Biomedical subjects

Andy Kho

Publications and source records attributed to Andy Kho.

6 recordsLinked to original sources

PubMed2006

Complex behavior in driven unidirectionally coupled overdamped Duffing elements.

It is well known that overdamped unforced dynamical systems do not oscillate. However, well-designed coupling schemes, together with the appropriate choice of initial conditions, can induce oscillations (corresponding to transitions between the stable steady states of each nonlinear element) when a control parameter exceeds a threshold value. In recent publications [A. Bulsara, Phys. Rev. E 70, 036103 (2004); V. In, ibid. 72, 045104 (2005)], we demonstrated this behavior in a specific prototype system, a soft-potential mean-field description of the dynamics in a hysteretic "single-domain" ferromagnetic sample. These oscillations are now finding utility in the detection of very weak "target" magnetic signals, via their effect on the oscillation characteristics--e.g., the frequency and asymmetry of the oscillation wave forms. We explore the underlying dynamics of a related system, coupled bistable "standard quartic" dynamic elements; the system shows similarities to, but also significant differences from, our earlier work. dc as well as time-periodic target signals are considered; the latter are shown to induce complex oscillatory behavior in different regimes of the parameter space. In turn, this behavior can be harnessed to quantify the target signal.

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PubMed2005

Complex dynamics in unidirectionally coupled overdamped bistable systems subject to a time-periodic external signal.

Recently, we have studied the emergence of oscillatory behavior in overdamped undriven nonlinear dynamic systems subject to carefully crafted coupling schemes and operating conditions [V. In, Phys. Rev. E 68, 045102(R) (2003).] The theoretical ideas have been validated in an experimental setup of N = 3 coupled ferromagnetic cores subject to a dc external magnetic "target" signal; the oscillations (corresponding to the periodic switching of each core between its stable steady states of magnetization) are triggered when the coupling constant crosses a threshold value, with the oscillation frequency exhibiting a characteristic scaling behavior with the "separation" of the coupling constant from its threshold value, as well as with the external signal amplitude. Here, we consider the system response to a time-periodic signal. We demonstrate experimentally that, depending on the signal amplitude and frequency, the response can be either synchronized to the signal frequency or to one-third this frequency. These phenomena afford unique techniques for time-periodic signal detection and characterization for a large class of sensors.

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PubMed2005

Multifrequency synthesis using two coupled nonlinear oscillator arrays.

We illustrate a scheme that exploits the theory of symmetry-breaking bifurcations for generating a spatio-temporal pattern in which one of two interconnected arrays, each with N Van der Pol oscillators, oscillates at N times the frequency of the other. A bifurcation analysis demonstrates that this type of frequency generation cannot be realized without the mutual interaction between the two arrays. It is also demonstrated that the mechanism for generating these frequencies between the two arrays is different from that of a master-slave interaction, a synchronization effect, or that of subharmonic and ultraharmonic solutions generated by forced systems. This kind of frequency generation scheme can find applications in the developed field of nonlinear antenna and radar systems.

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PubMed2004

Emergent oscillations in unidirectionally coupled overdamped bistable systems.

It is well known that overdamped unforced dynamical systems do not oscillate. However, well-designed coupling schemes, together with the appropriate choice of initial conditions, can induce oscillations when a control parameter exceeds a threshold value. In a recent publication [Phys. Rev. E 68, 045102 (2003)]], we demonstrated this behavior in a specific prototype system, a soft-potential mean-field description of the dynamics in a hysteretic "single-domain" ferromagnetic sample. The previous analysis of this work showed that N (odd) unidirectionally coupled elements with cyclic boundary conditions would, in fact, oscillate when a control parameter-in this case the coupling strength-exceeded a critical value. These oscillations are now finding utility in the detection of very weak "target" signals, via their effect on the oscillation characteristics, e.g., the frequency and asymmetry of the oscillation wave forms. In this paper we explore the underlying dynamics of this system. Scaling laws that govern the oscillation frequency in the vicinity of the critical point, as well as the zero-crossing intervals in the presence of a symmetry-breaking target dc signal, are derived; these quantities are germane to signal detection and analysis.

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PubMed2003

Experimental observation of multifrequency patterns in arrays of coupled nonlinear oscillators.

Frequency-related oscillations in coupled oscillator systems, in which one or more oscillators oscillate at different frequencies than the other oscillators, have been studied using group theoretical methods by Armbruster and Chossat [Phys. Lett. A 254, 269 (1999)] and more recently by Golubitsky and Stewart [in Geometry, Mechanics, and Dynamics, edited by P. Newton, P. Holmes, and A. Weinstein (Springer, New York, 2002), p. 243]. We demonstrate, experimentally, via electronic circuits, the existence of frequency-related oscillations in a network of two arrays of N oscillators, per array, coupled to one another. Under certain conditions, one of the arrays can be induced to oscillate at N times the frequency of the other array. This type of behavior is different from the one observed in a driven system because it is dictated mainly by the symmetry of the coupled system.

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PubMed2003

Coupling-induced oscillations in overdamped bistable systems.

It is well known that overdamped and unforced dynamical systems do not oscillate. However, well-designed coupling schemes, together with the appropriate choice of initial conditions, can induce oscillations when a control parameter exceeds a threshold value. We demonstrate this effect in a specific system, a soft-potential mean-field description of the dynamics in a (hysteretic) single-domain ferromagnetic sample. Using a specific (unidirectional, with cyclic boundary conditions) coupling scheme, together with nonidentical initial conditions, one can cause the coupled system of N elements (N odd) to oscillate when the coupling coefficient is swept through a critical value. The ensuing oscillations could find utility in the detection of very weak "target" signals, via their effect on the oscillation characteristics.

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