy
transmission should be expected over a 200 Hz bandwidth on a structure some 3-
5m long.
Although the investigation described in this project is based on satellite
structures, progress in the general area of integrated active / passive vibration
control will be of major benefit in all those fields where reduced noise
transmission is important, such as in aircraft and car design. Work in this area is
also in keeping with UK and EU goals in terms of improving quality of life and
reducing pollution (lighter vehicles consume less fuel and pollute less). A
vibration isolation capability of this type will therefore be of very great benefit to
the UK car, aerospace and marine construction industries where lighter-weight
and cheaper designs could be produced with improved noise performance,
leading to increased prospects for sales.
Scientific/Technological Relevance
Structure borne noise and vibration control is an aspect of design where there are
few design synthesis techniques available and one that is relevant to almost all
lightweight engineering structures. This is an area where many traditional
techniques have been tried with relatively little success. Moreover, structures
such as satellites, cars, aeroplanes, ships, etc., all suffer from exposure to noise
and vibration sources. These sources often excite unwanted structural vibrations
which can cause damage or the transport of vibrational energy to distant parts of
the structure where they cannot be tolerated. For example, the reaction wheels
and cryogenic coolers in satellites always vibrate to some degree and, despite
isolation treatments, excite motions of their mounting points. Since most space
structures have inherently low damping characteristics such motions may well be
relatively large. This vibrational energy can then flow through the structure and
cause significant motions of the mountings points of sensitive instruments. These
in turn fail to meet their design specifications resulting in failure or reduced value
of the mission. The most common treatment for such problems is to use anti-
vibration mountings or to coat the structural elements with heavy viscoelastic
damping materials with consequent weight and cost penalties. Moreover, the
effectiveness of such treatments diminishes with the vibration levels which makes
continuously improving noise and vibration targets difficult to meet. Clearly, if
the vibrational energy could be contained near to the points of excitation there
would be a reduced need for damping treatments and, additionally, they could be
concentrated in regions where they were most effective. This is precisely the aim
of the vibration isolators used between most pieces of equipment and their
supporting structure. However, such isolators cannot deliver the desired
behaviour in all situations, particularly for sensitive equipment. The upshot of
this problem is the need for some kind of widely applicable, generic structural
filter design capability that can be used to build desirable characteristics into a
structure, retaining its ability to carry static loads while blocking higher
frequency motions. To gain maximum benefit from the available technologies
such a capability would ideally be based on a integrated active / passive
approach, with these two techniques being used in tandem and together tackling
the widest possible range of excitation frequencies.
Analysis of the vibrational energy flows around complex structures is dominated
by the many resonances exhibited by such structures and also the large number of
physical parameters needed to specify typical structural designs (many thousands
in a full satellite structural model). Moreover, experience shows that even the
most detailed finite element models do not accurately predict the behaviour of
real structures over wide frequency ranges (because of the inevitable differences
between the structure modelled and that actually built). This means that the
approaches used when choosing design configurations must be robust enough to
tolerate such mis-matches. That to be used for the passive optimization aspect of
the work is based on statistical energy analysis models combined with genetic
algorithm optimization methods. This is an approach that has already been
shown to work in purely passive approaches to noise isolation[19]. In this
approach, prediction is aimed at the frequency and space averaged energy levels
of the structure at one end for given excitation levels at the other. The method
does not, however, work effectively at very low frequencies because the method
relies on the wavelengths of the vibrations being controlled being of a similar
order to the changes introduced in the geometry of the structure. Fortunately, this
is a range of frequencies ideally suited to active vibration control and this is why
a mixed active / passive approach is thought to be so potentially beneficial.
Active control holds great promise for the reduction of vibration caused by the
relatively low order modes of a structure because the volume and mass required
for conventional passive vibration control of these modes would be very high.
Moreover, robust active control has been the subject of enormous research effort
over the last 10-15 years, the result of which has been the in-depth development
of a range of controller analysis / design methods based on a nominal linear
model approximation to the system dynamics, e.g., Hinf, mixed H2/Hinf and loop
transfer recovery approaches[20]. These (and other approaches) are now very
mature areas in systems theory terms and increasing effort is now being directed
towards applications. This work is supported by the widespread availability of
Matlab compatible software with the onward ability to generate executable C
code for software based implementation of the resulting controller(s). Such
control schemes require a feedback strategy whose stability and performance are
relatively insensitive to typical changes in the response of the structure and
vibration environment. Various actuators and sensors have been designed for this
application, including piezoceramic stack actuators[9], reaction-mass
actuators[21], active struts[13] as actuators and accelerometers, strain gauges and
rate gyros as sensors. The design of the feedback control loops which connect
these actuators and sensors requires a knowledge of the dynamics of the structure
to provide effective active control. Ideally the gain of these feedback loops
would be large, to provide high attenuation of vibration disturbances, but the
stability of these loops is then very sensitive to small changes in the response of
the structure. These may be caused by geometric distortion, heating or, in space
missions, the change from 1g conditions on the ground to 0g in space[22]. To
ensure that the stability of the controller is robust to these changes, a good model
of the uncertainty in the structural response is required, in addition to a good
model of the structural response itself. Fortunately, the models needed for the
higher frequency, passive aspects of the study can be directly applied at lower
frequencies where they give very high resolution results.
Nonetheless, given the inherent modelling uncertainties that arise when working
in this field it is considered essential that modest `proof of concept' experiments
be performed, based on the theoretical designs produced. This will involve the
construction of an initial base-line structure which, for a satellite boom, will be of
the order of 3-5m long and have, for example, 10 identical bays. The response of
this structure will be measured when suspended in the laboratory, both at low
frequencies, to characterise the lower modes which are to be actively controlled,
and at high frequencies, to characterise the vibration transmission characteristics
which will be modified by structural changes. Initial active control experiments
will also be conducted on this base-line design.
The construction of the optimised structures will require more complicated
fabrication facilities to ensure that they accurately match the required designs. It
is anticipated that these optimised structures will be built on a similar scale and
with a similar number of bays to the base-line configuration. The results of the
low and high frequency structural response tests will then be directly comparable
between the base-line and optimised structures, so that the improvement in the
low frequency control performance and high frequency vibration transmission
can be established directly.
Clearly, the use of combined active / passive approaches will bring its own
difficulties and it is considered essential that the two methods are not considered
in isolation. Instead a sophisticated computational model of the satellite structure
with its active control system will be built and genetic algorithm optimization
methods applied to the complete system. The optimizer would try to minimize the
frequency averaged vibration transmission across the whole spectrum of noise
frequencies while the control system of the active part of the design would strive
to deal with just the low frequency components. Variations in the locations of
both sensors and actuators will form a key part of the design process and would
be controlled by the GA at the same time as it adjusted the structural geometry.
Thus the final design will inherently allow for aspects of both the passive and
active components of the system. Although computationally expensive, such an
approach is within the capabilities of the large scale clustered / parallel
computing facilities available to the CEDC at Southampton University. This,
combined with the University's 5 and 5* class expertise in all the technologies
necessary for the study, makes Southampton an ideal location for work of this
kind.
PROGRAMME AND METHODOLOGY
To investigate an inter-disciplinary approach to vibration control and to assess the
capabilities of active / passive systems designed following these ideas will
require a mixed theoretical, computational and experimental approach. Thus, the
programme will start with the design and construction of a base-line model
structure (an interferometer constructed to large model scale: typically around
10% of full size) in close collaboration with MMS. Then, while this is being built
and tested, attention will move to the design of geometrically optimized versions
of the structure. This process will also incorporate selection of suitable methods
for implementing control strategies in this field and also for analysing such
structures by the most efficient means, given the particular needs of active control
systems design. Having fixed on the appropriate methods a custom built
computational model will be built and coupled to the existing OPTIONS[5]
design exploration system. This system will allow selection and tuning of the
most appropriate variant of genetic algorithm for the work in hand. The
OPTIONS codes will then allow parallel implementation of the models on the
high performance computing platforms available to the group.
Having carried out a series of design studies and traded off noise isolation
characteristics against the other requirements of the design such as
manufacturability, deployability, weight, strength, etc., an example passively
optimized structure will be built for test. Following this, an integrated active /
passive design will be produced and built. These structures, together with
practical implementations of the control systems, will then be evaluated
experimentally so as to provide `proof of concept' for the approach to be
followed. They will clearly demonstrate what gains can be achieved solely by
active and also by simply passive means and what by a combined active / passive
approach. At the same time, further wholly computational studies will
investigate the utility of the approach for the related structural configuration of a
synthetic aperture radar (SAR), again in collaboration with MMS.
This list of activities is addressed by the following programme which will be
carried out mainly by the research assistant and research student appointed to the
project (see also the attached project plan) :-
Months Research Programme Stages
1-4 Familiarization with literature (and WWW) by appointed researcher
(RA). Establish working relationships between the RA and staff at
MMS (W100).
Establish sensible overall design parameters for an interferometer
structural boom in keeping with the proposed Darwin mission in
collaboration with MMS (W200).
3-7 Produce design specification for the base-line boom and have this built
in the University's workshops (W300).
Interface the computational model of the passive only structure to the
OPTIONS design exploration suite and carry out trial design runs with
various search engines (W400).
Produce six monthly report (W500).
8-11 Commission experimental facilities (transducers, shakers, active
elements, etc.) using the base-line structure and measure its passive
performance over a variety of input conditions to provide validation
data for the structural model (W600).
12 Prepare annual report and conference paper and present intermediate
results to design staff within the collaborating company (W700).
13-15 Use available high performance computing systems to produce suitable
optimized boom designs making use of experimental data from the
base-line structure. Develop practical designs in conjunction with
MMS staff (W800).
16-18 Prepare detail specifications for passive only optimized structural
design and have this built in the University's workshops (W900).
Establish sensible overall design parameters for synthetic aperture radar
(SAR) structure in collaboration with MMS (W1000).
Construct customized computational model of the structural dynamics
and potential active control system (W1100).
Produce six monthly report (W1200).
19-21 Carry out initial tests on passive only optimized structural design in
purely passive mode and compare with computational model (W1300).
Have active control system prototyped by University technician staff in
accordance with the design work already carried out (W1400).
22-24 Commission active control system using base-line structure (W1500).
Prepare annual report and journal paper and present intermediate results
to staff within the collaborating company (W1600).
25-30 Use available high performance computing systems to produce
integrated active / passive optimized boom designs using modified
code. Develop practical designs in conjunction with MMS staff
(W1700).
Design base-line and optimized SAR in collaboration with staff from
MMS (W1800).
Carry out full tests on the passive only optimized boom but with active
control. Compare results with those for base-line structure (W1900).
Produce six monthly report (W2000).
31-35 Prepare detail specifications for integrated active / passive optimized
structural design and have this built in the University's workshops
(W2100).
Carry out full tests on the integrated active / passive boom. Compare
results with those for base-line and passive only structure (W2200).
Refine computational models to establish maximum likely noise
isolation figure that might be achieved using this approach for both
interferometer and SAR booms, in the light of the experimental studies
(W2300).
36 Prepare final report, journal papers and electronic digests describing the
work. Brief collaborating company on further development of the ideas
explored during the programme (W2400).
RELEVANCE TO BENEFICIARIES
The work planned here is primarily aimed at future aerospace structures which
are vibration critical. However, the use of integrated active / passive techniques
of this kind has applications over a wide range of other fields. It is a common
need of modern vehicle designers, whether they are working on satellites, aircraft,
trains, cars or ships, to reduce noise and vibration levels. These reductions are
necessary to meet increasingly severe noise pollution and habitability legislation
as well as to improve customer satisfaction, and thus market share, for the
products concerned.
The principal beneficiaries of this research will be organizations working in the
aerospace sector such as the collaborating company, Matra Marconi Space, but
also including British Aerospace and Westland Helicopters who both produce
light weight structures with vibration sensitive payloads or where there are very
severe vibration noise sources (and who the investigators are also collaborating
with on other, related projects). More indirectly, the UK car and truck industry
will also benefit from advances in structural noise control techniques.
Matra Marconi Space (MMS) consider progress in this field to be critical to the
success of a number of satellite proposals currently under active consideration.
Consequently, they have indicated their willingness to contribute £40k in cash,
£25k in staff effort and £5k in kind to a three year programme of work. Since
MMS have particular interests in the ESA Horizon 2000+ Darwin mission this
will be the main domain in which the project will be set. The major deliverables
of the project being (1) to provide designers at MMS with access to methods that
will enable the home aerospace sector to enhance its competitiveness over
manufacturers outside the UK and (2) to demonstrate to aerospace and other
designers how such active / passive vibration control methods could be integrated
into the design process.
DISSEMINATION AND EXPLOITATION
The results of this work will be fed directly back into MMS by the active
collaboration of their staff during the project. It is intended that the work be based
on realistic design studies and that the new methods developed be assessed by
MMS design staff. This will be achieved by the close involvement of MMS staff
during the design and practical elements of the work. Such exercises will be
backed up by seminars and extended technology transfer sessions carried out by
the research staff of the project for the benefit of practising design staff. It must
be stressed, however, that MMS are extremely keen to see the results of this work
published in the open literature and they will only seek to delay those
publications which contain valuable IPR and then only until such time as
satisfactory protection has been obtained. It is therefore planned that the research
staff involved attend and give presentations at the AIAA Smart Structures
conference in St. Loius, MO., USA, April 1999 and also at the International
Conference on Genetic Algorithms in the USA in the spring of 2001. It also the
normal practice of those involved to publish journal papers describing their
research at regular intervals and to make their work available on the world wide
web. Southampton University also has an active technology transfer and IPR
exploitation programme managed by its Office of Innovation and Research
Support. They will be closely involved in developing any IPR arising from the
programme.
RESEARCH STAFF
This work will be supervised by Prof. A.J. Keane acting as P.I. in collaboration
with Prof. S.J. Elliott and Drs. M.J. Brennan and E. Rogers. as co-investigators,
assisted by a full-time post-doctoral research assistant, preferably with some
skills in the areas of structural dynamics, control systems design, optimization or
experimentation together with a research student working on related topics. The
main role of the P.I. will be to give strategic guidance on the direction of the
work, to supervise liaison with the industrial collaborators and to provide input
based on previous experience of using evolutionary optimizers in a passive noise
control context. Prof. Elliott and Dr. Brennan will lead on the experimental side
of the study while Dr. Rogers will provide input on active control theory. The
R.A. will carry out the required code development, design work, experimentation
and interaction with MMS staff. The student will, of course, be primarily focused
on completing a PhD thesis, but this activity will provide background support to
the main project and forms an important aspect of the project, particularly given
the broad spectrum of activities encompassed. At this stage the most promising
avenue of related research suitable for the student appears to be in the field of
structural modelling techniques for control systems design[23] although, as
always with PhD studies, it is difficult to predict the precise nature of the work to
be covered. The association with the main project will, of course, provide a very
good platform for such research.
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This article may be found at
http://www.soton.ac.uk/~ajk/mms_fps.html
Correspondence to Andy.Keane@soton.ac.uk, or Prof. A.J. Keane, Department of
Mechanical Engineering, University of Southampton, Highfield, Southampton, SO17 1BJ, U.K.
Tel +44-1703-592944, FAX +44-1703-593230.
