Hawkins' isothermal model developed to study noise in bipolar junction transistors (BJTs) is modified to investigate bias-dependent noise in heterojunction bipolar transistors (HBTs) by incorporating thermal effects. It is shown that the inclusion of thermal effects into the high-frequency noise model of HBTs is necessary as the temperature of the device may become very different from the ambient temperature, especially at high bias current. Calculation of the noise figure by including the thermal effect shows that the isothermal calculation may underestimate the noise figure at high bias current. It is observed that noise at low bias is ideality factor n dependent whereas high bias noise is insensitive to the variation of n. Moreover, the common base current gain plays a major role in the calculation of the minimum noise figure. The excellent fit obtained between the theoretical calculation and the measured data are attributed to the inclusion of the bias-dependent junction heating as well as C/sub De/ and C/sub bc/ into the present calculation. 相似文献
Logos are specially designed marks that identify goods, services, and organizations using distinguished characters, graphs, signals, and colors. Identifying logos can facilitate scene understanding, intelligent navigation, and object recognition. Although numerous logo recognition methods have been proposed for printed logos, a few methods have been specifically designed for logos in photos. Furthermore, most recognition methods use codebook-based approaches for the logos in photos. A codebook-based method is concerned with the generation of visual words for all the logo models. When new logos are added, the codebook reconstruction is required if effectiveness is a crucial factor. Moreover, logo detection in natural scenes is difficult because of perspective tilt and non-rigid deformation. Therefore, this study develops an extendable, but discriminating, model-based logo detection method. The proposed logo detection method is based on a support vector machine (SVM) using edge-based histograms of oriented gradient (HOGE) as features through multi-scale sliding window scanning. Thereafter, anti-distortion affine scale invariant feature transform (ASIFT) is used for logo verification with constraints on the ASIFT matching pairs and neighbors. The experimental results using the public Flickr-Logo database confirm that the proposed method has a higher retrieval and precision accuracy compared to existing model-based methods.
Reliability-based design optimization (RBDO) incorporates probabilistic analysis into optimization process so that an optimum
design has a great chance of staying in the feasible design space when the inevitable variability in design variables/parameters
is considered. One of the biggest drawbacks of applying RBDO to practical problem is its high computational cost that is often
impractical to industries. In search of the most suitable RBDO method for industrial applications, we first evaluated several
existing RBDO approaches in details such as the double-loop RBDO, the sequential optimization and reliability assessment,
and the response surface method. Then, based on industry needs, a platform incorporating/integrating the existing algorithm
of optimization and reliability analysis is built for a practical RBDO problem. Effectiveness of the proposed RBDO approach
is demonstrated using a simple cantilever beam problem and a more complicated industry problem. 相似文献
There have been many publications dealing with preventive maintenance policies for stochastically deteriorating systems. However, most of them study systems having one device subject to failure. Much of the limited research dealing with multi-device production systems has been conducted through simulation experiments. In this paper, the preventive maintenance problem for serial production systems is formulated as a mathematical model. This formulation permits management to analyse the impact of a preventive maintenance policy on a serial production system without resorting to simulation. Numerical examples are used to provide managerial implications for maintaining a serial production system. The results show that the operating characteristics of the stations are interrelated; therefore, it is important to examine the joint effects of a maintenance policy on the various stations of the production system simultaneously rather than study each station separately. 相似文献
For an unreliable production system, process monitoring, quality control, and maintenance are essential activities for assuring the proper operation of the production system and for managing the quality of the system's outputs. Although these activities are interrelated, they are seldom studied simultaneously. This paper interrelates and synthesizes the economic design of these activities and the management of an unreliable production system described in terms of an M/G/1 queue. A mathematical model is developed to determine the optimal policies of these activities when both the quality and quantity issues in the system are considered. Then, managerial insights are provided regarding the role and interrelationship of these activities. Finally, hypothetical data are used to illustrate the impact of these activities on the operating characteristics of the production system 相似文献
