Dr Oberst works as Senior Lecturer at the newly founded Centre for Audio, Acoustics and Vibration (CAAV). In 2017 he received the highly prestigious Junior Research Prize awarded by the European Association of Structural Dynamics (EASD) in the category "Development of Methodologies for Structural Dynamics". This prize follows the JSPS Award (2016) nominated by the Australian Academy of Science and an Australia Award/Endeavour Postdoctoral Research Fellowship (Australian Government, 2015) to visit the Imperial College London/Rolls-Royce Vibration University Technology Centre for 6 months. He worked as Chief Investigator on a prestigious DFG Priority Program (SPP1897) in Applied and Theoretical Mechanics at the Technical University Munich in 2016 and as Research Associate and Space Engineer (Mechanical) at the UNSW Canberra (2011-2016). Dr Oberst received his PhD in Mechanical Engineering from the University of New South Wales in Dec 2011 on his research on friction-induced instabilities in automotive brake systems.
- 2017 European Association of Structural Dynamics (EASD) Junior Research Prize in "Development of Methodologies for Structural Dynamics"
- 2016 JSPS Fellowship /Australian Academy of Science (revoked Fellowship to take a permanent position as Senior Lecturer)
- 2016 Grant, DFG Priority Program "Calm, Smooth and Smart" (SPP1897) - sole CI
- 2015 Australia Award/ Endeavour Postdoctoral Research Fellowship (Australian Goverment, 2015)
Can supervise: YES
Current research: Dr Oberst's research focuses on studying ‘Complex Dynamics’ using acoustic and vibration signals. In detail this research includes
- Nonlinear dynamics and Nonlinear time series analysis
- Bioacoustics and insect communication using microvibrations
- Mechanical engineering (friction-induced vibrations: brake squeal, hip squeak,...)
- Uncertainty analysis and statistics applied to dynamical systems
- Nonlinear oscillations in hydro-thermal geological systems modelled as open-flow chemical reactors
Overview of Sebastian's research activities and papers
PhD scholorships (domestic & international, 3-years, fully-funded) are available on
Teaching experience in Dynamics, Thermofluids, Introduction into vibration, Acoustic Noise
Zhang, Z, Oberst, S & Lai, J 2019, 'A non-linear friction work formulation for the analysis of self-excited vibrations', Journal of Sound and Vibration, vol. 443, pp. 328-340.View/Download from: UTS OPUS or Publisher's site
Even though much research has been devoted to understand friction-induced vibrations, its root cause is not yet fully understood. Reliable prediction of friction-induced unstable vibrations such as in brake squeal or hip squeak remains a challenge because of nonlinearities involved and because the complex eigenvalue analysis (CEA) widely used in industry is linear. The energy fed back into the system by friction has been shown to be useful for analysis of measurements and numerical simulations. In numerical simulations, the linearised method of feed-in energy, calculated purely based on friction work has provided some insights into the physical mechanism for instabilities. However, the dynamics due to friction-induced instabilities is highly nonlinear and damping may offset some or all of the excess friction energy provided to the system. By using a nonlinear 2-DOF dry friction oscillator, a nonlinear friction work formulation is proposed to demonstrate that in combination with viscous damping the energy budget provides an improved analysis capability over linearised friction work. The results highlight the potential of nonlinear friction work as a reliable tool to study friction-induced instabilities to gain deeper physical insights into squeal triggering mechanisms and to better understand the over- and under-predictive character inherent to linear methods.
Stender, M, Oberst, S & Hoffmann, N 2019, 'Recovery of Differential Equations from Impulse Response Time Series Data for Model Identification and Feature Extraction', Vibration, vol. 2, no. 1, pp. 25-46.View/Download from: UTS OPUS or Publisher's site
Time recordings of impulse-type oscillation responses are short and highly transient. These characteristics may complicate the usage of classical spectral signal processing techniques for (a) describing the dynamics and (b) deriving discriminative features from the data. However, common model identification and validation techniques mostly rely on steady-state recordings, characteristic spectral properties and non-transient behavior. In this work, a recent method, which allows reconstructing differential equations from time series data, is extended for higher degrees of automation. With special focus on short and strongly damped oscillations, an optimization procedure is proposed that fine-tunes the reconstructed dynamical models with respect to model simplicity and error reduction. This framework is analyzed with particular focus on the amount of information available to the reconstruction, noise contamination and nonlinearities contained in the time series input. Using the example of a mechanical oscillator, we illustrate how the optimized reconstruction method can be used to identify a suitable model and how to extract features from uni-variate and multivariate time series recordings in an engineering-compliant environment. Moreover, the determined minimal models allow for identifying the qualitative nature of the underlying dynamical systems as well as testing for the degree and strength of nonlinearity. The reconstructed differential equations would then be potentially available for classical numerical studies, such as bifurcation analysis. These results represent a physically interpretable enhancement of data-driven modeling approaches in structural dynamics.
Animals use cues to find their food, in microhabitats within their physiological tolerances. Termites build and modify their microhabitat, to transform hostile environments into benign ones, which raises questions about the relative importance of cues. Termites are desiccation intolerant and foraging termites are attracted to water, so most research has considered moisture to be a cue. However, termites can also transport water to food, and so moisture may play other roles than previously considered. To examine the role of moisture, we compared Coptotermes acinaciformis termite foraging decisions in laboratory experiments when they were offered dry and moist wood, with and without load. Without load, termites preferred moist wood and ate it without any building, whereas they moistened dry wood after wrapping it in a layer of clay. For the 'With load' units, termites substituted some of the wood for load-bearing clay walls, and kept the wood drier than on the unloaded units. As drier wood has higher compressive strength and higher rigidity, it allows more of the wood to be consumed. These results suggest that moisture plays a more important role in termite ecology than previously thought. Termites manipulate the moisture content according to the situational context and use it for multiple purposes: increased moisture levels soften the fibre, which facilitates foraging, yet keeping the wood dry provides higher structural stability against buckling which is especially important when foraging on wood under load.
Melnikov, A, Chiang, YK, Quan, L, Oberst, S, Alú, A, Marburg, S & Powell, D 2019, 'Acoustic meta-atom with experimentally verified maximum Willis coupling', Nature Communications, vol. 10, pp. 3148-3148.View/Download from: UTS OPUS
Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. Recent work has shown that the efficiency of many acoustic metamaterials can be enhanced by controlling an additional parameter known as Willis coupling, which is analogous to bianisotropy in electromagnetic metamaterials. The magnitude of Willis coupling in a passive acoustic meta-atom has been shown theoretically to have an upper limit, however the feasibility of reaching this limit has not been experimentally investigated. Here we introduce a meta-atom with Willis coupling which closely approaches this theoretical limit, that is much simpler and less prone to thermo-viscous losses than previously reported structures. We perform two-dimensional experiments to measure the strong Willis coupling, supported by numerical calculations. Our meta-atom geometry is readily modeled analytically, enabling the strength of Willis coupling and its
peak frequency to be easily controlled. Together with its ease of fabrication, this will facilitate the design of future high efficiency acoustic devices.
Stender, M, Oberst, S, Tiedemann, M & Hoffmann, N 2019, 'Complex machine dynamics: systematic recurrence quantification analysis of disk brake vibration data', Nonlinear Dynamics, pp. 1-10.View/Download from: Publisher's site
Complex machine dynamics, as caused by friction-induced vibrations and related to brake squeal, have gained significant attention in research and industry for decades. Today, remedies heavily rely on experimental testing due to the low prediction quality of numerical models. However, there is considerable lack of in-depth studies in characterizing self-excited oscillations encoded in scalar measurements. We complement previous works on phase-space reconstruction and recurrence plots analysis to a larger data base by applying a novel systematic approach using a large
data base. This framework considers appropriate delay embedding, time series partitioning into squealing and non-squealing parts and comparison to operational parameters of the brake system. By means of recurrence plot analysis, we illustrate that friction-excited vibrations are multi-scale in nature. Results confirm the existence of low-dimensional attractors in squealing regimes with increasing values of determinism and periodicity with rising vibration levels. It is shown that the squeal propensity can be directly linked to recurrence quantification measures. Using determinism and the clustering coefficient as metrics, we show for the first time that is possible to predict instabilities in regions of non-squealing conditions.
LU, S, Oberst, S, Zhang, G & Luo, Z 2019, 'Bifurcation analysis of dynamic pricing processes with nonlinear external reference effects', Communications in Nonlinear Science and Numerical Simulation, vol. 79, pp. 104929-104929.
Dynamic pricing has been widely implemented to hedge against
volatile demand. One challenging problem is the study of optimal price
choices under the influence of this volatility. Stochastic demand is a
prevalent assumption when it comes to model the volatility on pricing
decisions. However, the demand volatility might also be produced by
deterministic chaos, which has been rarely studied in this field of
research to-date. Disregarding deterministic dynamics may not only cause
revenue losses in practice but might also mislead regulators about the
underlying mechanisms used by market participants.
To improve pricing decisions and price regulations, we propose a
deterministic dynamic pricing process, of which the optimisation
objective is the revenue. Consumer expectations and discrete price
choices are considered to mimic a real pricing decision. Contradicted
expectations are quantitatively modelled on the consumer purchasing
decisions. Due to asymmetry in the perceptions of gains or losses, the
model becomes non-smooth. Period adding bifurcations, codimension-2
points and coexisting solutions can be observed.
Results highlight that an optimal pricing strategy should agree with the
dynamics of consumer expectations - which we show for the first time.
Based on that an optimal irregular pricing strategy is introduced: a
decision maker can make the first return iteration of each optimal price
choice non-periodic to follow non-periodic expectations when confronting
finite price choices. These results may justify implementing irregular
pricing strategies in the case of practical pricing decisions. Here, the
existence of coexisting solutions will assist to identify potential
market manipulations within a monopoly market. This not only contributes
to a fresh look on a volatile market but also spotlights the important
role of initial conditions to pricing decision and price regulations.
Stender, M, Tiedemann, M, Hoffmann, N & Oberst, S 2018, 'Impact of an irregular friction formulation on dynamics of a minimal model for brake squeal', Mechanical Systems and Signal Processing, vol. 107, pp. 439-451.View/Download from: UTS OPUS
Friction-induced vibrations are of major concern in the design of reliable, efficient and comfortable technical systems. Well-known examples for systems susceptible to self-excitation can be found in fluid structure interaction, disk brake squeal, rotor dynamics, hip implants noise and many more. While damping elements and amplitude reduction are well-understood in linear systems, nonlinear systems and especially self-excited dynamics still constitute a challenge for damping element design. Additionally, complex dynamical systems exhibit deterministic chaotic cores which add severe sensitivity to initial conditions to the system response. Especially the complex friction interface dynamics remain a challenging task for measurements and modeling. Today, mostly simple and regular friction models are investigated in the field of self-excited brake system vibrations. This work aims at investigating the effect of high-frequency irregular interface dynamics on the nonlinear dynamical response of a self-excited structure. Special focus is put on the characterization of the system response time series.
A low-dimensional minimal model is studied which features self-excitation, gyroscopic effects and friction-induced damping. Additionally, the employed friction formulation exhibits temperature as inner variable and superposed chaotic fluctuations governed by a Lorenz attractor. The time scale of the irregular fluctuations is chosen one order smaller than the overall system dynamics. The influence of those fluctuations on the structural response is studied in various ways, i.e. in time domain and by means of recurrence analysis. The separate time scales are studied in detail and regimes of dynamic interactions are identified. The results of the irregular friction formulation indicate dynamic interactions on multiple time scales, which trigger larger vibration amplitudes as compared to regular friction formulations conventionally studied in the field of friction-induced vibr...
Oberst, S, Tuttle, S, Griffith, D, Lambert, A & Boyce, R 2018, 'Experimental validation of tape springs to be used as thin-walled space structures', Journal of Sound and Vibration, vol. 419, pp. 558-570.View/Download from: UTS OPUS or Publisher's site
With the advent of standardised launch geometries and off-the-shelf payloads, space programs utilising nano-satellite platforms are growing worldwide. Thin-walled, flexible and self-deployable structures are commonly used for antennae, instrument booms or solar panels owing to their lightweight, ideal packaging characteristics and near zero energy consumption. However their behaviour in space, in particular in Low Earth Orbits with continually changing environmental conditions, raises many questions. Accurate numerical models, which are often not available due to the difficulty of experimental testing under 1g-conditions, are needed to answer these questions.
In this study, we present on-earth experimental validations, as a starting point to study the response of a tape spring as a representative of thin-walled flexible structures under static and vibrational loading. Material parameters of tape springs in a singly (straight, open cylinder) and a doubly curved design, are compared to each other by combining finite element calculations, with experimental laser vibrometry within a single and multi-stage model updating approach. While the determination of the Young's modulus is unproblematic, the damping is found to be inversely proportional to deployment length. With updated material properties the buckling instability margin is calculated using different slenderness ratios. Results indicate a high sensitivity of thin-walled structures to miniscule perturbations, which makes proper experimental testing a key requirement for stability prediction on thin-elastic space structures. The doubly curved tape spring provides closer agreement with experimental results than a straight tape spring design.
Oberst, S & Tuttle, S 2018, 'Nonlinear dynamics of thin-walled elastic structures for applications in space', Mechanical Systems and Signal Processing, vol. 110, pp. 469-484.View/Download from: UTS OPUS or Publisher's site
Driven by the need for multi-functionality and increasing demands for low mass and compact-stowing, unfolding, self-deploying or –morphing smart mechanical structures have become popular space engineering
designs for flexible appendages. Extensive research has been conducted on the use of tape springs as hinge deployment mechanisms for space booms, solar sails, or optical membranes or directly for used as antennas.
However, the vibrational behaviour of tape springs and its related dynamics have rarely been addressed in detail, even though missions are underway with similarly flexible appendages installed.
By conducting quasi-static bending tests on a tape spring antenna, we evidence hysteresis behaviours in both the opposite- and equal sense bending directions. Apart from the well-known snap-through buckling, the
structure exhibits torsional buckling in the equal sense bending direction before collapsing. Micro-vibrational excitation triggers nonlinear jump phenomena and the period-doubling route to chaos. Using a computational tape spring model and simplified environmental loads similar to those encountered in near-Earth orbits, coupling between the first bending and torsional modes generates a dynamic instability which is predicted by a complex eigenvalue analysis step. The current study highlights that high perturbation sensitivity and system-inherent nonlinearities can lead to stability issues.
In the course of designing a spacecraft with thin-walled appendages, system-level trade-offs are routinely performed. Since it is unclear how severely the vibrations of flexible appendages might affect their proper
functioning or the control of the spacecraft, it is of paramount importance to validate experimentally thin-walled structures thoroughly for their dynamic and stability behaviours.
Oberst, S, Baetz, J, Campbell, G, Lampe, F, Lai, JCS, Hoffmann, N & Morlock, MM 2018, 'Vibro-acoustic and nonlinear analysis of cadavric femoral bone impaction in cavity preparations', International Journal of Mechanical Sciences, vol. 144, pp. 739-745.View/Download from: UTS OPUS
Oberst, S, Niven, RK, Lester, DR, Ord, A, Hobbs, B & Hoffmann, N 2018, 'Detection of unstable periodic orbits in mineralising geological systems', Chaos, vol. 28, no. 8, pp. 085711-085721.View/Download from: UTS OPUS
Worldwide, mineral exploration is suffering from rising capital costs, due to the depletion of readily recoverable reserves and the need to discover and assess more inaccessible or geologically complex deposits. For gold exploration, this problem is particular acute. We propose an innovative approach to mineral exploration and orebody characterisation, based on the analysis of geological core data as a spatial dynamical system, using the mathematical tools of dynamical systems analysis. This approach is highly relevant for orogenic gold deposits, which - in contrast to systems formed at chemical equilibrium - exhibit many features of nonlinear dynamical systems, including episodic fluctuations on various length and time scales. Feedback relationships between thermo-chemical and deformation processes produce recurrent fluid temperatures and pressures, and the deposition of vein-filling minerals such as pyrite and gold. We therefore relax the typical assumption of chemical equilibrium and analyse the underlying processes as aseismic, non-adiabatic and inherent to a hydrothermal, nonlinear dynamical open-flow chemical reactor. These processes are approximated using the Gray-Scott model (GSM) of reaction-diffusion as complex toy system, which captures some of the features of the underlying mineralisation processes, including the spatio-temporal Turing patterns of unsteady chemical reactions. By use of this analysis, we demonstrate the capability of recurrence plots, recurrence power spectra and recurrence time probabilities to detect underlying unstable periodic orbits, as one sign of deterministic dynamics, and their robustness for the analysis of data contaminated by noise. Recurrence plot based quantification is then applied to three mineral concentrations in core data from the Sunrise Dam gold deposit in the Yilgarn region of Western Australia. Using a moving window, we reveal the episodic recurring low-dimensional dynamic structures and the period doubling rout...
Jordaan, J, Punzet, S, Melnikov, A, Sanches, A, Oberst, S, Marburg, S & Powell, DA 2018, 'Measuring monopole and dipole polarizability of acoustic meta-atoms', Applied Physics Letters, vol. 113, no. 22.View/Download from: UTS OPUS or Publisher's site
© 2018 Author(s). We present a method to extract monopole and dipole polarizability from experimental measurements of two-dimensional acoustic meta-atoms. In contrast to extraction from numerical results, this enables all second-order effects and uncertainties in material properties to be accounted for. We apply the technique to 3D-printed labyrinthine meta-atoms of a variety of geometries. We show that the polarizability of structures with a shorter acoustic path length agrees well with numerical results. However, those with longer path lengths suffer strong additional damping, which we attribute to the strong viscous and thermal losses in narrow channels.
Oberst, S, Bann, G, Lai, JCS & Evans, TA 2017, 'Cryptic termites avoid predatory ants by eavesdropping on vibrational cues from their footsteps', ECOLOGY LETTERS, vol. 20, no. 2, pp. 212-221.View/Download from: UTS OPUS or Publisher's site
Oberst, S, Marburg, S & Hoffmann, N 2017, 'Determining periodic orbits via nonlinear filtering and recurrence spectra in the presence of noise', Procedia Engineering, vol. 199C, pp. 772-777.View/Download from: UTS OPUS or Publisher's site
Oberst, S, Lai, JCS & Evans, TA 2016, 'Termites utilise clay to build structural supports and so increase foraging resources', SCIENTIFIC REPORTS, vol. 6.View/Download from: UTS OPUS or Publisher's site
Zhang, Z, Oberst, S & Lai, JCS 2016, 'On the potential of uncertainty analysis for prediction of brake squeal propensity', JOURNAL OF SOUND AND VIBRATION, vol. 377, pp. 123-132.View/Download from: UTS OPUS or Publisher's site
Zhang, Z, Oberst, S & Lai, JCS 2016, 'Instability analysis of friction oscillators with uncertainty in the friction law distribution', PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART C-JOURNAL OF MECHANICAL ENGINEERING SCIENCE, vol. 230, no. 6, pp. 948-958.View/Download from: UTS OPUS or Publisher's site
Oberst, S, Zhang, Z & Lai, JCS 2016, 'The Role of Nonlinearity and Uncertainty in Assessing Disc Brake Squeal Propensity', SAE INTERNATIONAL JOURNAL OF PASSENGER CARS-MECHANICAL SYSTEMS, vol. 9, no. 3, pp. 980-986.View/Download from: UTS OPUS or Publisher's site
Oberst, S & Lai, JCS 2015, 'A statistical approach to estimate the LYAPUNOV spectrum in disc brake squeal', JOURNAL OF SOUND AND VIBRATION, vol. 334, pp. 120-135.View/Download from: UTS OPUS or Publisher's site
Oberst, S, Nava-Baro, E, Lai, JCS & Evans, TA 2015, 'An Innovative Signal Processing Method to Extract Ants' Walking Signals', ACOUSTICS AUSTRALIA, vol. 43, no. 1, pp. 87-96.View/Download from: UTS OPUS or Publisher's site
Oberst, S & Lai, JCS 2015, 'Pad-mode-induced instantaneous mode instability for simple models of brake systems', MECHANICAL SYSTEMS AND SIGNAL PROCESSING, vol. 62-63, pp. 490-505.View/Download from: UTS OPUS or Publisher's site
Oberst, S, Lai, JCS & Marburg, S 2013, 'Guidelines for numerical vibration and acoustic analysis of disc brake squeal using simple models of brake systems', JOURNAL OF SOUND AND VIBRATION, vol. 332, no. 9, pp. 2284-2299.View/Download from: UTS OPUS or Publisher's site
Merz, S, Oberst, S, Dyleiko, PG, Kessissoglou, N, Tso, YK & Marburg, S 2007, 'Document Development of coupled FE/BE models to investigate the structural and acoustic responses of a submerged vessel', Journal of Computational Acoustics, vol. 15, no. 1, pp. 23-47.
LU, S, Oberst, S, Zhang, G & Luo, Z 2019, 'Novel order patterns recurrence plot-based quantification measures to unveil deterministic dynamics from stochastic processes' in Valenzuela, O, Rojas, F, Pomares, H & Rojas, I (eds), Theory and Applications of Time Series Analysis, Springer International Publishing, Cham, Switzerland.
Gong, S, Oberst, S & Wang, X 2019, 'Dynamic analysis of vibrating flip-flow screens equipped with support and shear rubber springs', Recent Advances in Structural Dynamics, Lyon, France.View/Download from: UTS OPUS
Vibrating flip-flow screens provide an effective means of screening highly viscous or fine materials, and the dynamic characteristics of the main and the floating screen frames are largely responsible for a flip-flow screen's screen performance and its processing capacity. An accurate dynamic model of the rubber shear springs used within the frame of the screen is critical for its dynamic analysis – to understand deficiencies and improve its performance. In this paper, the Sjöberg model is used to predict the frequency-and amplitude-dependent behaviour of the rubber shear springs. A friction model represents the amplitude dependency of the rubber shear springs. The fractional derivative model is used to describe its frequency dependency with its elasticity being represented by a linear spring. This model is further validated by cyclic tests of the rubber shear springs. Furthermore, dynamic response of the VFFS have been analysed using the Sjöberg model and the Kelvin-Voigt model, respectively. Experimental results indicate that dynamic response of VFFS can be better predict using the Sjöberg model than Kelvin-Voigt model in time region as well as in the frequency domain.
Vibrating flip-flow screens provide an effective solution for screening highly viscous or fine materials. However,
yet, only linear theory has been applied to their design. Yet, to understand deficiencies and to improve performance an
accurate model especially of the rubber shear springs equipped in screen frames is critical for its dynamics to predict e.g.
frequency- and amplitude-dependent behaviour. In this paper, the amplitude dependency of the rubber shear spring is
represented by employing a friction model in which parameters are fitted to an affine function rather constant values used for
the classic Berg's model; the fractional derivative model is used to describe its frequency dependency and compared to
conventional dashpot and Maxwell models with its elasticity being represented by a nonlinear spring. The experimentally
validated results indicate that the proposed model with a nonlinear spring, friction and fractional derivative model is able to
more accurately describe the dynamic characteristics of a rubber shear spring compared with other models.
Halkon, B, Rauter, A, Oberst, S & Marburg, S 2019, 'Research and development of an air-puff excitation system for lightweight structures', 8th International Operational Modal Analysis Conference 2019, Copenhagen.View/Download from: UTS OPUS
Lightweight, thin-walled structures appear in numerous engineering and natural structures. Due to their sensitivity, vibration excitation by, now traditional, contacting techniques, such as modally-tuned impact hammers or electrodynamic shakers, to investigate their dynamics is challenging since it typically adds substantial mass and/or stiffness at the excitation location. The research presented in this article, therefore, is intended to yield a system for the non-contact excitation of thin-walled structures through small, controlled blasts of air. An air-puff system, consisting of two fast-acting solenoid controlled valves, a small air outlet nozzle and bespoke control software with a programmable valve control sequence, is researched and developed. The excitation impulse characteristics are investigated
experimentally and described in detail for varying input control parameters. Ultimately, suitability of the system for the excitation of thin-walled structures is explored, for both a 3D-printed micro-satellite panel and a natural bee honeycomb, with promising results when compared to that of an impact hammer.
LU, S, Oberst, S, Zhang, G & Luo, Z 2019, 'Period adding bifurcations in dynamic pricing processes', IEEE CIFEr 2019: 2019 IEEE Conference on Computational Intelligence for Financial Engineering and Economics, Shenzhen.View/Download from: UTS OPUS
Price information enables consumers to anticipate
a price and to make purchasing decisions based on their price
expectations, which are critical for agents with pricing decisions
or price regulations. A company with pricing decisions can
aim to optimise the short-term or the long-term revenue, each
of which leads to different pricing strategies thereby different
price expectations. The choices between the two optimisation
objectives consider the maximal revenue and the robustness of
a chosen pricing strategy against market volatility. However
the robustness is rarely identified in a volatile market. Here,
we investigate the robustness of optimal pricing strategies with
the short-term or long-term optimisation objectives through the
analysis of nonlinear dynamics of price expectations. Bifurcation
diagrams and period diagrams are introduced to compare their
change in dynamics. Our results highlight that period adding
bifurcations occur during the dynamic pricing processes studied.
These bifurcations would challenge the robustness of an optimal
pricing strategy. The consideration of the long-term revenue
allows a company to charge a higher price, which in turn
increases the revenue. However, the consideration of the shortterm
revenue can avoid period adding bifurcations, contributing
to a robust pricing strategy. This allows a company to harvest
a good revenue through a robust pricing strategy in a volatile
market and to satisfy regulations of a control in price volatility.
Oberst, S, Baetz, J, Campbell, G, Lampe, F, Lai, JCS, Hoffmann, N & Morlock, MM 2017, 'Vibro-acoustic and nonlinear analysis of cadavric femoral bone impactionin cavity preparations', MATEC Web of Conferences, International Congress on Engineering Vibration, EDP Sciences, Sofia, Bulgaria, pp. 1-6.View/Download from: UTS OPUS
Owing to an ageing population, the impact of unhealthy lifestyle, or simply congenital or gender
specific issues (dysplasia), degenerative bone and joint disease (osteoarthritis) at the hip pose an increasing
problem in many countries. Osteoarthritis is painful and causes mobility restrictions; amelioration is often only
achieved by replacing the complete hip joint in a total hip arthroplasty (THA). Despite significant orthopaedic
progress related to THA, the success of the surgical process relies heavily on the judgement, experience, skills
and techniques used of the surgeon. One common way of implanting the stem into the femur is press fitting
uncemented stem designs into a prepared cavity. By using a range of compaction broaches, which are impacted
into the femur, the cavity for the implant is formed. However, the surgeon decides whether to change the size of
the broach, how hard and fast it is impacted or when to stop the excavation process, merely based on acoustic,
haptic or visual cues which are subjective. It is known that non-ideal cavity preparations increase the risk of
peri-prosthetic fractures especially in elderly people.
This study reports on a simulated hip replacement surgery on a cadaver and the analysis of impaction forces
and the microphone signals during compaction. The recorded transient signals of impaction forces and acoustic
pressures (≈ 80 µs - 2 ms) are statistically analysed for their trend, which shows increasing heteroscedasticity
in the force-pressure relationship between broach sizes.
TIKHONOV regularisation, as inverse deconvolution technique, is applied to calculate the acoustic transfer
functions from the acoustic responses and their mechanical impacts. The extracted spectra highlight that system
characteristics altered during the cavity preparation process: in the high-frequency range the number of
resonances increased with impacts and broach size. By applying nonlinear time series analysis the system dynamics
increase in compl...
Oberst, S, Baetz, J, Campbell, G, Lampe, F, Lai, JCS, Hoffmann, N & Morlock, M 2018, 'Vibro-acoustic and nonlinear analysis of cadavric femoral bone impaction for cavity preparation in hip implants', MATEC Web of Conferences, EDP Sciences.View/Download from: UTS OPUS
Gong, S, Wang, Z & Oberst, S 2018, 'Non-linear analysis of vibrating flip-flow screens', MATEC Web of Conferences, International Conference on Design and Manufacturing Engineering, EDP Sciences, Monash University, Melbourne, Australia, pp. 04007-04010.View/Download from: UTS OPUS or Publisher's site
Vibrating flip-flow screens provide an effective solution for the screening of highly viscous or fine materials. Apart from other factors, the vibration characteristics of the main and floating screen frames are largely responsible for the flip-flow screen's sifting performance and its processing capacity. In this paper, the vibration characteristics of a vibrating flip-flow screen with linear and nonlinear springs are compared. Analytical results highlight that increasing the relative amplitude and avoiding undesirable resonances of the main and the floating screen frames can be realised to improve the screen's performance. The materials on the screen panel have less an effect on the vibration characteristics of the vibrating flip-flow screen with nonlinear springs than using linear springs. Other design parameters which influence the performance of vibrating flip-flow screens are discussed.
Zhang, Z, Oberst, S & Lai, JCS 2018, 'Instability analysis of brake squeal with uncertain contact conditions', International Congress on Sound and Vibration, Hiroshima, Japan.View/Download from: UTS OPUS
Oberst, S, Lai, JCS & Evans, TA 2018, 'Excitation signal extraction of ant walking pattern under the influence of noise using a biomechanical bipedal mathematical model', 2nd International Symposium on Biotremology, San Michele all'Adige, Italy.
Melnikov, A, Quann, L, Alú, A, Oberst, S, Marburg, S & Powell, D 2018, 'Theory for Willis coupling prediction of acoustic meta-atoms', Symposium on Acoustic Metamaterials, Xatavia, Spain.
Oberst, S, Lim, S, Romão, AC, Lai, JCS, Stender, M, Hoffmann, NP & Evans, TA 2018, 'A coupled mono-bipedal biomechanical surrogate model to mimic ants walking and running gait analysed using recurrence plot quantification analysis', Colloquium on Irregular Oscillations and Signal Processing,, Hamburg, Germany.
Stender, M, Oberst, S & Hoffmann, NP 2018, 'Reconstruction of differential equations from time-series data for feature engineering and model identification', Colloquium on Irregular Oscillations and Signal Processing, Hamburg, Germany.
LU, S, Oberst, S, Zhang, G & Luo, Z 2018, 'Comparing complex dynamics using machine learning-reconstructed attracting sets', Colloquium on Irregular Engineering Oscillations and Signal Processing, TUHH, Hamburg, Germany.
LU, S, Oberst, S, Zhang, G & Luo, Z 2018, 'Order patterns recurrence plots and new quantifications to unveil nonlinear dynamics from stochastic systems', International Conference on Time Series and Forecasting 2018, Granada, Spain.View/Download from: UTS OPUS
The purpose of this paper is to emphasise that hydrothermal mineralising systems are giant chemical reactors that operate as nonlinear dynamical systems held far from equilibrium by the infux of energy and mass. Te nonlinear behaviour leaves its mark as apparently stochastic distributions of alteration assemblages, mineralisation and structures are deterministic and contain all the information required to understand why the ore body is small or large and whether it is well or poorly endowed.
Ord, A, Oberst, S, Niven, R & Hobbs, B 2017, 'What do we do with all these data? Ore Exploration Using Modern Technology', AIG Bulletin, Gold17@Rotorua, Australian Institute of Geoscientists, Rotorua, New Zealand., pp. 66-69.View/Download from: UTS OPUS
Oberst, S, Niven, R, Ord, A, Hobbs, B & Lester, D 2017, 'Application of recurrence plots to orebody exploration data', Target 2017, At University Club, University of Western Australia.View/Download from: UTS OPUS
A recurring question in mineral exploration is to predict the spatial distribution of ore-bearing
minerals, to determine both (i) the economic value of a prospective deposit, and (ii) the optimal
locations for mineral discovery (Jébrak M, 1997). In hydrothermal gold deposits such as the
Imperial deposit, in the Yilgarn of Western Australia, the gold exhibits a highly variable spatial
distribution, causing great difficulties for ore recovery and mine planning; the hypothesis is that
such variability results from orebody formation by a non-equilibrium open chemical reactor
process with complex chemical-flow-heat-mechanical couplings (Ord et al., 2012, 2016; Lester et
al., 2012). With this insight, an important goal is to develop new computational methods to analyse
drill-core data to extract key features of the underlying dynamical system, so as to shed light on
the orebody formation mechanism(s). In order to discover whether prediction is possible in such
systems we need to establish whether the patterns of alteration and mineralisation are intrinsically
random or have an underlying deterministic origin.
Oberst, S, Lester, D, Niven, RK, Ord, A, Hobbs, BE & Hoffmann, NP 2017, 'Application of recurrence plot quantification to mineralising systems in geology', 7th International Symposium on Recurrence Plots, São Paulo.View/Download from: UTS OPUS
Stender, M, Oberst, S & Hoffmann, N 2017, 'Why mechanical machines should be treated as complex systems', Recurrence Plot Symposium, Sāo Paulo, Brazil.
Nonlinear time and recurrence analysis have proven to be highly successful for a very diverse field of nonlinear dynamical processes. Typically, this data driven method is applied to systems that cannot be modelled in a bottom-up manner (i.e. model-based) due to complexities or uncertainties involved but can be quantified via measurements. Well-known examples range from natural sciences, medicine and biology (physiology) to finance and earth science to only mention a few. Focus is on analyzing the output, i.e. observations instead of trying to mathematically detail the systems.
Friction brakes, turbo machinery and many other highly sophisticated mechanical machines reveal a plethora of unwanted and possibly safety critical noise and vibration phenomena, which today's engineers can only partly relate to operational conditions or system configurations. Analysis approaches are mostly limited to the concept of assembling models of little parts: geometrical, material and dynamical properties of each part are determined, from which a mathematical model of the complete machine is created following the pure form of linearity (superposition principle). This bottom-up approach has been applied successfully in many disciplines with the aid of advanced material models, discretization strategies, model order reduction methods and high performance computing. However, system inherent nonlinearities and their interaction, multi-physics, different time and length scales and unknown operational conditions often cause large discrepancies between experimental results and the mathematical models for numerous engineering structures, making high-fidelity modelling of a synthesized machine an ambitious task.
Considering those modeling challenges in machine dynamics and the limited modeling improvements made in recent years, we pose the provocative question: why do we not as engineers look at a machine and its dynamics as if it was a brain, a stock market or a planet's climate system, i....
Lai, JCS, Oberst, S & Evans, TA 2017, 'Termites thrive by using vibrations', International Congress on Sound and Vibration 2017, Curran Associates, Inc., London, UK, pp. 1-6.View/Download from: UTS OPUS
Termites are notoriously cryptic: infestations in houses are often discovered when an apparently intact timber object collapses, the archetypal 'falling through the floor boards'. This behaviour is adaptive because remaining undiscovered is the termite's primary defence against predators. However, being cryptic severely limits the ability of termites to explore their environment and assess potential food sources. Despite being blind and hidden, termites can assess a piece of wood swiftly after contacting only a small part of it (sometimes as little as a few square millimetres). Although termite soldiers have been known to produce vibratory alarm signals to warn conspecific workers, it is not until our sustained research over the last 10 years that termites have been shown to use vibrations as a principal tool for communications, making foraging decisions and detection. In this paper, some of our recent discoveries on the use of vibrations by termites will be discussed. These include the discovery of (a) two drywood termite species, Cryptotermes (Cr.) domesticus and Cr. secundus, using vibration signals produced as a by-product of their feeding to assess food size; (b) the ability of Cr. secundus and Coptotermes (Co.) acinaciformis to discriminate material properties based on vibration signals; and (c) the amazing ability of the subterranean termites Co. acinaciformis to distinguish unloaded wood from loaded wood. Food size is just one factor termites consider when foraging; competitors and predators are also important factors. Our results show that Cr. secundus worker termites are able to discriminate their own species from the subterranean species, Co. acinaciformis, by eavesdropping on their competitors, while Co. acinaciformis can detect and avoid one of their main predators, ants of the species Iridomyrmex pupuereus, using only vibrations caused by ants walking.
(PDF) Termites thrive by vibrations. Available from: https://www.researchgate.net/publication/31...
Oberst, S, Lai, JCS & Evans, TA 2017, 'Extracting critical information from ant and termite substrate vibrations', Invertebrate Sound and Vibration 2017, Ebsdorfergrund-Rauischholzhausen, Germany.
Ord, A, Hobbs, B, Munro, M, Oberst, S & Niven, RK 2017, 'Evidence-based geological models: how to comprehend big data', 18th Annual Conference of the International Association for Mathematical Geosciences, Freemantle, Australia, pp. 1-1.
Hobbs, B, Ord, A, Oberst, S & Niven, RK 2017, 'Nonlinear episodic and chaotic behaviour of orogenic gold systems', 18th Annual Conference of the International Association for Mathematical Geosciences, Freemantle, Australia, pp. 1-1.
Oberst, S, Zhang, Z, Campbell, G, Morlock, M, Lai, JCS & Hoffmann, N 2016, 'Towards the understanding of hip squeak in total hip arthroplasty using analytical contact models with uncertainty', Proceedings of the 45th International Congress on Noise Control Engineering, Internoise Congress, http://pub.dega-akustik.de/IN2016/data/index.html, Hamburg, Germany, pp. 5539-5549.View/Download from: UTS OPUS
© 2016, German Acoustical Society (DEGA). All rights reserved.Osteoarthritis in hip joints affects patients' quality of life such that often only costly orthopaedic surgeries i.e. total hip arthroplasty (THA) provide relief. Common implant materials are metal alloys, steel or titanium-based, plastics such as ultra-high molecular weight polyethylene, or biocompatible alumina and composite ceramics. Hard-on-hard (HoH) bearing articulations, i.e. ceramic-on-ceramic, or hard-on-soft combinations are used. HoH implants have been known to suffer from squeaking, a phenomenon commonly encountered in friction-induced self-excited vibrations. However, the frictional contact mechanics, its dynamics related to impingement, the effect of socket position, stem configuration, bearing size and patient characteristics are poorly understood. This study gives an overview of the state of the art biomechanical research related to squeaking in THA, with a focus on the effects of friction, stability, related wear and lubrication. An analytical model is proposed to study the onset of friction-induced vibrations in a simplified hemispherical hip stem rubbing in its bearing by varying the contact area. Preliminary results of the complex eigenvalue analysis and stick-slip motion analysis indicate that an increased contact fosters the development of instabilities, even at very small values of the friction coefficient owing to large local contact pressures.
Zhang, Z, Oberst, S & Lai, JCS 2016, 'Influence of contact condition and sliding speed on friction-induced instability', Proceedings of the 23rd International Congress on Sound and Vibration, International Congress on Sound and Vibration: From Ancient to Modern Acoustics (ICSV), International Institute of Acoustics and Vibration, Athens, Greece.View/Download from: UTS OPUS
Brake squeal, defined as audible noise above 1 kHz, is triggered by energy provided in the contact area between the pad and the disc and friction-induced instabilities. Owing to customers' demand of reducing vehicle noise and the increasing use of light composite materials in cars, squealing brakes remain a major concern to the automotive industry because of warranty-related claims. The prediction of disc brake squeal propensity is as challenging as ever. Although friction-induced instabilities are inherently nonlinear and during squeal the brake system's operating and environmental conditions keep changing, mostly linear and steady state methods are used for the analysis of brake squeal propensity. While many different instability mechanisms have been identified, their interactions and the resulting dynamics are not yet fully understood. Linear instability predictions suffer from over- and under-predictions and have to be complemented by extensive noise dynamometer or in vehicle tests. Recent studies indicate that frictional contact is multi-scaled in nature, highly sensitive and inhomogeneous. Very high local pressures and partial contact separations in the contact interface further complicate its numerical modelling. By studying an analytical model of 3 × 3 friction oscillators using three different friction laws (Amonton-Coulomb, the velocity-dependent and the LuGre friction model) in point contact with a sliding rigid plate and incorporating uncertainties in the contact condition, robustly unstable vibration modes have been identified in our previous research. Here, the number and the combination of friction oscillators engaged in contact are randomised to model imperfect contact. In addition, the effect of the variation in the plate's sliding velocity on the in-stability analysis is investigated with randomised friction coefficient of the Amonton-Coulomb friction model. Results of instability prediction and net work calculations are used to illustrate the s...
Williams, JJR, Zhang, Z, Oberst, S & Lai, JCS 2015, 'Model updating of brake components' influence on instability predictions', 22nd International Congress on Sound and Vibration, ICSV 2015, International Congress on Sound and Vibration, Florence, Italy.View/Download from: UTS OPUS or Publisher's site
Customers perceive brake squeal as a major annoyance in their automobiles' acoustic performance. Squeal is self-excited, friction induced audible noise above 1 kHz and one of the strongest cost drivers in noise vibration and harshness departments of automotive manufacturers. In order to reduce expensive and time-consuming dynamometer and road vehicle tests, numerical complex eigenvalue analysis has become popular in predicting brake squeal. However, one difficulty in assessing the prediction quality apart from the linearisation of the system is the complexity of the brake system to be modelled. Using structural finite elements the computer model is often insufficiently detailed, insufficiently damped or insufficiently experimentally validated so that instabilities causing brake squeal are over-predicted. Here we present the process of updating components of a brake system's squeal prediction and the improvement in modelling using updated material parameters and a Rayleigh damping model by applying a rigorous mesh refinement study and different friction laws.
Zhang, Z, Oberst, S & Lai, JCS 2015, 'Instability analysis of coupled friction oscillators with uncertainties in contact conditions', 22nd International Congress on Sound and Vibration, ICSV 2015, International Congress on Sound and Vibration, Florence, Italy.View/Download from: UTS OPUS or Publisher's site
Although brake squeal is a significant noise, vibration and harshness (NVH) issue which incurs significant cost in the automotive industry, its prediction is still difficult. This is because brake squeal is essentially a nonlinear phenomenon and traditional complex eigenvalue analysis (CEA) is a linear method. In addition, there are many uncertainties in a brake system such as material properties, operating and contact conditions which cannot be determined accurately with confidence. Here, the influence of uncertainties in contact conditions on the instability of an analytical model consisting of 3x3 coupled oscillators in point contact with a sliding rigid plate is analysed. The uncertainties in contact conditions considered are: percentage of contact, stiffness and friction laws for the contact (Amonton-Coulomb, relative velocity dependent and LuGre law). The instability is analysed in the frequency domain by randomising these three uncertainty parameters. The results will be discussed with a view to applying this approach to the analysis of the squeal propensity for a full brake system.
Zhang, Z, Oberst, S, Williams, JJR & Lai, JCS 2015, 'Improving Brake squeal propensitiy prediction by model updating', Acoustics 2015 Hunter Valley, Conference of the Australian Acoustical Society, Hunter Valley, NSW, Australia.View/Download from: UTS OPUS
Brake squeal as a significant warranty-claim related costs problem to the automotive industry is difficult to model numerically and analyse because of inherent nonlinearities, uncertainties in material properties, contact and boundary conditions, and system complexity. Often, model components are linearised and not experimentally validated. Sophisticated contact or friction models as well as stiffness in joints are often not considered owing to difficulties in experimental validation. In this study, a full brake system is modally updated at the component level and then at the subassembly level (pad assembly alone, pad in bracket). Squeal prediction using the complex eigenvalue analysis on a finite element model of the system is compared to squeal results from a noise dynamometer test. The results are discussed with respect to further refinement of the modelling approach and improvements to brake squeal prediction.
Oberst, S, Griffin, D, Tuttle, S, Lambert, A & Boyce, RR 2015, 'Analysis of thin curved flexible structures for space applications', Acoustics 2015 Hunter Valley, Conference of the Australian Acoustical Society, Hunter Valley, NSW, Australia.View/Download from: UTS OPUS
With the advent of affordable nano-satellite designs (off-the-shelf payloads, standardised launch geometries), increasingly enterprises, governmental agencies and universities have started developing their own space programs to explore the environment of Low Earth Orbits. Thin, flexible and unfolding/deployable structures are common space engineering antenna and solar panel designs owing to their lightweight and ideal packaging characteristics, which are, however, difficult to experimentally validate in a 1-g environment. Further, curvatures or discontinuities to increase functionality without violating prioritised design criteria may lead to system-level trade-offs: stability issues arising from buckling in combination with micro-vibrations which feed back to the satellite's attitude behaviour. It appears that the literature lacks a systematic investigation of these aspects. On-Earth experimental validations (static experiments, model updating) are the starting point for studying the response to static/dynamic loading of thin curved flexible structures such as deployable high frequency antennas. Linear and nonlinear buckling modes owing to varying loadings (aerodynamic drag, solar radiation pressure, residual gravity and magnetic body forces) are found together with a high sensitivity to torsional modes' frequency changes under micro-vibrational forcing.
Zhang, Z, Oberst, S & Lai, JCS 2014, 'Instability prediction of brake squeal by nonlinear stability analysis', INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control, INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control.View/Download from: UTS OPUS
Prediction of brake squeal as unwanted high frequency noise above 1 kHz remains a challenging problem despite substantial research efforts in the past two decades. Brake squeal, triggered by friction-induced self-excited vibration, can be caused by many different and interacting mechanisms with nonlinear origins in material properties and boundary conditions. Although brake squeal is essentially a nonlinear phenomenon, the standard industrial practice for prediction of brake squeal relies on the linear complex eigenvalue analysis which may under-predict or over-predict the number of unstable vibration modes. Brake squeal can be considered in nonlinear dynamics terms to be caused by a friction-induced self-excitation driven into instability and oscillating in a limit cycle through super-critical Andronov-Hopf bifurcations. In this paper, a nonlinear stability analysis that may be applied to a full brake system is examined using an unforced 4-DOF friction oscillator with cubic nonlinearity. The local bifurcation behaviour of this model is studied using the normal form theory and the nonlinear stability boundary is evaluated. Differences between results of linear and nonlinear analyses are discussed and the limitations of the linear analysis are highlighted. The energy provided by friction and consumed by damping is calculated by multiple scales method to provide a physical explanation for instability generation.
Oberst, S, Nava-Baro, E, Lai, JCS & Evans, TA 2014, 'An innovative signal processing technique for the extraction of ants' walking signals', INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control, INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control.
Eusocial insects such as bees, ants and termites communicate multi-modally using chemical, visual, tactile and vibrational cues. While much work has been done on chemical and visual communications, the tactile and vibrational communication channel is somewhat neglected. Recent research indicates that structural vibrations caused by ants can be used to identify their activity level. However, these structural vibrations are caused by the response of the substrate excited by ants walking. The objective of this study is to determine the footprint of ants walking by separating the response of the substrate from the walking signal. The vibration of the substrate (in this case, a wooden veneer) caused by ants walking is measured by a laser vibrometer in an experimental setup isolated from environmental vibrations. By filtering the recorded vibration signal using a technique based on the dynamics in phase space followed by deconvolution from the response of the veneer using TIKHONOV regularisation, the ant's walking signal is extracted and its nature determined.
Zhang, Z, Oberst, S & Lai, JCS 2014, 'A stochastic approach to predicting brake squeal propensity', 21st International Congress on Sound and Vibration 2014, ICSV 2014, 21st International Congress on Sound and Vibration 2014, ICSV 2014, pp. 629-636.View/Download from: UTS OPUS
Brake squeal as a significant noise, vibration and harshness (NVH) issue to the automotive industry is triggered by friction-induced self-excited vibration. Validating theoretical predictions using analytical or numerical models against experiments is difficult because the test results are often not repeatable even under apparently similar operating conditions. The poor repeatability of brake squeal could be attributed to the nonlinearity of the dynamics involved and the uncertainties associated with material properties, boundary conditions (such as contact pressure, temperature, stiffness, exact area of contact) and operating conditions. In this paper, a stochastic approach to predicting brake squeal propensity is examined using an analytical model of a popular 4-DOF friction oscillator with constant friction coefficient. Instability of this model is first estimated using the conventional linear complex eigenvalue analysis (CEA) and compared with calculations of positive friction work. The sensitivity of this deterministic model to variations of parameters such as spring stiffness and damping coefficient is studied. To account for uncertainties in the exact values of parameters, the analytical model is studied using polynomial chaos expansions with beta distribution on a set of Jacobi polynomials. Probabilities for instabilities based on positive friction work are determined and the implications for estimating squeal propensity in a full brake system are discussed.
Baro, EN, Oberst, S, Lai, JCS & Evans, TA 2013, 'A signal processing method for extracting vibration signals due to ants' activities', 42nd International Congress and Exposition on Noise Control Engineering 2013, INTER-NOISE 2013: Noise Control for Quality of Life, International Congress and Exposition on Noise Control Engineering, Innsbruck, Austria, pp. 3631-3640.View/Download from: UTS OPUS
Many software algorithms have been developed to track ants by analysing recorded videos. On the other hand, the feasibility of using vibrations measured at the substrate to classify ants' behaviour has not been examined before. A method is developed to separate vibrations owing to ants' activities from the substrate's response through a filtering/de-convolution procedure. This involves estimating the frequency response of the substrate and applying wavelet analysis to the measured vibrations. A number of responses due to ants' behaviours have been observed: Ants shaking, falling, carrying stones, walking, scratching/biting, tapping hind legs, grooming, and antennation/feeding. Vibrations produced by ants falling, carrying stones, walking and scratching/biting are measurable (i.e, above background noise levels). The proposed method is shown to be successful in classifying activities due to ants falling, ants carrying stones and to a lesser extent ants' scratching/biting. With further refinement, it seems feasible to use vibrations and the proposed algorithm to measure ants' behaviours in bioassays. Copyright© (2013) by Austrian Noise Abatement Association (OAL).
Zhang, Z, Oberst, S & Lai, JCS 2013, 'Application of polynomial chaos expansions to analytical models of friction oscillators', Annual Conference of the Australian Acoustical Society 2013, Acoustics 2013: Science, Technology and Amenity, Annual Conference of the Australian Acoustical Society, Victor Harbor, Australia, pp. 408-414.View/Download from: UTS OPUS
Despite past substantial research efforts, the prediction of brake squeal propensity remains a largely unresolved problem. The standard practice to predict the brake squeal propensity is to analyse dynamic instabilities using the complex eigenvalue analysis. However, it is well known that not every predicted unstable vibration mode will lead to squeal and vice-versa. Owing to nonlinearity and problem complexity (e.g. operating conditions), treating brake squeal with uncertainty seems appealing. Another indicator of brake squeal propensity, not often used, is based on negative dissipated energy. In this study, uncertainty analysis induced by polynomial chaos expansions is examined for 1-dof and 4-dof friction models. Results are compared with dissipated energy calculations and standard complex eigenvalue analysis. The potential of this approach for the prediction of brake squeal propensity is discussed. © (2013) by the Australian Acoustical Society.
Oberst, S & Lai, JCS 2013, 'The role of pad-mode instabilities in disc brake squeal', 20th International Congress on Sound and Vibration 2013, ICSV 2013, International Congress on Sound and Vibration, Bangkok, Thailand, pp. 2861-2868.View/Download from: UTS OPUS
Automotive disc brake squeal remains an economically significant and technically challenging problem to solve, owing to customer complaints' associated warranty costs and the many interacting parameters. While industrial practice aims at identifying unstable vibration modes using complex eigenvalue analysis, in this paper, we show how to identify pad-mode instabilities using vibration forced response analysis complemented by acoustic radiation calculations for simplified brake systems in the form of a pad-on-plate model. Our recent results indicate that pad-mode instabilities might trigger so-called instantaneous mode squeal without the necessity of mode coupling. Pad-mode instabilities, which complex eigenvalue analysis fails to detect, are revealed by the dissipated energy spectrum at frequencies where the dissipated energy is negative (i.e. providing energy instead of dissipating energy). Pad-modes seem to radiate locally higher sound pressure depending on the phase shift between the structural vibration and the sound pressure while exciting the underlying plate's or disc's modes. Pad-mode instabilities are shown to be one mechanism of brake squeal. In order to identify pad-mode instabilities, it is beneficial to perform a full range of vibration analysis which includes complex eigenvalue value analysis, forced response and dissipated energy spectra as well as acoustic radiation calculations for a range of different parameters such as friction coefficient, operating pressure, temperature and contact conditions.
Oberst, SM & Lai, J 2013, 'The role of pad-modes and nonlinearity in instantaneous mode squeal', Proceedings of Meetings on Acoustics, Meetings on Acoustics.View/Download from: UTS OPUS or Publisher's site
Disc brake squeal is a major source of customer dissatisfaction and related warranty costs for automobile manufacturers. Although mode coupling is recognised as a mechanism often found in squealing brakes, recent research results show that friction induced pad-mode instabilities could be the cause of instantaneous mode squeal reported in the literature. In this paper, the nonlinear characteristics of instantaneous mode squeal initiated by pad-mode instabilities are studied by analysing phase space plots of vibrations and sound pressure for a numerical model of a pad-on-plate system as the friction coefficient increases. Results show tat as the friction coefficient increases from 0.05 to 0.65, attractors of vibration in the phase space transits from limit cycle to quasi-periodic, showing signs of approaching chaotic behaviour. It is shown here that the correlation of the sound pressure behaviour in the phase-space with structural vibration is crucial to understanding the role of pad modes and nonlinearity in instantaneous mode squeal. © 2013 Acoustical Society of America.
Oberst, S & Lai, JCS 2012, 'Analysis of disc brake squeal: Progress and challenges', 19th International Congress on Sound and Vibration 2012, ICSV 2012, pp. 2874-2881.
Brake squeal noise has been the subject of intense research efforts owing to concerns of car manufacturers caused by complaints lodged and warranty claim related costs arising from dissatisfied customers. Brake squeal is known to be fugitive, and often not repeatable, even under apparently similar operating conditions. The production of brake squeal is dependent on a large number of interacting parameters, such as the mechanical properties of the brake lining materials, contact conditions, wear, operating pressure and temperature which contribute to its often observed nonrepeatability. In this paper, an overview of the state-of-the-art understanding of brake squeal mechanisms and numerical analysis methods (primarily based on finite element analysis) for the prediction of brake squeal propensity is presented. The question of nonlinearity of brake squeal is raised in terms of analysing the mechanisms and how present solution methods reflect this degree of nonlinearity. This is complemented by a description of current industrial practice in the treatment of brake squeal which is, generally, managed on a case-by-case, trial-and-error basis using expensive equipment and time-consuming noise dynamometer and/or on-vehicle tests. The gaps between theory and industrial practice and, hence, challenges for brake squeal research are identified. Recommendations for bridging these gaps and improving the usefulness of current numerical methods for practical industrial use are proposed.
The prediction of disc brake squeal propensity remains difficult despite significant progress made in the last two decades towards understanding its nature. Most of the numerical analysis of brake squeal is based on linear methods that have found some success in guiding the development of brakes in industry. One popular approach is the complex eigenvalue analysis using finite element models to predict unstable vibration modes. However, the complex eigenvalue analysis may over-predict or under-predict the number of unstable vibration modes and not all predicted unstable vibration modes will result in squeal. Therefore, extensive brake testing in noise dynamometers is required in order to ensure that the noise performance of brakes is acceptable. Although the analysis of brake squeal propensity is primarily based on linear approaches, it has been recognised that the operation of a brake contains a number of nonlinearities such as the excitation through the friction contact between the disc and pad, material properties, and operating conditions. The purpose of this paper is to provide an overview on nonlinearity as one mechanism of the cause of brake squeal and to discuss how such knowledge could be used to develop alternative strategies in numerical prediction of brake squeal propensity. © European Acoustics Association.
Oberst, S & Lai, JCS 2011, 'Nonlinear friction coupling in disc brake squeal', 18th International Congress on Sound and Vibration 2011, ICSV 2011, International Congress on Sound and Vibration, International Institute of Acoustics & Vibration, Rio de Janeiro, Brazil, pp. 1748-1755.
Friction-induced noise, such as disc brake squeal as a research area of practical importance to the automotive industry, has been investigated for many years. In recent years research focus was on dynamic instabilities such as mode coupling as the most prominent squeal mechanism, rather than on physical or geometric instabilities as trigger mechanisms. However, many trigger mechanisms such as contact and friction nonlinearity are still poorly understood. In brake squeal analysis, pad-dynamics is poorly understood and in-plane radial and out-of-plane pad vibrations are often neglected as most research has been directed towards understanding the dynamics of the rotor as the main structure radiating sound. However, recent research has shown that transient radial in-plane vibrations of the pad might be a novel squeal mechanism. In this study, the transient nature of these radial in-plane vibrations and their influence on the overall vibration and dynamic behaviour is investigated. For this purpose a sinusoidally driven in-plane sliding of a friction-coupled 2-dof oscillator (in the form of a slider over a moving belt) is formulated based on the most stable configuration of a 1-dof dry friction oscillator with continuous/ locking contact using a friction law with constant/static-kinetic/velocitydependent friction coefficient. It is found that due to nonlinear friction-coupling, steady-state in-plane radial vibrations induce a broadband spectrum in the overall dynamics of the friction oscillator with slightly fractal POINCARE section for a small belt's angle. The results indicate the importance of in-plane pad-vibrations as a possible trigger squeal mechanism. Copyright © (2011) by the International Institute of Acoustics & Vibration.
Oberst, S & Lai, JCS 2010, 'Acoustic radiation of friction-induced pad-mode instability in disc brake squeal', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 2158-2168.
Since the early 1920s, disc brake squeal has been an issue for the automobile industry due to dissatisfied customer's complaints and the accompanying warranty costs. Despite a good deal of progress having been made in predicting brake squeal propensity, not all mechanisms are known and brake squeal remains unpredictable and highly fugitive. In recent years, research has been focused on brake squeal due to the mode-coupling type of instability, leaving out the primary friction-induced mechanisms such as stick-slip. In this paper, the acoustic radiation of simplified brake systems, in the form of a pad rubbing on both a plate and disc, is investigated. The radiation efficiency and acoustic power are calculated using the acoustic boundary element method, specifically ESI's Fast Multipole Solver (DFMM) implemented in VAOne. Results show that there exist some frequencies at which squeal occurs but which are predicted by the complex eigenvalue method. These frequencies do not correspond to the frequencies of the rotor modes and are here referred to as 'instantaneous' pad-modes causing a friction-induced instability. The frequencies of these instantaneous modes are dependent on the material properties of the pad and the contact conditions. Radiation efficiency due to pressure variations changes less, than due to friction coefficient variations. Further, it is shown, that pad-modes are acoustically relevant and especially active at lower pressures.
Oberst, S & Lai, JCS 2010, 'Numerical methods for simulating brake squeal noise', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 1505-1516.
Due to significantly reduced interior noise as a result of reduction of noise from internal combustion engine and tyre-road contact noise and the use of lightweight composite materials for the car body, disc brake squeal has become increasingly a concern to automotive industry because of the high costs in warranty related claims. While it is now almost standard practice to use the complex eigenvalue method in commercial finite element codes to predict unstable vibration modes, not all predicted unstable vibration modes will squeal and vice versa. There are very few attempts to calculate the acoustic radiation from predicted unstable vibration modes. Guidelines on how to predict brake squeal propensity with confidence are yet to be established. In this study, three numerical aspects important for the prediction of brake squeal propensity are examined: how to select an appropriate mesh; comparisons of methods available in ABAQUS 6.8.-4 for harmonic forced response analysis; and comparisons of boundary element methods (BEM) for acoustic radiation calculations in LMS VL Acoustics and ESI VA. In the mesh study, results indicate that the mesh has to be sufficiently fine to predict mesh independent unstable modes. While linear and quadratic tetrahedral elements offer the best option in meshing more realistic structures, only quadratic tetrahedral elements should be used for solutions to be mesh independent. Otherwise, linear hexahedral elements represent an alternative but are not as easy to apply to complex structures. In the forced response study, the modal, subspace and direct steady-state response analysis in ABAQUS are compared to each other with the FRF synthesis case in LMS/VL Acoustics. Results show that only the direct method can take into account friction effects fully. In the numerical analysis with acoustic boundary elements, the following methods are compared in terms of performance and accuracy for a model of a sphere, a cat's eye radiator, a pad-on-plate ...
Oberst, S & Lai, JCS 2010, 'Numerical study of friction-induced pad-mode instability in disc brake squeal', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 2146-2157.
Disc brake squeal as a major source of customer dissatisfaction is known to be friction-induced due to the highly non-linear contact of the surfaces between the disc and the pads. Brake squeal remains fugitive and difficult to predict also to some of its squeal frequencies have varying character and cannot always be associated with component modes. By means of structural finite element analysis, a simplified brake system in the form of a pin-on-disc is firstly approximated by a block sliding on a plate. By varying pressure and the friction coefficient, no mode coupling instability is observed and the mechanism extracted is purely of friction-induced nature. Especially in-plane pad motion in direction of and perpendicular to the sliding direction seem to feed-in most of the energy. These modes and their variability due to pressure variation, changes of lining material's elastic components and increased friction coefficient are studied in the following by means of the plate model. Then, it is shown, that these pad modes also exist for a pad-on-disc model with isotropic lining material. A second pad-on-plate model with more realistic lining material is developed which considers changes of elastic constants due to pressure variations. It is found, that changes in elastic properties of the lining material influence significantly the vibrations of the pad modes. The kinetic energy spectrum lifts up with changing pressure and stiffness and that combined effects of pressure synchronised with changing material properties are more severe than could be assumed by the complex eigenvalue method alone. By means of inverse Fourier transform of the response spectrum and non-linear time series analysis it is possible to detect the instability of the pad-on-plate model. The results show that friction-induced instabilities result from non-binding forces between pad and disc, with energy transfer from pad to disc causing dynamic instability, might trigger mode coupling or amplify un...
Oberst, S & Lai, JCS 2010, 'Uncertainty modelling for detecting friction-induced pad-mode instabilities in disc brake squeal', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 1517-1528.
Since the early 1930s, brake squeal has been a problem for NVH departments and the high-pitched noise causes customers to complain and lodge costly warranty claims. Due to its friction-induced nature, material properties and operating conditions, the problem of brake squeal is non-linear and highly complex. In the past, research has been focussed on mode-coupling instability predicted by the complex eigenvalue analysis (CEA). However, for unstable modes not detected by CEA, friction-induced energy fed back by the pad modes, due to the friction coefficient, pressure variations and non-linear material properties, has been shown, by means of non-linear time series analyses and the acoustic boundary element method, to cause friction-induced pad squeal or to amplify the mode coupling of brake components for a pad-on-plate system. It is suggested that pad mode instabilities be treated as a stochastic process defined by a random 3-parameter-space: the mean changes in kinetic energy, frequency and acoustic power caused by changes in pressure or the friction coefficient. It is shown that, for a pad-on-plate system and a pad-on-disc simplified brake system, this stochastic approach enables the probability to be calculated for a specified increase in kinetic energy or a specified change in frequencies, thus allowing the assessment of brake squeal propensity and the development of strategies for controlling brake squeal. Copyright © (2010) by the International Congress on Acoustics.
Oberst, S & Lai, JCS 2009, 'Non-linear analysis of brake squeal', 16th International Congress on Sound and Vibration 2009, ICSV 2009, pp. 1116-1123.
Transportation noise has received increasingly more attention in the last decade from the environmental point of view but also from customers. In the case of cars, manufacturers have focussed on reducing the overall car's noise level. As a result, engine, gearboxes and the overall drive train have seen a reduction sound emission. Because of this overall reduction, brake squeal becomes even more audible. Brake squeal is a constant annoyance for customers and a significant cost factor for the car industry due to warranty claims. Brake squeal is self-sustained friction-induced noise and various mechanisms have been shown to be capable of causing unstable vibration behaviour, for example, sprag-slip, stick-slip or mode coupling. Usually disc brake squeal is analysed by means of linear methods such as the complex eigenvalue method employed in finite element analysis as virtually an industry standard. However, brake squeal is intrinsically a transient and non-linear process, the geometry is complex, the material properties and contact conditions between brake pads and the brake disc are difficult to be determined exactly. As a result, prediction of brake squeal propensity is difficult to be realised in the near future. In this study, a new approach to brake squeal is undertaken by performing non-linear time series analysis of data obtained from an accelerometer and a microphone of a brake system in a dynamometer. New insight into brake squeal will be discussed.
Oberst, S & Lai, JCS 2009, 'Numerical prediction of brake squeal propensity using acoustic power calculation', Annual Conference of the Australian Acoustical Society 2009 - Acoustics 2009: Research to Consulting, pp. 111-118.
Both low- and high-frequency disc brake squeal, first studied some 80 years ago, remain of concern to automotive NVH departments due to customer warranty claims. Despite both intensive and extensive research, disc brake squeal is still not well understood. It is a very complex problem which involves many different disciplines, such as tribology, structural vibration, acoustic radiation and dynamic instabilities. While there has been considerable research in the first two areas (tribology and vibration analysis), the prediction of brake squeal through acoustic radiation calculations using numerical methods has remained largely unexplored. In this paper, the influence of the geometrical designs of brake pad on brake squeal is studied using a simplified brake setup consisting of an annular disc in contact with one brake pad. The various configurations of a brake pad studied here has been influenced by those used in the industrial testing of a full brake system. In this study, unstable vibration modes were first identified by the conventional complex eigenvalue analysis of a finite element model of the simplified brake system. Then, the acoustic power was calculated for a range of frequencies and friction coefficients using the acoustic boundary element method. It is shown that the performance of the various pads, in terms of brake squeal propensity caused by their geometric differences, could be ranked based on contour plots of acoustic power with friction coefficient and frequency as the independent variables. These results indicate that the inclusion of acoustic power calculations, following a complex eigenvalue analysis of unstable vibration modes, provides improved prediction of brake squeal propensity.
Oberst, S & Lai, JCS 2008, 'New approaches for understanding the mechanisms of brake squeal', Annual Conference of the Australian Acoustical Society, AAS'08, pp. 265-272.
Brake squeal has become an increasing concern for the automotive industry because of associated warranty costs and the requirement for the continued reduction of interior vehicle noise. Low and high frequency noises in car brakes, often referred to as brake squeal, are known to be a result of nonlinearity, unstable behaviour and bifurcations leading to limit cycle behaviour. By using the data from a separate experimental study designed to determine the influence of the geometric parameters of brake pads (such as the number and location of slots) on brake squeal noise, we examine two new approaches for providing improved understanding of the brake squeal phenomenon: statistical and nonlinear dynamics analyses. Results of the statistical analysis indicate that the performances of certain pad designs correlate with their levels of nonlinearity. The nonlinear time series analysis reveals that, in the experimental data, not only are limit cycle behaviours present but also a route to chaotic solutions can be observed.
Merz, S, Oberst, S, Dylejko, PG, Kessissoglou, NJ, Tso, YK & Marburg, S 2005, 'Development of coupled FE/BE models to investigate the structural and acoustic responses of a submerged vessel', JOURNAL OF COMPUTATIONAL ACOUSTICS, 7th International Conference on Theoretical and Computational Acoustics, WORLD SCIENTIFIC PUBL CO PTE LTD, Hangzhou, PEOPLES R CHINA, pp. 23-47.View/Download from: Publisher's site
Oberst, S, Stender, M, Baetz, J, Campbell, G, Lampe, F, Lai, JCS, Morlock, M & Hoffmann, N 2018, 'Extracting differential equations from measured vibro-acoustic impulse responses in cavity preparation of total hip arthroplasty'.
Hobbs, B, Ord, A, Oberst, S & Niven, RK 2017, 'Nonlinear episodic and chaotic behaviour of orogenic gold systems'.
- UNSW (Australia): Prof Joseph C.S. Lai; A/Prof Robert Niven; A/Prof Nicole Kessissoglou
- UWA (Australia) A/Prof Theodore A. Evans
- TU Munich (Germany): Prof Steffen Marburg
- Imperial College London & TU Hamburg-Harbug: Prof Norbert Hoffmann
- Cambridge University (Zoology, Neuroscience): Dr Caroline C.G. Fabre