5,448 research outputs found
Implementation of a Software-Defined Acoustic Modem for MBPS Wireless Underwater Communication
Wireless communication technologies such as cellular networks, GPS and Wi-Fi
are ubiquitous on land, but underwater, they are inadequate. The radio
waves that these technologies use to carry information wirelessly over land
can only penetrate a few centimeters of saltwater. This paper will discuss
the implementation of an acoustic modem that uses sound waves, as whales
and dolphins do, for sending information and that can support data rates
beyond 1 Mbps, 1000 times faster than existing commercial systems. We developed
and implemented a physical layer protocol specifically tailored to the
needs of a reliable high-speed acoustic communication link. First a custom
hyperbolic chirp waveform is sent for robust receiver synchronization and
initialization, then a stream of QAM symbols follows. Some of these QAM
symbols are known a priori, such that the receiver algorithms can adapt to
the time-varying reverberation and Doppler structure of the acoustic communication
channel. The modem is implemented in software on the BeagleBone
Black (BBB), a cheap single board computer, and an attached analog frontend
(AFE) daughter card is used for signal generation and acquisition. The
signal processing software runs in a non-preemptive operating system (Debian)
on an ARM processor. The programmable real-time units (PRUs) on
the BBB are utilized to interface with the AFE. The PRUs stream signals
from and to the AFE in real time and communicate with the ARM processor
through fast direct memory access transfers.Ope
The Effect of Credit Guarantees on Survival and Performance of SMEs in Korea
This study evaluates the impact of provision of credit guarantee in Korea at the firm level. The data is assembled from two public funds providing credit guarantees covering the period 2000 to 2003. The sample firms consist of SMEs mainly. To measure the effects of credit guarantee, the relationship between credit guarantees, survival of firms, and their productive performance is analyzed. Since the data is collected as repeated cross sections and firms are not identified over time, the analysis is carried out by using a pseudo panel data approach. The pseudo panel data is created using time invariant firm characteristics. The result from regression analysis conducted indicates that the amounts of credit guarantee and the number of times a firm receives credit guarantees have effects on their survival and growth. The amounts of credit guarantee increase the growth of sales and productivity while frequency of credit guarantees decreases business failure. Size and age play a decisive role in survival of firms and their employment growth as well. Moreover, survival and performance of firms are different across periods, industries, and locations.Credit guarantee; SMEs; Pseudo panel data; survival; performance
Display screens with variable refresh rate
Device displays of a mobile device such as a smartphone or tablet have capabilities to support different refresh rates. For optimal operation, the refresh rate of the screen of a mobile device can be refreshed based on the image or frame of video being displayed and on certain other conditions. This disclosure describes low-complexity, low-latency techniques to seamlessly transition between different screen refresh rates with minimal-to-zero changes in display settings
Analysis of electrical signatures in synchronous generators characterized by bearing faults
Synchronous generators play a vital role in power systems. One of the major mechanical
faults in synchronous generators is related to bearings. The popular vibration
analysis method has been utilized to detect bearing faults for years. However, bearing
health monitoring based on vibration analysis is expensive. One of the reasons
is because vibration analysis requires costly vibration sensors and the extra costs
associated with its proper installation and maintenance. This limitation prevents
continuous bearing condition monitoring, which gives better performance for rolling
element bearing fault detection, compared to the periodic monitoring method that
is a typical practice for bearing maintenance in industry. Therefore, a cost effective
alternative is necessary. In this study, a sensorless bearing fault detection method
for synchronous generators is proposed based on the analysis of electrical signatures,
and its bearing fault detection capability is demonstrated.
Experiments with staged bearing faults are conducted to validate the effectiveness
of the proposed fault detection method. First, a generator test bed with an in-
situ bearing damage device is designed and built. Next, multiple bearing damage
experiments are carried out in two vastly different operating conditions in order to
obtain statistically significant results. During each experiment, artificially induced
bearing current causes accelerated damage to the front bearing of the generator.
This in-situ bearing damage process entirely eliminates the necessity of disassembly and reassembly of the experimental setup that causes armature spectral distortions.
The electrical fault indicator is computed based on stator voltage signatures
without the knowledge of machine and bearing specific parameters. Experimental
results are compared using the electrical indicator and a vibration indicator that is
calculated based on measured vibration data. The results indicate that the electrical
indicator can be used to analyze health degradation of rolling element bearings in
synchronous generators in most instances. Though the vibration indicator enables
early bearing fault detection, it is found that the electrical fault indicator is also
capable of detecting bearing faults well before catastrophic bearing failure
Relaxation of superfluid turbulence in highly oblate Bose-Einstein condensates
We investigate thermal relaxation of superfluid turbulence in a highly oblate
Bose-Einstein condensate. We generate turbulent flow in the condensate by
sweeping the center region of the condensate with a repulsive optical
potential. The turbulent condensate shows a spatially disordered distribution
of quantized vortices and the vortex number of the condensate exhibits
nonexponential decay behavior which we attribute to the vortex pair
annihilation. The vortex-antivortex collisions in the condensate are identified
with crescent-shaped, coalesced vortex cores. We observe that the
nonexponential decay of the vortex number is quantitatively well described by a
rate equation consisting of one-body and two-body decay terms. In our
measurement, we find that the local two-body decay rate is closely proportional
to , where is the temperature and is the chemical potential.Comment: 7 pages, 9 figure
Observation of a Geometric Hall Effect in a Spinor Bose-Einstein Condensate with a Skyrmion Spin Texture
For a spin-carrying particle moving in a spatially varying magnetic field,
effective electromagnetic forces can arise due to the geometric phase
associated with adiabatic spin rotation of the particle. We report the
observation of a geometric Hall effect in a spinor Bose-Einstein condensate
with a skyrmion spin texture. Under translational oscillations of the spin
texture, the condensate resonantly develops a circular motion in a harmonic
trap, demonstrating the existence of an effective Lorentz force. When the
condensate circulates, quantized vortices are nucleated in the boundary region
of the condensate and the vortex number increases over 100 without significant
heating. We attribute the vortex nucleation to the shearing effect of the
effective Lorentz force from the inhomogeneous effective magnetic field.Comment: 9 pages, 11 figure
Development of a Multi-material Stereolithography System
Researchers continue to explore possibilities for expanding additive manufacturing (AM) technologies into direct product manufacturing. One limitation is in the materials available for use in AM that can meet the needs of end-use applications. Stereolithography (SL) is an AM technology well known for its precision and high quality surface finish capabilities. SL builds parts by selectively crosslinking or solidifying photo-curable liquid resins, and the resin industry has been continuously developing new resins with improved performance characteristics. This paper introduces a unique SL machine that can fabricate parts out of multiple SL materials. The technology is based on using multiple vats positioned on a rotating vat carousel that contain different photo-curable materials. To change the material during the process, the build platform is raised out of the current vat, a new vat with a different material is rotated under the platform, and the platform is submerged into the new vat so that the new material can be used. This paper introduces a new vat exchange mechanism, cleaning process, recoating process, resin leveling mechanism and process planning technologies for the implementation of multiple material SL. An overview of the system framework is provided and the system integration and control software is described. In addition, several multiple material test parts are designed, fabricated, and described
- …