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
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
西北电网联络线调度下AGC指令复杂多变对火电机组稳定运行不利,为了改善火电机组协调控制前馈系统在AGC指令频繁小幅度波动时造成锅炉燃料量大幅度摆动的问题,研究了一种可变滤波时间的非线性滤波器;依据AGC指令变化幅度设计一种变滤波...西北电网联络线调度下AGC指令复杂多变对火电机组稳定运行不利,为了改善火电机组协调控制前馈系统在AGC指令频繁小幅度波动时造成锅炉燃料量大幅度摆动的问题,研究了一种可变滤波时间的非线性滤波器;依据AGC指令变化幅度设计一种变滤波时间的非线性滤波器,对其动态微分部分输出信号进行增益修正和滤波处理;采集西北某电厂660 MW 24 h AGC指令进行算例分析,通过对比改进前后的燃料量指令证明该非线性滤波器可以实现:AGC指令大幅度变化时,前馈足额快速输出,以维持机前压力稳定;AGC指令小幅度频繁波动时,减小前馈输出幅值并增加滤波时间以减少燃料量波动。通过对比改进前后的燃料量指令,证明了可变滤波时间系数的滤波器具有应用价值。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.
文摘西北电网联络线调度下AGC指令复杂多变对火电机组稳定运行不利,为了改善火电机组协调控制前馈系统在AGC指令频繁小幅度波动时造成锅炉燃料量大幅度摆动的问题,研究了一种可变滤波时间的非线性滤波器;依据AGC指令变化幅度设计一种变滤波时间的非线性滤波器,对其动态微分部分输出信号进行增益修正和滤波处理;采集西北某电厂660 MW 24 h AGC指令进行算例分析,通过对比改进前后的燃料量指令证明该非线性滤波器可以实现:AGC指令大幅度变化时,前馈足额快速输出,以维持机前压力稳定;AGC指令小幅度频繁波动时,减小前馈输出幅值并增加滤波时间以减少燃料量波动。通过对比改进前后的燃料量指令,证明了可变滤波时间系数的滤波器具有应用价值。
