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信號時滯對NMPC路徑跟蹤系統的影響機理與消減方法

Influence mechanism and elimination method of signal time delay on NMPC-based path tracking systems

  • 摘要: 目前已有一些針對路徑跟蹤控制中信號時滯問題的研究工作,但這些工作大多針對某種特定的控制方法,而在路徑跟蹤控制方法中,非線性模型預測控制(Nonlinear model predictive control, NMPC)具有能夠顯式處理系統約束、便于實現多目標優化、能夠有效利用被控對象前方參考路徑信息等優勢,但是針對NMPC路徑跟蹤控制系統中時滯問題的研究較不成熟,制約了這種控制方法的實際應用。為解決上述問題,開展了以下研究工作。首先構建了能夠較好地孤立出時滯影響的類車機器人路徑跟蹤控制系統。接著分析了信號時滯對NMPC路徑跟蹤控制系統的影響機理,即時滯會導致控制器產生的控制信號不能適應類車機器人在執行控制信號時所處的位置。然后提出了基于增長NMPC預測時域的時滯影響消減方法,即在迭代周期不變的情況下,在無時滯系統較優預測步數的基礎上增加二倍時滯周期比以上的整數。最后通過計算機仿真和實驗驗證了提出方法的有效性。仿真和實驗結果表明,信號時滯對NMPC路徑跟蹤控制系統存在影響,未考慮時滯的NMPC控制算法能夠在無時滯系統中實現高精確性路徑跟蹤,而在有時滯系統中控制失效。通過增長預測時域可以有效消減信號時滯的影響,在信號時滯為0.2s以上的仿真與實驗系統中,基于該方法的NMPC控制器可以保證路徑跟蹤控制的位移誤差幅值不超過0.1002m,航向誤差幅值不超過0.0291rad。

     

    Abstract: There have been some research works on the signal time delay (STD) problem in path tracking control, but most of these works are aimed at a specific control method. Among the path tracking control methods, nonlinear model predictive control (NMPC) has the advantage of dealing with the system's constraints explicitly, realizing multi-objective optimization easily, and effectively utilizing the reference path information in front of the controlled object, etc. However, the research on the STD problem in the path tracking control system based on NMPC is relatively immature, which restricts the practical application of this control method. The following research has been carried out to address the above issues. First, a path tracking control system for car-like robots that can better isolate the influence of STD is constructed. Next, the mechanism of STD on the NMPC path tracking control system is analyzed, i.e., the STD causes the position of the car-like robot to be used for generating the control signals by the controller to not correspond to the position of the car-like robot at the time of execution of those control signals. Then, an STD influence abatement method based on growing the NMPC prediction horizon is proposed, i.e., with the iteration period unchanged, the number of the better prediction steps of the STD-free system is added by an integer number above the ratio of the STD and period by a factor of two. Finally, the effectiveness of the proposed method is verified by computer simulation and experiment. The simulation and experiment results show that the STD affects the NMPC path tracking control system, and the NMPC control algorithm that does not consider STD can achieve highly accurate path tracking in STD-free systems, whereas control failure in STD systems. The influence of the STD can be effectively reduced by increasing the prediction horizon. The NMPC controller based on this method can ensure that the displacement error amplitude of the path tracking control is not more than 0.1002m and the heading error amplitude is not more than 0.0291rad in the simulation and experiment systems where the STD is not smaller than 0.2s.

     

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