Dynamic modeling and nonlinear control strategy for an underactuated quad rotor rotorcraft

In this paper, a nonlinear dynamic MIMO model of a 6-DOF underactuated quad rotor rotorcraft is derived based on Newton-Euler formalism. The derivation comprises determining equations of motion of the quad rotor in three dimensions and seeking to approximate the actuation forces through modeling of the aerodynamic coefficients and electric motor dynamics. The derived model is dynamically unstable, so a sequential nonlinear control strategy is implemented for the quad rotor. The control strategy includes exact feedback linearization technique, using the geometric methods of nonlinear control. The performance of the nonlinear control algorithm is evaluated using simulation and the results show the effectiveness of the proposed control strategy for the quad rotor rotorcraft near quasi-stationary flight.     

Cryptanalysis of an image encryption scheme based on the Hill cipher

This paper studies the security of an image encryption scheme based on the Hill cipher （Ismail et al., 2006） and reports its following problems： （1） There is a simple necessary and sufficient condition that makes a number of secret keys invalid; （2） It is insensitive to the change of the secret key; （3） It is insensitive to the change of the plain-image; （4） It can be broken with only one known/chosen plaintext; （5） It has some other minor defects. The proposed cryptanalysis discourages any use of the scheme in practice.     

SOFC temperature evaluation based on an adaptive fuzzy controller

The operating temperature of a solid oxide fuel cell （SOFC） stack is a very important parameter to be controlled, which impacts the performance of the SOFC due to thermal cycling. In this paper, an adaptive fuzzy control method based on an affine nonlinear temperature model is developed to control the temperature of the SOFC within a specified range. Fuzzy logic systems are used to approximate nonlinear functions in the SOFC system and an adaptive technique is employed to construct the controller. Compared with the traditional fuzzy and proportion-integral-derivative （PID） control, the simulation results show that the designed adaptive fuzzy control method performed much better. So it is feasible to build an adaptive fuzzy controller for temperature control of the SOFC.     

Time-domain training sequences design for MIMO OFDM channel estimation

This paper describes a Least Squares （LS） channel estimation scheme for MIMO OFDM systems based on time-domain training sequence. We first compute the minimum mean square error （MSE） of the LS channel estimation, and then derive the optimal criteria of the training sequence with respect to the minimum MSE. It is shown that optimal time-domain training sequence should satisfy two criteria. First, the autocorrelation of the sequence transmitted from the same antenna is an impulse function in a region longer than the channel maximum delay. Second, the cross-correlation between sequences transmitted from different antennas is zero in this region. Simulation results show that the estimator using optimal time-domain training sequences has better performance than that using optimal frequency training sequence at low signal-to-noise ratio （SNR）. To reduce the training overhead, a suboptimal training sequence is also proposed. Comparing with optimal training sequence, it has low computation complexity and high transmission efficiency at the expense of little performance degradation.     

高分子抑制剂LS-YW在钻井施工过程中的应用

介绍了高分子抑制剂LS-YW的组成与作用机理.室内实验和现场应用表明,以LS-YW为主的钻井液性能稳定,抑制能力强,防塌效果好,可减少井下复杂情况的发生,缩短钻井周期,具有良好的社会和经济效益.     

浅析语用预设在广告语言中的应用及其效果

广告是经济全球化和信息时代的一个重要社会现象.语用预设在广告语言中起着十分重要的作用.语用预设具有单向性、主观性和隐蔽性特点,这些特点使得预设现象经常被用来增强广告的说服效果.语用预设的单向性促使广告语言及时抓住自己的目标对象,语用预设的主观性能增强其说服力,语用预设的隐蔽性使其具有表达言外之意之功效.应用语用预设设计出的广告必能达到既实现广告主的意图又满足消费者需求的双赢效果.     

浅谈乙醇汽油使用对车辆性能影响及对策

对发展乙醇汽油燃料的优势及现状进行了综合评述,提出了乙醇汽油对车辆性能的影响和对策,为我国推广使用车用乙醇汽油工作提供理论参考。     

数据库设计中的几个问题

对于一个数据库应用系统,数据库设计的合理与否,会直接影响系统实际运行的性能.因此在数据库设计阶段,需要对数据类型、索引等进行合理的使用与设计.     

三洋XACTI VPC-C1数字相机

它更像是一款不用录像带的数字摄像机，手枪似的握把造型拿起来非常顺手。     

浅析Oracle数据库的性能优化

性能优化是数据库系统维护的重要组成部分。根据对Oracle数据库长期的监测与维护经验,提出数据库优化的若干方面的建议。