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Biomedical subjects

Stephen J Elliott

Publications and source records attributed to Stephen J Elliott.

10 recordsLinked to original sources
PubMed2006

Active vibration control using an inertial actuator with internal damping.

Collocated direct velocity feedback with ideal point force actuators mounted on structures is unconditionally stable and generates active damping. When inertial actuators are used to generate the control force, the system can become unstable even for moderate velocity feedback gains due to an additional -180 degree phase lag introduced by the fundamental axial resonant mode of the inertial actuator. In this study a relative velocity sensor is used to implement an inner velocity feedback loop that generates internal damping in a lightweight, electrodynamic, inertial actuator. Simulation results for a model problem with the actuator mounted on a clamped plate show that, when internal relative velocity feedback is used in addition to a conventional external velocity feedback loop, there is an optimum combination of internal and external velocity feedback gains, which, for a given gain margin, maximizes vibration reduction. These predictions are validated in experiments with a specially built lightweight inertial actuator.

Journal Article↗
PubMed2006

An active headrest for personal audio.

There is an increasing need for personal audio systems, which generate sounds that are clearly audible to one listener but are not audible to other listeners nearby. Of particular interest in this paper are listeners sitting in adjacent seats in aircraft or land vehicles. Although personal audio could then be achieved with headsets, it would be safer and more comfortable if loudspeakers in the seat headrests could be actively controlled to generate an acceptable level of acoustic isolation. In this paper a number of approaches to this problem are investigated, but the most successful involves a pair of loudspeakers on one side of the headrest, driven together to reproduce an audio signal for a listener in that seat and also to attenuate the pressures in the adjacent seat. The performance of this technique is investigated using simple analytic models and also with a practical implementation, tested in an anechoic chamber and a small room. It is found that significant attenuations, of between 5 and 25 dB, can be achieved in the crosstalk between the seats for frequencies up to about 2 kHz.

Acoustics↗
PubMed2005

Dynamic nonlinear cochlear model predictions of click-evoked otoacoustic emission suppression.

A comprehensive set of results from 2-click suppression experiments on otoacoustic emissions (OAEs) have been presented by Kapadia and Lutman [Kapadia, S., Lutman, M.E., 2000a. Nonlinear temporal interactions in click-evoked otoacoustic emissions. I. Assumed model and polarity-symmetry. Hear. Res. 146, 89-100]. They found that the degree of suppression of an OAE evoked by a test click varied systematically with the timing and the level of a suppressor click, being greatest for suppressor clicks occurring some time before the test click, particularly at lower levels of suppression. Kapadia and Lutman also showed that although the general shape of the graph of suppression against suppressor click timing could be predicted by a static power law model, this did not predict the asymmetry with respect to the timing of the suppressor click. A generalised automatic gain control (AGC) is presented as a simple example of a dynamic nonlinear system. Its steady state nonlinear behaviour, as quantified by its level curve, and its dynamic behaviour, as quantified by its transient response, can be independently set by the feedback gain law and detector time constant, respectively. The previously reported suppression results, with the asymmetry in the timing, are found to be predicted better by such an AGC having a level curve with a slope of about 0.5 dB/dB, and a detector time constant of about twice the period at the characteristic frequency. Although this gives adequate predictions for high suppression levels, it under predicts the suppression and the asymmetry for lower levels. Further research is required to establish whether simple peripheral feedback models can explain OAE suppression of this type.

Acoustic Stimulation↗
PubMed2005

Smart panels with velocity feedback control systems using triangularly shaped strain actuators.

In this paper we present a theoretical study on the active structural acoustic control of a new smart panel with sixteen triangularly shaped piezoelectric patch actuators, having their base edges evenly distributed along the perimeter of the panel, and velocity sensors positioned at the vertices opposite the base edges. The performance is assessed and contrasted with that of a conventional smart panel using a 4 x 4 array of square piezoelectric patch actuators evenly distributed over the surface of the panel with velocity sensors at their centers. For both systems the control effectiveness and stability of MIMO decentralized or SISO direct velocity feedback control architectures have been analyzed. The two control systems are arranged to generate active damping which reduces the response and sound radiation of the panel in the lightly damped and well separated low-frequency resonances. In particular the new control system can be seen as a set of sixteen "active wedges" which absorb energy from the incident flexural waves to the borders of the panel so that the panel could be considered anechoic. This study shows that the new arrangement with triangularly shaped actuators can achieve better control than the corresponding system using square actuators.

Journal Article↗
PubMed2005

Using the short-time correlation coefficient to compare transient- and derived, noise-evoked otoacoustic emission temporal waveforms.

Transient-evoked otoacoustic emissions (TEOAEs) and derived, noise-evoked otoacoustic emissions (derived-NEOAEs) were measured in seven normally hearing subjects. The evoked OAEs were all recorded at three excitation levels chosen to ensure that the OAE level curve compressive region was reached. The short-time correlation coefficient (STCC) was used to compare the OAE waveforms at different excitation levels, and thus estimate the time over which the response exceeds the noise level. The short-time correlation for TEOAEs is significant for longer than it is for NEOAEs, particularly in some individuals, and the STCC allows this to be quantified. This suggests that derived NEOAEs do not display the highly synchronized dominant frequencies often seen in TEOAEs. This has been confirmed by examining the derived frequency responses for the two types of excitation. Conventional TEOAEs thus appear to measure a combination of two conceptually different processes, while NEOAEs measure just one.

Acoustic Stimulation↗
PubMed2005

A comparison of various nonlinear models of cochlear compression.

The vibration response of the basilar membrane in the cochlea to sinusoidal excitation displays a compressive nonlinearity, conventionally described using an input-output level curve. This displays a slope of 1 dB/dB at low levels and a slope m < 1 dB/dB at higher levels. Two classes of nonlinear systems have been considered as models of this response, one class with static power-law nonlinearity and one class with level-dependent properties (using either an automatic gain control or a Van der Pol oscillator). By carefully choosing their parameters, it is shown that all models can produce level curves that are similar to those measured on the basilar membrane. The models differ, however, in their distortion properties, transient responses, and instantaneous input-output characteristics. The static nonlinearities have a single-valued instantaneous characteristic that is the same at all input levels. The level-dependent systems are multi-valued with an almost linear characteristic, for a given amplitude of excitation, whose slope varies with the excitation level. This observation suggests that historical attempts to use functional modeling (i.e., Wiener of Volterra series) may be ill founded, as these methods are unable to represent level-dependent nonlinear systems with multi-valued characteristics of this kind.

Acoustic Stimulation↗
PubMed2003

Efficient estimation of a time-varying dimension parameter and its application to EEG analysis.

This paper considers the problem of estimating the dimension of nonstationary electroencephalogram (EEG) signals and describes the implementation of an efficient algorithm to calculate a time-varying dimension estimate. The algorithm allows the practical calculation of a dimension estimate and its statistical significance over large data sets with a high temporal resolution. The method is applied to EEG recordings from patients with temporal lobe epilepsy and in one case the results of the analysis are compared with those obtained from an existing method of computing the correlation density.

Algorithms↗
PubMed2002

Active vibroacoustic control with multiple local feedback loops.

When multiple actuators and sensors are used to control the vibration of a panel, or its sound radiation, they are usually positioned so that they couple into specific modes and are all connected together with a centralized control system. This paper investigates the physical effects of having a regular array of actuator and sensor pairs that are connected only by local feedback loops. An array of 4 x 4 force actuators and velocity sensors is first simulated, for which such a decentralized controller can be shown to be unconditionally stable. Significant reductions in both the kinetic energy of the panel and in its radiated sound power can be obtained for an optimal value of feedback gain, although higher values of feedback gain can induce extra resonances in the system and degrade the performance. A more practical transducer pair, consisting of a piezoelectric actuator and velocity sensor, is also investigated and the simulations suggest that a decentralized controller with this arrangement is also stable over a wide range of feedback gains. The resulting reductions in kinetic energy and sound power are not as great as with the force actuators, due to the extra resonances being more prominent and at lower frequencies, but are still worthwhile. This suggests that an array of independent modular systems, each of which included an actuator, a sensor, and a local feedback control loop, could be a simple and robust method of controlling broadband sound transmission when integrated into a panel.

Acoustics↗
PubMed2002

Coupling analysis of a matched piezoelectric sensor and actuator pair for vibration control of a smart beam.

This paper presents a theoretical and experimental study of the in-plane and out-of-plane coupling of a matched piezoelectric sensor/actuator pair bonded on a beam. Both the sensor and actuator are triangularly shaped polyvinylidene fluoride (PVDF) transducers and are intended to provide a compact sensor/actuator system for beam vibration control. The measured sensor-actuator frequency response function has shown an unpredicted increase in magnitude with frequency, which was found, to be due to in-plane vibration coupling. An analytical model has been developed to decompose the sensor-actuator response function into an in-plane contribution and an out-of-plane contribution. This in-plane coupling can limit the feedback control gains when a direct velocity feedback control is applied. A method called the j omega s compensation method is proposed to identify the effect of the in-plane vibration coupling at low frequencies. Even after this compensation, however, there was unexpected strong out-of-plane coupling at even modes, which may have been caused by a lack of accuracy in the shaping of the PVDF sensor and actuator. Numerical simulations have confirmed the sensitivity of the matched sensor/actuator pair with shaping errors.

Journal Article↗