29,703 research outputs found
Adding virtual measurements by signal injection
We propose a method to "create" a new measurement output by exciting the
system with a high-frequency oscillation. This new "virtual" measurement may be
useful to facilitate the design of a suitable control law. The approach is
especially interesting when the observability from the actual output
degenerates at a steady-state regime of interest. The proposed method is based
on second-order averaging and is illustrated by simulations on a simple
third-order system
An analysis of the benefits of signal injection for low-speed sensorless control of induction motors
We analyze why low-speed sensorless control of the IM is intrinsically
difficult, and what is gained by signal injection. The explanation relies on
the control-theoretic concept of observability applied to a general model of
the saturated IM. We show that the IM is not observable when the stator speed
is zero in the absence of signal injection, but that observability is restored
thanks to signal injection and magnetic saturation. The analysis also reveals
that existing sensorless algorithms based on signal injection may perform
poorly for some IMs under particular operating conditions. The approach is
illustrated by simulations and experimental data
An efficient method to include equality constraints in branch current distribution system state estimation
Distribution system state estimation is a fundamental tool for the management and control functions envisaged for future distribution grids. The design of accurate and efficient algorithms is essential to provide estimates compliant with the needed accuracy requirements and to allow the real-time operation of the different applications. To achieve such requirements, peculiarities of the distribution systems have to be duly taken into account. Branch current-based estimators are an efficient solution for performing state estimation in radial or weakly meshed networks. In this paper, a simple technique, which exploits the particular formulation of the branch current estimators, is proposed to deal with zero injection and mesh constraints. Tests performed on an unbalanced IEEE 123-bus network show the capability of the proposed method to further improve efficiency performance of branch current estimators
Impedance-compensated grid synchronisation for extending the stability range of weak grids with voltage source converters
This paper demonstrates how the range of stable power transfer in weak grids with voltage source converters (VSCs) can be extended by modifying the grid synchronisation mechanism of a conventional synchronous reference frame phase locked loop (PLL). By introducing an impedance-conditioning term in the PLL, the VSC control system can be virtually synchronised to a stronger point in the grid to counteract the instability effects caused by high grid impedance. To verify the effectiveness of the proposed approach, the maximum static power transfer capability and the small-signal stability range of a system with a VSC HVDC terminal connected to a weak grid are calculated from an analytical model with different levels of impedance-conditioning in the PLL. Such calculations are presented for two different configurations of the VSC control system, showing how both the static power transfer capability and the small-signal stability range can be significantly improved. The validity of the stability assessment is verified by time-domain simulations in the Matlab/Simulink environment.Peer ReviewedPostprint (published version
Electrode level Monte Carlo model of radiation damage effects on astronomical CCDs
Current optical space telescopes rely upon silicon Charge Coupled Devices
(CCDs) to detect and image the incoming photons. The performance of a CCD
detector depends on its ability to transfer electrons through the silicon
efficiently, so that the signal from every pixel may be read out through a
single amplifier. This process of electron transfer is highly susceptible to
the effects of solar proton damage (or non-ionizing radiation damage). This is
because charged particles passing through the CCD displace silicon atoms,
introducing energy levels into the semi-conductor bandgap which act as
localized electron traps. The reduction in Charge Transfer Efficiency (CTE)
leads to signal loss and image smearing. The European Space Agency's
astrometric Gaia mission will make extensive use of CCDs to create the most
complete and accurate stereoscopic map to date of the Milky Way. In the context
of the Gaia mission CTE is referred to with the complementary quantity Charge
Transfer Inefficiency (CTI = 1-CTE). CTI is an extremely important issue that
threatens Gaia's performances. We present here a detailed Monte Carlo model
which has been developed to simulate the operation of a damaged CCD at the
pixel electrode level. This model implements a new approach to both the charge
density distribution within a pixel and the charge capture and release
probabilities, which allows the reproduction of CTI effects on a variety of
measurements for a large signal level range in particular for signals of the
order of a few electrons. A running version of the model as well as a brief
documentation and a few examples are readily available at
http://www.strw.leidenuniv.nl/~prodhomme/cemga.php as part of the CEMGA java
package (CTI Effects Models for Gaia).Comment: Accepted by MNRAS on 13 February 2011. 15 pages, 7 figures and 5
table
A lowânoise highâspeed diode laser current controller
We describe a new diode laser current controller which features low current noise, excellent dc stability, and the capacity for highâspeed modulation. While it is simple and inexpensive to construct, the controller compares favorably with the best presently available commercial diode laser current controllers
FPGA-based implementation of the back-EMF symmetric-threshold-tracking sensorless commutation method for brushless DC-machines
The operation of brushless DC permanent-magnet machines requires information of the rotor position to steer the semiconductor switches of the power-supply module which is commonly referred to as Brushless Commutation. Different sensorless techniques have been proposed to estimate the rotor position using current and voltage measurements of the machine. Detection of the back-electromotive force (EMF) zero-crossing moments is one of the methods most used to achieve sensorless control by predicting the commutation moments. Most of the techniques based on this phenomenon have the inherit disadvantage of an indirect detection of commutation moments. This is the result of the commutation moment occurring 30 electrical degrees after the zero-crossing of the induced back-emf in the unexcited phase. Often, the time difference between the zero crossing of the back-emf and the optimal current commutation is assumed constant. This assumption can be valid for steady-state operation, however a varying time difference should be taken into account during transient operation of the BLDC machine. This uncertainty degrades the performance of the drive during transients. To overcome this problem which improves the performance while keeping the simplicity of the back-emf zero-crossing detection method an enhancement is proposed. The proposed sensorless method operates parameterless in a way it uses none of the brushless dc-machine parameters. In this paper different aspects of experimental implementation of the new method as well as various aspects of the FPGA programming are discussed. Proposed control method is implemented within a Xilinx Spartan 3E XC3S500E board
Muon g-2: Review of Theory and Experiment
A review of the experimental and theoretical determinations of the anomalous
magnetic moment of the muon is given. The anomaly is defined by a=(g-2)/2,
where the Land\'e g-factor is the proportionality constant that relates the
spin to the magnetic moment. For the muon, as well as for the electron and
tauon, the anomaly a differs slightly from zero (of order 10^{-3}) because of
radiative corrections. In the Standard Model, contributions to the anomaly come
from virtual `loops' containing photons and the known massive particles. The
relative contribution from heavy particles scales as the square of the lepton
mass over the heavy mass, leading to small differences in the anomaly for e,
\mu, and \tau. If there are heavy new particles outside the Standard Model
which couple to photons and/or leptons, the relative effect on the muon anomaly
will be \sim (m_\mu/ m_e)^2 \approx 43\times 10^3 larger compared with the
electron anomaly. Because both the theoretical and experimental values of the
muon anomaly are determined to high precision, it is an excellent place to
search for the effects of new physics, or to constrain speculative extensions
to the Standard Model. Details of the current theoretical evaluation, and of
the series of experiments that culminates with E821 at the Brookhaven National
Laboratory are given. At present the theoretical and the experimental values
are known with a similar relative precision of 0.5 ppm. There is, however, a
3.4 standard deviation difference between the two, strongly suggesting the need
for continued experimental and theoretical studyComment: 103 pages, 57 figures, submitted to Reports on Progress in Physics
Final version as published, several minor clarifications to text and a number
of references were correcte
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