Steady-state error conditions for use in the design of look-ahead digital control systems

Steady-state error conditions relating to step and ramp inputs often are used in designing control systems. The purpose of this correspondence is to develop such steady-state error conditions as they apply to the design of a class of look-ahead or preview control systems in which future knowledge of system inputs is utilized.     

Finite-settling-time control of continuous-time plants

A design method of ripple-free finite-settling-time control of continuous-time plants is presented in this paper. The method is applicable to a vast kind of plants including nonminimum phase plants. It is shown that deadbeat tracking design in the discrete-time domain yields a ripple-free finite-settling-time control for a step input. The design method relies on frequency-domain pole-assignment techniques.     

On the relative dominance of paging algorithms

In this paper, we give a finer separation of several known paging algorithms using a new technique called relative interval analysis. This technique compares the fault rate of two paging algorithms across the entire range of inputs of a given size, rather than in the worst case alone. Using this technique, we characterize the relative performance of LRU and LRU-2, as well as LRU and FWF, among others. We also show that look-ahead is beneficial for a paging algorithm, a fact that is well known in practice but it was, until recently, not verified by theory.     

Output feedback performance recovery in the presence of uncertainties

We propose an output-feedback tracking controller for uncertain, nonaffine, nonlinear systems. The output feedback controller results in a closed-loop system with a three-time-scale structure; an extended high-gain observer estimates unmeasured states and uncertainties in the fastest time scale and dynamic inversion is used to deal with nonaffine inputs and input uncertainties in the intermediate time scale while the plant dynamics evolves in the slowest time scale. The dynamic inversion algorithm is based on sector conditions and results in exponential convergence of the inputs. Together with the extended high-gain observer, dynamic inversion results in performance recovery of a target system. The singular perturbation method is used to analyze the stability and performance of the system and numerical simulations are used to demonstrate the effectiveness of the control design.     

Application of the analytical hierarchy process for real-time scheduling and part routing in advanced manufacturing systems

This paper presents a framework that employs the analytical hierarchy process (AHP) in advanced manufacturing systems for real-time scheduling and part routing. The proposed multicriteria decision-making framework brings a new perspective to real-time scheduling and part routing decisions, by implementing pairwise comparison of possible future states of a manufacturing system. The framework includes an extended finite state machine and a scheduler model to facilitate dynamic, short-term decision making. The scheduler model, which is developed on the basis of control theory, uses AHP to assess possible future states in a limited look-ahead horizon by comparing the performance measures of each state. The multicriteria decision-making framework developed in this study is implemented in a simulation environment to validate it for real-time manufacturing system control and investigate its performance under a range of look-ahead horizons. Simulation results indicate that the proposed framework performs better for a mid-range horizon for most of the commonly used performance measures.     

On near optimal neural control of multiple-input nonlinear systems

It has been a common consensus that general techniques for stabilization of nonlinear systems are available only for some special classes of nonlinear systems. Control design for nonlinear systems with uncertain components is usually carried out on a per system basis, especially when physical control constraints, and certain control performance measures such as optimum time control are imposed. Elegant adaptive control techniques are difficult to apply to this type of problems. A new neural network based control design is proposed and presented in this paper to deal with a special class of uncertain nonlinear systems with multiple inputs. The desired system dynamics are analyzed and utilized in the process of the proposed intelligent control design. The theoretical results are provided to justify the design procedures. The simulation study is conducted on a second-order bilinear system with two inputs and uncertainties on its parameters. The simulation results indicate that the proposed design approach is effective.     

基于多Agent强化学习的多站点CSPS系统的协作Look-ahead控制

研究多站点传送带给料生产加工站(Conveyor-serviced production station, CSPS)系统的最优控制问题, 其优化目标是通过合理选择每个CSPS的Look-ahead控制策略, 实现整个系统的工件处理率最大.本文首先根据多Agent系统的反应扩散思想, 对每个Agent的原始性能函数进行改进, 引入了具有扩散功能的局域信息交互项(原始项看作具有反应功能); 并运用性能势理论, 构建一种适用于平均和折扣两种性能准则的Wolf-PHC多Agent学习算法, 以求解决策时刻不同步的多站点的协作Look-ahead控制策略. 最后,论文通过仿真实验验证了该算法的有效性,学习结果表明, 通过性能函数的改进,各工作站的负载平衡性得到改善, 整个系统的工件处理率也明显提高.     

Worst Case Power Generating Capabilities of Nonlinear Systems

In this paper we analyze the worst case power generating capabilities of a class of nonlinear systems which exhibit a power gain property. This class of systems includes systems which exhibit persistent excitation in the absence of inputs. Examples include limit cycle systems and chaotic systems. In order to capture the power generating capability of a nonlinear system, we define a worst case average cost per unit time performance index. This performance index, called the available power, is in effect the most power that can be generated by a system via the application of any input. The main result of the paper is that the input which achieves this worst case performance is typically a persistent input whose power is given explicitly by a function of the derivative of the available power with respect to the power gain of the system. A natural corollary of this result is that the available power may be recast as an optimization over power inputs. Date received: 24 February 2000. Date revised: 11 May 2001.     

Deadbeat sampled system direct synthesis

A direct synthesis method is presented for obtaining a discrete compensator to produce a finite-settling-time transient in response to a prescribed input with no intersample transient terms after the finite settling time. The method is applicable to a single constant-frequency sampler in a feedback system with any type of hold for any polynomial input. The method uses a fictitious sampler at the output with sample timeT/mto scan the intersample times and to obtain criteria for deadbeat transients. The criteria are derived from root locus considerations but are applied to Laplace transforms containing both functions ofsand functions ofzso that the only operations are analytic ones.     

Modelling and optimal controller design of networked control systems with multiple delays

In this paper we discuss the modelling and control of networked control systems (NCS) where sensors, actuators and controllers are distributed and interconnected by a common communication network. Multiple distributed communication delays as well as multiple inputs and multiple outputs (MIMO) are considered in the modelling algorithm. In addition, the asynchronous sampling mechanisms of distributed sensors are characterized to obtain the actual time delays between sensors and the controller. Due to the characteristics of a network architecture, piecewise constant plant inputs are assumed and discrete-time models of plant and controller dynamics are adopted to analyse the stability and performance of a closed-loop NCS. The analysis result is used to verify the stability and performance of an NCS without considering the impact of multiple time delays in the controller design. In addition, the proposed NCS model is used as a foundation for optimal controller design. The proposed control algorithm utilizes the information of delayed signals and improves the control performance of a control system encountering distributed communication delays. Several simulation studies are provided to verify the control performance of the proposed controller design.     

On look-ahead heuristics in disjunctive logic programming

Disjunctive logic programming (DLP), also called answer set programming (ASP), is a convenient programming paradigm which allows for solving problems in a simple and highly declarative way. The language of DLP is very expressive and able to represent even problems of high complexity (every problem in the complexity class ${{\Sigma}_{2}^{P}} = {\rm NP}^{{\rm NP}}$ ). During the last decade, efficient systems supporting DLP have become available. Virtually all of these systems internally rely on variants of the Davis–Putnam procedure (for deciding propositional satisfiability [SAT]), combined with a suitable model checker. The heuristic for the selection of the branching literal (i.e., the criterion determining the literal to be assumed true at a given stage of the computation) dramatically affects the performance of a DLP system. While heuristics for SAT have received a fair deal of research, only little work on heuristics for DLP has been done so far. In this paper, we design, implement, optimize, and experiment with a number of heuristics for DLP. We focus on different look-ahead heuristics, also called “dynamic heuristics” (the DLP equivalent of unit propagation [UP] heuristics for SAT). These are branching rules where the heuristic value of a literal Q depends on the result of taking Q true and computing its consequences. We motivate and formally define a number of look-ahead heuristics for DLP programs. Furthermore, since look-ahead heuristics are computationally expensive, we design two techniques for optimizing the burden of their computation. We implement all the proposed heuristics and optimization techniques in DLV—the state-of-the-art implementation of disjunctive logic programming, and we carry out experiments, thoroughly comparing the heuristics and optimization techniques on a large number of instances of well-known benchmark problems. The results of these experiments are very interesting, showing that the proposed techniques significantly improve the performance of the DLV system.     

Periodic stabilization of look-ahead filters in VLSI implementation

A linear look-ahead filter is a model of digital dynamical systems and infinite impulse response filters for fast pipeline processing in very large scale integration (VLSI) implementation of the digital systems. Two essential problems to be dealt with in the design of look-ahead filters are stability and computational complexity of the filter. In this paper, a new periodic scheme is proposed to stabilize the d-step look-ahead filter and provide minimum amount of computation in digital implementation of the filter.     

Look-ahead and look-behind shortcuts in large item category hierarchies: The impact on search performance

Websites use shortcuts to facilitate navigation of large hierarchies of item categories. Two common types of shortcuts used for this purpose are location breadcrumbs and down-to-child/up-to-parent links; frequently both are employed simultaneously. The combined used of these shortcuts provide proximal cues which enable the user to look-ahead and look-behind in the navigational structure. In this study, the impact of shortcut usage on search performance on a known-item search task is estimated. A controlled experiment was conducted using a realistic hypertext hierarchy of item categories. The results indicate that greater use of shortcuts decreases both time on task and lostness for the user, and that the decrease is associated with increased depth in the hierarchy. These findings provide insight into possible performance trade-offs involved in website designs that include look-ahead shortcuts for navigating large item category hierarchies.     

A performance criterion

A natural measure of performance is defined for control systems. The input to the system is known to the extent that it belongs to a known function space. Lack of performance is then defined as the least upper bound of the absolute error that occurs over all time and all inputs. The main problems of computing a value of the controller parameter that satisfies prescribed performance are considered and useful expressions are derived for such computations. In particular, a simple formula is derived for calculating performance. The paper discusses the implications of the new measure of performance in control theory and design.     

Design and analysis of a look-ahead scheduling scheme to support pause-resume for video-on-demand applications

In a video-on-demand (VOD) system, subscribers can choose both the movie they wish to view and the time they wish to view it. In such an environment there are invariably hot videos which are requested by many viewers. The requirement that each viewer be able to independently pause the video at any instant and later resume the viewing with little delay can cause difficulties in batching viewers for each showing. Under batching, a single video stream is shared by multiple concurrent viewers and a resume request has to wait for additional stream capacity to become available before actual resumption can occur. The conventional approach to the support of on-demand pause-resume provides one video access stream to disks for each video request. This can greatly increase the disk arm requirements of a VOD system. In this paper, we propose a more efficient mechanism to support the pause-resume feature usinglook-ahead scheduling withlook-aside buffering. The idea is to use buffering to increase the number of concurrent viewers supportable. The concept of look-ahead scheduling is not to back up each viewer with a real stream capacity so he can pause and resume at any time, but rather with a (look-ahead) stream that is currently being used for another showing which is close to completion. Before the look-ahead stream becomes available, the pause and resume features have to be supported by the original stream through (look-aside) buffering of the missed content. It is shown via simulations that the proposed scheme can provide a substantially greater throughput than the approach without batching. Furthermore, for a given amount of buffer, the improvement in throughput grows more than linearly with the stream capacity of the server. In other words, the look-ahead stream scheduling scheme operates with good economy of scale because it is easier to form look-ahead streams for video servers with larger stream capacity.     

Effects of a target movement direction cue on head-tracking performance

A review of the literature has shown that most investigations of head-tracking performance have used symmetrically shaped targets. This paper identifies a problem in using circular targets to represent the movement of complex targets (i.e. targets giving directional cues). Two experiments investigated the effects of a target direction cue on head-tracking performance. In the first experiment, practice did not improve performance when tracking either with or without a ‘look-ahead trace’ showing all target positions 160 ms into the future. A second experiment utilized a ‘look-ahead trace’ showing target positions with eight different lead-times (0-560 ms). With lead-times of 160 ms or more, significant improvements in tracking performance and subjective difficulty ratings were obtained. Tracking responses were also significantly affected. The results suggest caution when performance data obtained with a symmetrical target are generalized to predict tracking performance with a real target giving cues to the direction of movement. The look-ahead trace offers a systematic means of bridging the gap between a symmetrical target and a real target having direction of movement cues.     

Quantitative feedback theory for multiple-input-multiple output feedback systems with control input failures†

In quantitative feedback theory (QFT), plant parameter and disturbance uncertainties are the reasons for using feedback. The system design is tuned to quantitative statements of these parameters and of the performance tolerances. Available design freedom is used to minimize the cost of feedback which is in the bandwidths of the loop transfer functions. This paper extends QFT to 2 × 2 linear time invariant (LTI) multiple-input-multiple-output plants, in which total failure of some control inputs is possible. Maximum possible achievement of the performance specifications is determined, with single fixed LTI compensation networks. A detailed design example is included.     

基于多目标约束的鲁棒故障检测滤波器设计

针对包含未知输入的线性时不变系统,研究了其鲁棒故障检测滤波器设计问题。由于故障信号往往分布在有限频域段内,设计的目标包括使特定有限频域段上的复合性能指标最小化以及满足区域极点配置的要求。一个基于LMI的方法被提出用于解决该设计问题。该方法的优点在于求解过程中可以获取给定有限频域段上的频域指标的真实值,并可求得满足目标的最优解。因此,设计的故障检测滤波器可以获得良好的故障检测性能。一个基于某国产歼击机的设计实例验证了该方法的有效性。     

智能交通系统车道保持纵横向耦合控制

考虑车辆纵横向运动之间的相互影响,采用位置预瞄和固定车辆间距跟随策略,对基于一列车队的自动化公路系统车道保持纵横向耦合控制进行了研究.利用车载前后双位置传感器检测车辆位置偏差,基于车辆纵横向动力学耦合模型,推导了基于预瞄的车道保持控制系统数学模型;采用非奇异的终端滑模控制技术,设计了车道保持纵横向耦合控制规律.通过构造李雅普诺夫函数,结合相平面方法,分析了控制系统的有限时间收敛性.采用6车辆编队,通过计算机仿真,对文中设计的控制规律进行了验证.仿真结果显示,车队中每个被控车辆在纵向上跟随期望状态的同时能够实现对期望车道轨迹的理想跟踪,跟踪误差精度不超过0.05 m.     

TLA: Temporal look-ahead processor allocation method for heterogeneous multi-cluster systems

In a heterogeneous multi-cluster (HMC) system, processor allocation is responsible for choosing available processors among clusters for job execution. Traditionally, processor allocation in HMC considers only resource fragmentation or processor heterogeneity, which leads to heuristics such as Best-Fit (BF) and Fastest-First (FF). However, those heuristics only favor certain types of workloads and cannot be changed adaptively. In this paper, a temporal look-ahead (TLA) method is proposed, which uses an allocation simulation process to guide the decision of processor allocation. Thus, the allocation decision is made dynamically according to the current workload and system configurations. We evaluate the performance of TLA by simulations, with different workloads and system configurations, in terms of average turnaround time. Simulation results indicate that, with precise runtime information, TLA outperforms traditional processor allocation methods and has up to an 87% performance improvement.