Critical Performance Parameters Research Articles

Experimental audit of the mixing-plane approach to turbomachinery analysis and a review of alternative multi-row techniques

AbstractDetailed numerical comparisons of pressure and flow angle measurements made in stage three of a four-stage, large scale, low speed, axial compressor are presented. The measurements are in both the rotating and stationary frame and were obtained as part of a BRITE/EURAM collaborative study of cantilevered and shrouded-stator compressor configurations. The numerical analysis is 3D, considers three blade rows simultaneously and incorporates multiple row effects by use of a conservative mixing-plane model allowing circumferential variation at the mixing plane.The paper discusses the early results of a study sponsored by Alstom Gas Turbines to examine steady-state, multiple blade-row modelling techniques. Growth in the endwall flow region due to multi-row effects is revealed from both the numerical and experimental results. The numerical simulation is conducted without altering blade gap spacings to assist numerical stability; the axial gap is increasingly being seen as a critical performance parameter for multiple row analysis. The limitations inherent in an approach using mixing-planes are presented and a review of alternative, more rigourous, treatments of these effects is then discussed. These treatments attempt to retain the unsteady flow structure in a steady-state model by the derivation of so-called deterministic stresses.

General Dynamic Routing with Per-Packet Delay Guarantees of O(Distance + 1/Session Rate)

A central issue in the design of modern communication networks is that of providing performance guarantees. This issue is particularly important if the networks support real-time traffic such as voice and video. The most critical performance parameter to bound is the delay experienced by a packet as it travels from its source to its destination. We study dynamic routing in a connection-oriented packet-switching network. We consider a network with arbitrary topology on which a set of sessions is defined. For each session i, packets are injected at a rate ri to follow a predetermined path of length di . Due to limited bandwidth, only one packet at a time may advance on an edge (link). Session paths may overlap subject to the constraint that the total rate of sessions using any particular edge is at most $1-\varepsilon$ for any constant $\varepsilon \in (0,1)$. We address the problem of scheduling the sessions at each switch, so as to minimize worst-case packet delay and queue buildup at the switches. We show the existence of a periodic schedule that achieves a delay bound of O(1/ri+di) with only constant-size queues at the switches. This bound is asymptotically optimal for periodic schedules. A consequence of this result is an asymptotically optimal schedule for the static routing problem, wherein all packets are present at the outset. We obtain a delay bound of O(ci + di) for packets on path Pi, where di is the number of edges in Pi and ci is the maximum congestion along edges in Pi. This improves upon the previous known bound of O(c + d), where d = maxidi and c = maxici . We also present a simple distributed algorithm that, with high probability, delivers every session-i packet to its destination within O(1/ri+di\log(m/rmin )) steps of its injection, where r min is the minimum session rate and m is the number of edges in the network. Our results can be generalized to (leaky-bucket constrained) bursty traffic, where session i tolerates a burst size of bi. In this case, our delay bounds become O(bi/ri + di) and O(bi/ri+di\log(m/rmin )), respectively.

Ohmic Heating of InGaN LEDs during Operation: Determination of the Junction Temperature and Its Influence on Device Performance

Heat generated by ohmic losses is a critical parameter for performance and lifetime of light-emitting diodes (LEDs). The temperature of InGaN MQW LEDs during operation has been investigated depending on the forward current using three independent methods. First, the temperature of the active region was derived from the electroluminescence spectra of the devices. Second, temperature dependent micro-Raman scattering by phonons was used to determine the local temperature. Finally, a finite element simulation was performed to get a full temperature profile of the device. While the first method yields the temperature of the active region, the latter two can map the thermal distribution. All three independent methods reveal maximum operation temperatures about 120 to 130 °C at a forward current of 30 mA, corresponding to a power density of 705 W cm—2.

Ohmic Heating of InGaN LEDs during Operation: Determination of the Junction Temperature and Its Influence on Device Performance

Heat generated by ohmic losses is a critical parameter for performance and lifetime of light-emitting diodes (LEDs). The temperature of InGaN MQW LEDs during operation has been investigated depending on the forward current using three independent methods. First, the temperature of the active region was derived from the electroluminescence spectra of the devices. Second, temperature dependent micro-Raman scattering by phonons was used to determine the local temperature. Finally, a finite element simulation was performed to get a full temperature profile of the device. While the first method yields the temperature of the active region, the latter two can map the thermal distribution. All three independent methods reveal maximum operation temperatures about 120 to 130 °C at a forward current of 30 mA, corresponding to a power density of 705 W cm—2.

Characteristics of a piezoelectric miniature motor

The performance of a piezoelectric inchworm motor with a diameter of 4 mm has been evaluated on a miniature scale. The experiments have shown that some of the desired characteristics for a miniature motor such as a high output torque and linear control of rotational velocity with frequency can be achieved using a simple construction. The experimentally measured maximum output torque of 1.4 mN μm is one of the highest presented for a motor of this size. To prevent electrode failure in this first miniature prototype the maximum speed was limited to 4.2 rpm. The most critical parameter for motor performance is height deviations between piezoceramic elements which therefore should be minimised. Further, results show a reasonable agreement with a previously derived analytical model. An optimised motor design is expected to have maximum output torque and speed values as high as 10 mN μm and 120 rpm.

X-ray imaging using amorphous selenium: Determination of Swank factor by pulse height spectroscopy

The use of photoconductors, especially amorphous selenium (a-Se), in x-ray imaging is currently of interest. A critical performance parameter of an imaging detector is the Swank factor for degradation of the signal to noise ratio, or DQE(0), due to variations in the detector response. The Swank factor is evaluated from measured pulse height spectra generated by the absorption of monoenergetic x-ray photons. The spectra show an additional width over previous theoretical expectations, but the Swank factor is still close to the high values previously predicted theoretically.

Practical spread spectrum pulse compression for ultrasonic tissue imaging

Spread spectrum pulse compression is a signal processing algorithm that enhances critical system performance parameters such as signal-to-noise ratio, peak power requirements, minimum detectable signal, and total dynamic range. For this research, a digital, real-time, Barker coded, bi-phase modulator was designed and constructed, as well as a simple ultrasonic test tank containing both synthetic targets and excised goat's liver. Upon reception and demodulation of the spread spectrum ultrasonic echo, cross-correlation with a sidelobe suppression filter was performed. Due to limitations such as narrow bandwidth, and very short minimum ranges, a practical ultrasonic pulse compression system must be restricted to short code lengths. For 13 bit Barker code compression, the expected increase in signal-to-noise ratio of 11 dB was realized; at the same time greater than 30 dB of instantaneous dynamic range was maintained.

Low noise multiwasher superconducting interferometer

The dc-superconducting quantum interference device (SQUID) is a low-noise converter from magnetic flux to voltage which can have, in principle, an energy sensitivity near the quantum limit of ℏ/2. A critical parameter for the ideal performance is the device inductance, which must be kept as small as possible. Minimizing the SQUID inductance, however, is a major concern for a practical device; this requirement implies a small SQUID ring and hence magnetic coupling with an external signal is more difficult to achieve. Here we present an original scheme (called multiwasher) to circumvent this problem, and its implementation in an all-refractory thin-film device. Our scheme not only provides good magnetic coupling with a large input coil (0.5 μH) and very low SQUID inductance, but also shielding from outside uniform fields, such as those generated by ambient disturbances. The measured coupled spectral energy sensitivity in the white region at about 1 kHz is 28ℏ at 4.2 K and 5.5ℏ in a pumped helium bath at 0.9 K. The flux noise spectral density at 0.1 Hz and 0.9 K is Φn=1×10−6 Φ0/Hz.

Three modes of measurement: theory and practice

Today's conventional wisdom appears to assert that a balanced approach is the one best way of measuring business performance. The aim of this paper is to challenge this wisdom. It is argued that performance measures can be used in three distinct ways: as a means of control versus critical performance parameters; as a means of checking health; and as a means of challenging the assumptions that underpin the strategy of the business. It is also argued that each of these modes of measurement is appropriate for all organisations; that each requires different performance review processes to be established; and that all organisations should therefore consider operating three distinct performance review processes. One which allows the critical performance parameters to be controlled. A second which enables the health of the business to be checked. And a third which encourages the assumptions underpinning the organisation's strategy to be challenged. The paper draws on both academic literature and organisational practice to support these arguments. The three modes of measurement – control, check and challenge – are defined and their application demonstrated. In closing a framework is presented, which encapsulates the arguments put forth in the paper and summarises the implications for the theory and practice of business performance measurement.

Mechanism and electrocatalysis in the direct methanol fuel cell

A unified treatment of the mechanism of electrocatalysis of methanol oxidation on platinum and platinum-containing alloys is put forward. The effect of various alloys is shown to be interpretable within this overall model, and methods for the systematic improvement of the rate of electro-oxidation of methanol are described. Finally, some recent results for model direct methanol fuel cells are given, showing that critical performance parameters for commercial exploitation are now achievable with modern catalytic formulations and cell designs.

Departmental audit in surgical anatomical pathology

Internal auditing of performance by pathology providers is a necessary component of total quality management. In this study a peer review of 10% of departmental surgical anatomical pathology accessions received over a seven month period was performed. A number of critical performance parameters were analysed including turn-around times, accuracy of reports and technical proficiency. The results demonstrated an approximate 2% significant error rate in macroscopic and microscopic descriptions, technically good quality sections and stains and generally satisfactory turn-around times. The value and costing of such an audit and changes initiated by the audit are discussed.

Point source resolution performance of a germanium Compton camera

One of the critical performance parameters for a Compton camera imager is the point source location accuracy. This paper describes analysis of both experimental and simulated data to evaluate the point source resolving power of a germanium Compton camera imager. The experimental data are taken from a set of experiments performed in 1993 at Naval Research Laboratory. Subsequent design studies have utilized Monte Carlo simulations to predict the performance parameters for a number of detector geometries. Presented in this paper are the results from one of those configurations.

Silicide thin films and their applications in microelectronics

Abstract Silicides are compounds of silicon with other elements in the periodic table. In this paper, usefulness of silicides and their thin films in microelectronics is discussed with emphasis on electrically conducting transition-metal silicides. Following a brief description of the device structure, contact resistance and interconnection delay (silicides are used to reduce these critical performance parameters of devices and integrated circuits), desired materials characteristics, preparation of thin films of silicides, a brief review of reaction kinetics, epitaxial silicides, and stability of silicides are discussed. It is shown that the silicides have found application in microelectronics (even on GaAs) because of their large silicon content, high electrical conductivity, high temperature stability, and corrosion resistance. They can be prepared easily and reliably and show potential for continued application in spite of ever-decreasing device/circuit dimensions in silicon integrated circuits. They offer a possibility of fabricating three-dimensional device/circuit structures and significantly faster heterostructure devices.

Desensitizing structural-control design

A new method for targeted robustness optimization is presented. This method allows the analyst to 'target' the most critical performance parameter (or family of design requirements with almost the same criticality) for enhanced protection from uncertainties in the model. To create this protection, this method uses knowledge about the uncertainties in some of the parameters of the model - this information is usually available to the structural analyst. The power of the targeted robustness method is demonstrated numerically using an active damping example. The results show that the most critical damping ratio is protected - its sensitivity to the uncertainties is far below those of non-critical damping ratios.

Performance evaluation of the generalized type-II hybrid ARQ scheme with noisy feedback on Markov channels

The generalized type-II hybrid automatic repeat request (ARQ) scheme on Markov channels is examined in order to study the effect of feedback channel errors on the performance of ARQ systems. It is shown that it is possible to derive expressions for certain critical performance parameters, such as the throughput efficiency, the probabilities of packet loss, undetected error, and correct delivery. To provide a means of comparison, a parallel set of expressions is provided under the usual assumption of an error-free feedback channel. By use of simulations, the ARQ system performance is examined under noiseless feedback and noisy feedback. It is found that feedback channel noise can result in the loss of packets, an increase in the number of undetected errors, and the occurrence of unnecessary transmissions. To enhance the performance of the GH-II ARQ scheme, a predictor is used and found to lower the probability of undetected error, reduce the number of unnecessary transmissions, and increase the throughput efficiency. >

Modeling process latitude in UV projection lithography

The degree to which critical performance parameters are stable against small variations in process parameters is called process latitude. Advanced computer models of UV microlithography are used to study the impact of the notching phenomenon (exposure enhancement near steps in the exposure plane) on process latitude. It is shown that notching effects give rise to a rapid degradation of resist development process latitude.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis and simulation of the SLD WIC PADS hybrid preamplifier circuitry

Computer simulation techniques are used to analyze critical performance parameters of the SLD PADS preamplifier hybrid, including its interactions with the detector system. Simulation results are presented and verified with measured performance. The use of the computer simulation of the detector/preamplifier combination is shown to be a reasonably accurate predictor of preamplifier performance, but more importantly, it allows one to demonstrate, analyze, and quantify selected aspects of the hybrid's behavior in the system. In particular, the analysis confirms wide tolerances on system timing and demonstrates that self-calibration can compensate for the effects of large variation in detector capacitance. The selection of 6 mu s for fixed time sampling is shown to be entirely reasonable for this system. >

Predictions Of Stray Light Modeling On The Ultimate Performance Of Acousto-Optic Processors

Measured values of critical optical processor performance parameters such as crosstalk and dynamic range generally fall short of the values predicted on the basis of first order design, ray tracing, and diffraction calculations. Normally, the procedure used to design an optical system such as a coherent optical processor does not take into account optical scatter, reflections, and incoherent light in the source. Stray light factors are of critical importance in those systems designed for spectrum analysis or correlation in which the light is collected on a closely spaced array of pixels. If the (bidirectional) transmission distribution function (BTDF) of optical elements were known very near specular (0.01° to 0.001°) and a suitable means of calculating its effect were available, stray light performance could be included in the design procedure. Instruments are now available that can measure the BTDF near the required angles, and an optical design code is also available that can make use of BTDF data. BTDF data for samples of acousto-optic material have been measured close to 0.001°, and the possibility of smaller angles is discussed. The new analysis code has been used to model several simple configurations found in acousto-optic processors. The results are compared with normal calculations (CODE V) and with some measured performance data.

Microstrip log-periodic antenna array with endfire beam

Endfire action with a very wide-bandwidth, low-profile antenna array has not yet been achieved. After showing that a conventional microstrip log-periodic array cannot be successfully scanned from broadside to endfire, a new configuration involving quarter-wavelength shorted patches is described. First-order theory indicates that good endfire action should be possible, and this is confirmed by preliminary measurements on an array designed for a 25% bandwidth. Radiation pattern control is seen to be the most critical performance parameter.

Combinatorial digital logic using charge-coupled devices

Describes a new approach to circuit design that allows charge-coupled devices to perform combinatorial digital logic. These circuits use charge packets, floating gates, and conventional NMOS circuitry in a way that combines the low power, high packing density of CCDs with some of the high-speed combinatorial logic capabilities of conventional NMOS circuits. Since only a few transfers are involved in the operation of these circuits, charge transfer efficiency is not a critical parameter. A CCD ripple adder is described that has been designed, fabricated, and tested, and a charge control analysis has been used to estimate its ultimate speed capabilities. An arithmetic logic unit design is also described. The combinatorial CCD circuits are particularly well suited to previous digital CCD logic approaches in that they allow elimination of power and space-consuming storage registers. These circuits are most useful in applications requiring large, regular, pipelined architectures such as systolic arrays where the critical performance parameter is throughput per unit power.