In data stream management systems (DSMSs), how to maintain the quality of queries is a difficult problem because both the processing cost and data arrival rates are highly unpredictable. When the system is overloaded,...In data stream management systems (DSMSs), how to maintain the quality of queries is a difficult problem because both the processing cost and data arrival rates are highly unpredictable. When the system is overloaded, quality degrades significantly and thus load shedding becomes necessary. Unlike processing overloading in the general way which is only by a feedback control (FB) loop to obtain a good and stable performance over data streams, a feedback plus feed-forward control (FFC) strategy is introduced in DSMSs, which have a good quality of service (QoS) in the aspects of miss ratio and processing delay. In this paper, a quality adaptation framework is proposed, in which the control-theory-based techniques are leveraged to adjust the application behavior with the considerations of the current system status. Compared to previous solutions, the FFC strategy achieves a good quality with a waste of fewer resources.展开更多
Load of an automatic feed mechanism is composed of the stretching force of feed belt at the entrance to lower flexible guidance and the friction force between feed belt and flexible guidance. A mathematical model for ...Load of an automatic feed mechanism is composed of the stretching force of feed belt at the entrance to lower flexible guidance and the friction force between feed belt and flexible guidance. A mathematical model for computing the load was presented. An optimization problem was formulated to determine the attitude of the flexible guidance based on the principle that the potential energy stored in the system was the minimum at the equilibrium. Then the friction force was obtained according to the attitude of guide leaves and the moving velocity of the feed belt and the friction factor. Consequently, the load of the automatic feed mechanism can be calculated. Finally, an example was given to compute the load when the horizontal and elevating firing angles of the automation were respectively 45° and 30°. The computing result can be a criterion to determine the designing parameters of automat.展开更多
When boost power factor correction(PFC) circuit works with large scale load fluctuations, it is easy to cause a higher total harmonic distortion and a lower power factor because of traditional controllers and inductor...When boost power factor correction(PFC) circuit works with large scale load fluctuations, it is easy to cause a higher total harmonic distortion and a lower power factor because of traditional controllers and inductor current mode. To solve this problem, this paper proposes a PFC control system, which can operate with load fluctuations up to 1 000 W by using duty cycle feed-forward control theory to achieve smooth switching mode. The duty cycles in the next period of the control system are pre-estimated in the current cycle, which enhances the speeds of AD samplers and switching frequency, and reduces the cost and volume of the equipment to some extent. Introductions of system decoupling and feed-forward of input-voltage greatly improve the system performance. Both theoretical simulation and experimental results prove the advantage of the proposed scheme.展开更多
<div style="text-align:justify;"> A novel method for tri-band microstrip array antenna with improvement in the bandwidth by incorporating wide slots and additional resonators which is gap-coupled to th...<div style="text-align:justify;"> A novel method for tri-band microstrip array antenna with improvement in the bandwidth by incorporating wide slots and additional resonators which is gap-coupled to the non-radiating edges of a radiating element is presented. The experimental results show that two element slot-loaded gap-coupled microstrip array antenna gives a ?10 dB return loss band-width for three bands. The design specifications, radiation patterns and gain of the proposed antennas are presented and described. </div>展开更多
In this communication the numerical modelling for input impedance of an annular ring antenna is done. The cavity model is used for the analysis and the post is loaded asymmetrically at a point from the centre of the r...In this communication the numerical modelling for input impedance of an annular ring antenna is done. The cavity model is used for the analysis and the post is loaded asymmetrically at a point from the centre of the ring. The numeri-cal model formed is verified with the results of IE3D model based on method of moments. The loading of antenna can be used to design broadband antennas by enhancing the bandwidth of antennas.展开更多
针对双PWM变频器整流侧与逆变侧独立控制时,直流侧必须利用大电容稳压,导致系统成本增加且寿命缩短的问题,提出一种整流侧采用引入新型开关矢量表的直接功率控制(Direct Power Control,DPC)方案。该方案逆变侧采用转子磁场定向矢量控制...针对双PWM变频器整流侧与逆变侧独立控制时,直流侧必须利用大电容稳压,导致系统成本增加且寿命缩短的问题,提出一种整流侧采用引入新型开关矢量表的直接功率控制(Direct Power Control,DPC)方案。该方案逆变侧采用转子磁场定向矢量控制,并采用负载功率前馈控制策略,以实现双PWM变频器的协调控制。仿真实验结果表明,相比独立控制的双PWM变频调速系统,采用该一体化协调控制策略的双PWM变频调速系统,不仅能减小网侧电流的谐波,还可较好地抑制负载突变时直流电压的波动,加快整流侧和逆变侧的动态响应,大大提高系统的抗扰能力,从而减小电容体积并降低成本。展开更多
基金Supported by the National Key R&D Program of China(2016YFC1401900)the National Science Foundation of China(61173029,61672144)
文摘In data stream management systems (DSMSs), how to maintain the quality of queries is a difficult problem because both the processing cost and data arrival rates are highly unpredictable. When the system is overloaded, quality degrades significantly and thus load shedding becomes necessary. Unlike processing overloading in the general way which is only by a feedback control (FB) loop to obtain a good and stable performance over data streams, a feedback plus feed-forward control (FFC) strategy is introduced in DSMSs, which have a good quality of service (QoS) in the aspects of miss ratio and processing delay. In this paper, a quality adaptation framework is proposed, in which the control-theory-based techniques are leveraged to adjust the application behavior with the considerations of the current system status. Compared to previous solutions, the FFC strategy achieves a good quality with a waste of fewer resources.
基金Project supported by the Seventh Research Institute of China State Shipbuilding Corporation
文摘Load of an automatic feed mechanism is composed of the stretching force of feed belt at the entrance to lower flexible guidance and the friction force between feed belt and flexible guidance. A mathematical model for computing the load was presented. An optimization problem was formulated to determine the attitude of the flexible guidance based on the principle that the potential energy stored in the system was the minimum at the equilibrium. Then the friction force was obtained according to the attitude of guide leaves and the moving velocity of the feed belt and the friction factor. Consequently, the load of the automatic feed mechanism can be calculated. Finally, an example was given to compute the load when the horizontal and elevating firing angles of the automation were respectively 45° and 30°. The computing result can be a criterion to determine the designing parameters of automat.
基金Supported by the National Basic Research Program of China("973"Program,No.2009CB219700)
文摘When boost power factor correction(PFC) circuit works with large scale load fluctuations, it is easy to cause a higher total harmonic distortion and a lower power factor because of traditional controllers and inductor current mode. To solve this problem, this paper proposes a PFC control system, which can operate with load fluctuations up to 1 000 W by using duty cycle feed-forward control theory to achieve smooth switching mode. The duty cycles in the next period of the control system are pre-estimated in the current cycle, which enhances the speeds of AD samplers and switching frequency, and reduces the cost and volume of the equipment to some extent. Introductions of system decoupling and feed-forward of input-voltage greatly improve the system performance. Both theoretical simulation and experimental results prove the advantage of the proposed scheme.
文摘<div style="text-align:justify;"> A novel method for tri-band microstrip array antenna with improvement in the bandwidth by incorporating wide slots and additional resonators which is gap-coupled to the non-radiating edges of a radiating element is presented. The experimental results show that two element slot-loaded gap-coupled microstrip array antenna gives a ?10 dB return loss band-width for three bands. The design specifications, radiation patterns and gain of the proposed antennas are presented and described. </div>
文摘In this communication the numerical modelling for input impedance of an annular ring antenna is done. The cavity model is used for the analysis and the post is loaded asymmetrically at a point from the centre of the ring. The numeri-cal model formed is verified with the results of IE3D model based on method of moments. The loading of antenna can be used to design broadband antennas by enhancing the bandwidth of antennas.
文摘针对双PWM变频器整流侧与逆变侧独立控制时,直流侧必须利用大电容稳压,导致系统成本增加且寿命缩短的问题,提出一种整流侧采用引入新型开关矢量表的直接功率控制(Direct Power Control,DPC)方案。该方案逆变侧采用转子磁场定向矢量控制,并采用负载功率前馈控制策略,以实现双PWM变频器的协调控制。仿真实验结果表明,相比独立控制的双PWM变频调速系统,采用该一体化协调控制策略的双PWM变频调速系统,不仅能减小网侧电流的谐波,还可较好地抑制负载突变时直流电压的波动,加快整流侧和逆变侧的动态响应,大大提高系统的抗扰能力,从而减小电容体积并降低成本。
