2,032 research outputs found
Ultra-low vibration pulse-tube cryocooler stabilized cryogenic sapphire oscillator with 10^-16 fractional frequency stability
A low maintenance long-term operational cryogenic sapphire oscillator has
been implemented at 11.2 GHz using an ultra-low-vibration cryostat and
pulse-tube cryocooler. It is currently the world's most stable microwave
oscillator employing a cryocooler. Its performance is explained in terms of
temperature and frequency stability. The phase noise and the Allan deviation of
frequency fluctuations have been evaluated by comparing it to an ultra-stable
liquid-helium cooled cryogenic sapphire oscillator in the same laboratory.
Assuming both contribute equally, the Allan deviation evaluated for the
cryocooled oscillator is sigma_y = 1 x 10^-15 tau^-1/2 for integration times 1
< tau < 10 s with a minimum sigma_y = 3.9 x 10^-16 at tau = 20 s. The long term
frequency drift is less than 5 x 10^-14/day. From the measured power spectral
density of phase fluctuations the single side band phase noise can be
represented by L_phi(f) = 10^-14.0/f^4+10^-11.6/f^3+10^-10.0/f^2+10^-10.2/f+
10^-11.0 for Fourier frequencies 10^-3<f<10^3 Hz in the single oscillator. As a
result L_phi approx -97.5 dBc/Hz at 1 Hz offset from the carrier.Comment: 8 pages, 10 figures, presented at European Frequency and Time Forum,
ESTEC, Noordwijk, Netherland, April 11-16th 2010 accepted in IEEE Trans. on
Micro. Theory & Technique
A Sequential Two-Step Algorithm for Fast Generation of Vehicle Racing Trajectories
The problem of maneuvering a vehicle through a race course in minimum time
requires computation of both longitudinal (brake and throttle) and lateral
(steering wheel) control inputs. Unfortunately, solving the resulting nonlinear
optimal control problem is typically computationally expensive and infeasible
for real-time trajectory planning. This paper presents an iterative algorithm
that divides the path generation task into two sequential subproblems that are
significantly easier to solve. Given an initial path through the race track,
the algorithm runs a forward-backward integration scheme to determine the
minimum-time longitudinal speed profile, subject to tire friction constraints.
With this fixed speed profile, the algorithm updates the vehicle's path by
solving a convex optimization problem that minimizes the resulting path
curvature while staying within track boundaries and obeying affine,
time-varying vehicle dynamics constraints. This two-step process is repeated
iteratively until the predicted lap time no longer improves. While providing no
guarantees of convergence or a globally optimal solution, the approach performs
very well when validated on the Thunderhill Raceway course in Willows, CA. The
predicted lap time converges after four to five iterations, with each iteration
over the full 4.5 km race course requiring only thirty seconds of computation
time on a laptop computer. The resulting trajectory is experimentally driven at
the race circuit with an autonomous Audi TTS test vehicle, and the resulting
lap time and racing line is comparable to both a nonlinear gradient descent
solution and a trajectory recorded from a professional racecar driver. The
experimental results indicate that the proposed method is a viable option for
online trajectory planning in the near future
Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators with 1 x 10^-16 frequency instability
Two nominally identical ultra-stable cryogenic microwave oscillators are
compared. Each incorporates a dielectric-sapphire resonator cooled to near 6 K
in an ultra-low vibration cryostat using a low-vibration pulse-tube cryocooler.
The phase noise for a single oscillator is measured at -105 dBc/Hz at 1 Hz
offset on the 11.2 GHz carrier. The oscillator fractional frequency stability
is characterized in terms of Allan deviation by 5.3 x 10^-16 tau^-1/2 + 9 x
10^-17 for integration times 0.1 s < tau < 1000 s and is limited by a flicker
frequency noise floor below 1 x 10^-16. This result is better than any other
microwave source even those generated from an optical comb phase-locked to a
room temperature ultra-stable optical cavity.Comment: 4 pages, 5 figure
Simulations Show that Vortex Flows could Heat the Chromosphere in Solar Plage
The relationship between vortex flows at different spatial scales and their
contribution to the energy balance in the chromosphere is not yet fully
understood. We perform three-dimensional (3D) radiation-magnetohydrodynamic
(MHD) simulations of a unipolar solar plage region at a spatial resolution of
10 km using the MURaM code. We use the swirling-strength criterion that mainly
detects the smallest vortices present in the simulation data. We additionally
degrade our simulation data to smooth-out the smaller vortices, so that also
the vortices at larger spatial scales can be detected. Vortex flows at various
spatial scales are found in our simulation data for different effective spatial
resolutions. We conclude that the observed large vortices are likely clusters
of much smaller ones that are not yet resolved by observations. We show that
the vertical Poynting flux decreases rapidly with reduced effective spatial
resolutions and is predominantly carried by the horizontal plasma motions
rather than vertical flows. Since the small-scale horizontal motions or the
smaller vortices carry most of the energy, the energy transported by vortices
deduced from low resolution data is grossly underestimated. In full resolution
simulation data, the Poynting flux contribution due to vortices is more than
adequate to compensate for the radiative losses in plage, indicating their
importance for chromospheric heating.Comment: 8 pages, 5 figures, accepted in ApJ
Cryogenic Sapphire Oscillator using a low-vibration design pulse-tube cryocooler: First results
A Cryogenic Sapphire Oscillator has been implemented at 11.2 GHz using a
low-vibration design pulse-tube cryocooler. Compared with a state-of-the-art
liquid helium cooled CSO in the same laboratory, the square root Allan variance
of their combined fractional frequency instability is for integration times s, dominated by
white frequency noise. The minimum for the two
oscillators was reached at s. Assuming equal contributions from
both CSOs, the single oscillator phase noise at 1 Hz offset from the carrier.Comment: 5 pages, 5 figures, accepted in IEEE Trans on Ultrasonics,
Ferroelectrics and Frequency Contro
A domain adaptive stochastic collocation approach for analysis of MEMS under uncertainties
This work proposes a domain adaptive stochastic collocation approach for uncertainty quantification, suitable for effective handling of discontinuities or sharp variations in the random domain. The basic idea of the proposed methodology is to adaptively decompose the random domain into subdomains. Within each subdomain, a sparse grid interpolant is constructed using the classical Smolyak construction [S. Smolyak, Quadrature and interpo- lation formulas for tensor products of certain classes of functions, Soviet Math. Dokl. 4 (1963) 240–243], to approximate the stochastic solution locally. The adaptive strategy is governed by the hierarchical surpluses, which are computed as part of the interpolation procedure. These hierarchical surpluses then serve as an error indicator for each subdo- main, and lead to subdivision whenever it becomes greater than a threshold value. The hierarchical surpluses also provide information about the more important dimensions, and accordingly the random elements can be split along those dimensions. The proposed adaptive approach is employed to quantify the effect of uncertainty in input parameters on the performance of micro-electromechanical systems (MEMS). Specifically, we study the effect of uncertain material properties and geometrical parameters on the pull-in behavior and actuation properties of a MEMS switch. Using the adaptive approach, we resolve the pull-in instability in MEMS switches. The results from the proposed approach are verified using Monte Carlo simulations and it is demonstrated that it computes the required statistics effectively
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