Sensitivity analysis of test methods for aspheric off-axis mirrors

Following the Hubble Space Telescope (HST), the next generation James Webb Space Telescope (JWST) is being developed to be launched in a few years. JWST will be a segmented mirror telescope with a design much like that developed for ground-based telescopes over the past 20 years. Several segmented mirror telescopes are currently in operation, and next generation ground-based telescopes of the 30-m class are also being designed using segmented primary mirrors. Regardless of size, segmented primary mirror telescopes often require the use of aspheric segment mirrors. One of the key factors in fabrication of aspheric segment mirrors is feasibility of testing off-axis surfaces with high accuracy. A couple of test methods have been investigated for aspheric off-axis segments. As a case study, we apply these test methods to secondary segmented mirror models of the Giant Magellan Telescope. We derive required dimensions of test set-ups and assess sensitivity of optical alignment. Characteristics of the test methods are also discussed.     

New split-winding doubly salient permanent magnet motor drive

A new split-winding doubly salient permanent magnet (DSPM) motor drive is proposed. This DSPM motor drive offers the advantages of high power density, high efficiency, and wide speed range. The corresponding operation, analysis, implementation, and experimentation are successively presented. Finally, experimental results are used to confirm that the proposed DSPM motor drive offers high efficiency over a wide output power range, exhibits good dynamic performance, and extends the constant-power operation range significantly.     

Equivalent ARMA model representation for RLG random errors

It is shown that a mixture of random noises can be represented by a single equivalent ARMA (autoregressive moving average) model that is simple to implement. We applied the scheme to model RLG (ring laser gyroscope) random errors. An identification result from real test data confirms the validity of this approach     

A digital quadrature demodulation system

Existing digital quadrature demodulation techniques sample the input at either 2B Hz or 4B Hz, select the even samples as the in-phase (I), and interpolate the odd samples to give the quadrature (Q), output. The signal bandwidth is B. We propose a demodulation system to produce I and Q samples at arbitrary sampling rate greater than 2B Hz. The system eliminates the IF downconversion step with a special sampling scheme. The even samples correspond to the I component, while the Q components are the filtered output. The filter can be a lowpass or least squares (LS) filter. The lowpass filter design is based on trade-offs between the filter length and the degree of oversampling. It produces similar results as previous work when the sampling rate is 2B Hz or 4B Hz. Unlike existing methods which assume the input is white, a LS filter, on the other hand, can make use of input signal characteristics to achieve a better result. The higher the correlation in the input the larger the improvement. The cost for LS filtering is a coefficient update step if the input is time varying. A scheme to cancel dc offset from analog to digital (A/D) converters is also given     

Concerns about Hash Cracking Aftereffect on Authentication Procedures in Applications of Cyberspace

In the Crypto 2004 Conference (Wang et al., 2004), one of the big news was that a fundamental technique in cryptography - one of the hash algorithms, Message Digest 5 (MD5) had been cracked. Soon after this event, it was announced that the secure hash algorithm (SHA-1) had been cracked as well. This series of information shocked the researches in the field of cryptography, and raised the attention of every related organization of security application mechanisms. Most agencies are still looking into the impact this may have, and are considering their future responses and solutions in the wake of these developments in the hash structure of cryptanalysis. Based on this information, this paper will discuss in detail the broken MD5 and SHA-1, which both are reported to have been cracked, and will investigate any impact, and propose recommendations for the applications in cyber-space, such as password authentication and forensic software, when using a hash algorithm     

Power sources and portable computers

The safety aspects of the nickel-metal-hydride and high-capacity nickel-cadmium batteries used in Apple Computer's current notebook computers are discussed. No problems are anticipated with nickel-cadmium packs, which have an excellent record for safety because the cell design parameters are well understood and there is ample safety margin under abusive conditions. At its early stage of development, the nickel-metal-hydride cell is not as electrochemically robust as the nickel-cadmium, and it does not have as wide an operational temperature range. The possible hydrogen release from nickel-metal-hydride batteries on charging at low temperatures can also be a potential explosion hazard. Nickel-metal-hydride is considered less environmentally harmful than nickel-cadmium and has escaped legislated recycling requirements     

Thermocapillary-driven convection in supported and floating-zone crystallization

Marangoni flow was induced in a thin disk of molten tin with an atomically clean surface by heating it from below in an ultra high vacuum apparatus. The radial distribution of temperature was measured and no temperature oscillations were observed. Strong mechanical disturbances introduced at a Marangoni number of about 4300 died out quickly with time indicating that the system was stable. Mathematical models indicate that strong flows in a two cell structure exist under the conditions studied, and that the velocity profile in the cell near the surface is of the bouindary layer type which varies rapidly with depth.     

Modeling quaternion errors in SDINS: computer frame approach

We propose new equivalent tilt error models which are applicable to the analysis of the terrestrial strapdown inertial navigation systems (SDINS), based on the quaternions. The currently available equivalent tilt error models, like the conventional Φ model of the gimbaled inertial navigation systems (GINS), are derived only by the true frame approach. The true frame approach has a computational disadvantage that it produces an error model where the attitude error equation is coupled with its position and velocity error equations. The motivation of this work is to solve this problem. As a result, two kinds of error models are derived. Among them, one is derived by the computer frame approach for practical onboard implementations. Thus, like the conventional GINS Ψ model, its attitude error equation is decoupled from the position and velocity error equations. The other is derived in order to show the relationship between the true frame approach and the computer frame approach which are applied to the quaternion-based SDINS. Thus, like the GINS δΘ model, it can be used to transform the error variables into each other which are calculated by the two different approaches     

Unified Voltage and Pitch Angle Controller for Wind-Driven Induction Generator System

A unified voltage and pitch angle controller (UVPC) for a wind-driven induction generator system is presented to reach effective voltage control and to stabilize generator speed and system frequency. The proposed UVPC comprises a linear optimal controller (LOC), a supplementary voltage regulator (SVR), and a supplementary pitch angle controller (SPC). An ac voltage regulator and a dc voltage regulator are designed in the SVR to generate the desired voltage magnitude and phase angle for the voltage-sourced inverter (VSI). On the other hand, the SPC is designed based on fuzzy logic inference rules in order to generate the desired pitch angle command for the variable blade pitch such that the mechanical power can be controlled in a very efficient manner. When the system is operated in the islanding condition, an approach based on the concept of relative rotating speed has been developed for the measurement of system frequency deviation. Simulation and experimental results reveal that both bus voltage and mechanical power can be effectively controlled by the proposed UVPC for the wind energy conversion system (WECS) either connected with or disconnected from the power grid when there is sufficient wind energy.