Contributors:Ali, Zunaib, Christofides, Nicholas, Lenos Hadjidemetriou, Kyriakides, Elias
The presence of dc offset and harmonics/interharmonics in grid voltage input signal of phaselocked
loop (PLL) results in inaccurate controller response. The inaccuracies are due to the low and high
frequencyoscillations that appear in the PLL estimated phase, amplitude and frequency. The importance of
DC offset and harmonic/interharmonic rejection capability for PLLs can be appreciated by international
standards that impose strict limitations for grid-tied converters. The suppression of fundamental frequencyoscillations caused by DC offset in the input signal must be carried out without compromising the dynamic
response of the system. The use of low pass filters for example results in undesirable, slow response. This
paper proposes an accurate and fast decoupling of fundamental frequencyoscillations using a mathematic
cancellation decoupling cell. Higher frequencyoscillations generated by harmonics/interharmonics are
eliminated by a different compensation network (HCN) that is also proposed in this paper. The performance
of conventional techniques is limited because they eliminate only specifically selected harmonics. The
proposed HCN module, however, eliminates any number of harmonics/interharmonics present in the grid
with the least computational complexity and without any prior knowledge. Furthermore, its advanced
features provide accurate synchronization under any abnormal grid condition at the lowest computational
complexity when compared to existing state-of-the-art PLLs. The advanced performance of the proposed
Harmonic-Interharmonic-DC-Offset (HIHDO) PLL is verified through simulation and experimental results.
self-oscillation... Volatile threshold switching, or current controlled negative differential resistance (CC-NDR), has been observed in a range of transition metal oxides. Threshold switching devices exhibit a large non-linear change in electrical conductivity, switching from an insulating to a metallic state under external stimuli. Compact, scalable and low power threshold switching devices are of significant interest for use in existing and emerging technologies, including as a selector element in high-density memory arrays and as solid-state oscillators for hardware-based neuromorphic computing. This thesis explores the threshold switching in amorphous NbOx and the properties of individual and coupled oscillators based on this response. The study begins with an investigation of threshold switching in Pt/NbOx/TiN devices as a function device area, NbOx film thickness and temperature, which provides important insight into the structure of the self-assembled switching region. The devices exhibit combined threshold-memory behaviour after an initial voltage-controlled forming process, but exhibit symmetric threshold switching when the RESET and SET currents are kept below a critical value. In this mode, the threshold and hold voltages are shown to be independent of the device area and film thickness, and the threshold power, while independent of device area, is shown to decrease with increasing film thickness. These results are shown to be consistent with a structure in which the threshold switching volume is confined, both laterally and vertically, to the region between the residual memory filament and the electrode, and where the memory filament has a core-shell structure comprising a metallic core and a semiconducting shell. The veracity of this structure is demonstrated by comparing experimental results with the predictions of a resistor network model, and detailed finite element simulations. The next study focuses on electrical self-oscillation of an NbOx threshold switching device incorporated into a Pearson-Anson circuit configuration. Measurements confirm stable operation of the oscillator at source voltages as low as 1.06 V, and demonstrate frequency control in the range from 2.5 to 20.5 MHz with maximum frequency tuning range of 18 MHz/V. The oscillator exhibit three distinct oscillation regimes: sporadic spiking, stable oscillation and damped oscillation. The oscillationfrequency, peak-to-peak amplitude and frequency are shown to be temperature and voltage dependent with stable oscillation achieved for temperatures up to ∼380 K. A physics-based threshold switching model with inclusion of device and circuit parameters is shown to explain the oscillation waveform and characteristic. The final study explores the oscillation dynamics of capacitively coupled Nb/Nb2O5 relaxation oscillators. The coupled system exhibits rich collective behaviour, from weak coupling to synchronisation, depending on the negative differential resistance response of the individual devices, the operating voltage and the coupling capacitance. These coupled oscillators are shown to exhibit stable frequency and phase locking states at source voltages as low as 2.2 V with MHz frequency tunable range. The numerical simulation of the coupled system highlights the role of source voltage, and circuit and device capacitance in controlling the coupling modes and dynamics.... coupled oscillators
Contributors:Parzen, G, Morton, P
Contributors:Fleming, J. A., Dyke, G. B.
Contributors:Roden, Gunnar I.
Office of Naval Research Contracts Nonr-477(10) and Nonr-477(37) Project NR-083-012
Contributors:Li, Larry, Juniper, Matthew
In the analysis of thermoacoustic systems, a flame is usually characterised
by the way its heat release responds to acoustic forcing. This
response depends on the hydrodynamic stability of the flame. Some
flames, such as a premixed bunsen flame, are hydrodynamically globally
stable. They respond only at the forcing frequency. Other flames,
such as a jet diffusion flame, are hydrodynamically globally unstable.
They oscillate at their own natural frequencies and are often assumed
to be insensitive to low-amplitude forcing at other frequencies.
If a hydrodynamically globally unstable flame really is insensitive to
forcing at other frequencies, then it should be possible to weaken
thermoacoustic oscillations by detuning the frequency of the natural
hydrodynamic mode from that of the natural acoustic modes. This
would be very beneficial for industrial combustors.
In this thesis, that assumption of insensitivity to forcing is tested
experimentally. This is done by acoustically forcing two different selfexcited
flows: a non-reacting jet and a reacting jet. Both jets have
regions of absolute instability at their base and this causes them to
exhibit varicose oscillations at discrete natural frequencies. The forcing
is applied around these frequencies, at varying amplitudes, and
the response examined over a range of frequencies (not just at the
forcing frequency). The overall system is then modelled as a forced
van der Pol oscillator.
The results show that, contrary to some expectations, a hydrodynamically
self-excited jet oscillating at one frequency is sensitive to
forcing at other frequencies. When forced at low amplitudes, the jet
responds at both frequencies as well as at several nearby frequencies,
and there is beating, indicating quasiperiodicity. When forced at
high amplitudes, however, it locks into the forcing. The critical forcing
amplitude required for lock-in increases with the deviation of the
forcing frequency from the natural frequency. This increase is linear,
indicating a Hopf bifurcation to a global mode.
The lock-in curve has a characteristic ∨ shape, but with two subtle
asymmetries about the natural frequency. The first asymmetry concerns
the forcing amplitude required for lock-in. In the non-reacting
jet, higher amplitudes are required when the forcing frequency is above
the natural frequency. In the reacting jet, lower amplitudes are required
when the forcing frequency is above the natural frequency. The
second asymmetry concerns the broadband response at lock-in. In the
non-reacting jet, this response is always weaker than the unforced response,
regardless of whether the forcing frequency is above or below
the natural frequency. In the reacting jet, that response is weaker
than the unforced response when the forcing frequency is above the
natural frequency, but is stronger than it when the forcing frequency
is below the natural frequency.
In the reacting jet, weakening the global instability – by adding coflow
or by diluting the fuel mixture – causes the flame to lock in at lower
forcing amplitudes. This finding, however, cannot be detected in the
flame describing function. That is because the flame describing function
captures the response at only the forcing frequency and ignores all
other frequencies, most notably those arising from the natural mode
and from its interactions with the forcing. Nevertheless, the flame describing
function does show a rise in gain below the natural frequency
and a drop above it, consistent with the broadband response.
Many of these features can be predicted by the forced van der Pol
oscillator. They include (i) the coexistence of the natural and forcing
frequencies before lock-in; (ii) the presence of multiple spectral peaks
around these competing frequencies, indicating quasiperiodicity; (iii)
the occurrence of lock-in above a critical forcing amplitude; (iv) the
∨-shaped lock-in curve; and (v) the reduced broadband response at
lock-in. There are, however, some features that cannot be predicted.
They include (i) the asymmetry of the forcing amplitude required
for lock-in, found in both jets; (ii) the asymmetry of the response at
lock-in, found in the reacting jet; and (iii) the interactions between
the fundamental and harmonics of both the natural and forcing frequencies,
found in both jets.
Contributors:S. Sarkar, R. Mandal
Cavity oscillation... The three-dimensional incompressible flow past a
rectangular open cavity is investigated, where the aspect ratio of the
cavity is considered as 4. The principle objective is to use large-eddy
simulation to resolve and control the large-scale structures, which are
largely responsible for flow oscillations in a cavity. The flow past an
open cavity is very common in aerospace applications and can be a
cause of acoustic source due to hydrodynamic instability of the shear
layer and its interactions with the downstream edge. The unsteady
Navier-stokes equations have been solved on a staggered mesh using
a symmetry-preserving central difference scheme. Synthetic jet has
been used as an active control to suppress the cavity oscillations in
wake mode for a Reynolds number of ReD = 3360. The effect of
synthetic jet has been studied by varying the jet amplitude and
frequency, which is placed at the upstream wall of the cavity. The
study indicates that there exits a frequency band, which is larger than
a critical value, is effective in attenuating cavity oscillations when
blowing ratio is more than 1.0.
A theory has been developed for small oscillations of a thin filament in the magnetosphere.
For the lowest-frequency even modes, the eigenfunctions are essentially buoyancy waves in the plasma sheet, but they are more like slow modes in the inner magnetosphere.
For the lowest-frequency even modes, the eigenfrequencies (radians/s) have peak values of approximately 0.07 s-1 between the inner plasma sheet and plasmapause, for an average magnetospheric field.
This includes software and model data used in this paper.
Submitted to JGR for review
Contributors:Zou, X, Seshia, Ashwin
Resonant MEMS accelerometers offer the potential for very high resolution and wide bandwidth measurements over a large input dynamic range. The read-out is implemented by constructing an oscillator with the resonator as the primary frequency determining element. The noise of this oscillator front-end typically determines the resolution of the device, and the noise floor is set by the modulation of operative noise processes by the system dynamics. The resonator element is typically operated in the linear regime to prevent the detrimental impact of resonator non-linearities on noise conversion limiting frequency stability. However, by operating at higher drive power levels it is possible to also increase the signal-to-noise ratio for sufficiently large input frequencies. This paper shows that improved device performance over a wide bandwidth is possible by employing appropriate amplitude and phase feedback schemes to optimally bias the resonator thus enabling both short-term and long-term measurements with an electrically tunable resolution.